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Retention /certified fixed orthodontic courses by Indian dental academy


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Retention /certified fixed orthodontic courses by Indian dental academy

  3. 3. Optimal outcome of the prosthetic treatment depends on the successful integration of the prosthesis with patients oral functions as well as the psychological acceptance of the prosthesis by the patient. These parameters require that the patients perceive their prosthesis as stationary & well retained during function & that the prosthesis & their effects on the face meet the psychodynamic requirements of the patients.
  4. 4. • COMPLETE DENTURES RETENTION • Retention (G.P.T) has been defined as “That quality inherent in the prosthesis acting to resist the forces of dislodgement along the path of placement.”Complete denture retention has been defined as “Resistance to removal of the denture base in a direction opposite to that of its insertion” and as “That quality inherent in the prosthesis acting to resist the forces of dislodgment along the path of insertion”. In effect, retention relates to the forces that are necessary to completely remove the denture from its basal seat.
  5. 5. • From this it is clear that then, ordinarily retention is regarded as a property of the denture rather than that of the patient. If a denture is easily dislodged during speech or eating, the embarrassment experienced can be mentally traumatic. A retentive denture contributes dramatically to the patients acceptance of the finished prosthesis.
  6. 6. Historically, prosthodontists have sought to improve the quality of denture treatment through an understanding and application of the factors involved in retention. Disagreements regarding the relative importance of various factors do exist. Improvements in denture base materials and in fabrication techniques stemmed from the simple realization that better fit meant great comfort and more efficient function.
  7. 7. • The prospect of retention without mechanical aid was not seriously considered in the 18th and early 19th centuries. With the advent of the well-fitting base came the awareness that bases could be, and the gradual acceptance that they should be, retained by non-mechanical means, inevitably various efforts were made to explain and better understand the physics of the observed clinical phenomena. • Hence complete denture retention may be described as that state of a denture where functional forces are unable to destroy the attachment between denture and mucoperiosteum.
  8. 8. It is difficult to consider stability and retention as separate entities because of the close dependency of the one upon the other. A lack of either property will influence the effectiveness of the other to a great extent. Although evaluation precedence of a over both good are indispensable denture, retention as prerequisite in denture construction. stability the in the takes paramount
  9. 9. Retention in a denture not only enhances its stability, but also it helps to meet the various psychologic problems encountered by the patient during the learning or the re-educating period. Such psychologic problems might include the fear, apprehension, and embarrassment caused by unsatisfactory denture retention. Thus, retention supplementing stability will achieve finished dentures which satisfies the physical, physiologic and psychologic needs of the patient.
  10. 10. • FACTORS GOVERNING RETENTION OF COMPLETE DENTURES: I. Positive physical factors: • Adhesion • Cohesion • Interfacial force • Atmospheric pressure • Gravity II. Physiological factors: • Role of saliva • Neuro muscular control • Ridge characteristics and relationships
  11. 11. III. Mechanical factors: • Leverage • Occlusion • Shape of polished surface • Under cuts • Surface roughness IV. Psychological factors V. Surgical factors • Ridge extension • Implant supported dentures.
  12. 12. POSITIVE PHYSICAL FACTORS Adhesion: Adhesion is the physical force involved in the attraction between unlike molecules. A drop of water introduced on the surface of a solid glass plate will resist movement away from the glass in proportion to the adhesion between the unlike materials. Fuller, A London dentist has been stated to mention the word “Adhesion” in retention to maxillary denture retention. Adhesion of salvia to the mucous membrane and the denture base is achieved through ionic forces between charged salivary glycoproteins and surface epithelium or acylic resin. By promoting the contact of saliva to both oral tissue and dentures base, adhesion works to enhance further the retentive force of interfacial surface tension.
  13. 13. The quality of denture adhesion depends on the: i. Close adaptation supporting tissues: of denture base to Impression techniques determine the degree of intimate contact with the tissue at rest and during function. It also prevents ingress of air which decreases the retention. ii. Fluidity of saliva: Saliva should be search in nature and wet the complete denture surface area.
  14. 14. iii. Area of contacting surface: Greater the area of contact, greater will be the retention. Mandibular dentures cover less surface area than maxillary prostheses and, therefore, are subject to a lower magnitude of adhesive (and other) retentive forces. Similarly, patients with small jaws or very flat alveolar ridges (small basal seats) cannot expect retention to be as great as can patients with large jaws or prominent alveoli. Thus, the dentures (and hence the impressions that serve as the patient analogue for their fabrication) should be extended to the limits of the health and function of the oral tissues, and efforts should at all times be made to preserve the alveolar height to maximize retention.
  15. 15. iv. Shape of denture bearing area: The forces act most powerfully at right angles to the surface. So, flat palate will provide good surface adhesion. A version of adhesion is observed between denture bases and mucous membranes themselves, which is the situation in patients with xerostamia (sparse or absent saliva). The denture base materials seen to stick to the dry mucous membrane of the basal seat and other oral surfaces. Such adhesion is not very effective for retaining dentures, and predisposes to mucosal abrasions and ulcerations due to lack of salivary lubrication.
  16. 16. It is annoying to the patients to have denture bases stick to lips, cheek and tongue. An ethanol free rinse containing aloe or lanolin, or a water soluble, lubricating jelly, can be helpful in this situation. For patients whose mouths are dry due to irradiation or auto immune disorders like Sjogrens syndrome, salivary stimulation through a prescription of 5-10mg of oral Pilocarpine 3 times daily can be beneficial if patient can tolerate likely side effects of increased perspiration and excess lacrimation.
  17. 17. Cohesion: Cohesion is the physical attraction of like molecules for each other. It is a retentive force because it occurs within the layer of fluid (saliva) that is present between the denture base and the mucosa, and works to maintain the integrity of the interposed fluid. The property of cohesion is effective in direct proportion to the area covered by the denture, if other factors are equal. According to Jacobson and Krol, cohesion is a physical factor of electromagnetic force acting between the molecules of the same material. A molecule within a fluid has an attraction exerted on it on all sides by neighboring molecules.
  18. 18. Forces of cohesion are responsible for maintaining the continuity of a water droplet when placed in contact with another material. Normal saliva is not very cohesive, so that most of the retentive force of the denture – mucosa interface comes from adhesive and interfacial factors unless the interposed saliva is modified (as it an be with the use of denture adhesive).
