Articulators and Orthognathic Surgery


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Articulators and Orthognathic Surgery

  1. 1. Articulators and OrthognathicSurgeryThe ProblemBy Robert E. Brossman, A.B., D.D.S., M.S.#3 Crossings MallWheeling, WV 26003A problem often arises in the application of cephalometric pre-surgicalplanning to clinical reality in orthognathic surgical cases. Basically, theproblem arises from the fact that that cephalometric planning works toosimply. Without this appreciation of the situation, it is easy to be misledwhen examining and planning changes to the maxillofacial skeleton basedon movements of segments of cephalometric tracings alone. Tracings ,afterall, are only line drawings representing the approximate outlines ofanatomic structures. They slide around easily, and can be placed justabout anywhere on top of other tracings. The problem arises becausetracings are simple two dimensional objects, and a whole treatment planis being programmed on a two dimensional representation of the patient.Orthognathic surgical treatment planning becomes a little more difficult inreality and many orthodontists and surgeons have been frustrated by theeffect of errors in surgical result brought on by inadequate appreciation ofthe problems of determining the ultimate position of three dimensionalobjects in three dimensional spaces. Any system of location, from themicroinches or millimeters commonly used in machine shops, to the feet-yards-miles-meters-kilometers associated with flying 747s, depends ondefning a reference starting point from which accurate determination ofthe ultimate location for a critical hole, to the actual position of theairplane on the face of the earth depends. Surgical planning of anorthognathic case should encompass a reliable starting reference as thefirst, and most critical step. What constitutes a reliable reference pointthat may be used as a starting reference is what this article is about to tryto determine.When you examine the human face from the viewpoint of trying todetermine an easily determined and reproduceable starting point fromwhich to project change, the first thing you notice is that there does notseem to be any easily used benchmark points. It would be wonderful ifthere was some common anatomic landmarks like a bilateral set of hornsprojecting from everyones forehead, but unfortunately, we arent puttogether that way. Facing reality, we have to accept the fact that we have alimited set of somewhat reproduceable anatomic landmarks to work with,and these are almost always limited to things like the nose, ears, eyes,
  2. 2. nostrils, chin, and maybe, the teeth ( as you can get at them through thelips). The rest of the face is sort of an amorphous series of curves andhollows that do not readily lend themselves to developing criticalreference points.At one time the only readily available method that gave any registrationpoints at all was a set of plaster casts of the dentition made with wax orplaster impressions. These could be utilized in several ways to projectmultidimensional changes in the spacial orientation of the mandible,because the model representing the maxilla was assumed to be, andactually was, a solid, fixed reference object from which to project thesechanges. Outside of relatively primative surgical procedures targetting themandible, the simplest three dimensional change would have beensomething like increasing the vertical relation of the jaws, or opening thebite. The earliest surgeries to reduce mandibular prognathism obviouslydepended on relating the position of the mandible to a fixed maxilla, andthe best operators of the time probably pre-planned their surgeries usingplaster models registered by hand. If a sufficiently harmoniousrelationship between the opposing arches was seen to occur, then anysurgical procedure that effectively separated the mandibular corpus fromthe rami and allowed the dentition to reach the new position would do thetrick. Once that was accomplished, the surgical challenge shifted to oneof adequately fixing the parts together in a manner that would maintainthe new position throughout the period of time necessary for the patientto heal.At one point , some past researcher must have decided on the use of theexternal auditory meatus as fairly obvious and readily reproduciblebilateral landmarks for