A method for evaluating skeletal relapsing force during maxillomandibular fixation after orthognathic surgery: A preliminary report Eiichi Komori, DDS,* Narumi Sagara, DDS,* and Kimihiko Aigase, DDS** Tokyo. Japan Although skeletal relapse after orthognathic surgery can be considered primarily the result of unbalanced tension in the surgically modified stomatognathic system (i.e., skeletal relapsing force [SRF] acting on the mandibular segment), no study of SRF has been reported and its nature still remains unclarified. This article presents a practicable method for evaluating SRF during the fixation period with the use of a foil strain gauge fixed to the upper arch wire between the central incisors, with a preliminary result after mandibular advancement by sagittal split ramus osteotomy to demonstrate the value of this method for studying SRF. The pattern and degree of SRF were shown to vary during the postoperative period and in accordance with experimental situations, including the mandibular resting posture, swallowing, clenching, and speaking. On day 3, a constant SRF of 550 gm existed even in the resting posture and was increased by 450 gm to a maximum of about 1 kg on swallowing; clenching and speaking also produced an increase. Thus it was inferred that SRF arose not only from surgically stretched perimandibular connective tissues but also from intermittent physiologic muscle actions. The constant SRF subsided gradually with time, and after day 33, swallowing as well as clenching no longer increased the SRF, which indicates that the critical period for postoperative skeletal stability was the first few weeks, during which muscle readaptation and primary bone healing occurred. It was concluded that this method of evaluating SRF, which is capable of immediately revealing any tendency to relapse or the progress of soft tissue adaptation and bone healing, could be used as an effective research tool. (AM J ORTHODDENTOFACORTHOP 1991 ;100:38-46.)

Although sagittal split ramus osteotomy is a versatile and thus widely practiced procedure for correction of malocclusions associated with mandibular deformities, skeletal relapse continues to be the most noteworthy and troublesome complication.t,2 The amount of skeletal relapse reported after mandibular advancement of this procedure has been as much as 80% of the advancement, and most of this relapse occurs as early as during the period of maxillomandibular fixation or in the first several postoperative weeks. 3-~ Diagnostic advances, modification of the surgical procedure, and several other measures have been taken to improve skeletal stability, 5"6'z~° Among these techniques, skeletal maxillomandibular fixation with suspension wires and rigid internal fixation with bone screws are currently accepted as the most effective From the Department of Dentistry, Jikei University School of Medicine, Tokyo, Japan, *Research associate, **Research associate in chief. 8/1/18083

38

means for prevention of relapse. 12 Nevertheless, Ellis and Gallo, 13 in a study of 20 patients who had the skeletal fixation, found a mean horizontal relapse of 9% of the advancement during the fixation period. In an analysis of 20 patients who had this fixation, Thomas et al. J4 also reported a measurable degree of relapse (1.14 mm relapse after 4.75 mm advancement, on average) during the same period. As for rigid fixation, although it has gained popularity in recent years, there are also several reports of its instability. Van Sickels, Larsen, and Thrash ~5 found that relapse began to be seen within the first 6 weeks when the mandible was advanced farther than 6 mm, and they concluded that relapse could overcome the rigid fixation. Doyle ~6studied relapse related to various types of intersegmental fixation technique with a mean 3t/2-year follow-up and noted 25% moderate relapse with the use of lag screws. Kirkpatrick et al. ~7observed an average of 8% relapse after 6 months (measured at B point), whereas Krekmanov and Lilja 18 observed about 12% horizontal relapse 1 year after surgery (measured at gnathion). Similarly, in relation to fixation with bone plates, Rubens