  19. 19. Thick high mucin saliva is more viscous than thin watery saliva – yet thick secretions usually do not result in increased retention for the following reason – Watery, serous saliva can be interposed in a thinner area than the more cohesive mucin secretions. Stefans law makes it clear, all other factors being equal, that increase in fluid viscosity cannot be accompanied by an equal increase in film thickness if displacement force is to be kept same. The combined effect of adhesion and cohesion will provide better retention.
  20. 20. Interfacial force: It is the resistance to separation of two parallel surfaces that is imparted by a film of liquid between them. A discussion of interfacial forces is best broken into separate comments on interfacial surface tension and viscous tension.
  21. 21. Interfacial surface tension: It is defined as the force that maintains the surface continuity of a fluid. It results from a thin layer of fluid that is present between two parallel planes of rigid material. It is dependent on the ability of the fluid to “wet” the rigid surrounding material. The cohesive attraction between molecules is balanced in equilibrium within the fluid. At the surface, the absence of the neighboring molecules creates the one sided attraction imbalance that causes surface tension. If the surrounding material has low surface tension, as oral mucosa does, fluid will maximize its contact with the material, thereby wetting it readily and spreading out in a thin film.
  22. 22. If the material, with the result that it will form beads on the material’s surface. Most denture base materials have higher surface tension than oral mucosa, but once coated by salivary pellicle they display low surface tension that promotes maximizing the surface area between liquid and base. The thin fluid film between denture base and the mucosa of the basal seat therefore furnishes a retentive force by virtue of the tendency of fluid to maximize its contact with both surfaces. It is a relatively small force when considered alone but by interaction with other physical factors, it becomes an important determinant.
  23. 23. Another method to explain the role of surface tension in denture retention is by describing capillary attraction or capillarity. Capillarity is what cause a liquid to rise in a capillary tube, because in this physical setting the liquid will maximize its contact with the walls of the capillary tube, thereby rising along the tube wall at the interface between liquid and air. When the adaptation of the denture base to the mucosa on which it rests is sufficiently close, the space filled with a thin film of saliva acts like a capillary tube in that the liquid seeks to increase its contact with both the denture and the mucosal surface. In this way, capillarity will help to retain the denture.
  24. 24. Interfacial surface tension may not play as important a role in retaining the mandibular denture as it does for the maxillary one. Interfacial surface tension is dependent on the existence of a liquid / air interface at the terminus of the liquid/ solid contact; if the two plates with interposed fluid are immersed in the same fluid, there will be no resistance to pulling them apart. In many patients, there is sufficient saliva to keep the external borders of the mandible denture awash in saliva, thereby eliminating the effect of interfacial surface tension. This is not so in the maxilla. A simple system to explain interfacial surface tension involves the attraction of two glass slabs placed in direct apposition with an interposed fluid film. In 1948, Stanitz27 used this model to explain the part played by the fluid film in denture retention.
  25. 25. In review, the phenomenon of surface tension is defined as the force that maintains the surface continuity of a fluid. This results from the imbalance in cohesive forces between molecules present at the surface. The cohesive attraction between molecules is balanced in equilibrium within the fluid. At the surface the absence of neighbouring molecules creates the onesided attraction and imbalance that causes a free potential energy called surface tension.
  26. 26. When water rises vertically in a column within a capillary tube standing in an open container of water, the fluid pressure within the water at the height of the column is les that at the base. The pressure at the base of the column is equal to atmospheric pressure, and therefore the pressure at the height of the column is less than atmospheric pressure. This phenomenon creates a pressure gradient across the meniscus. Although it is the forces of adhesion and cohesion that cause the water to rise in the tube, it is the forces of surface tension that maintain the difference in pressure across the meniscus.
  27. 27. Atmospheric pressure (Pa) is in equilibrium with fluid pressure exerted on molecules within capillary tube at level of liquid in container. Therefore pressure on molecules along dotted line (A) is equal to Pa. Fluid pressure exerted on molecules at higher level (B) is less than at level A in proportion to distance between A and B. Because B is less than A, B is less than Pa, which indicates presence of a pressure gradient across meniscus which is maintained by surface tension.
  28. 28. Interfacial viscous tension: It refers to the force holding two parallel plates together that is due to the viscosity of the interposed liquid. Viscous tension is described by Stefan’s Law. For two parallel, circular plates of r that are separated by a Newtonian (incompressible) liquid of viscosity k, and thickness h, this principle states that the force (F) necessary to pull the plates apart at a velocity V in a direction perpendicular to the radius will be F = (3/2) Π kr4 H3 x V
  29. 29. The relationship expressed by Stefan’s law makes it clear that the viscous force increases proportionally to the increase in the viscosity of the interposed fluid. The viscous force drops off readily as the distance between the plates (i.e., the thickness of the interposed medium) increases. The force increases proportionally to the square of the area of the opposing surfaces. When applied to denture retention, the equation demonstrates the essential importance of an optimal adaptation between the denture and the basal seat (a minimal h), the advantage of maximizing the surface area covered by the denture (a maximum r), and the theoretical improvement in retention made possible by increasing the viscosity of the medium between the denture and its seat. It also explains why a slow, steady displacing action (small V) may encounter less resistance and, therefore, be more effective at removing a denture than is a sharp attempt at displacement (large V). In application, interfacial forces are further enhanced through ionic forces developed between the fluid and the surrounding surfaces (adhesion) and the forces holding the fluid molecules to each other (cohesion).
  30. 30. Atmospheric pressure: Atmospheric pressure is the physical factor of hydrostatic pressure due to the weight of the atmosphere on the earth’s surface. At seal level this force amounts to 14.7 psi. Atmospheric pressure can act to resist dislodging forces applied to dentures, if the dentures have an effective seal around their borders. This resistance force has been called “suction” because it is a resistance to the removal of dentures from their basal seat; but there is no suction or negative pressure, except when another force is applied (suction alone applied to the soft tissues of the oral cavity for even a short time would cause serious damage to the health of the soft tissues under negative pressure).
  31. 31. For example, a suction cup pressed against a pane of glass stays in place because the rubber of the squeezed cup elastically seeks to return to a larger shape, thereby causing air pressure within the cup to be less than the pressure outside the cup. A denture cannot be distorted like a suction cup, but oral mucosa can be. When a force is exerted perpendicular to and away from the basal seat of a properly extended and fully seated denture, pressure between the prosthesis and the basal tissues drops below the ambient pressure, resisting displacement. Retention due to atmospheric pressure is directly proportional to the area covered by the denture base. For atmospheric pressure to be effective, the denture must have a perfect seal around its entire border. Proper border molding with physiological, selective pressure techniques is essential for taking advantage of this retentive mechanism.