relating the jaws. Actually, the auditory meatusisnt a bad landmark to use for orthognathic surgical purposes, but itshould be obvious that a system of location that uses both of them stilllacks an important third point needed to accurately locate a skeletal ordental unit in three dimensional space. A commonly used third point ofreference is the bridge of the nose after fabricating a small acrylic saddleto make the point somewhat more reproducible. This lack of accuracyleaves something to desire, but it is often close enough for clinicalpurposes. More precise accuracy would require placement of a temporaryosseous implant in the area of the glabella.Most bias against the use of the ear canals comes from the people whowant to determine the arc of closure and hinge axes of the condyles of themandible for restorative purposes. Although someone obviouslyattempted to use them for prosthetic, or restorative purposes, we nowappreciate that these points have only a rough relationship with the actualcenter of rotation of the mandibular condyles. Hall (8) went into the earlierresearches in this area in the twenties. Without extensive review of theliterature, of which there was precious little, it would probably be safe tosay that earliest attempts to mimic the action of the mandiblemechanically used the external auditory meati as convenient places tostick in rods on a simple facebow, and simple hinge articulators were
  3. 3. probably used to coordinate this information into the fabrication of moresophisticated dentures. That things didnt exactly work out to everyonessatisfaction was probably what lead to the early development of the hinge-axis facebow we know today. The information gained about the actualcenter of rotation of the condyles naturally lead to instrumentation thattracked the condyles during all phases of functional motion, and this inturn resulted in the design of more and more adjustable instruments toput this information to use in the laboratory.One reason that the earliest dentures pictured in dental historical textswere often fabricated with curved springs between upper and lower wasthat the actual bite relationship of the upper and lower denture was achanging and dynamic balance that actually depended on the wearersoral muscles, condyles, and frame of mind. Of course, the denturesthemselves were formidibly crude devices that fit the alveolar ridgesabout as closely as the carvers skills would allow. Front teeth were oftenno more than small piano keys wired to the underlying base, and posteriorcusp anatomy was generally flat-plane. Afterall, who needed theaggravation of carving detailed anatomy in blocks of walrus tusk, whaletooth, hippopotamus tusk, or elephant tusk ivory when the things werelucky to come into close approximation during function.The world owes atremendous debt to those early denture patients who had the grit to facethe world with a pair of those well named "choppers" firmly gripped intheir mouths.George Washington should also be remembered as one of the fathers ofdenture prosthetics - not particularly as a dental researcher, but as one ofthe historic figures who actually tried to wear some of those early devices- an experimental animal in the history of dentistry. One thing for sure, weknow that these pioneers of prosthodontics inspired a lot of effort in theR&D area toward making improvements in both materials and methods offabrication. Maybe I should amend that to include the dental practitionersof those bygone days who realized they also had to look forward towearing the same prosthetic appliances when their time came. Necessitytruly is the mother of invention.Complicating What Went BeforeIn the fine black art of maxillary impaction surgery, a means of predictingwhere the mandible could end up after the maxilla is detached from therest of the skull is needed by the surgeon, and especially, the patient.From the cephalometric tracing, the condyle(s) are superimposed overone-another, but are usually somewhat detectable on the head plate films.If you are truly trusting, you could arbitraily determine the average centerof the condyles and place a mark on the tracing film you can use toeffectively, but remember, totally arbitrarily, rotate the mandible tracing toa new, and thus, more desireable position. Sometimes this even works outOK for all concerned in real life.
