Full moth rehabilitation/certified fixed orthodontic courses by Indian dental academy

959 views
837 views

Published on

The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide range of dental certified courses in different formats.


Indian dental academy provides dental crown & Bridge,rotary endodontics,fixed orthodontics,
Dental implants courses.for details pls visit www.indiandentalacademy.com ,or call
0091-9248678078

Published in: Education
0 Comments
2 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
959
On SlideShare
0
From Embeds
0
Number of Embeds
20
Actions
Shares
0
Downloads
57
Comments
0
Likes
2
Embeds 0
No embeds

No notes for slide

Full moth rehabilitation/certified fixed orthodontic courses by Indian dental academy

  1. 1. CONTENTS 1. INTRODUCTION 2. INDICATIONS FOR OCCLUSAL REHABILITATION 3. GOALS OF FULL MOUTH REHABILITATION 4. ANATOMY AND PHYSIOLOGY OF MASTICATORY MECHANISM 5. THE HINGE AXIS 6. CENTRIC RELATION 7. VERTICAL DIMENSION 8. FUNCTIONAL ASPECTS OF COMPLETE MOUTH REHABILITATION 9. DIAGNOSIS AND TREATMENT PLANNING 10. PREPARATION OF THE MOUTH FOR REHABILITATION ll.PMS PHILOSOPHY 12. SELECTING INSTRUMENTS FOR OCCLUSAL REHABILITTION 13. MOUNTING MODELS 14. FUNCTION AND IMPORTANCE OF ANTERIOR GUIDANCE 15. PRINCIPLES OF OBTAINING OCCLUSSION IN OCCLUSAL REHABILITATION 16. RATIONALE AND TECHNIQUE OF BIO MECHANICAL OCCLUSAL REHABILITATION 1.7. RESTORING LOWER ANTERIOR TEETH 1
  2. 2. 18. RESTORING UPPER ANTERIOR TEETH 19. THH PLANE OF OCCLUSION 20. POSTERIOR OCCLUSAL MORPHOLOGY 2!. RESTORING LOWER POSTERIOR TEETH 22.WAXING TECHNIQUE FOR LOWER POSTERIOR TEETH 23. RESTORING UPPER POSTERIOR TEETH 24. FUNCTIONALLY GENERATED PATH 25. PROCEDURAL STEPS IN RESTORING OCCLUSION 26. REVIEW OF LITERATURE 27. SUMMARY 2
  3. 3. INTRODUCTION Planning and executing the restorative rehabilitation of a decimated occlusion is probably one of the most intellectually and technically demanding tasks facing a restorative dentist. The term "occlusal rehabilitation has been defined as the restoration of the functional integrity of the dental arches by the use of inlays, crowns, bridges and partial dentures". Occlusal rehabilitation therefore involves restoring the dentate or a partially dentate mouth. The aim is to provide an orderly pattern of occlusal contact and articulation that will optimize oral function, occlusal stability and esthetics. Occlusal adjustment by grinding may be required, as part of the rehabilitation but does not constitute rehabilitation per se. Occlusal rehabilitation is discussed in the context of cases where restorations are supported by natural teeth and doesn't include the restoration of the fully edentulous arch or maxillofacial defects, nor does it include the use of osseointegrated implants. Definition: Full mouth rehabilitation entails the performance of all the procedures necessary to produce healthy, esthetic, well functioning, and selfmaintaining masticatory mechanism. 3
  4. 4. INDICATIONS FOR OCCLUSAL REHABILITATION The reasons for undertaking occlusal rehabilitation may include the restoration of multiple teeth, which are missing, worn, broken-down or decayed. Increasingly occlusal rehabilitation is also required to replace improperly designed and executed crown and bridge work. In certain circumstances treatment of temporomandibular disorders may also be considered an indication for rehabilitation, but great caution is advisable in such cases. Regardless of the clinical reason, the decision to carryout any treatment should be based upon achieving oral health, function, esthetics and comfort, and treatment should be planned around these rather than the technical possibilities. If these goals are to be achieved certain biological considerations are necessary when planning and carringout occlusal rehabilitation. They are 1. The indications for reorganizing the occlusion 2. The choice of an appropriate occlusal scheme 3. The occlusal vertical dimension 4. The need (or otherwise) to replace missing teeth 5. The effects of the material used on occlusal stability control of parafunction and TMD The indications for reorganizing occlusion: When undertaking relatively small amounts of restorative treatment, for example up to two or three units of crown and bridge work, it is often acceptable, and it is often advisable to adopt a confirmative approach that is to construct the restoration to conform with the patient's existing intercuspal position. 4
  5. 5. The alternative strategy is to reorganize the occlusion by establishing a new occlusal scheme around a stable condylar position. The condylar position usually chosen is termed "centric relation' (CR). The decision to re organize a patient's occlusion may be made on the grounds either that the existing IP is unacceptable and needs to be changed, or where a very large amount of treatment is to be undertaken and the operator has the opportunity to optimize patient's occlusion. The decision should (and can) only be made after a detailed and careful examination of the occlusion, preferably with the use of accurate study casts mounted-in a semi adjustable articulator in the retruded arc of closure. Mounted casts should allow the discrepancy (slide) between CR and IP to be analysed as vertical, horizontal and lateral components both at tooth and condylar level. Moreover, adjustments can be tried and potential restorations waxed allowing the feasibility and difficulty of reorganization to be judged properly. It must be borne in mind that jaw movement will be simulated only partially by any type of articulator. Nevertheless, the semi adjustable articulators are an invaluable supplement to diagnosis and can save time with occlusa! adjustments when restorations are fitted. Reorganization maybe considered when the existing IP is considered unsatisfactory for any of the following reasons: Repeated fracture or failure of teeth or restorations: Clinical experience suggests that persistently failing restorations (for example crown and bridge debonding) are very commonly attributed to unfavorable occlusal loading which may be improved by reorganization. Bruxism: An optimally constructed occlusion will better be able to deal with the forces generated in parafunction. 5
  6. 6. Lack of interoeclusal space for restoration: Reorganising the occlusion to eliminate a large horizontal component of slide between CR and IP can create a valuable interoeclusal space for the restoration of worn.anterior teeth. Alternatively, the occlusion may be reorganised at an increased vertical dimension necessitating occlusal coverage for at least one arch. Trauma from occlusion: This may be soft tissue trauma (due to teeth impinging on the cheek or alveolar ridge) or periodonlal trauma (due to excessive or aberrantly directed occlusal forces) the latter may have an accelerating effect on periodontal disease although the evidence is conflicting. Reorganisation of the occlusion to direct forces axially and eliminate interferences and premature contacts can reduce tooth mobility. However, the overall gain in periodontal attachment is marginal and should be considered as no more than adjunct to periodontal management. Unacceptable function: Poor tooth to tooth contact with tilting and overeruption of teeth may create problems with masticatory function, particularly when large number of teeth have been lost. Unacceptable esthetics: Alteration in the clinical crown height may be necessary to improve esthetics, and this may be made possible by constructing the restorations to a reorganised occlusion, possibly at an increased vertical dimension. The presence of TMD: The link between the occlusion and TMD is controversial. 6
  7. 7. Reasons for Full Mouth Rehabilitation: The most common reason for doing full mouth rehabilitation is to obtain and maintain the health of periodontal tissues. Clinical periodontal findings are correlated with radiographs to determine the extent and character of any disease findings must then be correlated to function of the mouth in examining the function of the mouth, many factors must be considered. The most important factor is discrepancy known as the "premature contact" a contact between an upper and lower tooth that prevents or interferes with the normal path of closure of mandible. The area that receives the force of closure after the patient "skids" off the prematurity may relate to a greater degree of disease. This area receives the force in the form of a rebound as the normal path of closure is interfered with by the premature contact. The various excursions of the mandible must be examined to determine how much harmony exists between the jaw movement and the tooth contacts, and the teeth that receive most of the load in the different positions should be noted. A definite correlation between the malfunction and the clinical periodontal findings is usually possible. This correlation generally precludes all other potential causes of the pcriodontal condition. Even so, in order to attack the problem from every conceivable angle, we must consider and investigate each of the other factors. In conjunction with malfunction, we must consider oral habits that could have a bearing on the condition present. These may include such things as bruxism. lip-chewing, thread-biting, tongue habits, and soon. Temporomandibular joint disturbance is another reason for full mouth rehabilitation. This may be difficult to diagnose, and great care must be taken to determine the etiological factors involved. Frequently, there is a poor 7
  8. 8. relationship between the articulation and the movements of the joint -a disharmony of function. Sometimes there is a muscular dysfunction that is caused by some irritant or a nervous affliction. This muscular dysfunction may be caused by the poor articulation, which produces muscle spasms, and these in turn may be interpreted as a joint disturbance. In joint cases, the periodontal condition is usually very good, which is probably why the joint has been injured instead of the periodontium. When a disharmony exists, we must ascertain whether the patient is injuring his joint as a result of malfunction, a bad habit, or an emotional disturbance. Emotional disturbances are very difficult to deal with and often require the assistance of a competent psychiatrist. Still another reason for full mouth rehabilitation is the need for extensive dentistry. In such cases, some teeth are missing, others are worn down, and there are old fillings that need replacing. Usually, the patients have little periodontal involvement and no joint symptoms. These are the easiest cases to treat, and the beginner should limit his or her full mouth rehabilitation to them. As long as extensive dentistry is necessary, why not work on the case as a whole so that ail the parts will be related to each other and to the function of the individual? By far, the most difficult patients to treat are the few who have succeeded in developing a severe periodontal condition as well as a malfunctioning joint. Even though the joint may be asymptomatic, it may exhibit a behavior pattern that is troublesome to deal with. It may be mobile, and the dentistry may require frequent adjustment as the joint begins to function properly with possible heaiing. In addition, tooth settling and migration will increase the discrepancies. These are the cases that try men's souls, they require one to proceed with extreme care and to anticipate the possible contingencies. 8
  9. 9. Sometimes it is impossible to predict the extent of treatment necessary in order to resolve the condition. The treatment of the emotionally disturbed patient with a joint probiem is probably the most exasperating. Although it is possible to treat the condition physiologically, the emotional disturbance is another problem. Patients with beautifully functioning masticatory mechanisms may still be able to produce joint symptoms almost at will. It is unpleasant to have to suggest psychiatric treatment, but if the dentist is convinced that this is the problem, then he owes it to the patient and to himself to recommend such a course. Which Patients Should Not Be Treated by Full Mouth Rehabilitation? Frequently, friends and relatives of one's rehabilitation patients will request similar treatment. There are many malfunctioning months that do not need extensive dentistry and have no joint symptoms. These cases are best left alone. Some mouths that have the potential to break themselves down, never actually produce the destruction, for some reason. We are not justified in prescribing a full mouth rehabilitation unless there is definite evidence of tissue breakdown. One may argue, as many have, that it should be undertaken as a preventive measure. But there are many malfunctioning mouths that do not break down, proving that we cannot predict such things. If there is need for extensive dentistry, then by all means it should be carefully correlated to the rest of the mouth by complete rehabilitation. Some times one or two "good" teeth may have to be operated on in order to satisfactorily accomplish our objective. Ideally, dental procedures should be directed toward the prevention of such conditions: in short, no pathology -no treatment. 9
