Adaptation of the suprahyoid muscle complex to mandibular advancement surgery

The suprahyoid musculature has been implicated as one of the major factors responsible for relapse after mandibular advancement surgery. Previous studies have also indicated that the muscle and connective tissues comprising the suprahyoid complex must adapt to increased length brought about by mandibular advancement for skeletal stability to be achieved. The purpose of this study was to provide quantitative data concerning the immediate changes and long-term adaptations that take place within the suprahyoid complex over a 2-year period after mandibular advancement. Mandibular advancement was performed on ten adult Macaca mulatta monkeys with and without suprahyoid myotomy (n = 5/group). Six animals were used as controls. Mandibular length and changes in the length of the various anatomic regions of the suprahyoid complex were evaluated radiographically with the aid of radiopaque bone, muscle, and tendon markers implanted preoperatively. The results for the nonmyotomy group showed that the suprahyoid complex was elongated approximately two thirds the amount of mandibular lengthening, the major immediate adaptations within the suprahyoid complex after the surgical procedure occurred at the muscle-bone interface and the muscle-tendon interface, the change in length at the muscle-tendon junction was maintained throughout the 2-year follow-up period, indicating that significant long-term adaptations took place primarily at that location, and no significant short-term changes or long-term adaptations were seen within the anterior digastric muscle or the intermediate digastric tendon. Within the myotomy group, it was found that the suprahyoid complex recoiled immediately after myotomy such that the anterior belly of the digastric muscle became separated from the advanced distal mandibular segment by more than twice the amount of mandibular lengthening, the anterior digastric muscle remained essentially at this posterior position throughout the 2-year follow-up period, and though not significant, there was a trend for a decrease in the length of the anterior digastric muscle belly. On the basis of these results, it was concluded that both short-term changes and long-term adaptations to lengthening of the suprahyoid complex as a result of mandibular lengthening occur primarily within the connective tissues comprising the muscle-tendon and muscle-bone interfaces, not within the muscle fibers themselves.     

Short-term stability and muscle adaptation after mandibular advancement surgery with and without suprahyoid myotomy in juvenile Macaca mulatta

The purpose of this study was to examine the short-term adaptations that occur within the mandible and anterior digastric muscle complex after mandibular advancement with and without suprahyoid myotomy in 20 juvenile rhesus monkeys. The results showed that the animals that did not undergo myotomy experienced relapse equivalent to 13% of the surgical advancement. Those animals that underwent a myotomy of the digastric muscle complex showed complete stability of the surgical lengthening of the mandible. Both groups of animals grew normally after the fixation period when compared to age-matched control animals. Analysis of adaptations within the digastric muscle complex was performed with the use of radiopaque muscle and tendon markers. The results showed an immediate lengthening of the entire digastric muscle complex with mandibular advancement surgery in the group that underwent advancement without myotomy. Further analysis showed that most lengthening in these animals occurred at the connective tissue interfaces of the complex--at the muscle-bone and muscle-tendon interfaces. No significant changes in sarcomere or fiber length were found in the group that did not undergo myotomy, although there was a significant shortening of muscle fibres resulting from loss of serial sarcomeres in the myotomy group. Comparison of histochemical characteristics of the anterior digastric muscle before and after surgery revealed the following findings: (1) there were no significant differences in percentage of composition between control and experimental muscles; (2) despite fixation of the jaws and myotomy, there was no evidence of atrophy of the anterior digastric muscle at any experimental interval; and (3) the type I fibers of the anterior digastric muscle underwent significant stretch-induced hypertrophy after lengthening. The results of this study support the hypothesis that tension produced by stretching of the connective tissues associated with the digastric muscle complex can contribute to postsurgical relapse of the surgically advanced mandible. However, no adverse effect on future growth of the mandible was observed from stretching the digastric muscle complex by mandibular advancement surgery in juvenile subjects.     

Electromyography of the suprahyoid musculature following mandibular advancement with and without rigid fixation

The purpose of this investigation was to determine if the activity of the suprahyoid musculature changes following advancement of the mandible and the use of rigid or nonrigid fixation. Ten monkeys underwent mandibular advancement; six underwent 6 weeks of maxillomandibular fixation (MMF), and four had rigid fixation without MMF. Electromyography (EMG) of the suprahyoid musculature was performed preoperatively, and at 3, 7, and 10 weeks postoperatively. The results of this study fail to demonstrate an increase in suprahyoid EMG activity following mandibular advancement. Furthermore, there were no differences between the groups with different types of fixation.     