  32. 32. A conclusive clinical study by Snyder et al, in 1945 demonstrated the effect of reduced atmospheric pressure on the retention of maxillary complete dentures constructed for seven patients. Measurements made in a pressure chamber at 4.7 psi simulating a 30,000 – foot ascent above the earth demonstrated a decrease in denture retention. With a 70% decrease in atmospheric pressure, a 50% decrease in retention was noted. At sea level the force of atmospheric pressure acts with approximately 14.7 psi against the external surface of the denture provided no air or gaseous pressure exists between the denture base and the tissue surface.
  33. 33. Gravity: The weight of the prosthesis constitutes a gravitational force that is insignificant in comparison with the other forces acting on a denture. But if a maxillary denture is fabricated wholly or partially of a material that increases its weight appreciably (eg., A metal base or precious metal posterior occlusal surfaces), the weight of the prosthesis may work to unseat, it, if other retentive forces themselves are suboptimal. Increasing the weight of the mandibular denture (through addition of a metallic base, insert or occlusal surfaces) may seem theoretically capable of taking advantage of gravity. Anecdoteal evidence suggest that this may indeed prove beneficial in case where other retentive forces and factors of retention into play when constructing a lower denture, gravity aids in providing the necessary force to maintain the prosthesis in place at rest. Grunewald recommended gold base complete dentures of a weight similar to that of the lost teeth and alveolar tissues. Such a technique would enhance the effectiveness of gravity on the retentive properties of the prosthesis.
  34. 34. PHYSIOLOGIC FACTORS Role of saliva: Atmospheric pressure, or “suction” as it is incorrectly called, contributes very little to the retention of a denture until an attempt is made to move it away from the tissue. Then, provided that the saliva film remains intact, a reduction in pressure occurs between denture and tissues and atmospheric pressure resists displacement of the denture. If the saliva film breaks down and air enters he space between denture and tissues, the denture is no longer retained. Hence it is important to exclude as much air as possible from the saliva film.
  35. 35. Some dissolved air always remains, but if air bubbles are present these expand rapidly and connect up with the atmosphere thus producing a loss of retention. As pockets remain in areas of poor adaptation, deep relief chambers or places where the denture has been adjusted away from a deep undercut. When only a small space has been created, it fills with saliva. A larger space, however, usually contains air and this reduces denture retention.
  36. 36. The internal pressure within a liquid is slightly less than that of the surrounding atmosphere, hence the formation of a meniscus at the interface. It is argued, therefore, that there is slight retentive pressure from the atmosphere provided that the saliva film is intact. However other writers suggest that no true meniscus is present, as the entire surface of the denture is coated with saliva.
  37. 37. Viscosity and volume of saliva: The relationship between the denture and the tissues is a dynamic one. Whenever the denture tries to move, the viscosity of the interposed saliva film resist or dampens this movement. Viscosity of the saliva depends upon its mucin content. Parotid secretion is mainly serous and therefore the secretion of the mandibular and the sublingual glands is the more important for denture retention.
  38. 38. Mucin is also secreted from the palatal glands and lies between denture and tissues, flowing slowly to the periphery. Here it remains in the sulcus or on the soft palate until it is swallowed. A thin film of saliva resist flow much more readily than a thicker film. Resistance to flow varies inversely as the cube of the film thickness. Thus by halving the gap between denture and tissue, the retentive force due to resistance to saliva flow is increased eightfold.
  39. 39. If the saliva itself has a high viscosity, it resists flow more effectively. Hence the use of denture fixatives or adhesives produce a large increase in the viscosity of saliva. Unfortunately with the forces usually generated in the mouth, a saliva of such high viscosity cannot be compressed to a thin film. Therefore a marked increase in viscosity is necessary to produce an effect similar to that of decreasing the film thickness. The danger of using denture adhesives is that whilst a thin film of saliva is developed initially, this gradually increases in thickness and the patient finds difficulty in reducing it down again to a narrow section.
  40. 40. Unless there is plenty of further highly viscous saliva available in the mouth, then as soon as the denture is tilted or is moved away from the tissues, there is a shortage of saliva, air enters the space, and the denture falls. This is what usually happens when a denture adhesive is in use as the adhesive – thickened saliva beyond the border of the denture is swallowed and does not remain in the mouth.
  41. 41. If a denture is loaded at one side, then the saliva film is thinned on this side and the denture may attempt to move away from the tissues on the opposite side. Here any extra saliva from the sulcus can flow in and so maintain the continuity of the saliva film in the increasing space. Adequate saliva volume is necessary and retention is therefore poor in the mouths of patients whose saliva volume is low. The rate at which displacement of the denture is attempted is also important. To a force applied suddenly, there is little time for saliva flow to occur and the denture is displaced. If a much smaller force is applied continuously, however, flow takes place and the denture remains in place. To study the influence that saliva might have on the adhesion between on upper denture and the mucosa, we may consider the adhesion mechanism between two glass plates with a thin layer of fluid between them. Let us suppose that the fluid consists of distilled water.
  42. 42. If the plates are held horizontally, the intermediate layer of fluid in the periphery of the plate will be limited by a free layer of fluid, the so called meniscus. The form of this meniscus depends on the pressure within the fluid at the time of the examination. If the plates are closer to one another (greater pressure in the fluid compared with the atmospheric pressure), the meniscus will bulge out and attempts to separate the plates will cause and inward bulge of the fluid meniscus (reduced pressure). The following physical factors must be considered in this respect, viz., adhesion, cohesion, and surface tension. Measurable factors active in the adhesion of two glass plates are adhesion and surface tension. The surface tension plays a role in the fixation has already been stressed by various authors, including Schultze (1921), who formulated the equation. K= 2axy b Where K = fixation force a = surface tension coefficient y = surface of the plates between the plates b = distance
  43. 43. Measurement of the force necessary to separate two glass plates with an intermediate layer of water and fresh mixed saliva, respectively, will show that separation requires a greater force if the intermediate layer consists of saliva. How can this be explained? Since the surface tension of saliva is lower than that of the water?. The meniscus created by the surface tension will act as spring all around the edges of the plates, and the tension of that spring will be directly correlated to the coefficient of the surface tension. This is a very important factor that holds the plates together. When a separating force exceeds the elasticity modulus of the fluid meniscus, the meniscus breaks and an intense flow in the intermediate layer of fluid will occur. This divides the layer of fluid into two parts, each of which adheres to the glass plates.