Vo/ume 100

Number 1

et al. j° reported 18.7% relapse at pogonion with a follow-up period of 6 to 14 months. Accordingly, although relapse has been minimized to a certain degree, the use of any of these fixation techniques may not necessarily be synonymous with predictable skeletal stability at the present time. In fact, regardless of fixation techniques, relapse could also occur through condylar osteolysis and remodeling as a result of increased load on the condyle. ~6.2o Skeletal relapse can primarily be considered the resulting change in position of the mandibular segments in response to an imbalance in force acting on them. Since the existing stomatognathic system is in a state o f inherent functional balance, surgical manipulation o f the system would naturally create an imbalance in force through alterations in dimension and biomechanits. 2~ Regardless of whether it is physiologic in nature, any sort of force that influences postoperative skeletal stability may be referred to as skeletal relapsing force (SRF). Prevention of such force, as well as development of a suitable fixation technique, is important for the minimization of relapse. In this respect, failure to maintain the proximal segment in its presurgical position and the advancement of the distal segment have often been cited as two major factors that create SRF because of stretching of the perimandibular soft tissues, which then tend to regress to their original anatomic state.* Wade," Schendel and Epker, 2s and Lake et al?"6 assumed that condylar distraction from the glenoid fossa (positional change in the proximal segment) was the most significant parameter of relapse. In contrast, however, Will et al. 27 failed to find any significant correlation between surgical and fixation condylar movement and the amount of relapse during the fixation period. In fact, they showed that considerable relapse could occur in the absence of condylar distraction. Similarly, relapse is less predictable in relation to the amount of advancement, showing wide individual variation on a percentage basis, although there is a general tendency for the absolute amount of relapse to increase as the amount o f repositioning inc r e a s e s . 4"7"8"~'26'27 Furthermore, opinion about the true efficacy of suprahyoid myotomy in human subjects for release of the stretching after advancement is not yet unanimous among investigators. 25"2~-~ Accordingly, in spite of many cephalometric studies on relapse, no satisfactory explanation of relapse has yet been given, and the factors that influence SRF still appear to require investigation. Although radiographic cephalometry has been used *References 3, 5, 6, 9, 10, 13, 14, 22-24

Evaluation of skeletal relapsing force

39

as the sole means of studying relapse because of its usefulness for measurement of relapse that has occurred over a given interval, it m a y not be entirely satisfactory for studying factors related to SRF, because it is incapable of showing SRF itself, which may be fluctuating at all times and because the degree of relapse measured by it m a y not directly reflect the relapse potential, or SRF, because of the recent development of fixation techniques. This article therefore presents a method for evaluating the SRF that is present during the period of maxillomandibular fixation, together with some preliminary results of evaluation made after mandibular advancement, to demonstrate the potential value of this method for studying factors of relapse.

MATERIALS AND METHODS Method for evaluating SRF When maxillomandibular fixation is maintained by tying the upper and lower orthodontic arch wires together with ligature wires at the interproximal spaces, a certain degree of arch wire deflection by the ligature wire could take place, depending on the maxillomandibular tension, which in turn may reflect the degree of SRF. This deflection was used as the basis for evaluating SRF. After securing of the maxillomandibular fixation if it was loose, either a ligature wire or a piece of surgical silk was placed between the middle parts of the upper and lower arch wires to maintain the least possible degree of tension without play (Fig. 1, A). A waterproof foil strain gauge (N11-FA-2120-11-W; NEC San-el Instruments Ltd., Tokyo, Japan) was then attached to the upper arch wire between the central incisor brackets, with the use of two orthodontic brackets in reverse to make the strain gauge removable (Fig. 1, B). On application of this setup, the degree of SRF acting at the middle of the arch wire was considered to be zero and was calibrated as such (Fig. 1, C). Next, the maxillomandibular ligatures, except for this middle one. were released so that the fixation was then maintained by only the middle ligature, with the expectation that most of the SRF would be concentrated at the middle of the upper arch wire by means of the ligature, creating a measurable deflection of the arch wire (Fig. l, D). The degree of this deflection was detected with the strain gauge in position and was converted into voltage readings with a 5370 bridge box (NEC San-el Instruments, Ltd.). Readings from the box were amplified with a 6M82 strain amplifier (NEC San-ei Instruments, Ltd.) and recorded with a WTR331 pen-writing oscillographic recorder (Graphtee Corp., Tokyo, Japan). The degree of the force that created the deflection was calibrated with a known weight at the time of every recording.

Experimental situations SRF was evaluated at 3, 10, I8, 33, and 44 days after

surgery in the following four experimental situations. Evaluation was repeated at least three times for each situation, and a representative record was adopted. During evaluation.

