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.

Pathophysiology of cleft lip muscle.

Although attention has been focused for decades on the correction of cleft lip deformities, our knowledge about the etiology of such deformities has remained presumptive. Sixty-six muscle biopsy specimens from cleft lip infants were obtained at the time of primary closure. Histochemical stains, histographic analysis, and electron microscopy were performed. A nonneurogenic muscle atrophy was seen that varied in severity, with muscle fibers near the cleft being the most atrophic and disorganized. Muscle fibers stained with the modified Gomori trichrome technique also demonstrated "ragged red" fibers typical of a mitochondrial myopathy. Electron microscopy confirmed large accumulations of mitochondria distorting the fibrils. These mitochondria also were increased in size and densely packed with cristae. This study thus demonstrates that the muscles in cleft lip deformities are not normal. Instead, they reflect either myopathy in the facial mesenchymal mitochondrion or at least a delay in maturation. We hypothesize that some of the morphologic deformities associated with cleft lip may cause a failure of mesenchymal reinforcement of the facial processes at a critical time in development.

Is the myosin type altered in the aging platysma?

The myosin light chains of platysma samples originating from 2 to 86 year old patients of the maxillo-facial surgery clinic were investigated. The platysma contained both fast and slow myosin. No change in the proportion of myosin type in relation to age was found. Female patients often displayed less fast myosin than male patients. Each time when the platysma muscle activity has been reduced a decrease in the amount of slow myosin light chains was observed. In the reported study, variations of the fast and slow myosin light chains resulted mainly from the platysma activity level, i.e. from the pathology requiring the surgery, rather than from the patients' age.