  4. 4. In actuality, the precise location of the point(s) about which the averagemandible will truly rotate to the desired position is a bit harder todetermine. This lack of precise location is attributable to the fact that themandible does not rotate about a point axis as does a simple hinge, but asa sliding hinge joint where the rotational axis is also free to move inposition in more than one plane. Back when maxillary impaction surgerywas in its infancy, a lot of surgeons often noticed that they had to do a lotof trimming and tuning to actually get all the parts to fit in the desiredplaces. The individual skills of the surgeon often determined the successor failure of the operation. The smarter ones began to realize that therewas probably a better way to do these procedures and they began to lookaround for the solution. Somewhere along the line a surgeon mentionedhis problem to an engineer.Mechanical engineers, being very practical people in most cases, werebound to approach the basic problem from a totally different directionthan surgeons. Engineers, by virtue of their occupations, were oftencalled on to provide practical solutions to problems that, on the surface,appeared very similar to the problems that were cropping up in the worldof facial remodeling surgery. One surgeon consulted his engineer friendabout his problems, and the engineer remembered the method ofdetermining the center of rotation of odd shaped objects on moving on asliding axis once figured out by a Frenchman named Rouleaux, around1875. The only limitation was that everyone was still basically thinkingand working in a two dimensional system of coordinates while stillpracticing surgery in a three dimensional patient. However, improvementswere seen in the end product, and the concept of incorporating the effectsof condylar rotation on the prediction tracings bagan to creep into thepicture.Navakari, published a rather extensive cephalometrically based study ofthe motion of the mandible between rest and occlusal position in 1956.This degree of motion is normally much more limited than the degree ofmotion involved in gnathologic surgery, but the basic idea seemed sound.He also utilized some of the engineering practices attributed to Rouleaux,especially the use of a tripod-like device to assist in the determination ofthe center of rotation on cephalometric tracings. The tripod he describeddoes away with the calculation method that Rouleauxs original studiesrequired, and truly put the mechanics of determination of the twodimensional center of rotation of the mandible well within the capabilitiesof the average orthodontist and surgeon. All it took was tracings ofidentical landmarks on two cephalometric films, one closed in occlusion,one in an open position somewhere between centric and maximumopening, and a Navakari tripod. Some orthodontists and surgeons evenincreased the accuracy of the technique by taking two x-ray exposures onone film.Applying the method of determination was simplified and the odd thingwas that it began to point out that the actual center of rotation was notwithin the confines of the traced condyle, but often ended up being in the
  5. 5. area of the mastoid process, or elsewhere. Of course, the seriousdeficiency in the whole process was the assumption that a threedimensional object like the human skull and mandible behaved exactlylike a two dimensional representation called a cephalometric tracing.Theproblem still remained that the Navakari method could only produce anapproximation of the hinge movement of the mandible on a twodimensional pre-surgical tracing, but it was an improvement overprevious guestimates and surgery without hard guidelines. The nextdegree in the solution to the problem eventually came down to the factthat some method of actually duplicating the hinge-axis of the pairedmandibular condyles had to be determined, preferably by using the actualmotion of the mandible as the standard model, then some method had tobe developed that allowed the clinician the luxury of actually making atransferable record of this motion that could then be transferred tomechanical instrument designed to duplicate jaw motion in space with aset of mounted models This transfer of information of jaw motion to aninstrument that duplicated the functional motion of the mandibularcondyles in the fossi allowed practical use of the information gained insome areas of clinical dentistry, but still left something to be desired forthe surgeon. The prosthetic and restorative people were still allowed theluxury of having a third leg of a triangle that remained fixed andimmovable in space. A maxilla, covered with teeth, or even a maxillacovered with just gums was still a lot easier to work with than a maxillathat wobbled all over the place.The weak link here was, and remains the fact that only one member of theduplicating instrument, or articulator as we will now call it, was freelymovable in several planes of space. Moving two jaws in all planes ofspace remained a serious affair when only one structure could bemanaged on the laboratory bench. Nevertheless, the fully adjustableanatomic articulator was a great improvement when it came to surgicalmovement in one or two planes, and with certain strategic props, it evenbecame somewhat valuable in planing surgical movements in all threeplanes of space. Basically, this remains