Autorotation of Mandible

Autorotation of the mandible: Effect of surgical superior repositioning of the maxilla on mandibular resting posture

2s Dr. Wessberg

George A. Wessberg, D.D.S.,* Michael C. Washburn, D.D.S.,** John P. LaBanc, D.D.S.,*** and Bruce N. Epker, D.D.S., Ph.D.**** Honolulu, Hawaii, Austin, Texas, and Fort Worth, Texas

Mandibular autorotation is a generally accepted cephalometric phenomenon that occurs when surgical superior repositioning of the maxilla is planned. This investigation was undertaken to determine whether autorotation of the mandible is a biologic phenomenon as well. Fifteen adults with vertical maxillary excess who underwent a mean surgical superior repositioning of 6.2 mm. were evaluated with cephalometric, kinesiometric, and electromyographic instrumentation immediately before and 3 months following surgery. Five persons selected at random from the original group (who also underwent a mean surgical superior repositioning of 6.2 mm.) were similarly evaluated 24 months following surgical intervention. The results of this study, which were analyzed by the Wilcoxon Matched-Pairs Signed-Rank test, revealed that a significant compensatory autorotation of the mandible occurred by the third month and was still constant 2 years later. We propose, on the basis of this preliminary evidence, that an “occlusal programming feedback mechanism” within the central nervous system mediated the compensatory autorotation of the mandible following surgical superior repositioning of the maxilla.

T he surgical-orthodontic correction of vertical maxillary excess via surgical superior repositioning of the maxilla is generally accepted as a treatment regimen on the basis of skeletal stability and esthetic soft-tissue changes. ‘W Yet the ability of the masticatory neuromusculature to adapt to such skeletal change has been a topic of controversy.7, * This controversy is centered upon the effect of surgical superior repositioning of the maxilla upon mandibular resting posture: Does the rest position of the mandible (rest vertical dimension) remain constant and result in a corresponding increase in the interocclusal distance (freeway space), or does the rest position of the mandible adapt to the surgical movement of the maxilla and result in a constant interocclusal distance? Preliminary studies of neuromuscular adaptation to surgical superior repositioning of the maxilla in human patients suggests that a compensatory anterosuperior autorotation of the mandible does maintain a constant interocclusal distance immediately following surgery.g However, the instrumentation employed for the investigation has not been universally accepted, and the long-term results of

*Oral and Maxillofacial Surgeon in private practice in Honolulu, Hawaii. **Oral and Maxillofacial Surgeon in private practice in Austin, Texas. ***Research Fellow, Oral and Maxillofacial Surgery, John Peter Smith Hospi- tal, Fort Worth, Texas. ****Director, Oral and Maxillofacial Surgery, John Peter Smith Hospital, Fort Worth, Texas.

OGQZ-9416/82/060465+08$00.80/O 0 1982 The C. V. Mosby Co.

masticatory neuromuscular adaptation have not been accurately documented. The purpose of this article is to delineate the merits of this instrumentation and to measure the effect of surgical superior repositioning of the maxilla on mandibular resting posture 24 months after surgical intervention.

REVIEW

Mandibular resting posture has been a topic of in- terest for more than three decades. During the past 5 years we have conducted several studies of the rest positions of the mandible in persons with diverse vertical dentofacial morphology and the subsequent effects of orthognathic surgery. s--12 The instrumentation employed for these pioneer studies was selected to permit indirect measurement of mandibular movement in order to minimize external influences upon the proprioceptors and mechanoreceptors of the stomatog- nathic system. The specific instruments used are a mandibular kinesiograph to measure actual mandibular displacement, transcutaneous electrical stimulation to induce neuromuscular relaxation, and direct masticatory electromyography to ascertain corresponding levels of muscle activity at respective positions of the mandible.r3-16 Since the capabilities and limitations of these instruments are often poorly understood or have not been precisely delineated, we conducted several preliminary studies to identify these important factors and eliminate the majority of these concerns.