40 Komori, Sagara, and Aigase

Orthop. July 1991

Am, J. Orthod. Dentc~w,

B

i

i

i~ ~ i

i

+ :

/! +'~ '~? ~)/ I ~ :):~ii:il/~:i/ ) ~~I~(:!i!~ ~II~I : ~:'!:

Fig. 1. A, Stable maxillomandibular fixation with an additional middle ligature in place. B, Foil strain gauge in combination with two orthodontic brackets. C, Setup for zero calibration; the gauge was attached to the upper arch wire by means of the brackets so as to cross the middle ligature. 13, Maxillomandibular fixation was maintained only with the middle ligature during evaluation.

the subject was seated comfortably upright in a standard dental chair with the head in line with the back, and some cotton rolls were inserted into the vestibule to separate the lips from the strain gauge. Resting posture of the mandible. SRF was evaluated with the mandible at rest to determine whether any constant SRF was present and, if so, to record its magnitude. The subject was instructed to keep his or her jaw completely at rest without any intentional movement. Evaluation was continued until the degree of SRF became constant. Saliva swallowing. The degree and pattern of SRF associated with saliva swallowing were evaluated to determine whether the action of swallowing plays any role in relapse. The subject was directed to swallow and then to keep the mandible at rest again. Voluntary clenching. The degree and pattern of SRF during voluntary clenching were evaluated to study the role of masticatory muscle activity in relapse. The subject was directed to clench the back teeth for a while and thereafter to rest the mandible again. No instruction as to the degree of clenching effort was given. Speaking. The subject was asked to say "oh" in order to

study the influence of speaking on the degree and pattern of SRF. No instruction as to the loudness of speaking or the degree of mouth-opening effort was given.

Experimental p a t i e n t The patient used for this preliminary study was a woman who was 23V2 years old at the time of surgery. She had a Class II, Division I malocclusion and retrognathism (Fig. 2 and Table I). Routine orthodontic preparation was undertaken with an 0.018 x 0.025 inch full edgewise appliance after removal of the upper first premolars. The surgical procedure consisted of the modified sagittal split ramus osteotomy and augmentation genioplasty as described by BelP and by Epkcr and Fish. '~ The mandibular distal segment was advanced by 8 mm in a horizontal direction and the chin by an additional 7 ram. Suprahyoid myotomy was not performed, although the degree of surgical stretching of the muscle calculated according to Wessberg, Schcndel, and Epker 2a was 45%, Maxillomandibular fixation, which consisted of dental wiring between the upper and lower orthodontic arch wires and skeletal wiring with bilateral maxillary peralveolar and circummandibular wires connected to their respective arch wires at

Volmne 100 Number 1

Evaluation of skeletal relapsing force 41

A

..........

L-

C

Fig. 2. Cephalometric superimpositions. Broken lines indicate relative changes, A, Surgical change; B, relapse during fixation; C, net result of surgery and relapse.

Table I. Presurgical c e p h a l o m e t r i c values in the patient, together with changes at surgery and during fixation

Parameter

Presurgery

Surgical change

Fixation change

Facial axis angle (degrees) Facial depth angle (degrees) Mandibular plane angle (degrees) Maxillary depth angle (degrees) Convexity (mm) Upper incisor inclination to Frankfort horizontal plane (degrees) Lower incisor inclination to mandibular plane (degrees)

71.5 73.0 48.0 85.0 12.5

+9.5 + 7.5 -7.0 0 -8.5

-2.0 - 1.5 +4.5 0 +1.5

95.0

-0.6

94.0

- 2.0

the canine regions, was maintained for 44 days after surgery. 32 Cephalometric changes at the time of surgery and during the period of fixation were as shown in Fig. 2 and Table I.

At surgery, anterior rotation of the proximal segment of 12° and posteroinferior condylar displacement of 2 mm occurred, and the mandibular plane angle decreased by 70. During the fixation, a horizontal relapse of 2 ram, or 13% of the total

42 Kornori, Sagara, and Aigase

Am. J. Orthod. Dentofac. Orthop. Ju/y

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advancement of 15 ram (measured at pogonion), occurred along with an increase of 4.5 ° in the mandibular plane angle and a 2 nun loss of posterior facial height. This degree and pattern Of relapse was almost comparable to that in previous studies of relapse that employed a similar type of skeletal fixation.

Control To provide a baseline for evaluating the patient's SRF, the degree and pattern of maxillomandibular tension that may be inherently produced by fixation of the intact maxilla and mandible were evaluated in a healthy 27-year-old female volunteer who had good dental occlusion because a presurgical

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Evaluation of skeletal relapsing force

Number 1

43

"1lOs x

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patient without stable centric occlusion in centric relation is unsuitable for maxillomandibular fixation. The volunteer was given temporary fixation with orthodontic appliances attached to the anterior teeth.

