3.1.1 Introduction
Orthognathic Surgery is a major form of surgical intervention which carries with it a degree of potential morbidity and indeed can be life threatening. However the disruption of established neuromuscular control of the facial skeleton predisposes to malocclusion and relapse. This is compounded by defective diagnosis, analysis, planning and surgical technique. Hence a precise rational is essential in the planning phase. The best outcome is achieved if the operation is carefully planned and executed correctly on the first occasion when surgery is undertaken. Further attempts will be adversely affected by the results of scarring from first operation. Furthermore the procedure is expensive, and a drain on resources if unsuccessful. Therefore it is essential that every effort is taken preoperatively to plan the final position and perhaps even more importantly that the planned position is achieved on the operating table.
It is repeatedly reported in the literature that the chances of carrying out preoperative planning successfully in the operating theatre are very poor (Pospisil 1987, Sarver and Weissman 1991, and McCance et al 1992). Thus there is a need to establish and validate a reliable method of assisting the surgeon in the reorientation of the jaws on the operating table. In most studies the actual results of the planned movements compared to the actual movements achieved are disappointing despite the determined efforts and commitment of the surgeon to adhere to the predetermined plan (Fried and Kahnberg 1987, Pospisil 1987, Stanchina et al 1988, Polido et al 1990, Kahnberg et al 1990, Polido et al 1991, Sarver and Weissman 1991 and McCance et al 1992).
When planned treatment objectives are at significance variance with the results actually achieved, as may be noted by both clinical evaluation and comparison of records during any stage of treatment, the reaction is either to blame the patient’s structural makeup or the technique. Such a reaction leads to obscure the understanding of the factors necessary to produce consistently good results and
failure to diagnose the specific etiological factors in a chain of events that produce the undesired result. One of the cornerstones of planning are the model surgery and occlusal wafer, which is a means of transferring the agreed treatment plan to the patient. Accurate model surgery relies upon the following chain of events:-
Accurate impressions, a correct facebow recording, an unmodified bite registration in centric relation, and transfer of the relevant clinical information to the articulator as the starting point of the model surgery, and also the accuracy of the occlusal wafers. The following stages were examined and validated.
3.1.2 Reliability of the Facebow
One of the most common errors in model surgery is in the mounting of models. Accurate transfer of the patients maxillary occlusal plane angle and the distance between the dentition and the intercondylar hinge axis, is important, especially when vertical movements of the maxilla and/or mandible are planned, because autorotation changes the position of the jaws in both vertical and horizontal dimensions. The more accurately the maxillary model is mounted with respect to the true hinge axis the more accurate will be the information provided during model surgery so the intermediate wafer will transfer this relation of the maxilla more accurately to the base of the skull, with the help of an unchanged mandible. Unfortunately very little attention has been paid to this aspect, resulting in very little research in this field. The facebow recording relates the maxillary cast to the condylar assemblies of the articulator and the Frankfort horizontal plane. This is assumed to reproduce the manner in which the patient’s maxilla relates to the terminal hinge axis of their mandibular condyles. An arbitrary hinge axis location is a common method of determining the axis for articulation and subsequent treatment planning. One method uses the earplug facebow and another a point
13mm anterior to the base of the tragus.
The results of previous studies that compare arbitrary with kinematic methods are conflicting: Walker (1980) observed that only 20% of the true axis locations found with a hinge axis locator were within a 5mm radius of the arbitrary location,
situated 13mm anterior to the tragus, while 60% were 6mm distant or more. Whereas Schallhorn (1957) reported that 95% of true hinge axis locations were within a 5mm radius of an arbitrary axis.
One of the objectives of this investigation was to evaluate the reliability of the facebow. The Dentatus and the Denar facebows, both used for surgical planning by this department were compared and their ability to achieve reproducibility in treatment planning studied.
3.1.3 The Occlusal Registration
An obvious and common error is the failure to obtain a bite registration in centric relation. Since the preoperative position of the mandible determines the final position of the maxilla, a postured mandible when centric relation is recorded will ultimately compromise the prescribed postoperative position of the maxilla.
