Muscular and mandibular adaptation after lengthening, detachment, and reattachment of the masseter muscle.

Abstract

This investigation was undertaken to study the skeletal and muscular adaptations in the mandible after lengthening, detachment, and reattachment of the masseter. Placing the bite-opening appliance and altering the length of the masseter, with and without surgical detachment and reattachment, resulted in a number of mascular and skeletal adaptations: First, a posterior and inferior rotation of the mandible was produced. In the experimental period, forward repositioning of the gonial angle and upward repositioning of the symphyseal region occurred. The amount of skeletal repositioning was greater in those animals in which the associated musculature was not detached. Second, if the masseter muscle was lengthened and not detached from the mandible, a gradual return of the muscle to its original length was observed. After 48 weeks, a 50% return occurred. Third, if a muscle was lengthened and then surgically detached, immediate shortening of the muscle occurred in an anterior and superior direction, with ultimate reattachment often at a length shorter than the original. Fourth, if a muscle was surgically detached and then surgically reattached without placement of a bite-opening appliance, a slight shortening of the muscle occurred. Fifth, if the bite was opened and the masseter was surgically detached and then surgically reattached, the muscle returned to its approximate original length and had a more predictable postsurgical positioning. It may be concluded that, in surgical procedures where lengthening of the masticatory muscles is mandatory, it is beneficial, if possible, to surgically detach these muscles from their insertions. In doing so, one would maintain the original orientation of the muscle and at the same time presumably impose less stress on the newly placed, surgically repositioned skeletal elements.