465

466 Wessberg et al Am. J. Orthl. Jaw 1982

The mandibular kinesiograph* was designed to measure mandibular displacement via an indirect method. The position of a small magnet attached to the mandibular incisors is recorded on a cathode-ray tube by means of an electronic transducer.‘” The reliability of this instrument under laboratory and clinical conditions has been documented by us and others.‘“. lZ The mandibular kinesiograph is becoming increas- ingly popular in clinical studies of mandibular movement because of its physiologic compatibility and relative accuracy (4.4 percent error from 0 to 15 mm.).” Ii

Transcutaneous electrical stimulation-I was intro- duced to dentistry as a method of inducing masticatory muscle relaxation. This instrument has been available for professional use for more than 15 years and is also becoming more popular among clinicians, despite criticism that the occlusal position (myocentric) generated by it lacks reproducibility. This controversy is not relevant to the present investigation, because we are using the transcutaneous electrical stimulator only to induce masticatory muscle relaxation. We verified the fact that muscle relaxation does occur, in fact, by mea- suring simultaneous masticatory electromyographs$ before and after 30 minutes of stimulation.‘O These measurements were repeated on three separate occa- sions at weekly intervals, and two definite mandibular rest positions-the clinical (CRPM) and the physiologic (PRPM)-were identified. The clinical rest position (rest vertical dimension) was induced by phonetics in a conventional manner and was found to create an interocclusal distance of 3.5 t 2.0 mm. The physiologic rest position (transcutaneous electrical stimulation position) was induced by the transcutaneous stimulator via surface electrodes at a rate of 40 impulses per min- ute to the preauricular areas of the face and was found to generate an interocclusal distance of 6.1 -+ 3.2 mm. It is important to note that transcutaneous neural stimulation is disconrinued just prior to measurement, so that the mandible is suspended in a passive rest position by relaxed masticatory muscles. (This should not be con- fused with the myocentric position!)

Masticatory electromyography§ has been employed to delineate the clinical and physiologic rest positions of the mandible by us and others.10, l2 We compared the mandibular rest positions created by transcuta-

*Model MK-5, Myotronics Research, Inc., Seattle, Wash. tMyomonitor, Model 52, Myotronics Research, Inc.. Seattle, Wash. $Grass Electroencephalograph. Model S- 16. Grass Instruments Company, Qumcy, Mass. $Myotron 220, Enting Physiological Instrument Company, Dorst, The Nether- lands.

neous electrical stimulation and by the point of minimal masticatory electromyographic activity and found no statistical difference between the two positions. lo

This review of the sophisticated electronic instrumentation employed for the indirect measurement of mandibular resting posture can be summarized from the perspective of the clinical and physiologic rest positions of the mandible. The clinical rest position is the posture assumed by the mandible following a swallow or after the creation of certain phonetic sounds. Since this posture predominates under clinical circumstances, the common theory suggests that the central nervous system governs this “adaptive holding position” to optimize masticatory function via occlusal programming from the existing dentition. Hence, one would expect that the clinical rest position of the mandible would be closest to the maxillary teeth (least interocclusal distance) and would generate increased masticatory muscle tonus. Thephysiologic rest position is also determined by the masticatory musculature but usually exists only under laboratory conditions because the muscles are in a state of equilibrium at their physiologic resting length with minimal masticatory muscle tonus. Hence, one would expect that the physiologic rest position of the mandible would be farthest from the maxillary dentition (greatest interocclusal distance) and would exhibit diminished electromyographic activity.