the state of the art for mostorthognathic surgery, but a lot of surgical skill and fine trimming is stillneeded in practical application. Several type of fully adjustablearticulators are available and poobahs of the orthognathic surgical worldoften outline their pros and cons in their textbooks. An example of whatone team of maxillofacial surgeons utilizing the basic fully adjustablearticulator wants in their laboratory instrumentation follows.In a discussion of instrumentation used to support pre-surgical planingthe articulators used should have certain features in common. Adescription of what is generally desirable in an articulator instrumentappeared in Bell, Proffit, and White, Surgical Correction of DentofacialDeformities. The condylar elements should be on the lower member of thearticulator. The intercondylar distance should be adjustable. The distancebetween the condylar elements must stay constant in eccentricmovements. The distance between the hinge axis and the mandibularfront teeth must remain constant during all excursions. Incisal guide
  6. 6. tables are adjustable. The axis orbital plane is represented by the uppermember of the articulator. All condylar guidance is taken from the uppermember. The facebow, which obviously does not have to be a hinge axistracing-type, functions to provide a reasonably accurate transfer of thehinge axis location to the articulator itself. All things considered, thisinstrumentation apparently does not have to be a gnathologic qualitydevice.The facebow is the most important link in the system. It has the criticalfunction of providing a fundamental orientation of the maxillary cast onthe articulator to the fixed simulated positions of the hinge axes glenoidfossi of the patient. The maxilla, and the rest of the head for that matter,are considered fixed, non-moving structures in this system by functionand definition. The facebow can additionally be oriented to the FrankfortHorizontal Plane, or the axis orbital plane, so it does supply someinformation that can be applied to the cephalometric tracing.Instrumentation of the type described above can duplicate condylarfunction during a very small portion of the mandibular functionalmovement, and can approximate excursive function to a degree, butbegins to have increasingly critical limitations when the surgical plangoes much beyond mild closure of vertical dimension during maxillaryosteotomies. In essence, this describes an instrument that is still moreuseful in prosthetic appliance construction.The FutureSome system has to be developed to 1.) accurately locate the hinge axesof the condyles during all ranges of condylar motion, 2.) accurately relatethe maxilla to the hinge axes and other constructional planes of the facialskeleton, 3.) accurately relate the mandible to the cranium in all threeplanes of space, and 4.) accurately locate the maxilla and mandible in allthree planes of space - when, and if, in the case of maxillary, mandibular,or combination osteotomy, they becomes freely movable.Obviously, the instrument to accomplish all of these goals is still indevelopment somewhere. Necessity remains the mother ofinvention.Perhaps one method may come from application ofcomputerized axial tomography. Today, it is possible to generate highlyaccurate three dimensional plastic models of the facial bones using astemplates the multiple "slices" of information derived from the CT Scan.Copyright 1995, R. E. BrossmanReferences:1. Shendel, S. A., Eisenfeld, J. M., Bell, W. M., and Epker, B. N. Superiorrepositioning of the maxilla: stability and soft tissue osseous relations.Am J Orthod Dentofac Orthop 1976, 70:663-674.
  7. 7. 2. Radney, L., Jacobs, J. Soft tissue changes associated with surgicaltotal maxillary intrusion. Am J Orthod Dentofac Orthop 1981; 80: 191-2123. Fish, L. C., Epker, B. N. Surgical-orthodontic cephalometric predictiontracings. J Clin Orthod 1980; 14: 36-52.4. Valinoti, J. R. The hinge axis angle. J Clin Orthod 1977; 9: 551-559.5. Sperry, T. P., Steinberg, M. J., and Gans, B. J. mandibular movementduring autorotation as a result of maxillary impaction surgery. Am JOrthod Dentofac Orthop 1982; 81:116-122.6. Navakari, K. An analyisis of the mandibular movement from rest toocclusal position. Acta Odontol Scand 1956; 14: Supplement 19.7. Grant, P. G. Biomechanical significance of the instantaneous center ofrotation: the human temporomandibular joint. J Biomech 1973; 6: 109-113.8. Hall, R. E. An analysis of the work and ideas of investigators andauthors of relations and movements of the mandible. J Am Dent Assoc1929; 16: 1642-1693.9. Brewka, R. E. Pantographic evaluation of cephalometric hinge axis. AmJ Orthod Dentofac Orthop 1980; 79: 1-19.10. Lepera, F. Determination of the hinge axis clutches on condyleposition. J. Prosthet Dent 1958; 8: 260-265.11. Rouleaux, F. The kinematics of machinery: outline of a theory ofmachines (translated by Kennedy, A. B. W.) Dover, 1875.12. Soudan, K. and Van Audekercke, R. methods, difficulties, andinaccuracies in the study of human joint kinematics and pathokinematicsby the instant axis concept. J. Biomech 1979; 12: 27-33.13. Panjabi, M. M. Centers and angles of rotation of body joints: a study oferrors and optimization. J Biomech 1979 12: 911-920.14. Bell, Proffit, and White. Surgical Correction of Dentofacial Deformity.W. B. Saunders, Philadelphia, 1980, p.