  10. 10. THE GOAL OF FULL MOUTH REHABILITATION History The modern practice of renewing and reorganizing (he teeth by prostheses began with the idea of'raising the bite" to rectify closure resulting from excessive wear of the occlusal surfaces. Later, such closure was associated with hearing loss, noted by Costen. This view, though later questioned, served to stimulate interest increasing the length of the patients own teeth and thus in increasing the vertical dimension. In correcting articular disturbances, the best procedure came to be the retention of the remaining natural teeth in so far as this was possible. To accomplish this, these teeth were rebuilt to harmonize with the movements of the joints in order to protect them from further injury. With our present understanding of traumatic occlusion and its deleterious effect upon the supporting structures, the procedure known as "bite raising" has shifted in emphasis and broadened in scope and is now designated by a term that describes it accurately. Full mouth reconstruction, jt now includes therapy which will, by improving the relationship of the teeth, improve the condition and health of the supporting structures. When the teeth have been realigned through full mouth reconstruction, the general tone of the supporting tissues invariably improves. What factors account for this improvement? Obviously, the removal of excessive lateral forces and the elimination of plunger cusps and similar forces attendant upon the realignment of full mouth reconstruction lessen continuous injury to the supporting structure. But these factors, though helpful in improving the condition of these structures, are less important than the increased stimulation of and circulation in the tissues that are brought about by the improved function. 10
  11. 11. The masticating apparatus that is normal, healthy, and functioning is able not only to carry out the work for which it is designed, but also to maintain itself in health. The various structures involved, through their form and arrangement, provide for both the synchronization of, and mutual protection against, all forces. When function is good, a generous blood circulation furnishes the tissue with the elements needed to keep them in a healthy condition. When function is disturbed by malocclusion, the relation between the mutually protective parts of the masticating apparatus is disrupted; moreover, because of lessened use, blood circulation is diminished. As indicated by O'Rourke, the force of a persons masticatory muscles remains fairly constant. It is the use of the force that changes under conditions of traumatic occlusion. The patient's ability or willingness to use his muscular force is dependent upon the comfort, or absence of pain, he experiences each time he brings his jaws together. Mutilated mouths with chronically inflamed supporting structures, due to traumatic occlusion, will support very little force without producing some discomfort. The result is continuous subnormal use of, or at best failure to make vigorous use of, the teeth and jaws. The vascular tissues of the periodontium can be stimulated only by the teeth in function. Such stimulation is lacking when this function is impaired by the inability of the patient to use the musculature in chewing because of the tenderness of these tissues. The results, in the words of Merritt, "are" atrophy of the alveolar process, malocclusion of the teeth, dental caries, impacted and missing teeth, periodontal lesions, and so on. Unfortunately, subnormal function lower vitality at the same time that it increases susceptibility to disease. Patients who have had full mouth rehabilitation commonly say that their mouths feel "stronger". The masticatory muscles have obviously not been 11
  12. 12. strengthened by therapy. What has happened is that out patients can exert greater force with comfort and without anticipation of pain than they could before and that therefore they do exert greater force. The therapeutic benefit of improved tooth arrangements and improved functioning have been indicated. The individual patient's reaction bears witness to these benefits and should inspire us, in terms of human satisfaction as well as of scientific progress, to strive continuously for improvement in the techniques of full mouth rehabilitation. It should be kept in mind that although the operations of all mouth rehabilitation procedures are performed on tooth units, they have one basic objective: the equalization of the forces directed against the supporting structures. Any disharmony at the occlusal or incisal aspects of a tooth will direct forces against these malaligned surfaces and thus subject the supporting structure to traumatic injuries. Similarly, any impairment of buccal or lingual harmony will be reflected in injury to the gingival tissue and subsequently to the deeper tissues involved in supporting the tooth. The proximal contact anatomy is also vital in maintaining the health of the underlying soft tissue. Poor contact relationships encourage food impaction with resultant periodonlal tissue loss. 12
  13. 13. ANATOMY AND PHYSIOLOGY OF THE MASTICATORY MECHANISM An understanding of the anatomy and physiology of the masticatory mechanism is essential to intelligent diagnosis and adequate treatment. If we know, how the normal masticating mechanism functions, we will be able to recognize its malfunction and be, in a position to correct it. A correlation of the anatomy of the parts and the function of the parts will help one understand the intricacies of the mechanism. The Osseous Structures: The masticating mechanism is primarily made up of three osseous structures: the temporal bones, the maxillae, and the mandible. The maxillae and the body of the mandible house the teeth -the instruments of mastication. The temporal bone and the condyle portion of the mandible form the contact or articulation between the osseous structures of the mechanism. In addition, the muscles that activate, the chewing mechanisms obtain their anchorage from and are attached to these osseous structures. Continuous contact is made between the mandibles and the temporal bones by means of the temporomandibular joints. The glenoid fossa of the tempoial bone is concave antero-posteriorly as well as mediolaterally. It is the shape of the anterior slope of the-fossa that determines the condyle paths (lateral, protrusive, and lateral-protrusive). The head of the condyfe is oval in shape, with its long axis at an oblique angle to the median axis of the skull. The synovial membranes and the meniscus are inter-posed between the fossa and the condyle(fig-l). In function, the head of the condyle rotates on the undersurface of the meniscus. It is in this compartment of the joint that the hinge-like action of the joint takes place. The hinge-like action is the center of action of the parts: the disc, the condyle head, and the synovial membranes. The upper surface of the 13
  14. 14. disc makes contact with the articular eminence of the glenoid fossa. It is the simultaneous sliding of the disc and condyle that produces the translatory movement of the temporomandibular joint. In other words, the head of the condyle rotates on the undersurface of the disc the condyle and the disc together translate in the fossa (anteroposteriorly, mediolaterally, or inbetween). The meniscus, or disc, consists of fibrocartilage, which is oval in shape and thinner at the center than at the circumference. "The inferior surface is concave and Fits on to the condyle of the lower jaw; while its superior surface is concavoconvex from before backward, and is in contact with the articular surface of the temporal bone" (Morris. 1933). The stress-bearing character of the disc is evident in the fact that ihe blood and nerve supply is in the periphery, the center being devoid of these tissues. Contact between the head of the condyle and the articular eminence is made by the cenier of the disc. The stress of mastication in the joint is absorbed in this relation. The meniscus has its bearing against the articular surface of the temporal bone(fig-2), which forms the anterior wall (articular eminence) of the glenoid fossa. It is in this relationship that the forces of mastication are absorbed by the temporomandibular joint. Lubrication of the joint is accomplished by the synovial membranes. Each compartment of the joint has its own synovial sac. The disc and the synovial membranes are neither compressible nor variable in the normal course of events, but serve as ball bearings between the skull and condyle. Function of Ligaments and Muscles: 14
  15. 15. Ligaments and muscles hold the temporomandibular joint and the chewing mechanism together. The ligaments limit the amount of movement of the mechanism and prevent the mechanism and joint from falling apart when the muscles are relaxed. They help to determine the position to the mandible when the muscles relax: thus, to some extent, physiological rest is govcrneu by these ligaments. The most important ligaments of the temporomandibular joint are the capsular and the temporomandibular. The temporomandibular ligament forms the lateral part of the capsule and reinforces it. The upper part of the ligament is broad and is attached to the zygoma and to the tubercle (articular eminence of the zygoma). It is inclined downward and backward and is inserted into the condyle and neck of the mandible laterally(fig-3). The fibers coming from the tubercle are short and nearly vertical. Together, the capsular and temporomandibular ligaments enclose the structures of the joint and tend to limit its1 movements. Sphenomandibular and stylomandibular ligaments are the two accessory ligaments that protect the joint during wide excursions (fig-4). These ligaments are loosely attached in the upper compartment of the joint to permit translator;' movements. They are more firmly ' attached in the lower compartment where the hinge-like action takes place. The temporomandibular joints are movable fulcrums activated by the muscles of mastication. These joints have some of the elements of a ball and socket. They glide forward and backward as well as sidewise. Actually, they can glide and rotate at the same time in the manner of a movable ball and socket. Muscles of mastication: In a discussion of the muscles of mastication, there is a tendency to speak of individual muscles and describe I heir separate actions, but muscles function in groups as kinematic chains. The Temporalis Muscle: 15
  16. 16. The temporal is muscle is a large, strong muscle of mastication. It has its origin in the temporal fossa on the side of the skull. Its origin covers a considerable area and. by means of an aponeurosis, it connects with its mate on the other side of the skull very much in the manner of a saddlebag. Its insertion is in the coronoid process of the mandible and reaches down to the ramus of the mandible should be noted that the insertion is anterior to the temporomandibular joint. Although the fibers of the temporal is muscle are described as vertical, oblique, and horizontal, contraction of any or all of these fibers has a definite tendency to elevate and relrude (he mandible. This is understandable if we recall that the temporomandibular joint is made up of the glenoid fossa, the anterior surface of which slopes upward and backward, and that the meniscus is interposed between the head of the condyle and this slope. Any contraction of a muscle attached in front of this upward slanting guide must have a tendency to brace the condyle head in a posterior and superior position(fig-5). The Masseter Muscle: The masseter muscle has its origin in the zygomatic arch. It arises in two heads: a superficial one from the outer border of the arch, and a deeper one from the inner and more posterior portion of the arch. Its insertion is in the outer angular region of the mandible. Fibers of the masseter muscle are almost at right angles to the occlusal surfaces(fig-5). The masseter is a very powerful muscle of mastication. Its contraction elevates the jaw and forcibly brings the teeth together. Like the temporalis muscle, its contractions tend to seat the condyle in a posterior-superior position in the glenotd fossa. Neither the temporalis nor the masseter has anything to do with lateral movements of the jaw. Their contractions primarily elevate the jaw and bring the teeth together. It is because of this action that the occlusal surfaces of the teeth must harmonize with the hinge-like action of the mandible. The 16