  44. 44. Thin fluid film exists (shaded area) between denture base and tissues of residual ridges. Meniscus develops at border of denture. Note that position of meniscus will depend on where soft tissue loses contact with denture border. Draping effect of cheeks may provide a meniscus along polished surface of denture border (A). When cheek is retracted, meniscus is developed at denture border (B). The flow of the fluid is, however, diminished by an increased viscosity. This explains why fresh saliva, despite its lower surface tension, gives stronger adhesion between the glass plates, i.e., the rate of flow is lowered by the high viscosity of the saliva owing to its mucoid content. The higher the viscosity, the lower the rate of flow and the greater the fixation power.
  45. 45. Conditions which increase retention. From an analysis of the effects of the saliva film on retention, there are three important factors. The thickness of the saliva film: Retention ∝ 1 Saliva film thickness3 The viscosity of the fluid film: Retention ∝ saliva viscosity. The larger the surface area, the better the retention: Retention ∝ area2
  46. 46. The best conditions for denture retention are therefore: • A fully extended denture (with border seal, including post dam). • A closely adapted denture (minimal saliva film thickness) • A saliva of medium viscosity which can be compressed to a very thin film by the normal intra-oral forces. • A saliva of adequate volume.
  47. 47. Neuro muscular control: Dentures are always foreign bodes in the mouth and when fitted for the first time, most muscular actions tend to expel them. Gradually however the wearer learns to differentiate between the food and denture Dentures are always foreign bodes in the mouth and when fitted for the first time, most muscular actions tend to expel them. Gradually however the wearer learns to differentiate between the food and denture and at first consciously but later subconsciously, to control and stabilize them. Neuromuscular control refers to the functional forces exerted by the musculature of the patient that can affect retention. This is primarily a learned biologic phenomenon. Certain characteristics can be incorporated into the external controls of the denture base to promote neuromuscular control.
  48. 48. The oral and facial musculature supply supplementary retentive forces, provided (i) the teeth are positioned in the ‘neutral zone’ between the cheeks and the tongue (ii) the polished surfaces of the dentures are properly shaped. (iii) the denture bases must be properly extended to cover the maximum area possible without interfering in the health and function of the structures that surround the denture (iv) the occlusal plane must be at the correct level and (3) the arch form of the teeth must be in the “neutral zone” between the tongue and the cheeks.
  49. 49. This is not to say that patients must hold their prosthetic teeth in place by conscious effort, only that the shape of the buccal and lingual flanges must make it possible for the musculature to fit automatically against the denture and thereby to reinforce the border seal. One of the objectives in impression making and arch form design is the harnessing of a patients unconscious tissue behaviour to enhance both retention and stability of the prosthesis. If the buccal flanges of the maxillary denture slope up and out from the occlusal surface of the teeth and the buccal flanges of the mandibular denture slope down and out from the occlusal plane, the contraction of the buccinator will tend to seat both dentures on their basal seats.
  50. 50. The lingual surfaces of the lingual flanges should slope towards the center of the mouth so that the tongue can fit against them and perfect the border seal on the lingual side of the denture. The base of the tongue is guided on top of the lingual flange by the lingual side of the distal end of the flange, which turns laterally toward the ramus. This part of the denture also helps ensure the border seal at the back end of the mandibular denture. The base of the tongue also may serve as an emergency retentive force for some patients. It rises up at the back and presses against the distal border of the maxillary denture during incision of food by the anterior teeth. This is done without conscious effort. It is seldom that a patient needs to be taught to do this. For the oral and facial musculature to be most effective in providing retention for complete dentures, the following conditions must be met:
  51. 51. Every prosthodontist recognize the ability of certain patients to wear their dentures and function without complaint despite the fact that they may be extremely ill fitting, unstable, or even broken. The biologic factor of neuromuscular control gradually becomes a major determinant in complete denture retention as experienced patients learn to alter their muscular function to harmonize with the prosthesis. The fields of oral perception, sensation and proprioception are currently being researched.
  52. 52. Individuals appear to vary in their ability to develop the motor coordination and conditioned reflexes necessary to manipulate intraoral prostheses. While some patients are able to adapt to restorations that seem to be unacceptable, others appear to have difficulty learning to control any dentures, regardless of the contours, design, or occlusion. It is muscular control that enables patients to function with dentures that rest on basal tissues that have undergone resorptive changes and no longer related to the intaglio of the denture base.
  53. 53. Studies by Brill et al demonstrate that older patients have more difficulty adjusting to new complete dentures. This may result from the progressive cerebral atrophy that affects related neurologic systems. They also demonstrated the dramatic denture decrease retention in that mandibular accompanied complete local anesthesia of the oral mucosa in experienced denture patients. This was especially marked in those patients with severely compromised residual ridge height and conformation.
  54. 54. The normal activity of muscle is dependent on different impulses originating in proprioceptors recording changes in muscle tendons and joints. More over they depend on other different impulses originating in exteroceptors, which record changes in the oral environment. The touch receptors found in the oral cavity, including those of the tongue, are particularly concerned with denture retention. It is believed that importance in these receptors properly are adjusted coordination of lips, cheeks and tongue. of great muscular
  55. 55.
  56. 56. Ridge characteristics and relationship : Types of alveolar ridges and palate formation and their significance on retention. Maxillary denture bearing area: 1. Well developed but not abnormally thick ridges and a palate with a moderate vault. 2. High V – shaped palate usually associated with thick bulky ridges. 3. Flat palate with small ridges and shallow sulci 4. Ridges exhibiting undercut areas:
  57. 57. Mandibular denture bearing area: 1. Broad and well –developed ridges: 2.Ridges exhibiting undercut areas: 3.Well –developed but narrow or knife-like ridges.
  58. 58. Ridge forms and retention of denture: their influence on In a discussion of physical considerations of retention, ridge forms and their influence on the retention of dentures should be considered. One should be able to predict the degree of retention by the shape of the ridge. Class IA: Inverted U-shaped ridge: The inverted U-shaped ridge permits a very retentive lower denture. The sides are parallel and when the denture slides upward it hugs them. At the crest, there is fairly lat surface being pulled apart at right angles to the surface. This resists separation.