The relative significance of the clinical and physiologic rest positions to the study of neuromuscular adaptation to orthognathic surgery should be much clearer to the reader. If orthognathic surgery produces changes in the interocclusal distance, one may expect a compromise in subsequent masticatory function. Theo- retically, if the interocclusal distance at the clinical re.st position of the mandible is not altered following surgical superior repositioning of the maxilla, a compensatory autorotation of the mandible was induced by neuromuscular programming to maximize masticatory function.“, I!’ Subsequent effects of orthognathic surgery on the physiologic rest position are also relevant because if the interocclusal distance at the physiologic rest position is not altered following surgery the masticatory muscles would have become physically shorter. However, if the interocclusal distance at the physiologic rest position increased following surgical superior repositioning of the maxilla, it would indicate that there was little or no physical shortening of the masticatory muscles and theoretically might compromise muscle efficiency on the basis of Starling’s law.

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MATERIALS AND METHODS Subjects

Autorotation of mandible 467

Twenty-five adult subjects were selected on the basis of specific cephalometric characteristics. Their age range was 16 to 34 years. The control group consisted of ten female subjects with normal dentofacial morphology and Class I occlusion and no prior orthodontic or orthognathic therapy. The persons in the control group underwent two recording sessions with a 3-month interval between the two trials. The surgery group consisted of fifteen persons (eleven females and four males) with variable amounts of vertical maxillary excess and variable malocclusions which had been aligned and coordinated with orthodontic therapy. Sur- gical superior repositioning of the maxilla to improve esthetics and jaw relationships was followed by inter- maxillary fixation for a maximum of 6 weeks. No man- dibulur surgery was performed in any of these persons. All fifteen subjects (PST 3) in this group underwent a recording session immediately before and 3 months following surgical superior repositioning of the maxilla. Only five persons (PST 24) were available for a final recording session 24 months after surgical treatment.

Instrumentation

Prior to each recording session lateral cephalometric radiographs of each subject were obtained with the Frankfort plane parallel to the floor and the mandible in the centric occlusion position. Acetate tracings of these cephalograms were made by the same investigator (G.A.W.) and entered into a DEC System- 10 computer by means of a 180-point facial diagram via an electronic digitizer. 2o The transcutaneous electrical stimulator, mandibular kinesiograph, and electromyographic instrumentation employed for this were the same as those tested and reviewed previously in this article.

Procedure

Electromyography . Each individual was adminis- tered 5 mg. of diazepam orally to alleviate anxiety and seated upright in a shielded recording room. Monopo- lar, fine wire electrodes* were implanted with 27 g hypodermic needles into the midportion of the anterior and posterior temporalis, superficial masseter, and medial pterygoid muscles bilaterally. The position for the anterior temporalis muscle was located by an anterior movement from the tragus of the ear along the ala-tragus line for 2 cm., then superiorly for 2 cm. on a perpendicular axis. The site for the posterior temporalis

*Stabholm alloy 4 p.pm.; 0.0025 mm. diameter: 124 ohms per foot, Califor- nia Fine Wire Co., Grover City, Calif.

Fig. 1. Individual placed in shielded recording room with the kinesiometric, electromyographic, and transcutaneous stimulation instrumentation. The numerous direct and surface electrodes, intraoral magnet, and magnetic sensors suspended from the eyeglass frame are stabilized in a manner that does not impair or influence mandibular movement.

muscle was along the ala-tragus line posteriorly 4 cm. from tragus, then superiorly for 2 cm. on a perpendicular axis. The site of the superficial masseter muscle was located 3 cm. anteriorly to tragus and 2 cm. inferiorly on a perpendicular axis from the ala-tragus line. The medial pterygoid muscle was palpated medial to the insertion at the mandibular angle, and the hypodermic needle was inserted in an anterior and superior direc- tion. A common cutaneous ground was taped on the midline of the forehead to complete the system. An immediate base line electromyographic recording was made to verify electrode placement and to isolate hy- pertonic activity as a result of traumatic insertion of the electrode. A period of 10 minutes was generally found to be adequate for the muscle trismus to subside.