  17. 17. masseter can snap the teeth together in any position from centric to protrusive. Having the occlusal surfaces of the teeth in harmony with this action permits a better dissipation of the forces of this muscle to the periodontal tissues of the teeth as they come together through a bolus of food. The External and Internal Pterygoid Muscles: The external and internal pterygoid muscles are responsible for the lateral movements of the mandible. The external pterygoid has its origin, by means of two heads, in the great wing of the sphenoid bone and the outer surface of the pterygoid plate. The uppermost fibers of this muscle are inserted in the articular disc through the articular capsule. The majority of the remaining fibers are inserted in the anterior surface of the neck of the mandible. The fibers of the external pterygoid muscle are in a horizontal and medial direction, and their contraction pulls the head of the condyle and the meniscus forward and medially. This action sets the mandible into position for chewing. If the external pterygoid on one side relaxes while the one on the other side contracts, the mandible will be moved into a lateral position. It guides the mandible into lateral position and steadies it while the subject bites (contraction of the temporal and masseter) in the lateral position, Contraction of the fibers of the external pterygoid also tends to act as a brake against the posterior pull of the temporalis muscle. It effects a muscular balance against any violent jamming of the head of the condyle posteriorly(fig-6). The internal pterygoid muscle originates from the palatine bone and the maxilla and from the internal surface of the pterygoid plate. Its fibers are inserted in the lower part of the inner surface of the ramus of the mandible at the angle. They run laterally, downward, and backward(fig-7). 17
  18. 18. Neuro-Muscular Coordination: The various structures and individual movements just described are coordinated by a complex integration of the nervous function. During mastication, sudden contact of a tooth with a hard object produces discomfort and reflexively opens mouth. This is called a nociceptive reaction and is partially responsible for protecting the chewing mechanism when there are premature contacts in the articulation. Reciprocal Inncrvation: The nociceptive reaction is able to protect the mechanism because of the phenomenon known as reciprocal innervation. This is the simultaneous activation of a flexor reflex and the inhibition extensor (stretching) reflex, and vice versa. Rhythmic chewing is made possible by the efficient reciprocal innervation of the masticatory muscles as they alternately depress and elevate the lower jaw. During mastication, proprioceptors in the muscles, tendons, and joints send messages through afferent fibers in the trigeminal nerve to the chief sensory nucleus of this nerve. Secondary fibers cross the brain stem, ascend to the thalamus, and finally arrive in the sensory cortex via tertiary tracts. In this manner, awareness of motion in the jaws and of the position of the mandible in relation to the maxillae during chewing movements is permitted. Some proprioceptive impulses pass from the chief sensory nucleus to the cerebellum, thence through a chain of neurons to the motor cortex. The motor cortex is thus informed of the position of the teeth and jaws. and its action makes possible the synchronous mastication movements. Motor activity, whelfter reflex or voluntary, demands little conscious effort: so it becomes necessary to have all the parts of the masticating mechanism working in harmony with each other to prevent its self-destruction. 18
  19. 19. Movements of Mastication: The masticatory movements of the mandible are automatic and occur under considerable force. Mastication begins with the incision of a morsel of food: To accomplish this, the mandible is dropped open by the contraction of the external pterygoids and the infrahyoid and digastric group of muscles. If the external pterygoids contract equally (which is improbable), the patient will execute a straight protrusive movement. More likely they will contract unequally, and a lateral protrusive position will be assumed. Now the incisor teeth have to be propelled through the food to cut it, and this is accomplished by the contraction of the elevators of the jaw: the lemporalis, masseter, and internal pterygoids. After some food has been grasped, mastication proceeds. The bolus is propelled into the mouth by the lips, tongue, and checks and probably is rolled onto the bicuspids, which cut it up further with the crushing and shearing action of their blades. The temporalis and masseter muscles partially relax allowing the food to be replaced on the chewing surfaces. The external and internal pterygoids are in a state of alternate relaxation and partial contraction, and the temporal is and masseter again contract to crush the food some more. By this time, and after several strokes, the bolus has reached the molar teeth, where now it will get a final milling before is swallowed. The masseter and temporalis muscles relax; the external and internal pterygoids on the same side contract while those on the opposite side relax, thus cocking the mandible in a lateral protrusive position. The food is now repositioned on the occlusal surfaces of the molars, and the real power of mastication is applied by the masseter, temporalis, and internal pterygoid muscles. As the masseter and temporalis muscles contract and crush the food, the alternate contractions of the internal pterygoids cause a wiping of the lower occlusal surfaces of the molars across 19
  20. 20. the upper occlusal surfaces in a finely triturating action that comminutes the food preparatory lo swallowing. More specifically, if the bolus of food is on the lower right first molar and ready for its final comminution, the temporalis and masseter muscles on both sides relax. The external and internal pterygoids on the right side relax; the external and internal pterygoids .on the iefi side contract and cock the mandible to execute a working occlusion on the right side. Now the temporalis and masseter muscles on both sides contract forcibly to crush through the food. The external pterygoid on the left side relaxes, permitting she mandible on the left side to return home. As the condyles both approach centric position, the internal pterygoid of the right side contracts, executing the Bennett movement. The masseter and temporalis on the right side soon relax, permitting the followthrough of the masticating stroke as the external pterygoid on the right side contracts. It must be remembered that during all of the jaw movements the condyles and menisci are moving together. Again we must emphasize the harmonious relation of the teeth to these movements that should exist if the investing structures are to be protected from destruction. Harmony of Form and Function: Because of the complexity, automation, and force fullness with which the chewing cycle is executed, it should be apparent that a high degree of harmony must exist between the form and function of the 'parts. Although nature has a buiit-in safety device in the proprioceptive reflex mechanism, repeated insults in the form of a premature contact may impose the learning of a new reflex pattern. It may not be as effective as it should be, and soon the additive trauma will begin to take its toll. Then too, with advancing age the sharpness of the protective reflex is lost, and more and more damage is done to the mechanism. 20
  21. 21. Protection by Proprioception: It is interesting to note that the protective proprioceptive reflexes operate best during normal function. The self-protective mechanisms are weak or missing during non functioning movements. It has also been shown that reflex activity is reduced during sleep, with the nonsecretion of the parotid gland. The protective proprioceptive reflex apparently fails to function during bruxism. This is one reason for correcting the malocclusion of patients who practice bruxism, for while it may not cure the habit, it will minimize the damage that is done. Up to now we have attempted to briefly outline the chewing movements and tc describe the anatomy of the masticatory mechanism. Very little has been said about the teeth, the chewing implements: but we have implied that a harmony of form and function is necessary. The objective of maintaining the health of the structures of the mechanism is of prime importance. To accomplish this, we strive to prevent any part of the mechanism from overworking or being abused. A certain amount of work has to be done in the form of chewing. For the moment, let us disregard any bad habits and consider only the normal use of the apparatus to masticate food. A certain amount of muscular force is necessary and available. How that muscular force is dissipated by the various components (the joints, teeth, and investing structures) is of extreme importance. For instance, if, in the chewing cycle previously described, a single tooth came into contact before the others, what would be the result? As the patient penetrated the food bolus, the premature tooth would receive all the muscular force exerted after penetration of the food. This force in turn would be transmitted to the periodontal tissues and in time would cause their destruction. 21
  22. 22. By harmony of form and function, then, we mean an equal distribution of the forces of mastication that will permit the periodontal tissues of all the teeth and the stress-bearing portion of the joints to equal) absorb this muscular force. Equal distribution of the functional forces over as much tissue and as great an area as possible will guarantee the health of the entire mechanism: this is the objective of our treatment. 22
  23. 23. THE HINGE AXIS The successful application of the hinge axis in dentistry was the greatest single contribution of the Gnathological Society. It was the cornerstone of all future accomplishments and still is the basis for articulation. The hinge-like action of the temporomandibular joint has been described by anatomists for over a hundred years. Its application to dentistry, however, had to wait for the Gnathological Society in the 1920s. Prior to that, Snow, Gysi, and others had been aware of the presence and importance of an opening and closing axis. Yet their methods were so crude that they concluded that the axis was somewhere below the condyles. This inaccuracy led them to believe that changing vertical dimensions was still a chair operation. The desirability of being able to reproduce the opening and closing component of jaw movements on an articulator must have been evident. That they were not able to accomplish this was the fault of the methods used and the fact that there was no articulator that could duplicate this movement. The Gnathological Society developed a means of attaching a face-bow rigidly to the mandibular teeth. This permitted accurate location of the opening and closing axis. Many refinements in equipment were, of course, -necessary to make this a practical procedure. For example, easy adjustment of the caiiper points was a "must." After it had been clinically demonstrated that there was a usable hinge axis, it became necessary to design an instrument that would duplicate this component. The articulator had to have an intercondylar axis that could be aligned with 'he axis located on the patient. 23