  59. 59. Class IB: Flat inverted U-Shaped ridge: The flat inverted U-shaped ridge presents shorter parallel walls. It, therefore, resist dislodgment for a short distance only when the pull is on a upward direction. After that the entire denture leaves the ridge. There is a tendency for the denture to slide since the flange are short. Class IC: U-Shaped: The U-shaped ridge presents very little retention in comparison with class IA, but it will resist displacement in a upper direction. It will have a tendency to slide. Articulation must be very accurate in these cases to prevent movement.
  60. 60. Class 2: Inverted V-shaped ridge: The inverted V-shaped ridge is probably the least retentive of all. The moment the denture moves in an upward direction it leaves the entire ridge at once. There is no parallelism. Also, the direction of force is such that separation from the ridge is easier. Class 3A : Parallel walled thin ridge: The retention is not very great in a parallel walled thin ridge because of the small area at the crest of the ridge. Although the parallel walls of the denture hug the sides of the ridge and create some vaccumatic space, this is too small to account for much of the retention.
  61. 61. Class 3B: Parallel walled ridge, broad crested: The broad crested parallel walled ridge is the most retentive of all. It is a combination of the longer parallel walls and the broad crest. The broader crest has the two features of creating a vacuumatic space as well as resisting separation with the direction of force at right angles. Class 4: Flabby tissue: Flabby ridges play very little part in disturbing stability and retention.
  62. 62.
  63. 63. Classification based on ridge relations and retention: The relations of the residual ridges may be classified as: 1. Protruded residual ridge. 2.Both residual ridges are protruded 3.Mandibular residual ridge is protruded. 4.Under developed mandible. 5.Retruded mandible.
  64. 64.
  65. 65. The maxillary and mandibular ridges should be observed at he appropriate occlusal vertical dimension. The amount of inter ridge distance should be noted. An excessive amount of space due to resorption will result in poor stability and retention because of the increased leverage. A small amount of interridge distance will lead to difficulty in setting teeth and maintaining a poor free way space. However this condition greatly enhances the stability of the dentures because the occlusal surfaces of the teeth are close to the ridge minimizing leverage, tilt and tongue forces. Ridges that are not parallel to each other will cause movement of the bases when the teeth occlude because of an unfavourable direction of forces.
  66. 66. The ridges should also be observed in their anteroposterior and lateral relationships. As the maxilla resorbs, the crest of the ridge appears to move downward, forward and laterally because it is wider at its inferior border than at its occlusal border. This condition could be further compromised by a prognathtic jaw relationship and this accentuates the importance of tooth placement to maintain esthetics and minimize undesirable leverage. When maxillo mandibular or ridge relations are not normal, they dictate a different occlusal relation of the teeth. One must remember that the medial and lateral positions of the teeth must provide acceptable anatomic and physiologic limits.
  67. 67. Peripheral seal Border seal: The border or periphery of the denture provides ample opportunity for the ingress of air. Correct adaptation of the sulcus tissues to the inside of the cheeks and lips insures a border or valve seal. During a small displacement of the denture, the soft tissue of lips and cheeks move inwards under atmospheric pressure and maintain contact with the denture, thus preventing the ingress of air. Overtrimming of the border to provide relief for frena allows easy ingress of air as does an under extension of the denture into the sulcus. Both extension and width of the border are important. If the denture flange does not fill the sulcus laterally, air collects in the space left and valve seal is lost. Lateral filling of the sulcus is particularly important in the upper tuberosity regions.
  68. 68.
  69. 69. In the lower jaw there is a relatively long border for a small area. Consequently the potentially for air leakage is high. Seal of the tissue against the lingual aspect of the lower denture is difficult to achieve as also is seal at the distal end of a lower denture where it covers part of the retromolar pad. Usually, therefore, border seal in the case of the lower denture is poor. In the upper jaw, the posterior palatal border is similarly the weakness part of the border seal, as only as slight movement of the denture away from the tissue allows the ingress of air and breakdown of the saliva film. A correctly positioned and shaped post dam is therefore essential to maintain retention. Increased pressure on the retromolar pads by the lower denture may have a similar effect.
  70. 70. MECHANICAL FACTORS LEVERAGE: The three classes of levers are well known and are used in many operations. Although many simple tasks, such as using a crowbar to lift a rock and squeezing on a nutcracker, are accomplished by simple levers in one of these three classes, the act of chewing with artificial dentures involves a multiple lever system. It is the problem of the dentistengineer to prevent or minimize leverage when its operation would be unfavourable and to establish or increase it when its operation would be beneficial. Anteroposterior point of application of muscle pull on mandible. This point should be known in order that the working occlusal surfaces may be placed in their most favourable positions on the lower alveolar ridge (The upper areas of support extend further, both anteriorly and Posteriorly, than the lower ones and ordinary need not be considered in this connection).
  71. 71. The point of application is controlled to some degree by the patient as a result of nerve impulses acting more or less independently on the various muscle fiber groups. However, the significant fact in connection with artificial denture restoration is that the point where muscle pull is applied on the mandible is always behind the center of the denture supporting areas and probably is always behind the distal ends of these areas. Thus, as pointed out previously, with the resistance on the anterior teeth the mandible is a leaver of the third class, the point of application of muscle pull lies between the condyle head and the resistant mass of food, resulting in pressure upon
  72. 72. Yielding of foundation – the tissues covered by the denture base are neither rigid nor uniformly yielding. They are neither permanently nor even temporarily stable in form. Under intermittent chewing force they change form, both because of their elasticity and because of changes in volume of the contained liquid. The fact that the denture base rests on a more or less yielding support causes it to become a lever under certain conditions. This presents the dentist – engineer with problems that he aims to solve with his knowledge of lever action.
  73. 73. Consider the analogy of a saucer resting on a glass surface the saucer remains stable when downward pressure is applied at any point within the circumference of the base. On the other hand, when downward pressure is applied at any point outside the supporting base the saucer becomes a lever, the edge of the base acts as a fulcrum and the opposite side of the saucer is raised. If instead of resting on a glass surface, the saucer rests on a yielding surface ( a layer of soft rubber, for example) tilting of the saucer result whenever downward force is applied at any point except the very center. The further from the center a force is applied, the greater will be the tilting effect. Even when pressure is applied inside the base but not in the exact center, the saucer becomes a lever.