Stability two years after mandibular advancement with and without suprahyoid myotomy: an experimental study

Ten adult rhesus monkeys underwent mandibular advancement surgery of 4-6 mm with and without suprahyoid myotomy. Serial lateral cephalograms using radiopaque bone markers were obtained during maxillomandibular fixation and for 96 weeks after release of fixation to determine the effects of suprahyoid myotomy on short-term and long-term adaptations in the advanced mandible. The non-myotomy group exhibited a significant reduction in the length of the advanced mandible (relapse) during the fixation period but showed no significant change in mandibular length after release of fixation. The myotomy group exhibited no relapse during the fixation period and after release of fixation displayed a slight but statistically significant increase in mandibular length. This supports the hypothesis that stretching of the suprahyoid musculature as a result of mandibular advancement surgery is a major factor leading to skeletal relapse.     

Stability of the mandible following advancement: a comparison of three postsurgical fixation techniques

This study examines short-term stability of the mandible following mandibular advancement surgery by means of three standard techniques of postsurgical fixation. Twenty-two adult female rhesus monkeys (Macaca mulatta) underwent sagittal ramus advancement osteotomy of approximately 4 to 6 mm. Six animals had dental maxillomandibular fixation alone. Six animals had dental plus skeletal maxillomandibular fixation with circummandibular wires connected to pyriform aperture wires. Ten animals had rigid internal fixation with bicortical bone screws between the proximal and distal segments without maxillomandibular fixation. Radiographic cephalograms with the aid of tantalum bone markers and dental amalgams were analyzed during the first 6 postoperative weeks to evaluate skeletal and dental stability. Rigid internal fixation and the use of dental plus skeletal maxillomandibular fixation were both equally effective in the prevention of postsurgical relapse. However, in the animals in which only dental maxillomandibular fixation was used, statistically significant changes (relapse) occurred when compared with either of the other groups.     

Craniofacial pattern of Class III deciduous dentition.

The purpose of this study was to investigate the morphological characteristics of the cranio-dento-facial complex of children with deciduous dentition and Class III malocclusion. Forty Chinese children in Taiwan with normal occlusion and 40 with Class III malocclusion in deciduous dentition were selected for cephalometric analysis. Mandibular length was significantly greater and the mandible was situated farther forward in the Class III group. The maxilla was also slightly backward in this group, perhaps in association with the shorter maxillary length. The mandibular incisors were tipped lingually to compensate for the intermaxillary skeletal dysplasia and the maxillary incisors were tipped lingually by the retroinclined mandibular incisors.     

Stability of the mandible after advancement and use of dental plus skeletal maxillomandibular fixation: an experimental investigation in Macaca mulatta

This study examined short-term stability of the mandible following advancement surgery and the use of skeletal suspension wires plus dental maxillomandibular fixation. Twenty-four adult female Macaca mulatta underwent bilateral sagittal ramus osteotomy and advancement of approximately 6 mm. All animals had dental maxillomandibular fixation secured by bonding the upper and lower teeth together with an orthodontic composite resin. In half of the animals, the use of circummandibular wires connected to pyriform aperture wires were additionally applied. Tantalum bone markers were placed and cephalograms analyzed during the first six postoperative weeks to evaluate skeletal stability. A statistically significant mean horizontal relapse at the mandibular symphysis occurred in the group without the skeletal wires, whereas no relapse occurred in the group with the skeletal wires. A significant difference in the vertical displacement of the anterior mandible occurred, with an inferior movement of the symphysis in the group without skeletal wires, and a superior movement of the symphysis in the group with skeletal wires. The results of this study indicate that the use of skeletal suspension wires is advantageous in the prevention of horizontal and vertical skeletal relapse.     