  24. 24. Definition of the Hinge Axis: What is the Hinge Axis? The head of the condyle rotates on the undersurface of the meniscus. While it rotates on the-meniscus, the meniscus and condyle can move on the surface of the articular eminence. The movement can be forward, to the side, or anything in between. While the meniscus and condyle are thus translating, the condyle can execute a pure hinge movement anywhere along this translation. Consequently, mandibular movements appear to be very complicated and confusing. It is practical to locate the center of vertical motion; it is also practical to locate the center of lateral motion. The center of vertical motion and the center of lateral motion are one and the same -the center of rotation and there is one in each condyle. The hinge axis is an imaginary line connecting the center of rotation of one condyle to the center of rotation of the other condyle(fig-8). The vertical opening and closing movements, as well as the pure lateral movements originate from the centers of rotation. Any combination of vertical and lateral movement has its center in the same point. The center of rotation of each condyle is constant to the condyle, and therefore to the mandible. The hinge axis (the imaginary line joining these centers) then is constant to the mandible (and teeth). As the mandible moves in its various excursions, the hinge axis moves right along with it. The mandible is capable of executing a hinge-like closure in any position(fig-9). This is one reason why the hinge axis is so important. It permits us to duplicate all the arcs of closure of the mandible on an instrument and thus tailor our cusps to harmonize with these arcs. One point of confusion about the hinge axis stems from the method of locating it. It is located in the rearmost position of the mandible - the terminal hinge position. It is located in this position because only here can it be repeatedly 24
  25. 25. separated from the other components of jaw motion. The patient, of course, does not function in this terminal hinge position. We purposely make him execute a terminal hinge closure so that we can locate the center. Once it is located, we endeavor to trace the path of this center to enable us to duplicate every possible combination of the two movements (rotation and translation) that the patient will use in function. By determining the hinge axis and transferring it to an articulator, it is possible to make casts of the mouth (teeth) in the exact dynamic relationship to each other that exists in the patients head. Only by use of the hinge axis is it possible to have teeth approach each other on an articulator exactly as they do in the mouth. The hinge axis permits us to have the vertical dimension under our control on the articulator and to duplicate all the eccentric relations and all the possible contacts of the teeth in these relations. We can study and diagnose tooth relations thoroughly; confident that they are exactly as they exist in the patient's mouth, and we can return our work (whether dentures or natural tooth reconstruction) to the instrument for correction with knowledge any changes in vertical relations will be harmonious when placed in the patients mouth. It is only by means of the hinge axis (and centric relation) that the teeth can be related accurately to the terminal hinge position. The Hinge Axis and Centric Relation: To secure a centric interocclusal record, we attempt to "freeze" the terminal hinge closure at a convenient opening. Without the hinge axis, we would be unable to secure an accurate centric interocclusat record because to obtain such a record, the recording medium must not be penetrated by the teeth or the occlusion rims. (The implication is that the mandible would deviate because of the guidance of the penetrating teeth or rims). In order to avoid penetration (at least in dentulous cases), we must obtain centric interocclusal record in an open relationship, and if we were not on the same arcs of closure, our efforts would 25
  26. 26. be useless. It is impossible to check a centric inlerocclusal record without an axis mounting. Technique for Locating the Hinge Axis (fig-10): The location and transference of the hinge axis are not very difficult procedures, but they must be carried out with great care because they form the foundation for many other procedures. A convenient type of facebow is used. It must be rigidly attached to the mandible so that it actually forms an extension of the mandible. A reference plate or clutch is cemented to the lower teeth with Truplastic. Graph-lined flags are placed on the side of the face over the condyle areas to eliminate any skin movement distraction. These flags may be attached to the maxillae by means of a crossbar and a maxillary clutch, or they may be held in place by a head frame or other contrivance. A crossbar is attached to the lower reference plate or dutch. Adjustable side arms are placed on the lower crossbar with the styli in the vicinity of the condyles. The patient must now be instructed in the hinge-type of movement. As previously indicated, this is not a normal movement for the patient it is for our convenience only. The patient must be coached to let his mouth drop open. This necessitates the relaxation of the external pterygoid muscles, and some patients may have difficulty in comprehending this movement. It helps sometimes to have the patient place his hand on our chin as we demonstrate the type of relaxed opening and closing desired. Possible Need for Bite Plane Therapy: If a patient has difficulty in executing a pure hinge movement, it may be necessary to train him in This abnormal opening and closing movement. Training can be accomplished by using the jig. 26
  27. 27. In some cases where joint pathology may be present it may be necessary to have the patient wear a bite appliance of some kind to disclude the teeth and allow the joint to return to a more normal condition. Three or four days will usually suffice, but sometimes several months of bite plane therapy may be required. The patient must be carefully monitored during any extended period of wearing a bite appliance for tooth movement. The patient naturally opens downward and forwards a combination of rotation and translation. We must separate the rotation from the translation so that we can locate the center of vertical opening. In addition, this opening and closing must be accomplished in the terminal hinge position, for here we can get repeated concentric arcs that will permit us to locate their center. Any other arcs will serve only to confuse the issue, at this point. What we actually have is a compass with bent rigid arms. The pivoting part of the compass is on the center of rotation in the patient's condyle. The stylus point is the tracing part of the compass. If we succeed in getting the tracing point exactly over the pivoting point, there will be no arcing of the tracing point. Geometrically, if we had two concentric arcs, and if we erected bisecting perpendiculars to the chords of these arcs, they would intersect at the center of the arcs(fig-11). However, there is no practical method for making such a plot. The trial and error method first used by Dr. McCollum is still the only practical way to locate the axis. When we succeed in getting the patient to execute a rhythmic opening and closing in the terminal position and the stylus point is arcing, we visualize where a center would have to be for scribing such an arc. Thus, we will have an idea of which way to move the stylus in order to reach the center. After making an adjustment in this direction, we try it again. As we approach the center, the arcs will become smaller and a little more opening will be required to magnify the arc. After several adjustments, we will be close to the center. A magnifying 27
  28. 28. glass should now be used to help us see whether there is still any arcing of the stylus tip. The graph lines will aid the eye in determining this. By viewing down one line and then down the crossing line, we can see whether there is any slight arcing. If there is, we continue adjusting until it disappears completely. We must learn to distinguish between the pure hinge movement and the movement with some translation. The patient will inadvertently make a translatory movement every third or fourth try. Some patients will be most cooperative; others will be exasperating. Nevertheless, we must arrive at an axis if the rest of the procedures are to be correct. The axis center must be located on each side. What we arc locating is the hinge action on the side of the face. It is a point on the hinge axis and not the actual center of rotation. The actual center is approximately 10 or 11mm medial to this location. Consequently, the location of this point must be made as close to the skin as possible. This means that the flag must be very close to the skin (fig-12). Marking the Axis Location on the Patient: When we are satisfied that we have located these points on the axis, we remove the flags from the patient. A marking medium, such as an indelible pencil, is rubbed on the end of the stylus. We make sure the patient is in the terminal hinge position and then have him move his head out of the headrest, making sure that he does not also move out of the terminal hinge position. The stylus is gently pushed against his face to transfer the paint to the skin. These marks are made permanent by using a special needle and a little pink marking dye sulfide of mercury (fig-13). In all of our subsequent transfers we must try to simulate these conditions -the skin in the same relaxed position and the stylus pins locked the same distance from the face as they were before the flags were removed. This is usually 1/16 28
  29. 29. of an inch from the skin. By doing so, we reduce to an absolute minimum any possible error in transference. In addition, the stylus pins must be locked and not moved until the mounting is completed. The articulator has to have an intercondylar axis that can be extended to these points so that the transfer is accurately lined up with the axis of the machine. Selection of a Face-Bow: From a purely theoretical point of view, an ordinary face-bow such as a Snow or Hanau can be used to locate the hinge axis. To attempt to use either one of them in actual practice, however, is impossibility. It is a bit more practical to use one of these bows as a transfer instrument, provided the styli are perfectly lined up one to the other. As a matter of fact, if the styli are perfectly lined up and we are able lock the bow by means of the universal joint in front so that the points of the styli are on the axis locations then it will not be necessary to have an articulator with an expandable intercondylar axis. Under these circumstances, it is possible to bring in the styli pins an equal degree towards the intercondylar axis of the articulator and still stay on the axis. However, it is far easier and more accurate to use a fully adjustable face-bow (i.e.. one with arms that can be independently adjusted by means of micrometer screws) for both the axis locations and transfers. By means of a face-bow transfer and the mounting frame, the upper cast can be properly mounted to the axis of the patient. The Hinge Axis and the Plane of Reference: The hinge axis is constant to the mandible, as has been indicated. The terminal hinge position, which is actually the centric relation, is constant to both the mandible and the maxillae. All our mountings are made in this relation. Therefore, the only practical way to maintain constant relationships throughout treatment is to use the axis points and a fixed third point at the base of the right 29
  30. 30. orbit as our plane of reference. Thus, the axis orbital plane gives us a constant position for the upper jaw, and a correct centric interocclusal record will establish the position of the lower jaw to the constant upper jaw. In this way, repeated mountings will have a constant, relation to our records and to the patient's centers of rotation. Discussion and Conclusion: Many have attempted to find fault with the hinge axis and to disprove it. Their criticisms cover such things as skin mobility, change of the axis, the introduction of errors by moving the stylus tip a slight degree, and the presence of a separate axis for each condyle. Actually, skin mobility is reduced to a minimum by the precaution of having the patient move his head out of the headrest when all references are made to the marks. Any changes that might occur over the years from loss of weight and the like would be minor. As far as change of the axis is concerned, the only changes observed have been in the joints with some pathology. If a patient has a painful joint, or if a patient does not execute a hinge-like closure after a few guided opening and closing movements, it would be desirable to do one of two things: either train the patient with the jig as you do when getting a centric relation record, or put the patient on a bite plane for several days. This will usually relieve the pain and give a smoother hinge-like movement. If there are symptoms in a temporomandibular joint, there may be some slight change in the axis location. Always plan to relocate the axis on such patients a year or two after the pathology has cleared up. 30