  74. 74. This principle applies also to a denture base but not to the same degree, for although the supporting tissues are never so rigid as glass, they are not often so yielding as soft rubber. Because of the yielding of the denture base’s support, the base becomes a lever. Often the upper base acts as one lever, the lower base is another lever, and the mandible itself as a third lever. Thus, we have a multiple lever system. It is impossible to eliminate leverage in this system ; our problem is to control it.
  75. 75. Leverage of the mandible is made more favourable by placing the working occlusal surfaces back in the mouth nearer the line of force, but leverage of the base on yielding tissues is made more favourable by placing these surfaces at the center of the foundation. Since these two ideals are in conflict, the occlusal areas on which closing force is exerted should be placed in most instances some where between the best position with respect to the bases. This is another situation that demands compromise on the part of the dentist –engineer who respects the requirements of the living tissues with which he is dealing.
  76. 76. Under cuts: Under cuts of the ridge are acceptable if they can be utilized fully by the denture. Unilateral under cuts can be accommodated by selecting a suitable path of insertion of the denture. Bilateral undercuts can only be used to the extent by which the soft tissues over them can be compressed. The resistance of the mucous and sub mucosa overlying the basal bone allows for the existence of modest undercuts that can enhance retention.
  77. 77.
  78. 78. Although exaggerated bony undercuts or less obvious ones covered by thin epithelium may compromise denture. Generally bilateral undercuts of 1-2 mm can be used. Trimming the denture to fit partially into the under cut area produces a space. If this is filled only with saliva, then little retention is lost. If space produced in large, however, an air bubble remains and this reduces retention. Some “undercuts” are only undercut in relationship to a linear path of insertion or relative to a presumed vertical path of insertion. But if the undercut area is seated first (usually in a direction that deviates form the vertical), and the remainder of the denture base can be brought into proximity with the basal seat on rotation of the prosthesis around the undercut part that is already seated, this “rotational path” will provide resistance to vertical displacement
  79. 79. Surface Roughness: In so far as increasing roughness would increase the interfacial area for adhesion between saliva and denture, the strength of that union would be improved. However, since, as stated above, failure does not occur at this site in this way, roughness is irrelevant and can be discounted.
  80. 80. PSYCHOLOGICAL FACTORS Psychological effects on retention: The dentures may have an adverse psychological effect on the patient, and the nervous influences that result may affect the salivary secretions and thus affect retention. Eventually, patients acquire an ability to retain their dentures by means of their oral musculature. This muscular stabilization of dentures is probably accompanied by a reduction in the physical forces used in retaining their dentures. Quite clearly, the physical forces of retention can be improved and reestablished, up to a point, by careful and frequent attention to the denture status. This is done by periodic inspection and by relining and rebasing procedures.
  81. 81. Dentures are always foreign bodies in the mouth and when fitted for the first time most muscular actions tend to expel them. Gradually, however, the wearer learns to differentiate between the food and the dentures and, at first consciously but later subconsciously, to control and stabilize them with the tongue and cheeks. The tongue, by resting on top of the lower denture and pressing it downwards and forwards, can control its tendency to rise, and also counterbalance to a large degree un stabilizing masticatory forces. The tongue can also be uncon­sciously trained to prevent the back edge of the upper denture dropping while the front teeth are incising.The muscular cheeks can be trained, again unconsciously, to press downwards on the buccal flanges of the lower denture, while still carrying out their function of placing food between the teeth.
  82. 82. SURGICAL FACTORS Ridge extension: Alveoloplasty, the surgical reshaping of the alveolar ridge, is indicated where uneven interseptal spines or bilateral bony undercuts exist. Bone removal should always be done with prudence because it is accompanied by varying degrees of bone resorption. The use of ridge augmentation with implant materials should be considered when removal of bony undercuts will results in a deficient ridge.
  83. 83. The use of these materials can both preserve bone and correct the anatomic defect. Removal of interseptal bone and gentle compression of the expanded socket is often all that is required to achieve the goal of a well contoured ridge with the conservation of bone. Excessive removal of bone during multiple extractions should be avoided as a ridge with an inverted “V” shape may result.
  84. 84. In the mandibular arch, an area which often requires alveoloplasty but is frequently overlooked is the lingual aspect at the posterior termination of the mylohyoid ride. The denture flange should extended below this projections area. The should be removal of bilateral.In these the bony atrophic mandible the alveolar processes, because of lateral resorption, frequently presents a thin bony ridge called a “Knife-edge ridge’. The overlying soft tissue is often rolled with base. a mobile fibrous
  85. 85. Denture tooth contact may cause pain and require extensive modification of the denture bas in this area. Previously, surgical procedures to remove or stabilize the mobile soft tissue and recontour the sharp bony ridge left the patient with less vertical tissue height and continued bone resorption frequently leading to a recurrence of the “knife-edge ridge”. Today, ridge augmentation with synthetic implant materials shows great promise to correct this inadequacy.
  86. 86. In the maxillary arch, an area which often requires an alveloplasty is the alveolar tubercles. The tubercles often present opposing bilateral buccal undercuts that become a problem in impression making and, if reproduced, with the insertion and removal of the denture. Removal of these undercuts by grinding form the tissue surface of the denture can lead to retention and food accumulation problems
  87. 87.
  88. 88. . If no undercuts are present in the anterior section of the arch, it is not always necessary to remove the undercuts from both tubercles. Before an undercut is allowed to remain one should be sure to check that adequate exists in a horizontal direction to allow free passage of the coronoid process without crushing or trapping sensitive cheek tissues between it and the denture base. In a vertical direction, the alveolar tubercles frequently approximate the retromolar papilla and pad area to the extent that adequate denture base coverage and correct placement of the occlusal plane is not possible.
  89. 89. Both bony and soft tissue removal should be accomplished where possible to allow adequate vertical height for the denture bases. Care should be exerted to avoid damaging the greater palatine artery or entering the maxillary sinus. Careful presurgical examination to determine the location of the sinus floor is necessary. Gentle repositioning of the sinus floor superiorly and medially can also be accomplished when an enlarged tubercle with a thin sinus wall is evident.