Kinesiogruphy. A 5 by 10 mm. positive magnet was affixed to the labial aspect of the mandibular incisors at the gingival margin with autopolymerizing acrylic resin. Placement in this manner permits un- obstructed closure of the mandible into centric occlusion so that the occlusal programming mechanism is not adversely affected. The light-weight eyeglass frame

468 Wessberg et al. A m J Orthod. June 1982

Fig. 2. Computerized composite cephalometric diagram of the ten persons in the control group. These persons demonstrate normal dentofacial morphology and Class I molar occlusion.

Table I. Control group mean values

CRPM PRPM

Text IOD E M G IOD E M G NO. (mm.) (PJ) (mm.) (WV)

1 3.5 -+ 2.0 0.5 ? 0.5 6.1 k 3.2 0.1 + 0.1 (1.5 ~ 7.5) (0.3 - 2.7) (1.5 - 10.2) (0 - 0.3)

2 3.6 k 1.4 0.4 k 0.4 6.1 + 2.6 0.2 r 0.1 (1.6 - 7.7) (0.3 - 2.3) (1.5 - 11.7) (0.1 - 0.4)

A 0.1 0.1 0.0 0.1

Key to abbreviations: CRPM = Clinical rest position of mandible. PRPM = Physiologic rest position of mandible.

IOD = Interocclusal distance. E M G = Masticatory electromyographic activity.

A = Difference between trials. k = One standard deviation. * = Statistically significant change at 0.05 level.

( ) = Range.

and sensor array of magnetometers were placed on the face of the subject and the kinesiograph was calibrated according to the manufacturer’s specifications.

Transcutaneous stimulator. Surface electrodes were placed over the mandibular sigmoid notches bilaterally. A midline ground was placed at the suboccipital area. A saline-base conducting gel was employed to improve depolarization. The transcutaneous electrical impulse was balanced and increased until a 0.5 mm. vertical deflection of the mandible was observed on the cathode-ray tube of the kinesiograph (Fig. 1).

Recording program

Regimen. Approximately 10 minutes following the application and calibration of the instrumentation, the subject was instructed to stand with his or her head in a

natural position. Recording of the clinical rest position of the mandible was achieved by instructing the patient to swallow, relax the jaw, and then tap the teeth into centric occlusion three times. This was repeated to verify approximate centric occlusion. The linear distance that the mandible moved from the clinical rest position to the centric occlusion position was measured on the cathode-ray tube of the kinesiograph. This was recorded as the interocclusal distance at the clinical rest position of the mandible. Simultaneous masticatory electromyographic activity was recorded when the mandible was in the clinical rest position.

Transcutaneous electrical stimulation to the preauricular areas of the face was next conducted for a mini- mum of 30 minutes while the subject was seated. This stimulation continued until a reproducible mandibular position appeared on the kinesiograph screen. The subject was again instructed to stand upright with his or her head in a natural position until a consistent physiologic rest position of the mandible was observed on the kinesiograph cathode-ray tube. The subject was again directed to tap the teeth into centric occlusion three times, and the linear distance displayed on the cathode-ray tube of the kinesiograph was measured as the interocclusal distance at the physiologic rest position of the mandible.

The masticatory electromyographic activity again recorded after the transcutaneous electrical stimulation was discontinued.

Tabulation of data. The technique for visual interpretation of electromyographic data reported by Ahlegren and associateP was employed for each separate recording session. Each interpretation was placed into a sealed envelope until the termination of the study (single-blind method).

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Table II. Surgery group mean values (PST 3)

L

PRE 1.9 -e 0.5 0.7 5 0.4 3.3 2 1.9 0.5 k 0.2 (0.8 - 4.7) (0.3 - 1.2) (1.1 - 6.2) (0.1 - 0.7)

PST 3 2.0 ? 0.3 0.7 + 0.5 4.9 + 2.3 0.3 c 0.2 (0.6 - 2.6) (0.2 - 1.3) (1.8 - 8.0) (0 - 0.5)

A 0.1 0.0 1.6* 0.2

Key to abbreviations: CRPM = Clinical rest position of mandible. PRPM = Physiologic rest position of mandible.