  31. 31. Why use the hinge axis in these cases? The answer is simple: it is still the only means of establishing a starting point to which we can repeatedly return and to which we can definitely relate our work as it progresses. In every normal joint case that we have rechecked over the years for demonstration purposes and to satisfy our own curiosity, we have always been able to relocate the axis within very acceptable limits, that is, by the thickness of the tattoo mark. The most ridiculous criticism is the charge that error is introduced because the stylus pin has to be moved through the thickness of the card covering the face. A Single Transverse Axis: The allegation that there is a separate axis for each condyle is mumbo-jumbo. The anatomy and physiology of the joints would not permit a two-axis arrangement. About 1950, Dr. William Branstad, Dr. Raymond Garvey, and Dr. Robert Okey conducted an experiment to determine whether there was one transverse axis through both condyles or an axis for each condyle. They found that there was one transverse axis. Dr. Arne Lauritzen, working with a study group, repeated the same experiment about 1957 and arrived at the same conclusion. Dr. Frank Celenza and V.O.Lucia repeated the experiment during the summer of 1959, with the same result. In the fall of 1959, the Hinge Axis Committee of the Greater New York Academy of Prosthodontics repeated this experiment and concluded that there was only one transverse axis through both condyles. This, in brief, was the experiment: Clutches were cemented to the patient's teeth. A crossbar, 36 inches long, was attached to the upper clutch, and another of the same length was attached to the 31
  32. 32. lower clutch. Four graph-lined flags were attached to the, upper bar for the purpose of accurately locating the center of rotation(fig-14). One flag was placed on each side of the face, close to the skin. The other two were attached near the ends of the upper bar, about 12 inches from the Hags against the face. Attached to the lower bar were four adjustable side arms, to be used in locating the center of rotation. Each side arm was placed against a flag. The center of rotation was located in each of the four areas, that is, each side arm was adjusted against its corresponding flag until there no longer was any arcing, but only rotation of the stylus point. When all four centers of rotation had been accurately located and the patient was held in centric relation (terminal hinge position), the cards on the flags were carefully marked with the tips of the styli. The upper bar with flags attached was then removed from the clutch. With a fine, heated instrument, a tiny hole, was burned through each card where it had been marked. When the four flags were held up to the light, it was possible to see the light through all four flag holes, proving that the four points had to be on a straight line(fig-15). Thus, it was concluded that there was only one transverse axis. To demonstrate this more emphatically, we set up the bar and flags and with a small penlight passed rays of light through the four holes. The camera at the other end of the four flags recorded the light rays coming through the four pinholes. In addition. a piece of dental floss was threaded through the holes. When pulled taut, it was perfectly straight. This was conclusive proof of the existence of only one transverse hinge axis. The existence of a usable hinge axis component to the temporomandibular joint movement is one of the greatest luxuries that we could have when treating the oral mechanism. 32
  33. 33. Proof Positive of a Usable Hinge Axis: On a patient whose axis was located, we proceeded to take centric relation records at increasing vertical dimensions and compared them with a split cast mounting. An accurate set of casts was made of the patient's teeth. The upper cast was prepared for a split cast to accurately examine the likeness of the various records. The CENTRIC RELATION Centric relation means many things to many people: to some, it means the contact of teeth after a jaw closure; to others, it means a closure in a particular position, the particular position having many interpretations, varying from an habitual closure to a forced retrusion, or somewhere in between. Still others identify it as the most retruded position from which right and left lateral excursions can be made. Some dentists refer to centric relation when they are talking about the mandible, disregarding the teeth depending upon their belief and understanding, they decide that the mandible is in centric position, and that if the teeth occlude in this mandibular relation, then the teeth are in centric relation. Other dentists describe "mandibular centricity" as a mandible-tomaxillae relationship at a certain vertical dimension. It is unfortunate that centric relation means so many things to so many people, because no other phase of dentistry is as important as a clear understanding of centric relation. Obviously, it should have one and only one connotation to the dentist. To understand centric relation and to appreciate its great importance, we must understand how the jaw functions. We must set about to make restorations that will function normally in that jaw: they must neither interfere with nor force a particular action on the chewing mechanism. In other words, the restorations must fit into the pattern of jaw movements: they should follow, without any detrimental effects, the movements of the masticating mechanism. Our present 33
  34. 34. concern is with the type of motion, how it takes place, and its bearing on the all-important subject of centric relation. This, of course, directs our attention to the temporomandibutar joint. For the moment, we may forget about the ether important structures -the muscles, tendons, ligaments, nerves, blood supply, and teeth and confine our consideration to the action of the temporomandibular joint. Location of the Centers of Rotation: It is possible to demonstrate beyond any doubt that there exists a recordable center of vertical rotation in the condyles. An imaginary line joining these centers has been termed the hinge axis. In practice, when we locate the point on the side of the face for the hinge axis, we are actually locating the hinge action in the facial plane (on the side of the face). This is not the true center of vertical motion, however, for that is located in the condyle. What we are locating is a point on a line-that has been extended from the centers of vertical motion. In other words, the point we locale on one side of the face is on the same line passing through the actual centers of vertical rotation in each condyle and through the point on the other side of the face. For this reason, when making a transfer, we must not move the points of the stylus in or out once we have located the point of hinge action. In practice, we must have a means of transferring these hinge- action points to a suitable articulator, the intercondylar axis of which can be lined up with these points. This is accomplished with the mounting frame. We can locate the centers of hinge action only when the condyle is in a position where it can repeatedly perform the hinge action. Because patients normally do not execute a hinge action in the most retruded position of the mandible, they must be educated to this movement. When we consider this, as well as the 34
  35. 35. many habits patients can acquire over the years and the conditioned reflex action forced by habits and tooth relations, it is easy to understand why some patients reluctantly produce the hinge action during treatment. This hinge action (and the imaginary line called the "hinge axis") is constant to the mandible. In other words, the vertical motion of the mandible (and condyles) is produced by the action of the heads of the condyles on the undersurface of the meniscus. Thus, as the condyle and the meniscus translatemove down the incline of the glenoid fossa or across the trough of the fossa in the Bennett movement the mandible can produce this hinge-like action in any position of the condyle. As a matter of fact, it will start to produce a hinge-iike action as it glides down or across the condyle path. We must remember that the hinge action is constant to the meniscus in any position in which it may find itself, but it is constant to the maxillae or fossae only when the condyle is executing the hinge action in the terminal position.. In addition to the centers of vertical (opening and closing) motion of the mandible, there exist centers of lateral rotation: The patient can make pure lateral movements that have centers of rotation located in the condyles. At one time, there was considerable confusion about these centers because they are seldom stationary. In other words, the centers themselves move as the mandible (condyle) is making the movement. The-path of these centers of lateral rotation on the rotating or working side is the Bennett path. The confusion arose because it was claimed that the center of lateral rotation was some where in back of each condyle or in the vicinity of the foramen magnum. The moving centers of lateral motion were called "loci." Actually, what was termed the center of lateral movement behind the condyies or "somewhere else" was the center of the locus. The center of the path that the center of lateral movement was making on the working side was, in fact, the center of the Bennett path. It is practical to locate the exact centers of lateral rotation by means of two gothic 35
  36. 36. arch tracings taken in the same plane in front of each condyle and on either side of the midline of the face and reproduce their path across the fossae (fig-16). When this has been done in conjunction with the location of the centers of vertical rotation (hinge action), then we have truly found centric relation. The terminal hinge action is the vertical component of centric relation; the centers of lateral rotation are the lateral components of centric relation. Why this is centric relation, we shall now attempt to explain. We shall also show why. it is so important. It might be stated categorically that unless we locate the centers of rotation, we are disregarding centric relation. This statement will immediately draw protests because, regardless of one's understanding of centric relation, all will agree that centric relation is essential to the practice of dentistry and cannot be ignored. Let us analyze what really happens: In the course of constructing occlusal surfaces for dentures, bridges, or natural teeth, we take a centric interocclusal record, using the material of our choice. The casts on which the restorations are going to be fabricated are mounted on some sort of instrument, and the case is constructed. In order for a centric interocclusal record to be usefui, it must register the maxillomandibular relationship without any tooth contact or tooth penetration of the recording medium. If tooth surfaces contact through the recording medium, we can be sure that the proprioceptive reflexes have crossed us up and caused us to record an improper relationship. It is apparent that one of two things must be done even to begin to get an accurate interocclusal record either it must be secured at the exact level of vertical dimension without tooth contact (a nice trick if it can be accomplished), or else the casts must be mounted on the articulator to the same opening axis that the mandible has to the maxillae of the patient. If the latter is 36