  90. 90. Exostoses are bony nodules located on the alveolar process of the mandible and the maxilla. The buccal aspect in the molar region of the mandible and the buccal aspect from the premolars Posteriorly to the alveolar tubercle in the maxilla are the most frequent locations. These exostoses usually present undercuts to the path of insertion and removal of the denture and should be removed by alveoloplasty techniques.
  91. 91. Tori are bony hyperostoses common to both maxilla and mandible. Small tori that do not act as fulcrum points under a denture may not require removal. The torus, however, even when small, may act as a fulcrum under a denture if the mucosal covering of the crest and slopes of the ridges are displaceable to a greater extent than the mucosal covering of the torus. In these instances, the denture base over the area must be relieved to compensate for the difference or the torus should be surgically removed. When a torus is large, grossly undercut, or located Posteriorly where the post palatal seal is to be placed, it should be surgically removed.
  92. 92.
  93. 93.
  94. 94. Genial tubercles are neither exostoses nor tori but are often prominent following advanced alveolar ridge resorption in the anterior area of the mandible. They are covered by thin tissue which will not bear the pressure of a denture flange located in this area. The superior portion of these prominences may be removed in a fashion similar to the mandibular torus. That portion of the genioglossus muscle with attached in the area is usually left free.
  95. 95. ANATOMIC INFLUENCES ON MAXILLARY DENTURE RETENTION Considerations unique to the maxillary complete denture include the incorporation of a posterior palatal seal to complete the border seal. The posterior palatal seal maintains tissue contact during base movement or soft palate function and compensates for processing changes. This critical area extends between the hamular notches along the flexure line of the soft palate.
  96. 96. The posterior palatal seal of the denture must extend horizontally beyond the supportive hard palate to include the muscular aponeurosis of the soft palate. This area is not susceptible to pressure atrophy and therefore allows moderate tissue displacement to maintain the thin fluid film. To obtain the proper amount of tissue displacement, the posterior palatal seal must be deeper as the palatal vault becomes steeper to compensate for greater processing error.
  97. 97. Patients exhibiting highly tapered steep palatal vaults, present a special problem. The processing error may be so severe that no amount of posterior palatal seal can compensate for the resulting deficiency in intimate tissue contact. In these situations a metal base or subsequent bench-cure reline procedure would be incorporated into the initial treatment plan. A region that often causes problems in maintaining border seal is the buccal space or retrozygomatic space. This varies in size and shape but must be filled to avoid ingress of air beneath the denture base.
  98. 98. Care should be taken to fill the entire buccal space during border molding and subsequently impression making as limited by the normal functional range of movement of the coronoid process. The remaining border of the maxillary denture benefits from a draping effect of the lips and cheek and is not usually a problem in maintaining border seal if overextension is avoided.
  99. 99. ANATOMIC INFLUENCES ON MANDIBULAR DENTURE RETENTION The mandibular denture generally presents the major problem with regard to retention. Reasons for this include a movable floor of the mouth, which causes difficulty in establishing a lingual border seal, and lack of ideal ridge height and conformation, which minimizes stability. denture
  100. 100. Intimate tissue contact of the mandibular denture can be achieved through sound impression procedures as outlined above. The elimination of dislodging forces by accurate border molding that prevents overextension can also be accomplished. Special attention to the triangular buccal frenum, and the mentalis muscle, which may active in the region of the labial flange, should accompany any border – molding procedure. The border seal of the entire facial flange of the denture depends on accurate border molding and is enhanced by the dropping effect of the lips and cheek.
  101. 101. A slight posterior seal may be necessary on the distal border of the mandibular denture at the point where the cheek no longer provides contact along the denture border. The denture base should cover the posterior extension of the firmly bound, keratinized tissue of the pear – shaped pad. Craddock coined the term “pear –shaped pad”, which refers to the area formed by the residual scar of the extracted third molar and the associated retro molar papilla.
  102. 102. Clinically the pear shaped pad is distinguishable by the lighter color and firmly bound nature of the overlying mucosa. Immediately distal to the area is the less keratinized more resilient, and more vascular retro molar pad. It contains glandular tissue and a sub mucosa layer that can tolerate a gentle posterior seal. Lamie and Krol suggest beading this region at the junction of the pear – shaped and retro molar pad to ensure peripheral dentures border. seal along the posterior
  103. 103. Mandibular lingual anatomic influences: The border seal along the distal extension of the lingual flange requires an understanding of the anatomy and dynamic muscle physiology of the region. The posterolateral portion of the retromylohoid curtain overlies the superior constrictor muscle, and the posteromedial aspect covers the palatoglossus muscle and lateral surface of the tongue. The inferior wall of the retromylohoid fossa overlies the submandibular gland, which fills the gap between the superior constrictor and the most distal attachment of the mylohyoid muscle. Border molding must allow for the muscular function in this region. It is possible that medial pterygoid contraction could influence the contours of the distolingual flange by causing a bulge in the posterior wall of the retromlohyoid space.
  104. 104. Adequate seal can be obtained by gently compressing the tissues of the lateral wall of the retromylohyoid fossa lingual to the retromolar pad and tucking the distolingual flange laterally against the mucosa overlying the superior constrictor muscle superiorly and the loose connective tissue of the mandible inferiorly. Maximum posterior extension into the fossa is not necessary. Once the border seal is established, further posterior extension adds little to the support, stability, or retention.
  105. 105. The contour and inferior extension of the lingual flange are dependent on the action and anatomy of the mylohoid muscle. The lingual flange slopes medially away from the mandible to allow for the action of the mylohoid muscle. This inclination also enhances the ability of the tongue to control the mandibular denture, providing a seating force to the denture.
  106. 106. The mandibular attachment of the mylohoid muscle extends anterioinferiorly along the mylohyoid ridge from the lingual tuberosity in the molar region to the genial tubercles at the midline. Posterior fibers extend vertically to attach to the hyoid bone, while the anterior fibers extend horizontally to meet the fibers of the contralateral side to form a midline tendinous raphe. This explains why the lingual flange can be made longer Posteriorly despite a more superior mylohyoid muscle attachment.
  107. 107. Certain authors believe that adequate inferior extension of the flange can provide continuous contact regardless of tongue position or mobility of the floor of the mouth. However, the inferior extension of the posterior determined by the displaceability lingual flange is of the soft tissue and underlying mylohyoid muscle when the floor of the mouth is at its most superior position. In addition, the flange is molded by a contracted mylohyoid muscle. At rest the level of the floor of the mouth may be inferior to the lingual flange and the mucosa may drop laterally away from the intaglio as the mylohyoid muscle relaxes.