RESULTS Patient The degree and pattern of SRF were found to vary during the postoperative period and according to the experimental situations used, as follows: 1. Day 3 (Fig. 3, A). When the mandible was at rest, a constant SRF of about 550 gm existed, showing a regular, slight fluctuation concomitant with respiration. Swallowing of saliva caused a transient increase in the SRF by 450 gm to a maximum of about i kg. Speaking also increased the SRF by about 100 gm, whereas clenching had no appreciable effect. The patient seemed unable to clench vigorously. 2. Day 10 (Fig. 3, B). The constant SRF at rest decreased by 150 gm to about 400 gm. Swallowing again increased the SRF by 300 gm, speaking by 150 gm, and clenching by 100 gm. 3. Day 18 (Fig. 3, C). The constant SRF decreased further by 150 gm to about 250 gm. Swallowing still increased the SRF to 500 to 550 gm at peak, whereas the effect of swallowing appeared to become somewhat shorter and sharper in comparison with that on days 3 and 10. Clenching and speaking increased the SRF by almost the same amount as that on day 10. 4. Day 33 (Fig. 3, D). The constant SRF decreased further to 200 gm. The effect of swallowing on this constant SRF was not as marked as before; at the beginning and end of its course, the SRF was reduced by 100 gin, whereas in midcourse it was increased by 100 gm. Clenching no longer increased the SRF but reduced the value completely to zero; speaking increased the value by 250 gin.

. Day 44 (Fig. 3, E). The value of the constant SRF fell to 100 to 150 gm. Swallowing reduced the SRF to almost zero at peak. Clenching also reduced the SRF to zero, but speaking still increased the value by 200 gm.

Control (Fig. 4) Maxillomandibular tension that may be comparable to the constant SRF during the time when the mandible is at rest was 100 to 150 gm. Swallowing and clenching reduced this tension to zero, whereas speaking increased it by 250 gm.

DISCUSSION Method of evaluating SRF Since skeletal relapse is a three-dimensional phenomenon, in theory several gauges in different positions may be needed to detect SRF in its entirety. In practice, however, the use of such an experimental system may be too complex and would involve patient discomfort. In this study, only one gauge was placed between the central incisors, since the fact that a skeletal suspension wire in the anterior mandible is most effective for minimizing skeletal relapse 5'6'12 was considered to indicate that values obtained with the gauge in this position would reflect the majority of SRF. The maxillomandibu!ar tension evaluated as SRF in this way may be comparable to forces that create anterior tooth movement during the fixation period rather than the actual SRF. However, since this tooth movement occurs to compensate for skeletal relapse, it would seem possible to consider the tension to reflect the actual SRF. The degree of arch wire deflection varies with the wire dimension and the span between the incisor brackets, as well as with the tension exerted on the wire by the ligature wire. Mobility and movement of the incisors may also affect the deflection. Therefore standardization of the appliances and calibration at every evaluation are necessary. In this study, an arch wire of

44

Komori, Sagara, and Aigase

0.018 × 0.025-inch full-dimension Elgiloy (Rocky Mountain Orthodontics, Denver, Colo.) was employed. Since the strain gauge was very sensitive to deflection, an arch wire of this stiffness and size worked satisfactorily without allowing appreciable slippage of occlusion during evaluation.

Evaluation of the preliminary result Although the elasticity of the surgically stretched perimandibular soft tissues has been cited as a major factor that creates SRF after mandibular advancement,* the actual degree of this force is not yet known. In this study, the apparent disparity in the degree of SRF with the mandible at rest between the patient (550 gm on day 3) and the control Subject (100 to 150 gm) would correspond to this force. A constant orthopedic force of this degree would seem sufficient to give rise to skeletal relapse with compensatory movements of the teeth, especially in patients without skeletal fixation. However, because of condylar distraction and anterior proximal segment rotation at the time of surgery (Fig. 2, A), an increased degree of this constant SRF would be expected in the patient. For clarification, it is necessary to evaIuate the constant SRF in several patients in whom the condyles are seated properly. It is likely that a relapsing force much greater than 550 gm existed i n this patient o n the day o f surgery and on postoperative day s 1 and 2, since the constant SRF apparently ameliorated with time after surgery. This finding is in agreement with previous reports that most of the relapse occurred rapidly within the first few postoperative weeks. 3-5.8.14.2-/Furthermore,judging from the observation that the mature muscle can readapt to its new functional length within a few weeks, 31'a3'~4the finding can be considered to justify the assumption that among the various perimandibular tissues, the stretched suprahyoid muscles p!ay a major role in creating the constant SRF.'[" The role of this muscle in SRF may be better understood when comparative SRF values at rest are determined in groups of patients with and without myotomy. In contrast to the previously mentioned passive tension induced by positional change of the segments, active muscle tension has received less attention. In this study, however, swallowing, clenching, and speaking all apparently increased the constant SRF on days 3, 10, and 18. The suprahyoid and masticatory muscles work synergistically for about 1 second during swallowing, whereas they are independent during both clenching and speaking. 35-37 The activated suprahyoid *References 3, 5, 6, 9, 10, 13, 14, 22-24 "PReferences 3, 5, 6, 9, 10, 13, 24, 31