  37. 37. done, then the centric interocclusal record can be secured in an open position to clear the tooth contacts; and when the registering medium is removed, the teeth on the casts can be approximated as they are in the mouth. It is also most desirable that we check our centric mounting because many hours of laboratory work will depend on this relationship. It is utterly impossible to check a centric interocclusa! record accurately unless hinge axis procedures and transfers have been used. In order to check a centric intcrocclusal record, it is necessary to take a second record, using all the care taken with the first one. It would be pure chance if the second record were of the exact thickness as the first. The wax might be softer, or the patient might close further. Whatever the reason, chances are against our getting records of exact thickness. Yet, unless we were on the same arc of closure on the articulator as in the mouth, the thick ness of the two records would have to be absolutely identical. The seating of a wax interocclusal record on casts can be quite deceptive. The second record might appear to fit between the casts without causing any malposition of the articulator parts. However, if we really want to determine whether our two centric interocclusal records are identical, we must resort to the following procedure frequently demonstrated in the clinics of Dr. Arne Lauritzen The Split Cast Technique: Before mounting the upper cast on an articulator, second section (the split cast) is carefully prepared. First, it is very important that the upper cast be poured with extreme accuracy, care being taken to avoid any bubble formation. The mounting side the upper cast is trued up on a model trimnifr. "V'notches are cut on the edges of the mounting side of the upper cast -two in front, two on the sides, and one in the posterior region. These notches are carefully made so that 37
  38. 38. they are truly wedge shaped. A piece of electrician's tape is wrapped around the periphery of the cast, producing a form into which the second section of the split cast is poured. Prior to this pour, the cast has been carefully lubricated with Kerr Separating Medium. Three knobs of stone are placed on top of the pour to serve as handles in the separation of the disc from the original cast. In pouring the disc, it is extremely important to prevent any bubbles from forming. When the disc pour has hardened, the cast is separated from the disc by removing the electrician's tape and using the stone knobs on the disc as a handle. Immediately after separation, the two parts are reassembled to prevent any dust or loose fragments of stone from adhering lo the contacting surfaces. The knobs are now cut down with a model trimmer, leaving just enough of them to engage the new mix of stone that will be used to fasten the disc and cast to the upper bow of the articulator. An impression (whether it is for a study cast, a master working cast, or a remount cast) is poured in stone. The excess stone is vibrated into the plastic mold groove former and quickly inverted and placed on top of the poured impression. Press it into place as you center and level the former. Do not invert the impression. When the stone has set, remove the former and replace it with the ring mold and secure it with periphery wax. Lubricate the grooved stone with a separating solution and vibrate a mix of stone of a different color into the ring mold. Level the surface with a spatula and place several knobs of excess stone for retention when mounting to the articulator. Remove the ring 'after the stone sets and separate the impression. Trim the sides on ? model trimmer and you are ready to mount it on the articulator by means of the facebow transfer. By means of a face-bow transfer, the upper cast and disc are accurately attached to the upper bow of the articulator. By means of our centric interoccltisal record, the lower cast is next attached to the lower bow of the 38
  39. 39. articulator. This completes the mounting of the split cast and the lower cast in what we believe to be a centric relation. If we now open the articulator, separate the disc from the upper cast, press the upper cast into the centric interocclusal record to be sure it is accurately seated in place, and then attempt to close the upper bow and disc into the "V" notches on the upper cast, we will soon find out whether: our mounting was accurate. If it is satisfactory, we proceed to check this mounting and interocclusal record with the second record taken. The first wax record is replaced by the second one; the upper cast is seated into the indentations; and again an attempt is made to close the disc into the "V" notches of the upper cast. It is amazing how often an apparently acceptable interocclusal record is inaccurate. This technique should be ample proof that a centric interocclusal record cannot be accurately checked unless the hinge axis and hinge transfer procedures are used. If these procedures are as far as we go, the restorations constructed on such casts will come together accurately in centric closure. If we add one more step and reproduce a protrusive path with a protrusive record, it is possible to have proper contacts in both the centric and protrusive relationships. Unfortunately, though, patients do not chew only in these positions. . How does a dentist manage without using the axis and a protrusive record? Like the dentist who simply takes a static closure, by proceeding to do a great deal of work in the mouth, grinding here and there until some surfaces come together. Considerable work is involved for an inferior result. The dentist who takes a hinge-closure record, relating it properly to an instrument by means of a face-bow, and then takes a protrusive record is only slightly better off because there are all the laterals to contend with. Even if one believes that the patient does not use his lateral excursions, the fact is that he will use them if he is permitted to. The apparent shortcuts -not locating an axis, 39
  40. 40. not reproducing all of the patient's movements are responsible for the creation of flat, useless occlusions. To avoid these headaches, we must locate the centers of rotation. In addition to locating the hinge axis and obtaining a proper centric interocclusal record, we must locate the centers of lateral rotation. This is accomplished by means of the twin gothic arch tracings. Moreover, we must trace the paths of these centers of lateral rotation. This is done with an extraoral tracing device, the pantograph: the pantograph is the only practical means of accomplishing this today. With the pantograph, we can trace the protrusive paths of the centers of rotation, as well as the right and left lateral paths. From the pantograph tracings made by the path of travel of the centers of rotation, we can reverse the procedure and duplicate the centers and their paths on an articulator capable of full adjustment. Now when the restorations are constructed and placed in the mouth, they will be in harmony with the patient's movements. It will not be necessary to grind them, with the resultant destruction of proper function. To recapitulate: A thorough understanding of centric relation is essential 10 the proper practice of dentistry. However, unless we locate the centers of rotation, we are disregarding centric relation, which entails the following: 1. Location of the hinge axis 2. Location of the centers of lateral rotation 3. Transference of the casts to the axis a) Face-bow transfer of the upper cast to the axis b) Relation of the lower cast to the upper by a correct centric interocclusal record. 40
  41. 41. Obtaining Centric Relation - Various Materials for Various Situations Methods of manipulation for centric relation: 1. One handed technique by Anderson and Tanner (fig-16a). 2. Anterior stop technique a) Lucia jig technique b) Leaf gauge technique advocated by Long (fig-l6b) 3. Central bearing point method (fig- 16c) 4. Bilateral manipulative technique (Dawson technique fig-l6d) Methods for taking centric bite records l. wax bite procedures 2. Anterior stop techniques 3. Use of preadapted bases 4. Central bearing point technique. The technique for obtaining a centric relation is secondary to an understanding of the phenomena. Various materials will produce acceptable results, but the important thing is to know what we have to get and to be aware that we have what we want. From our preceding discussion we know that we must locate the centers of rotation. By means of the two gothic arch tracings we are able to locate the centers of lateral rotation, and by means of the hinge axis location we are able to locate the centers of vertical rotation. Our practical problem now is to couple these two centers of rotation into the center of rotation. To do this, we must relate the lower jaw to the fixed member, the upper jaw. Having related the upper jaw (cast) to the center of vertical rotation by means of the face-bow and 41
  42. 42. having set the articulator for the centers of lateral rotation, it now remains for us to orient the lower jaw (cast) to these centers. We accomplish this by "freezing" the lower jaw (cast) in the terminal hinge closure at a convenient vertical dimension. This is the problem of obtaining centric relation. There are many factors that complicate this procedure; and patience and experience are required to complete the task satisfactorily. Among the complicating factors are the patient's reluctance to make a pure hinge closure; the patient's neuro-muscular pattern, which may have developed around a deflective occlusal contact; the natural tendency of many patients to go into a physiological rest position at the completion of any jaw movement; and the natural tendency of a patient to exercise his prehensile reflex whenever anything is placed between the teeth. Certain procedures and materials are required lo overcome these factors. The very first procedure is to practice with the patient until he is able to execute a pure hinge closure. Second, we must block out some of the neuro-muscular reflexes by preventing the teeth from coming together. We can accomplish this by using our thumbnail as a controllable anterior stop. Third, we must keep the patient under function, swinging up and down so that he cannot go into physiological rest. As long as the jaw is functioning, its bracing position is maintained. The natural prehensile reflex can be minimized if we have the patient close his eyes during these procedures. If he sees the wax wafer (the recording medium) approach his mouth, he will automatically begin to reach out to grasp it with his teeth; and this is not a centric closure. We must take care not to violate these precautionary procedures as we make our recording. This presents quite a problem because what is really needed is a magic material -a material that by its lack of resistance will not cause any unequal displacement of the joint or teeth; a material that will remain sufficiently soft 42
  43. 43. long enough to ensure a dynamic registration, but will "freeze" just as soon as all the procedures are completed. Two-Stage Registration: One method that has proved acceptable is a two- stage registration. A wafer is made of one sheet of DeLar wax and one sheet of Tenax wax. These are luted together. The reason for using two kinds of wax is to permit an easy indentation on one side and to provide a stiffer side that will act as a carrier. The wafer is placed vertically- in a water bath at 138°F. The anterior part is kept out of the water so that it will remain stiffer and offer some resistance anteriorly, thus ensuring the bracing position of the condyles. While the wax is softening, the patient is rehearsed in the terminal hinge closure. The patient is instructed to open and close his jaw without clenching his teeth together. By avoiding the tooth contacts, the patient does not receive the pcriodontal proprioception that could cause an abnormal reflex closure. This is what we are trying to avoid: we desire a pure hinge closure free of any "acquired" malpositions. This procedure will help the patient to execute a pure hinge closure. It permits the temporomandibular ligament to be extended to its normal position. It trains the patient to separate the rotation from the natural combination of rotation and translation that makes up all functional movements. The patient is rehearsed in the terminal hinge closure while the cheek retractors are in place. These conditions will simulate the actual taking of an interocclusal record. The patient is instructed to close his eyes, and when the wax wafer is sufficiently soft on the Tenax side, it is inserted into the mouth, with the Tenax side against the upper teeth. The patient is told to swing his jaw several times without closing on the wax, and then when we can "feel" the terminal hinge closure, lie is instructed to close lightly against the wafer. At this stage we are 43