  108. 108. In such situations the border seal occurs at the border of the lingual flange when the mylohyoid muscle is active. When it is inactive, with the tongue retracted or at rest, the seal may occur as high as along the contact of the intaglio with mucosa overlying the mylohyoid ridge. Fortunately, the tongue often occupies the entire superor to the floor of the mouth at rest. By contacting the lingual denture surface, it is able to promote a seal in this region and enhance retention. Accurate border molding and impression procedures ensure adequate border seal.
  109. 109. Mandibular anterior lingual influences: The most difficult region in which to obtain a border seal is the anterior lingual border. The mylohyoid muscle acts anteriorly as well as Posteriorly to raise the floor of the mouth, and the genioglossus muscle functions in the region underlying the lingual frenum. The superior fibers of the genioglossus muscle attach to the superior genial tubercles and function in depressing the body of the tongue. Activation of the inferior fibers of the genioglossus muscle that pulls the tip of the tongue posterosuperiorly, depresses the central part of the tongue to form a concavity during bolus formation, and causes the anterior floor of the mouth to reach its most superior position.
  110. 110. Several methods may be used to establish and maintain border seal throughout the functional range of movement of the anterior floor of the mouth. Some techniques recommend the horizontal extension of the anterior lingual flange sublingually. Here the lingual flange is extended inferiorly to contact the highest level of the floor of the mouth. The flange can then be extended
  111. 111. Posteriorly to contact the sublingual folds and there by establish a seal when the tongue is at rest and the floor of the mouth drops. Care is taken not to impinge on the submandibular or sublingual gland ducts. Another technique involves a similar method of border molding to determine the inferior extension of the flange. However, a slight displacement of the mucosa anteriorly can be tolerated and provides a seal when the muscular floor of the mouth is at rest. This is accomplished by adding a slight additional amount of softened border-molding material to the inner surface of the previously molded anterior lingual area and reseating the custom tray. Again, the tongue at rest aids in maintaining the border seal by contacting the polished lingual flange as well as the mandibular anterior teeth.
  112. 112. Relationship of the external surface and periphery to surrounding orofacial musculature: Some important yet easily overlooked determinants of both denture stability and retention involve the relationship of the polished surface of the denture base to the surrounding musculature on the denture base generally result in lateral and vertical dislodging forces. Certain factors involving the musculature and the polished surface of the denture can facilitate stability in two ways. First, the action of certain muscle groups must be permitted to occur without interference by the denture base so that they will not dislodge the prosthesis during function or compromise stability. Second, the dentist must recognize that normal functioning of some muscle groups can be used to enhance stability. Alterations in external denture base contours can lead to a dynamic seating and stabilizing action directed towards the prosthesis.
  113. 113. MECHANICAL AIDS TO RETENTION: There are certain devices which are intended to keep complete dentures in place but their perman­ent use should only be employed as a last resort, and some not at all. I] Springs: These are made of coiled stainless steel, gold-plated base metal or nylon and have their ends attached to swivels in the premolar areas on both sides of the upper and lower dentures. The -dentures are thus permanently attached to each other and are held in occlusion for insertion into the mouth: as soon as they are released the dentures are forced apart by the action of the springs and held in place .
  114. 114. Disadvantages: 1. The constant pressure may cause mucosal irrita­ tion and excessive alveolar resorption. 2. The inner surfaces of the cheek are frequently injured from frictional contact with the springs 3. Lateral movements are extremely restricted and hence the efficiency of the dentures impaired. 4. They are generally inefficient and unhygienic. is
  115. 115. II] Denture fixatives: The powder form consists of natural gums such as tragacanth or karaya with cellulose added. It is sprinkled on the moist, fitting surface of the denture which will then usually stick in place for several hours. Gradually the sticky jelly is pressed from underneath the denture or washed away by the saliva, and for this latter reason is rarely of any use for holding lower dentures. Even with prolonged use it does not appear to affect the mucosa. Creams and liquids, containing the same substances as are in artificial saliva, are also available.
  116. 116. Uses 1.To hold the upper record block in position when securing intra-oral record 2. To prolong the usefulness, for a short time, of an immediate upper denture which is becoming loose through alveolar resorption. . 3. To enable a patient to wear an old, ill-fitting denture, while a denture in normal use is upper being repaired. 4. Sometimes used by public speakers, such as actors or clergymen, to give them the assurance that the upper denture will not move while they are addressing their audiences.
  117. 117. Disadvantages: 1. It has an unpleasant feel as soon as it is pressed out from beneath the denture. 2. It is only a temporary expedient and the less accurate the fit of the denture the more rapidly is the fixative washed away. 3. It,is of little use for retaining lower dentures. III] Suction chambers: These often resemble relief areas in shape but differ from them in having a clearly defined outline instead of merging into the surrounding surface. When the denture is inserted the patient creates a partial vacuum in this chamber by sucking
  118. 118.
  119. 119. and swallowing and this small area of reduced pressure helps to keep the denture in place. The mucosa in this area of reduced pressure proliferates to form a mass of dentureinduced hyperplasia. For this reason the technique should not be used. Disadvantages: 1. It has an unpleasant feel as soon as it is pressed out from beneath the denture 2. It is only a temporary expedient and the less accurate the fit of the denture the more rapidly is the fixative washed away 3. It,is of little use for retaining lower dentures.
  120. 120. Suction chambers :These often resemble relief areas in shape but differ from them in having a clearly defined outline instead of merging into the surrounding surface . When the denture is inserted the patient creates a partial vacuum in this chamber by sucking. Rubber suction discs :Although these are still used in practice they are only included in this list in order to condemn them. They consist of a rubber disc which is affixed to a stud on the fitting surface of a denture. The partial vacuum created within the perimeter of this disc holds the upper denture suspended from the hard palate. They cause a constant irritation and serve no useful purpose.
  121. 121. IV] Magnets: From time to time the use of small magnets embedded beneath the molar and premolar teeth and arranged with similar poles opposite each other, has been advocated. In theory the repulsion effect will keep both dentures in place but in practice it will be found that, owing to magnetic force being inversely proportional to the square of the distance and also the small size of the magnets which it is possible to fit, the repulsive effect is undetectable when the dentures are separated by more than 1 or 2mm.
  122. 122. Thank you For more details please visit