Am. J. Orthod. Dentofac. Orthop. July I991

muscles could pull the anterior end of the distal segment downward and backward, whereas the masticatory muscles could pull the proximal segment forward and upward and, by means of the intraosseous wires, pull the posterior end of the distal segment upward, a.v,3sIn either case, the distal segment tends to rotate backward and downward. If this is accepted, then the transient increase in SRF observed on swallowing would be a net result of the combined action of the two muscle groups, and in this way swallowing could affect SRF to a much greater extent than clenching or speaking. In addition, since swallowing of saliva is an essential physiologic action and occurs as frequently as 600 times per day, 39 the contribution of swallowing to relapse during the fixation period would seem plausible. At the very least, it would be reasonable to implicate the masticatory muscle activity during swallowing in the loss of posterior facial height and intrusion of the molars that occur during the fixation period, for which tension from the suprahyoid muscles alone cannot account. 7'8'14'2~'38 In this patient especially, because condylar distraction was ev!dent, it is likely this masticatory activity played a greater role in the creation of relapse. After day 33, swallowing no longer had such a marked effect on the SRF as was found in the healthy subject. This change in SRF behavior during swallowing occurred almost in tandem with that during clenching, which indicates the importance of masticatory muscle activity for swallowing as well as for clenching. Because this activity has a vector that could bend the mandible at the pliable osteotomy site and encourage rotation of the distal segment, this change in SRF behavior during swallowing and clenching after day 33 would imply that the osteotomy site has healed to a degree that does not permit immediate bending of the mandible. This inference would be compatible with the observation by Bell and Schendel4° that in rhesus monkeys new bone and osteoid were deposited at the surgical site within 3 weeks and that bony bridging between the segments was evident within 6 weeks after the modified sagittal split ramus osteotorny. Comparative studies on SRF behavior at swallowing in patients with and without rigid internal fixation may clarify the role of mandibular pliability in relapse. In contrast to swallowing, clenching may not in itself be an important factor in relapse because it may be controllable. However, speaking may contribute to relapse, since it is impractical to restrict speaking throughout the fixation period and, although the contribution of an isolated speech sound (such as "oh") to the increase in SRF was relatively small, connected speech may generate a much more significant amount of SRF.

Volume t00 Number t I n conclusion, it was inferred f r o m these prelimin a r y findings that S R F was c o m p o s e d of not only a c o n s t a n t passive t e n s i o n that arises from surgical s t r e t c h i n g of the p e r i m a n d i b u l a r connective tissues but a l s o intermittent physiologic m u s c l e activity such as that w h i c h occurs during s w a l l o w i n g and speaking, and that t h e first few postoperative weeks were especially critical f o r skeletal stability b e c a u s e o f m u s c l e readaptation to a l t e r e d mandibular d i m e n s i o n and p r i m a r y bone healing to r e g a i n normal m a n d i b u l a r biomechanics.