Histochemical analysis of the masseter and temporalis muscles in macaca mulatta after mandibular advancement using rigid or nonrigid fixation

This study compared the histochemical characteristics of the temporalis and masseter muscles in monkeys who underwent mandibular advancement with and without immobilization of the mandible. The results showed, when compared with controls, that the animals who underwent maxillomandibular fixation (MMF) following advancement of the mandible had significant atrophy in both temporalis and masseter muscles. In contrast, much less myoatrophy was noted in the animals who underwent rigid internal fixation and no MMF following advancement of the mandible.     

Treatment of unilateral ankylosis of the temporo-mandibular joint when a class II skeletal relationship exists

If, in a case of ankylosis of the temporo-mandibular joint, a class II skeletal relationship exists, advancement of the mandible into a class I skeletal relationship should be an integral part of the treatment plan. The advantages are: better aesthetics (especially the appearance of the chin), more efficient action of the suprahyoid muscles (mouth openers), diminished contact of the angle of the jaw with the sterno-cleido-mastoid muscle, increased distance of the lower incisors from the axis of rotation of the mandible (resulting in increased incisal distance at the same rotational angle) and the possibility of correction of the sometimes traumatic occlusion against the palatal mucosa all in the same session. The method advocated in unilateral cases is the insertion of a costo-chondral graft on the affected side and a lengthening osteotomy on the opposite side with fixation into a class I skeletal relationship initially disregarding the incisal relationship.     

Role of suprahyoid musculature on mandibular morphology and growth orientation in rats.

This study was undertaken in rats to study the influence of the suprahyoid muscles on mandibular growth, morphology, and orientation. The aim was to investigate the effect of bilateral suprahyoid muscle myectomy on the skeletal growth and orientation of the mandible and on its orientation in rats. Forty-eight 4-week-old rats were divided into 2 experimental and 2 control groups as follows: A, 12 animals in which bilateral excision of the anterior digastric muscle was performed; B, 12 animals in which bilateral excision of the anterior digastric, transverse mandibular, and mylohyoid muscles was performed; C, 12 animals that were sham-operated bilaterally without any muscular excision; D, 12 control animals not subjected to any operation. The experimental period was 30 days. Lateral and dorsoventral radiographs were taken on days 1 and 30, after muscular excision. Cephalometric analysis was performed for each animal according to a method and procedure established in our laboratory and described earlier. The findings support the occurrence of decreased mandibular growth in the group in which bilateral digastric, transverse mandibular, and mylohyoid excision was performed as compared with controls. In addition, the mandible in the bilateral digastric excision group exhibited a more upward orientation. It was thus proven that the absence of the suprahyoid musculature does affect both skeletal growth and orientation of the mandible.     

Cephalometric analysis of adaptations after lengthening of the masseter muscle in adult rhesus monkeys, Macaca mulatta

This relationship between the function of the muscles of mastication and craniofacial form was investigated in young adult monkeys by increasing the functional length of the elevator muscles of the mandible non-invasively by a bite-opening splint cemented to the maxillary dentition. The major adaptations to increased vertical dimensional were (1) marked superior and some anterior displacement of the maxillary complex, (2) dental intrusion and (3) rotation of the mandible. These alterations produced a shortening of the lengthened masseter muscle, i.e. a reduction in the amount of muscle stretch brought about by the appliance. Migration of the masseteric insertion along the ramus did not contribute significantly to the pattern of adaptation. The role of the masseter muscle in craniofacial adaptations to altered vertical dimension was determined by detaching and re-attaching the insertion of the masseter muscle in one group of experimental animals. The myotomized monkeys experienced significantly less anterior displacement of the maxilla than the non-myotomized monkeys, indicating that the surgery may have lessened some of the anteriorly-directed distracting forces of the lengthened masseter. Masseter myotomy alone was not sufficient to eliminate the vertically-directed distracting forces of the remainder of the mandibular elevator muscles brought about by increasing the vertical dimension of the lower face.     