IOD = Interocclusal distance. EMG = Masticatory electromyographic activity. PRE = Presurgical trial.

PST 3 = Postsurgical trial at 3 months. A = Difference between trials. + = One standard deviation. * = Statistically significant change at 0.05 level.

( ) = Range.

RESULTS

The results of this investigation were analyzed by the W ilcoxon Matched-Pairs Signed-Rank test. The null hypothesis for these comparisons was that no significant difference was evident between the interocclusal distances or masticatory electromyographic ac- tivities.

Control group

The data from the ten subjects in the control group are represented as a composite diagram in Fig. 2 and mean values in Table I. The difference in the mean interocclusal distance between the two trials recorded with a 3-month interval was 0.1 mm. at the clinical rest position and 0.0 mm. at thephysiologic rest position of the mandible. Nonparametric analysis of these findings supported the null hypothesis that there was no significant difference in the respective mandibular rest positions (p > 0.05). There was also no significant difference in masticatory electromyographic activity between the two trials (p > 0.05).

Surgery group

Postsurgery 3 months (PST 3). The data from the fifteen subjects in this group are represented as a composite diagram in Fig. 3 and Table II. The mean surgical superior repositioning of the maxilla, as determined by the comparison of the presurgical composite cephalograms and the 3-month postsurgical composite cephalograms, was 6.2 mm. (range, 3.1 to 8.2 mm.). The difference in the mean interocclusal distance was 0.1 mm. at the clinical rest position and 1.6 mm. at the

Autorotation of‘ mandibk 469

-lb0 Fig. 3. Computerized composite cephalometric drawings of the fifteen persons in the surgery group (solid line). The composite drawing of the same group of persons is superimposed on sella-nasion to determine the amount of maxillary intrusion (6.2 mm.) 3 (PST 3) months after surgical intervention.

physiologic rest position of the mandible when the presurgical and 3-month postsurgical recordings were compared. Nonparametric analysis of these results demonstrated no significant difference in the mean interocclusal distance at the clinical rest position (p > 0.05). However, there was a signijcant difference in the mean interocclusal distance at the physiologic rest position of the mandible (p < 0.05). There was na significant difference in masticatory electromyographic activity between the two trials at either the clinical (p > 0.05) or the physiologic (p > 0.05) rest position.

Postsurgery 24 months (PST 24). The data from the five subjects in the parent group PST 3 were available for measurement 24 months after surgery and are represented in Fig. 4 and Table III. Coincidentally, the mean surgical superior repositioning of the maxilla of this smaller group was also 6.2 mm. (range, 3.5 to 10.5 mm.) when compared to the presurgical cephalograms of the same five persons. The difference in the mean interocclusal distance at the clinical rest position was 0.3 mm. and at the physiologic rest position was 5.1 mm. Nonparametric analysis of these results demonstrated no significant difference in the mean interocclusal distance at the clinical rest position (p > 0.05). However, there was a signi&-ant difference in the mean interocclusal distance at the physioogic rest

O

i

E

I

tI""ILI)'l""I"" 0 -100

Fig. 4. Computer-generated composite cephalograms of five persons from the surgery group are superimposed at sella- nasion to determine the amount of maxillary intrusion (6.2 mm.). The presurgical composite drawing (solid line) is superimposed upon the composite postsurgical drawing (broken line) obtained 24 months after surgical intervention (PST 24).

position (p < 0.5). There was no significant difference in the mean masticatory electromyographic activity at either the clinical or the physiologic rest positions of the mandible between the presurgical and the 24-month postsurgical recording sessions (p > 0.05).

DISCUSSION

The potential mechanisms for neuromuscular adaptation to orthognathic surgery have been delineated by McNamara and associates2’ on the basis of animal exper- imentation. These studies suggest that neuromuscular adaptation occurs in variable amounts within (1) the central nervous system, (2) tendons, (3) muscles, and (4) bones. The potential influence of these factors upon mandibular resting posture will be discussed.