  44. 44. chiefly interested in getting an accurate imprint of the upper teeth in the Tenax wax(fig-!7). The wafer is removed from the mouth and placed in water of room temperature. After partial chilling, it is trimmed to the outside edges of the tooth indentations to remove the bulk. We also remove the anterior portion, cutting it off across the center of the cuspids. There is a twofold reason for removing this part of the wafer: first, with the anterior teeth exposed, we can use our thumbnail as the anterior resistance; and second, without the anterior portion, there is that much less area to seat against the casts when we make the mounting. Consequently, should there be a slight discrepancy in the anterior part of the casts, it will not cause their malrelationship. In short, our only concern will be with the posterior areas. The wax wafer is now replaced on the upper teeth and held in place with the thumb and forefinger of the left hand. It must be evenly seated against the upper teeth. The patient is instructed to close into it again to correct any warpage. The wafer is then removed, and with a Bard Parker knife4 we trim away with the excess wax around the indentations on the Tenax side, leaving only the cusp tip indentations so that the cast may be accurately seated when we make our mounting. Again, we seat it on the upper teeth and have the patient close once more to eliminate any warpage that may have resulted from the trimming process. When we are satisfied that we have an accurate seating of the wafer against the upper teeth, we proceed to complete the interocclusal record. We remove the wafer and dry it with a blast of compressed air. Taking a sheet of AI u wax, we form a "pencil," melt it, and apply the softened wax to the underside (DeLar side) of the wafer, dripping it on as if using a candle. Aluwax melts at a lower temperature than DeLar or Tenax, and thus provides us with a soft surface that can be easily carried to the mouth without warping the 44
  45. 45. wafer(fig-18). We place the wafer on the upper teeth, holding it in place with our left thumb and forefinger. With our right thumb on the patient's chin, we guide the patient into the terminal hinge closure. During this procedure, the patient's eyes arc closed. We have him execute the terminal hinge closure, but do not allow him to contact the softened Aluwax until we are sure of the "swing." Gradually, we let him close more and more after each swing until the Aluwax is contacted. It may be necessary to add wax several times before we can obtain an acceptable interocclusal record(fig-!9). The Tests of an Accurate Interocclusal Record: There are several ways of determining whether an interocclusal record is accurate. 1. We should hold the wax wafer up to the light to see whether there is any penetration. If there is. it will not be correct. Likewise, if there are one or two thin spots, the chances are that it is incorrect. Areas of penetration or thin areas are likely to cause a slight deviation of the mandible -so slight that we may be unaware of it. Variations of thick and thin spots will offer variations in resistance and may cause as much inaccuracy as a penetration. 2. If the thickness is satisfactory, we place the wafer on the upper teeth and carefully examine it to determine whether the seat is accurate. There must not be any "give" in any area. 3. We have the patient close into the wafer, first guiding him as we did during the taking of the interocclusal record and then allowing him to close by his own muscular force. If there is a hesitation in finding the indentations, the interocclusal record is probably inaccurate, 4. If the foregoing requirements are satisfied, there is one final test lo make: we have the patient close into the wafer and hold it firmly; then we examine the 45
  46. 46. posterior portion for any play between the teeth. Both sides should be examined carefully. If the interocclusal record meets all these tests, we are justified in accepting it as correct. This may seem to be a long and tedious procedure but bear in mind that everything we have done previously and everything we do subsequently will depend absolutely upon this one procedure. An error in some other part of the operation may be tolerated but an error here is disastrous. Up until 1961, the preceding technique was reasonably successful. We still use this technique for a preliminary record, before locating the hinge axis. In 1961 and 1962 the "jig" was developed. There was nothing new in the principle of the jig. The late Ernest Granger used his thumbnail as an anterior resistance. His analogy of taking a centric record to the driving of a golf ball said a great deal. He described it. as an art -driving a golf ball well is not accomplished by many. The late Steve Brown used a wax wafer with chilled wax anteriorly to seat the condyles. Dr. Grubb and his technician, "Jonsey". used a gold casting on the lower teeth to maintain vertical and centric relation while they carved the restorations in the mouth. I am certain that Dr. Pete Dawson captures the correct centric relation with his jaw manipulation technique. Dr. Stuart uses a tongue blade for his anterior resistance. The Jig Technique: Constructing the Jig: It is preferable to make the jig on an upper study cast. Some clinicians make it in the mouth, but this can be dangerous because of the heat generated when the self-curing plastic cures. 46
  47. 47. Block out any undercuts in the anterior teeth of the cast with wax. Adapt tin foil over the anterior teeth of the prepared cast. Lubricate the tin foil with petroleum jelly. Make a mix of Dura Lay in a dappen dish. When the mix has a doughy consistency, place it on the tin foil and adapt it labially and lingually over the centrals. Labially it should extend just over the margin of the gums. Lingually it can extend onto the palate about 1/2 inch. The sides are tapered to the lingual 'and extend to the distal of the two centrals. Occlusally, ihe surface is a flat plateau, thick enough to have sufficient material to adjust and separate the teeth. As the Dura Lay polymerizes, keep removing and readapting it so that you have a well-fitting jig that can be removed from the model without breaking the model. The occlusal surface is not inclined -we don't want a wedge effect Some dentists have used the jig as an inclined plane. This is absolutely wrong! The wedge is one of the most powerful mechanical devices in existence. A wedge can split a mighty oak. The jig used as a wedge can displace the temporomandibular joint distally with great ease. The platform on the mandibular surface of the jig is just that a platform against which the lower anterior teeth will close. It acts as the third leg of a tripod -the other two legs are the condyles. The platform (mandibular surface of the jig) must not influence the direction of closure. It must not force the lower jaw to the right or to the left- It must not force the mandible forward or backward. It just stops the closure. A very, very slight posterior inclination will assist the patient in holding this position while the recording material sets. Even in very deep overbite cases, the contact area is a flat platform, not an incline. When the jig has cured, we trim it as shown in(fig-20). The labial frenum is cleared and the labial margin of the jig just goes beyond the tree margin of the gums. It should fit onto the anterior teeth without being displaced. 47
  48. 48. Three wax wafers are prepared on the upper study cast. Use one sheet of DeLar wax. Soften the wax in water and place it on the cast so that you can cut it to proper dimensions. Have the wax extend about % inch outside the buccal surfaces of the teeth. In the anterior region, cut out a "U" large enough to accommodate the Dura lay jig We are now ready to go to the patient. A DeLar wax wafer is softened in water at 1380 F. The wax wafer is placed in the mouth and the patient is guided into a closure (hopefully somewhere near centric relation). Before the wax solidifies, bend the corners of the wax over the labial of the cuspids{fig-21). These “ears" will serve to reposition the wafer after we cover the indentations with the zinc oxide and eugenol paste that we will use later. The indentations will be covered with the paste and will not help us to reseat the wafer in the same position in which it was imprinted. The "ears" will help us to reseat it in the same place as when it was formed. The three wafers are so prepared. Now we are ready to train the patient with the jig. This is one of the important functions of the jig-to break the patient's habitual closure. It prevents the teeth from reinforcing together, and thus it prevents the teeth from reinforcing the reflex act of closure. It short-circuits the proprioception that directs the engram of closure. Therefore, it is essential that the teeth are not allowed to come together during the training process. If they did contact, the reflex act would be reinforced and we would defeat our efforts. A piece of carbon paper is placed between the mandibular surface of the jig and the lower anterior teeth. The patient is instructed to move to the right, move to the left, move forward, and move backward. This has the tendency to free the jaw movement. The jig is removed and the patient is prevented from bringing his teeth together by placing a saliva ejector in his mouth. The jig is reduced in thickness with an abrasive rubber wheel. There usually is a gothic arch traced on the jig. Remove the tails of the gothic arch and slowly reduce the apex -the area of lower 48
  49. 49. tooth contact. This is area is ground flat -not inclined. The procedure is repeated again and again until the vertical is reduced, but there is still ample intcrocclusal space. This should be continued for about 20 minutes. When you are finished, place one of the wax wafers between the teeth, and with the patient closing firmly against the jig, the wax wafer must still be free to be moved up and down between the teeth. There must not be any contact between the teeth and the wax. When we take the zinc oxide and eugenol wash, the wafer should literally be floating between the teeth. In this way, there is no conduction of stimuli from the upper teeth to the lower teeth. We are now ready to take our final registration. The teeth are lubricated with petroleum jelly. The jig is secured on the anterior teeth with denture adhesive. A mix of Temp Bond or a bite registration paste is made and applied sparingly on the indentations of the wax wafer on both sides. The wax is sandwiched between the paste. Do not use too much paste. Place the paste-covered wax wafer in the mouth using the "cars" to seat the wafer on the upper teeth. The patient is guided into a hinge closure and instructed to hold this position firmly. The patient is closing firmly against the jig. Hold your thumb on the patient's chin with the index and middle fingers cradling the undersurface of the chin(fig-22). This will enable you to know if the patient relaxes before the paste sets. Keep reminding the patient to close firmly. This will place the condyles upward and on the posterior slope of the articular eminence -a position that most believe to be the correct position for the heads of the condyles. The record is carefully removed after the paste has set. In order to avoid warpage, a simple procedure is followed. The thumbs of both hand* are placed on the patient's 'chin, and the index fingers are placed on the outer edge of the wax record. The patient is instructed to gently separate his teeth. The wax 49