Early developmental traits in class II malocclusion

The occlusal traits of Class II occlusion in the deciduous dentition include distal terminal plane of the second deciduous molars, distal canine relation, large overjet, and large overbite. Other findings are narrow upper dental arch and maxillary base and poor anterior spacing. Skeletally, Class II children differ less from normal children. The cranial base, including the base flexure, and the maxilla are normal. The mandibular corpus and lower facial height are short, the gonial angle is large, and the dentoalveolar position of the mandible is retruded. The height of the ramus is normal, as is the skeletal position of the mandible, with the exception of the chin, which becomes slightly retruded after 5 years of age. As most skeletal traits of Class II occlusion develop later than the occlusal characteristics, it is suggested that no evidence can be found for a skeletal Class II growth pattern in the deciduous dentition. The deficient transversal growth of the maxilla and the sagittal growth of the mandible seem to cause the typical Class II occlusion. Further skeletal changes are likely to develop as secondary adaptations.     

Early developmental traits in Class II malocclusion

The occlusal traits of Class II occlusion in the deciduous dentition include distal terminal plane of the second deciduous molars, distal canine relation, large overjet, and large overbite. Other findings are narrow upper dental arch and maxillary base and poor anterior spacing. Skeletally, Class II children differ less from normal children. The cranial base, including the base flexure, and the maxilla are normal. The mandibular corpus and lower facial height are short, the gonial angle is large, and the dentoalveolar position of the mandible is retruded. The height of the ramus is normal, as is the skeletal position of the mandible, with the exception of the chin, which becomes slightly retruded after 5 years of age. As most skeletal traits of Class II occlusion develop later than the occlusal characteristics, it is suggested that no evidence can be found for a skeletal Class II growth pattern in the deciduous dentition. The deficient transversal growth of the maxilla and the sagittal growth of the mandible seem to cause the typical Class II occlusion. Further skeletal changes are likely to develop as secondary adaptations.     

The role of suprahyoid myotomy in surgical advancement of the mandible via sagittal split ramus osteotomies

The suprahyoid muscles have been implicated as primary effectors of relapse following surgical advancement of the deficient mandible. Accordingly, suprahyoid myotomy and/or the use of cervical collars have been recommended as adjunctive procedures to minimize postoperative relapse. This computerized morphometric evaluation of 16 patients revealed that suprahyoid myotomy is not essential to skeletal stability following surgical advancement of the mandible.     

Superior repositioning of the maxilla combined with mandibular advancement: mandibular RIF improves stability.

Postsurgical changes in 24 patients who had rigid internal fixation (RIF) of the mandible with screws after combined superior repositioning of the maxilla and mandibular advancement were compared with 53 patients who underwent the same surgery but who had intraosseous wire fixation, skeletal suspension wires, and 8 weeks of maxillomandibular fixation (MMF). During the first 8 weeks after surgery, the mean posterior relapse of the mandible was greater in the MMF group than in the RIF group (for example, -1.1 mm versus 0.15 mm at B point), and the percentage of patients with clinically significant vertical and horizontal changes was greater in the MMF group. By 1 year, there had been slight additional mean relapse in the MMF group (-1.5 mm net relapse at B point, with 42% of the patients showing 2 mm or more relapse). In the RIF group, the mandible was more likely to be repositioned forward than posteriorly (net mean change at B point, 0.7 mm forward; 33% had 2 mm or more forward movement). In the RIF group, all but one of the patients (96%) were judged to have an excellent clinical result; in the MMF group, the corresponding figure was 60%.     

Stability of simultaneous maxillary intrusion and mandibular advancement: A comparison of rigid and nonrigid fixation techniques

This study examines the short-term stability of bimaxillary surgery following Le Fort I impaction with simultaneous bilateral sagittal split osteotomies and mandibular advancement using two standard techniques of postsurgical fixation. Fifteen adults had skeletal plus dental maxillomandibular fixation, and fifteen adults had rigid internal fixation using bone plates in the maxilla and bicortical bone screws between the proximal and distal segments in the mandible. The group with rigid internal fixation did not undergo maxillomandibular fixation. Radiographic cephalograms were analyzed during the postsurgical period to evaluate skeletal and dental stability. There was no statistical difference in postsurgical stability with rigid internal fixation or skeletal plus dental maxillomandibular fixation other than the vertical position of the maxillary molar; the skeletal plus dental maxillomandibular fixation group had a significant amount of postsurgical intrusion of the maxillary molar when compared with the rigid internal fixation group. Although the other measures showed no statistically significant difference between the experimental groups, the amount of variability in postsurgical stability in the group with skeletal plus dental maxillomandibular fixation was greater than that found in the group with rigid internal fixation.     