Central nervous system. Several complex mechanisms within the central nervous system (reticular ac- tivating system, cerebral cortex, and extrapyramidal system) respond to afferent stimuli from the stomato- gnathic proprioceptors and mechanoreceptors to modify the gamma efferent neurons and alter the sensitivity of the masticatory muscle spindles to dictate the clinical rest position of the mandible. This neuromuscular feedback mechanism functions to protect the teeth and

Table III. Surgery group mean values (PST 24)

CRPM PRPM

IOD EMG IOD EMG l’riul (mm .) (PI (mm .) ( wi

Pre 1.2 t 0.9 0.4 ? 0.2 2.8 f 1.3 0.3 f 0.2 (0.2 - 6.3) (0.2 - 0.6) (I. 1 ~ 7.9) (0 - 0.5)

PST 1.5 -t I.0 0.5 t 0.2 7.9 + 1.6 O.? 2 0.2 24 (0.2 - 6.5) (1.3 - 0.6) (4.4 - 13.2) (0 ~ 0.5)

A 0.3 0. I s.1* 0.0

Key to ubbreviations. CRPM = Clinical rest position of mandible. PRPM = Physiologic rest position of mandible.

IOD = Interocclusal distance. EMG = Masticatory electromyographic activity. PRE = Presurgical trial.

PST24 = Postsurgical trial at 24 months. A = Difference between trials. t = One standard deviation. * = Statistically significant change at 0.05 level.

( ) = Range.

temporomandibular joints and optimizes masticatory function. 18, I!). 2:1

Tendons. Animal studies recently conducted by Ellis and CarlsonZ4 demonstrated the ability of tendons to change their length and/or to migrate on osseous surfaces in response to altered jaw position. The rate and magnitude of adaptations within the tendons are variable but may be of sufficient magnitude to alter the physiologic rest position of the mandible.

Muscles. GoldspinkZJ has demonstrated the ability of denervated skeletal muscles to change their physical length when immobilized at a new length. These adaptations occur via geometric rearrangement of muscle fibers and/or changes in fiber length by the addition or deletion of sarcomeres. The change of muscle length directly influences the physiologic rest position qf the mandible.

Bone. Skeletal adaptations occur gradually in comparison with the other factors, are localized to the surface of the tendon attachment, and exert a direct influence upon the physiologic rest position of the mandible.

This evaluation of mandibular resting posture following surgical superior repositioning of the maxilla has produced four relevant findings in regard to mandibular autorotation. Specifically, these findings are (1) early clinical adaptation, (2) early physiologic adaptation, (3) late clinical adaptation, and (4) late physiologic adaptation. The manner in which the data from the investigation pertain to these four aspects of mandibular autorotation will be summarized.

Early clinical adaptation. The earliest neuromus-

Autorotation qf mundihle 471

cular adaptations are most likely mediated by the central nervous system because dramatic changes in soft tissues or bone are not biologically probable. The data in Table I illustrate the constancy of the clinical rest position of the mandible. Certain authors suggest that the interocclusal distance should be constant because the central nervous system sensorimotor feedback mechanism which governs the clinical rest position re- sponds to afferent stimuli from periodontal mechanoreceptors. Inspection of the data in Table II demonstrates that complete compensatory autorotation of the mandible has occurred in the PST 3 group during the 3 months following surgery because the interocclusal distance at the clinical rest position of the mandible did not change in response to a 6.2 mm. surgical superior repositioning of the maxilla.

Early physiological adaptation. The animal studies by several authors support the suggestion that physical change in the resting length of the tendon muscle system may occur within a few months following orthognathic surgery. We described the basis on which these changes in the masticatory musculature would be man- ifested by an alteration in the interocclusal distance at the physiologic rest position of the mandible. The data in Table I show the physiologic rest position to be relatively constant over time, so the significant increase (1.6 mm.) in the interocclusal space at the physiologic rest position that appeared in the PST 3 group 3 months following a 6.2 mm. surgical superior repositioning of the maxilla indicates that some physical shortening of the masticatory tendon-muscle system may have occurred (Table II). However, this foreshortening was incomplete and quite likely in a transition phase of neuromuscular adaptation.

