Adaptation of the suprahyoid muscle complex to large mandibular advancements

The purpose of this study was to provide quantitative data concerning the changes and adaptations that take place within the suprahyoid complex to larger mandibular advancements. Mandibular advancement of 6.5 mm was performed on 12 adult rhesus monkeys. Six underwent maxillomandibular fixation (MMF) using the dentition, six underwent MMF using the dentition plus skeletal suspension wires. Mandibular position and changes in the length of the various anatomic regions of the suprahyoid complex were evaluated cephalometrically with the aid of radiopaque bone, muscle, and tendon markers implanted preoperatively. Relapse of the mandible in the dental MMF animals was 27% of the advancement, whereas there was none in the dental plus skeletal MMF group. Results of adaptations within the suprahyoid complex showed that 1) the suprahyoid complex was elongated slightly less than the mandible, and 2) the major adaptations (lengthening) occurred at the muscle-bone interface, the muscle-tendon interface, and within the belly of the anterior digastric muscle. On the basis of these results, it was concluded that adaptations within the suprahyoid complex to mandibular lengthening occur first at the connective tissue attachments of the muscle, and then within the muscle belly itself. The methods of dealing with the potentially distracting forces from the stretch within the suprahyoid complex are discussed.