Limitation and further study To our knowledge, no study on the nature of S R F h a s been reported, and thus that aspect has remained o n l y speculative, i n this respect, the method of evalu a t i n g SRF presented here m a y be o f interest, and m a n y i n f e r e n c e s could be m a d e from the preliminary results, t h o u g h it is clearly inadvisable to draw conclusions f r o m data based on o n l y a single patient. It was at least s h o w n that this m e t h o d could have potential as a new r e s e a r c h tool for studying relapse f r o m an aspect other t h a n cephalometry. In contrast, evaluation of S R F has t h e potential to provide i m m e d i a t e valuable information o n relapse, since the degree of S R F can directly indicate t h e t e n d e n c y for relapse and its pattern can indicate the progress of bone healing, as has b e e n shown, Further s t u d y on SRF i n v o l v i n g groups o f patients assigned a c c o r d i n g to various criteria, such as type o f osteotomy, f i x a t i o n technique e m p l o y e d , and degree of proximal s e g m e n t displacement a n d distal segment advancement, w i l l be necessary to o b t a i n m e a n i n g f u l data, which c o u l d prove valuable for a better understanding o f the etiologic factors o f skeletal relapse. FIEFEFIENGES 1. Turpin DL. Surgical mandibular advancement and stability. AM J ORTHOt)1983;84:171-3. 2. Martis CS. Complications after mandibular sagittal split osteotomy. J Oral Maxillofac Surg 1984;42:101-7. 3. McNeill RW, Henley JR, Sundberg RJ. Skeletal relapse during intermaxillary fixation. J Oral Surg 1973;3l:212-27. 4. Ire J, McNeill RW, West RA. Mandibular advancement:skeletal and dental changes during fixation. J Oral Surg 1977;35:881-6. 5. Bell WH. Surgical correction of dentofacial deformities. Philadelphia: WB Saunders, 1980:765-7. 6. Epker BN, Fish LC. Dentofacial deformities. Integrated orthodontic and surgical correction. St. Louis: CV Mosby, 1986: 183-7. 7. Will LA, Joondeph DR, Hohl TH, West RA. Condylar position following mandibular advancement: its relationship to relapse. J Oral Maxillofac Surg 1984;42:578-88. 8. Smith GC, Moloney FB, West RA. Mandibular advancement surgery: a study of the lower border wiring technique for osteosynthesis. Oral Surg Oral Med Oral Pathol 1985;60:467-75. 9. PoultonDR, WareWH. Surgieal-orthodontictreatment ofsevere mandibular retrusion. AM J ORTHOD1971;59:244-65.

Evahtation of skeletal relapsing force

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10. Poulton DR, Ware WH. Surgical-orthodontic treatment of severe mandibular retmsion (Part II). AM J ORT~OD 1973;63:237-55. 11. Wade DB. Surgleai-orthodontic stability in retrognathic patients: an implant study. Angle Orthod 1988;58:71-95. 12. Ellis E III, Reynolds S, Carlson DS. Stability of the mandible following advancement: a comparison of three postsurgical fixation techniques. AM J ORTrIODDENTOFACORTHOP1988;94:3849. 13. Ellis E III, Gallo WJ. Relapse following mandibular advancement with dental plus skeletal maxillomandibular fixation. J Oral Maxillofac Surg 1986;44:509-15. 14. Thomas PM, Tucker MR, Prewitt JR, Proffit WR. Early skeletal and dental changes following mandibular advancement and rigid internal fixation. Int J Adult Orthod Orthognath Surg 1986;1: 171-8. 15. Van Sickels JE, Larsen AJ, Thrash WJ. A retrospective study of relapse in rigidly fixated sagittal split osteotomies: contributing factors. AM J ORTHODDENTOFACORTHOP1988;93:413-8. 16. Doyle MG. Stability and complications in 50 consecutively treated surgical-orthodontic patients: a retrospective longitudinal analysis from private practice. Int J Adult Orthod Orthognath Surg 1986;1:23-36. 17. Kirkpatrick TB, Woods MG, Swift JQ, Markowitz NR. Skeletal stability following mandibular advancement and rigid fixation. J Oral Maxillofac Surg 1987;45:572-6. 18. Krekmanov L, Lilja J. Orthognathic surgery with no postoperative intermaxillary fixation. Stand J Plast Reconstr Surg Hand Surg 1987;21:189-97. 19. Rubens BC, Stoelinga PJW, Blijdorp PA, Schoenaers JHA, Politis C. Skeletal stability following sagittal split osteotomy using monoeortical miniplate internal fixation, lnt J Oral Maxillofac Surg 1988;17:371-6. 20. Worms FW, Speidel TM, Bevls RR, Waite DE. Fosttreatment stability and esthetics of orthognathic surgery. Angle Orthod 1980;50:251-73. 21. McNamara JA Jr, Carlson DS, Yellich GM, Hendricksen RP. Musculoskeletal adaptation following orthognathic surgery. In: McNamara JA Jr, Carlson DS, eds. Muscle adaptation in the craniofacial region. Monograph 8, Craniofacial Growth Series. Ann Arbor: Center for Human Growth and Development, University of Michigan, 1978:91-132. 22. Mayo KH, Ellis E IIL Stability of the mandible after advancement and use of dental plus skeletal maxillomandibular fixation: an experimental investigation in Macaca mulatta. J Oral Maxillofac Surg 1987;45:243-50. 23. Epker BN, Wessberg GA. Mechanisms of early skeletal relapse following surgical advancement of the mandible. Br J Oral Maxillofac Surg 1982;20:175-82. 24. Epker BN, Wolford LM, Fish LC. Mandibular deficiency syndrome. II. Surgical considerations for mandibular advancement. Oral Surg Oral Med Oral Pathol 1978;45:349-63. 25. Schendel SA, Epker BN. Results after mandibular advancement surgery: an analysis of 87 cases. J Oral Maxillofac Surg 1980;38:265-82. 26. Lake SL, McNeill RW, Little RM, West RA. Surgical mandibular advancement: a cephalomctric analysis of treatment response. AM J ORTHOD1981;80:376-94. 27. Wiil LA, West RA. Factors infiuencingthe stability of the sagittal split osteotomy for mandibular advancement, 3 Oral Maxillofac Surg 1989;47:813-8. 28. Wessberg GA, Schendel SA, Epker BN. The role of suprahyoid myotomy in surgical advancement of the mandible via sagittal split ramus osteotomies. ] Oral Maxillofac Surg 1982;40:273-7.

