articulators and orthognathic surgery 1642

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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, and

the 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 meatusisn't 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 temporary

osseous 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 mandible

mechanically used the external auditory meati as convenient places tostick in rods on a simple facebow, and simple hinge articulators were


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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 an

approximation 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 an

instrument 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 world

often 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 eccentric

movements. The distance between the hinge axis and the mandibularfront teeth must remain constant during all excursions. Incisal guide


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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, this

instrumentation 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 Future

Some 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 of

computerized 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. Brossman

References:

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.


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2. Radney, L., Jacobs, J. Soft tissue changes associated with surgicaltotal maxillary intrusion. Am J Orthod Dentofac Orthop 1981; 80: 191-212

3. 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 of

rotation: 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. 170.

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articulators and orthognathic surgery 1642

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