The use of occlusal stabilisation appliance for a period to eliminate this posturing habit and to disorientate their proprioceptive awareness of maximum intercuspal or adopted occlusion, may help to determine an accurate centric relation. Following orthognathic surgery the dental occlusion sometimes significantly differs from that predicted with treatment planning, and frequently, further adjustments of the occlusion are necessary to ameliorate these occlusal discrepancies. Normally the planning and the work-up is based on centric occlusion, variation in centric occlusion and centric relation may make some contribution in unplanned postoperative results. Occlusal registration for orthognathic work-up is usually done in an upright posture, which may differ from the supine position of a patient under general anaesthesia.
3.1.4 The mandibular autorotation.
The articulator’s inability to reproduce the patient’s hinge axis movements and the mandibular anteroposterior slide due to autorotation may be an other contributory factor for errors in osteotomy planning (Rekow et al 1985 and Nattestad et al
3.1.5 The Assessment of Maxillary Asymmetry
Perfect symmetry is an anatomical ideal but not always an aesthetic one. The elements of composition for the artist are such that he must strive consciously to balance the attractive forces of the different areas of right and left as well as top and bottom. Slight asymmetry gives life, character, and personality, overdone, the same device produces deformity. In craniofacial development a gross deformity may well be symmetrical, which will need to be judged against the so-called ’norms’.
Asymmetry in an orthognathic patient may consist of a displacement of the upper or lower dental midlines, or it may include the greater part of the upper or lower jaw. It is important to compare the dental midlines with the midlines of the nose and philtrum and with that of the chin.
Asymmetry may also include a tilt (cant) in the transverse occlusal plane, and require simultaneous levelling with bimaxillary osteotomies. Dental versus skeletal midlines can be observed on the P A cephalogram, but it is better judged from clinical examination of the patient. The problem with PA film is that slight twisting of patient’s head can make an apparent difference of 1-2mm in the cephalometric midline, which is enough to affect the determination of whether a midline discrepancy is caused by upper or lower teeth. Clinical judgement is likely to be more accurate, as with any other diagnostic point. Where there is a disagreement between the diagnostic records and clinical notes, the patient should be examined again to resolve the discrepancy.
Deviation of the dental midline from the facial midline is common and can usually be easily corrected by rotation of the maxilla at the time of Le Fort I osteotomy, but it must be planned at the model surgery stage. Without preoperative planning and the help of an accurate intermediate wafer, relocation of osteotomized maxilla coincidally with facial midline is difficult. This is especially so where nasal and chin asymmetry is already present, which will be accentuated by the less than ideal operating position with the nose being displaced by the anaesthetic and
feeding tube.
One of the objectives of this research is to set up a system which will allow the patients facial midline to be transferred to the articulator and help the surgeon to match the maxillary dental midline with it, using an intermediate wafer.
3.1.6 Model Surgery
The accuracy and reproducibility of a model surgery technique must also play some role in the outcome of an osteotomy ( Angle 1903, Lockwood 1974, Guyuron and Ross 1989, Parretta and Caruso 1983, Ellis 1990, Anwar and Harris 1991, Nattestad et al 1994).
3.1.7 The Occlusal Wafer
The thickness of the wafer may also be an unexplained error especially the intermediate wafer (Riply 1982, Block and Hoffman 1987, Telfer and Page 1990, Harris and Reynolds 1991, Proffit and White 1991, Cottrell and Wolford 1994).
3.1.8 Proprioception
Alteration of proprioception from the occlusion may alter the jaw relationship. This may be of clinical significance in the management of orthognathic patients with severe malocclusion, especially if posturing habits to mask the occlusal disharmony have been adopted. Proprioceptors in the temporomandibular complex include the muscle spindles in the muscles, Golgi’s and Pacinian corpuscles in the tendons, joints, periosteum, fascia and subcutaneous tissue. The nucleus concerned with proprioception is the mesencephalic nucleus. The proprioceptors are stimulated by the action of the body itself and provide information to the central nervous system to appreciate the sense of movement and position of the body parts, and to control muscle tone, thus, maintaining posture and regulating movement of the body parts.