Late clinical adaptation. Strong advocates of the ‘ ‘occlusal neuromuscular programming mechanism” would have us believe that the interocclusal distance at the clinical rest position remains constant throughout life to promote optima1 masticatory function. We have no data from the control group to support this hypothesis beyond 3 months. However, on the basis of our knowledge and experiences during the past 5 years, we strongly suspect this to be true. The data in Table III reveal that complete compensatory autorotation of the mandible is still evident in the smaller PST 24 group 2 years after surgery. In essence, the interocclusal distance at the clinical rest position was not significantly altered by a mean 6.2 mm. surgical superior repositioning of the maxilla, even 2 years postsurgically. This suggests that clinical autorotation of the mandible was not only complete but permanent.

Lute physiologic adaptation. Speculation regarding

the long-term physiologic response of the mandibular resting posture to surgical superior repositioning of the maxilla was difficult because these waters have not been previously charted. Whatever permanent compensatory response did occur would likely be stabilized within 2 years following surgery. The data in Table III reveal that there was a significant increase (5.1 mm.) in the interocclusal distance of the PST 24 group when the presurgical and 24-month postsurgical physiologic rest positions were compared. These data suggest that physiologic autorotation of the mandible, though evident immediately following immobilization, is minima1 after an extended period of time.

The mean masticatory electromyographic activity obtained from monitoring the anterior and posterior temporalis, superficial masseter, and media1 pterygoid muscles bilaterally provided little information of ben- efit to this discussion. Since there was no significant change in electromyographic activity at 3 or 24 months following surgery, we can only assume that the central nervous system maintains a neuromuscular equilibrium via slight change in natural head posture.

SUMMARY

What is the true relevance of this investigation? We suggest that two factors pertinent to successful surgical superior repositioning of the maxilla have been iso- lated. First, and most important, a complete compensatory clinical autorotation of the mandible does occur. In practical terms, this means that masticatory function will be at least maintained, if not improved, following surgery and that persons who undergo surgical intrusion of the maxilla will not walk around with their mouths open in an unfavorable manner. Second, the data from this study suggest that the greater the distance the maxilla is moved superiorly, the greater the dispar- ity between the functional centric occlusion and the physiologic rest position of the mandible. The significance of this phenomenon is borne out in Starling’s law, which states that optimum efficiency of skeletal muscle occurs at its physiologic resting length. Since the masticatory muscles of persons with vertical maxillary excess are of generally poor quality, a 15 mm. surgical superior repositioning of the maxilla to correct a severe vertical dentofacial deformity may reduce the efficiency of the muscles to the point where masticatory function is diminished significantly. Until our studies of masticatory bite force are conclusive in this matter, we caution surgeons to beware of this potential hazard in the treatment of the severe long-face deformity

472 Wessberg et al. Am. J. Orrhod. .Junr 1982

The authors wish to thank Dr. Stephen A. Schendel for technical assistance in computer morphomettic analysis. Re- quests for reprints should be addressed to Dr. Bruce Epker.

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23. Mohl, N. D.: Neuromuscular mechanisms in mandibular function, Dent. Clin. North Am. 22: 63-71, 1978.

24. Ellis, E., and Carlson, D. S.: The effect of suprahyoid myotomy on relapse of the surgically advanced mandible, Abstract pre- sented at 63rd annual meeting, American Association of Oral and Maxillofacial Surgeons, Sept. 20, 1981, Washington, D.C.

25. Goldspink, G.: The adaptation of muscle to a new functional length. In Anderson, D. J., and Matthews, B.: Mastication. Bristol, 1976, John Wright & Sons, Ltd., pp. 90-99.

Documents

Autorotation of Mandible