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29. Steinhauser EW. Advancement of the mandible by saglttal ramus split and suprahyoid myotomy. J Oral Maxillofac Surg 1973; 31:516-21. 30. Ellis E III, Carlson DS. Stability two years after mandibular advancement with and without suprahyoid myotomy: an experimental study. J Oral Maxillofac Surg 1983;41:426-37. 31. Carlson DS, Ellis E III, Dechow PC, Nemeth PA. Short-term stability and muscle adaptation after mandibular advancement surgery with and without suprahyoid myotomy in juvenile Macaca mulatta. Oral Surg Oral Med Oral Pathol 1989;68:135-49. 32. Komori E, Aigase K, Sugisak[ M, Tanabe H. Skeletal fixation versus skeletal relapse. AM J ORTHOD DENTOFAC ORTHOP 1987;92:412-21. 33. Goldspink G. The adaptation of muscle to a new functional length. In: Anderson DJ, Matthews B, eds. Mastication. Bristol, England: J Wright Ltd., '1976:90-9. 34. Reynolds ST, Ellis E III, Carlson DS. Adaptation of the suprahyoid muscle complex to large mandibular advancements. J Oral Maxillofac Surg 1988;46:1077-85. 35. M~ller E. The chewing apparatus: an electromyographie study of the action of the muscles of mastication and its correlation to

36. 37. 38.

39. 40.

facial morphology. Acta Physiol Scand 1966;69(suppl 280): 914, 59-150. Kawazoe T, Kinesiological studies of the rest position of the mandible. Shika Igaku (J Osaka Odontol Soc) 1972;35:474-507. Munro RR. Activity of the digastric muscle in swallowing and chewing. J Dent Res 1974;53:530-7. Sandor OKB, Stoelinga PJW, Tideman H, Leenen RJ. The role of the intraosseous osteosynthesis wire in saglttal split osteotomies for mandibular advancement. J Oral Maxillofac Surg 1984;42:231-7. Lear CSC, Flanagan JB Jr, Moorrees CFA. The frequency of deglutition in man. Arch Oral Biol 1965;10:83-99. Bell WH, Sehendel SA. Biologic basis for modification of the sagittal ramus split operation. J Oral Surg 1977;35;362-9.

Reprint requests to: Dr. Eiichi Komori Department of Dentistry Jikei University School of Medicine 3-25-8 Nishi-shinbashi Minato-ku, Tokyo 105, Japan

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1992--St. Louis, Mo., May 9 to 13, St. Louis Convention Center 1993--Toronto, Canada, May 16 to 19, Metropolitan Toronto Convention Center 1994--Orlando, Fla., May 1 to 4, Orange County Convention and Civic Center 1995--San Francisco, Calif., May 7 to 10, Moscone Convention Center 1996--Denver, Colo., May 12-15, Colorado Convention Center ,i

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A method for evaluating skeletal relapsing force during maxillomandibular fixation after orthognathic surgery: a preliminary report.

Although skeletal relapse after orthognathic surgery can be considered primarily the results of unbalanced tension in the surgically modified stomatog...
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