  50. 50. wafer is braced against the lower teeth -the lower jaw acts as a form. Then the patient is guided into closure again. Now the thumb and index fingers of the left hand are placed to support the wafer against the upper teeth, and the patient is again instructed to gently separate his teeth. The upper jaw now acts as the form to prevent the wafer from being distorted. The wax wafer is removed from the mouth and chilled. With a pair of surgical scissors (those with a serrated jaw to grab the set paste), the excess paste is removed. All we want are the cusp tip indentations so that we can accurately seat the casts into the wafers and see if they are perfectly seated. The trimmed wafer is returned to the mouth (with the jig in place) and the patient is guided into centric relation closure. This will correct any possible slight warpage that could have taken place in the removal and trimming. The two other wafers are treated the same way. After the third record has been taken and trimmed and reseated, we remove the jig and again guide the patient into centric closure (this time with out the jig). Remove the third wafer "and don't allow the patient to close. Insert each of the other two records, one at a time, and have the patient close into them (guided) without the jig in place. The records are now completed and we arc ready to go to the laboratory and make our mounting and check our centric relation records. There are some considerations for special situations. When taking a centric relation record for working (master) casts, it is necessary to use several thicknesses of wax for the record. The reason for this is that because of the increased interocclusal space after tooth preparations, the paste will not register the tips of the preparations unless the space is reduced by means of the thicker wafer. In other words, the paste will not stand up long enough to capture an imprint. In a remount record, a single wax wafer is sufficient, because now the restorations are in place and the interocclusal space is reduced. When anterior 50
  51. 51. teeth are missing, you might have to make a Duralay bridge. Your own ingenuity is your only limitation. When posterior teeth are missing, you may have to make a Forma Tray wafer with cones to contact the soft tissue in very small areas. A little thought will allow you to handle almost any situation. In the laboratory, the upper cast with the split cast wafer is attached to the upper bow of the articulator, related by a face-bow transfer. A centric relation record is used to relate the lower cast to the lower bow of the articulator. When the mounting in completed, we are read;' to verify our results. Open the articulator separating the two halves of the split cast arrangement. Make sure the centric relation record is satisfactorily seated between the upper and lower casts, and close the articulator. The split cast should come together perfectly. This assures a correct mounting. Next we must verify the centric relation records. Remove the record used for mounting the lower cast and replace it with one of the other records. Again, make sure of correct seating of the casts in the record and close the articulator. The split cast should go together as it did with the first record. Repeat with the third record. If ail three records check out the same, there can be no doubt about the accuracy of the centric relation you obtained. The Jones Bite Frame: Another method of taking an interocclusal record is with the Jones Bite Frame. After the patient has been "trained" with the bite jig. the recording is made with a zinc oxide and eugenol paste, taken in a gauze "sandwich." This is a rather tricky, but accurate, procedure. A Jones Adjustable Bite Frame is used to carry gauze strips. These are glued to a thin wire insulation known as "spaghetti" in the radio trade. The insulation tubes with the gauze strips attached are cut into 1 1/2 -inch lengths. These are slipped on the wire frame, which is adjusted 51
  52. 52. according to the size of the patient's jaw. This is best done on a cast of the upper jaw. A zinc oxide and eugeno! paste, such as Opotow's Mandibular Paste? or Kerr's Registration Paste is used. The mixed paste is placed on the surface of the gauze, and the gauze, which is about 3 inches long, is wrapped around the outside wire of the frame and then around the inside wire until there is no loose end. In other words, the gauze, with paste on its surface, is wrapped around the buccal and lingual wires of the frame. This neatly wraps the paste between the gauze and between the buccal and lingual wires. A slight amount of paste may be applied on the upper and lower surface of this roll, on both sides of the frame. It will take a little practice to learn to manipulate the paste without becoming entangled in it. The patient is retracted, after being trained with the jig, and the loaded bite frame is inserted between the teeth. The patient closes firmly against the jig until the paste sets (fig-23). The lower jaw is supported by your thumb and first two fingers to ensure against patient relaxation. The set gauze "sandwich" is removed and carefully trimmed (fig-24), This record is used tc accurately relate the lower cast to the upper cast. It is best used to remount procedures where the actual restorations are involved. VERTICAL DIMENSION A simple rule to help us determine the vertical dimension of occlusion on patients with natural teeth is: do not change the vertical dimension of occlusion that the patient has when the teeth are intcrcuspated in maximum contact. Another rule that can be used with natural teeth to keep out of trouble is: do not open the bite. Bite raising refers to increasing the vertical dimension of occlusion. It is usual!; done for one of the following reasons: 52
  53. 53. 1. To relieve a temporomandibular joint syndrome 2. To restore "lost" vertical dimension in a severely worn occlusion 3. To get rid of facial wrinkles None of these reasons is valid: Opening the vertical in each of these situations is an invitation to problems. U is almost always contraindicated. Some facts should be understood about each of these problems before any treatment is considered. Bite raising for temporomandibular joint syndrome: The vertical dimension has nothing to do with temporomandibular joint syndromes. The pain-dysfunction syndrome can be solved at any vertical dimension up to the point of condylar translation and down to the point of coronoid impingement. As long as the condyles are free to go to their terminal hinge position, the syndrome can be relieved. Correcting the occlusion at an increased vertical may eliminate Lhe joint pain but it almost always results in depression of the teeth, instability of the occlusion, and excessive stresses on the periodontium. Besides, the temporomandibular joint syndrome often recurs as the teeth shift under the added stress. Restoring "lost" vertical dimension: More study is needed, put much clinical evidence indicates that even severely worn occlusions do not lose vertical dimension. Restoring "lost" vertical dimension in a worn occlusion really amounts to opening the bite because wear does not normally produce a loss of vertical dimension. Patients can wear their teeth down to the gum line and still not lose vertical dimension, because the eruptive process matches the wear to maintain the original vertical dimension. 53
  54. 54. This process of eruption and alveolar development may continue throughout life as teeth are worn because of the continual addition of layers of cementum on the root and concurrent passive vertical development of the alveolar process. So even with wear the jaw-to-jaw relationship remains the same when the teeth are together. Opening the bite to eliminate facial wrinkles: On patients with natural opposing teeth, this procedure may have very detrimental effects. When the masticatory and facial muscles are at rest, the teeth should not be in contact. Increasing the vertical dimension to the extent of stretching the wrinkles out puts such an unnatural demand on the stretched muscles that It may actually accelerate further wrinkling. The increased length of the teeth positions them in continuous interference to both normal contracting and resting lengths of the muscles. Such continuous stretch stimulation may cause reflex contraction of the muscles with damaging results to the teeth and supporting structures. The stresses exerted on the teeth are amplified by unfavorable crown root ratios that result from increasing the length of the clinical crowns. Furthermore, the effect on the continuously stretched muscle is to "age" it faster and produce worse wrinkles. Patients who have previously had bite raising procedures to eliminate wrinkles are often very insistent about further increases. As the teeth depress or the wrinkles return, they express the need for more and more increase in vertical dimension. Some patients tell us they were more comfortable when the bite was first raised and they would like to regain that comfort. It is difficult not to give in to such z request because it sounds so reasonable. If we understand that their early comfort was the result of an improved occlusal relationship rather than the increased vertical dimension, we can almost always regain the comfort by equilibration without further increase of vertical dimension. 54
  55. 55. The patient must be made to understand that the muscles should be allowed to position the jaw without interference from the teeth. "Support" from the teeth at an opened vertical dimension constitutes an interference to the contracted muscle in a normal power stroke. Why not increase vertical dimension? Occlusions get into trouble primarily from stress. The safest approach when restoring an occlusion is to keep the teeth from interfering with normal muscle activity. When a muscle is neither hypotonic nor hypertonic, it is said to be "at rest". Even resting muscle is in a mild state of contraction. This mild contraction of antagonistic muscles is necessary to maintain the posture and position of the bony parts. We cannot contract one muscle beyond its resting length without affecting its antagonistic muscle to some degree. The antagonist must release and give the contracting muscle its way or it may respond by isometrically contracting more forcefully itself to counterbalance the effect of its antagonist. Either way, the harmony of resting muscle is disturbed. Any restoration, appliance or denture that interferes with the optimum lengths of the resting muscles serves as a stimulus that produces hypertonicity. Such hypertonicity may result clinically in destructive clenching or bruxism patterns. Many years 'ago, Niswonger defined the rest or postural position as "that position of the mandible in which it is involuntarily suspended by the reciprocal coordination of the muscles of mastication and the depressor muscles with the upper and lower (teeth) separated". He referred to this as a neutral position of the mandible. The rest position has often been a popular starting point for determining the occlusal vertical dimension, but it is an unreliable approach because the dimension between the rest position and occlusal contact is not a consistent 55
  56. 56. measurement for different patients. The rest position itself is not consistent. Atwood found variations as great as 4mm, at the same sitting and even greater variations at different sittings. Finding the vertical dimension of the rest position and then arbitrarily closing a specific amount is a very unsatisfactory approach. If the occlusal vertical dimension can be established in harmony with the optimum length of contracting muscles, the muscles will be free to rest at whatever length is comfortable. The practical approach therefore is to concentrate on accurately recording the occlusal vertical dimension and allowing the freeway space to be the natural result of the difference between the optimum length of contracted muscles and the length of the muscles at rest. Stoneking has proposed that the definition for occlusal vertical dimension be: "The vertical relationship of the dental arches when there is maximum inlercuspation of the natural teeth, and the mandibular muscles arc contracting through their maximum power cycle". Some muscles may contract as much as 50% to 75% of their natural length. Mahan has pointed out that the maximum force with which muscle resists elongation is applied when it is completely committed to contraction. It is also apparent that an increase in the vertical dimension would interfere with the optimum length of contracting muscle in its power stroke. Several studies have shown that there is a significant relationship between the power point" of muscular contraction and repcatable phonetic and comfort measurements. Tueller, using electronic means on dentures, found an average variation of less than 0.5mm from the vertical established at the muscular power point when compared with either preextraction records or phonetic methods. 56

×