Long-term changes of hyoid bone and pharyngeal airway size following advancement of the mandible

OBJECTIVE: To determine long-term changes in hyoid bone position and pharyngeal airway size after mandibular advancement, including evaluation of the relationship between length of suprahyoidal musculature and skeletal relapse. STUDY DESIGN: A cephalometric follow-up study (12 years) of 15 patients who underwent mandibular advancement surgery. RESULTS: The final position of the hyoid bone was more posterior than it had been preoperatively. Suprahyoidal musculature continuously lengthened from preoperatively to 12 years postoperatively. Total skeletal relapse at B-point and pogonion correlated significantly with postoperative stretch of suprahyoidal musculature. The upper and middle pharyngeal airways were narrower than their preoperative values. CONCLUSIONS: Mandibular changes influence hyoid bone position during the entire postoperative period, whereas stretching of suprahyoidal musculature seems to contribute to skeletal relapse. Mandibular advancement surgery alone possibly does not achieve a stable increase of pharyngeal airway size over a long-term period of 12 years.     

Relapse following mandibular advancement with dental plus skeletal maxillomandibular fixation

This study examines the short-term stability of the mandible following mandibular advancement surgery in which skeletal suspension wires were used in addition to dental maxillomandibular fixation. Twenty adults underwent sagittal ramus osteotomies. No concomitant surgical procedures were performed. Maxillomandibular fixation consisted of wiring between the upper and lower orthodontic brackets and circummandibular wires connected to the piriform aperture or anterior nasal spine wires for eight weeks. Cephalograms were analyzed during this period to evaluate skeletal stability. A statistically insignificant mean horizontal relapse of 8.9% was found at pogonion during the period of fixation. Significant vertical intrusion of the anterior mandible occurred, however, with a mean superior movement of pogonion of 0.83 mm (P less than or equal to 0.05). Dental changes noted were uprighting of the maxillary incisors and flaring of the mandibular incisors. In comparison with the results of other studies in which dental maxillomandibular fixation was used alone, the results of this study indicate that the use of skeletal suspension wires is advantageous in the prevention of horizontal skeletal relapse.     

Mandibular distraction osteogenesis by sagittal split osteotomy and intraoral curvilinear distraction

Mandibular distraction combining the sagittal split ramus osteotomy and an intraoral curved distractor is efficacious to lengthen the mandibular body and ramus. The sagittal split permits immediate rotation of the proximal segment and lengthening while still providing a large surface interface for regenerative bone. The buried curved distractor enables mandibular lengthening along the normal logarithmic grow spiral as shown by Moss and Rickets. In addition, it is well tolerated by the patient and can remain in place for a long duration to provide support. In summary, this combination of surgical techniques provides certain advantages for distraction of the mandible.     

Dental arch widths and mandibular-maxillary base width in Class III malocclusions with low, average and high MP-SN angles

OBJECTIVE: To analyze the development of the dental arches and skeletal mandibular-maxillary bases in untreated Class III malocclusions with low averages and high mandibular plane angles in subjects aged 10 to 14. MATERIALS AND METHODS: The records of 50 untreated Japanese girls with Class III malocclusions at age 10 were selected from the files of patients pending orthodontic surgery. The patients included those with low (< or =27 degrees), average (27 degrees through 37 degrees) and high (>37 degrees) mandibular plane angles. The maxillary skeletal base width, biantegonial width, and maxillary and mandibular intermolar width were determined on posteroanterior cephalograms obtained at annual intervals when subjects were between 10 and 14 years of age. The difference between the maxillary and mandibular intermolar width was also calculated and reported. RESULTS: All skeletal and dental transverse widths in the high-angle group were significantly smaller than those in the low-angle group (P < .05) from ages 10 to 14. On the other hand, the maxillary to mandibular molar difference was the same for the three groups (P > .05) at each age. The deviations in molar differences did increase from age 10 to age 14 in all three groups. CONCLUSION: Mandibular plane angles might play a stronger role in the transverse skeletal growth of the maxilla and the mandible than the transverse dental growth of the maxilla and the mandible.