Voh#ne 100 Reviews and abstracts 97 Number 1

Relationship between masticatory muscle function and bite force: Morphological and physiological implications Steven J. Lindauer Farmington, Connecticut: University of Connecticut, 1989

Masticatory muscles are believed to influence and, in turn, be influenced by facial growth and subsequent orthodontic and dentothcial orthopedic treatment. An understanding of how these muscles function and of the relationship between muscle function and cranio- facial morphology is critical to the field of orthodontics.

The rationale for undertaking this investigation was based on the following factors: (1) ' There is experi- mental evidence to support a link between facial form and muscle function; (2) mechancial models of man- dibular function predict that differences in muscle ac- tivity are associated with variations in craniofacial mor- phology; (3) previous studies have shown correlations between facial form and muscle activity, but poor re- liability of the methods used in previous investigations make their results difficult to interpret; and (4) the use of EMG-force function characteristics to assess muscle function provides a potentially more accurate means by which to quantify masticatory muscle activity.

The purpose of this investigation was to determine the relationship between force-related masticatory mus- cle function and craniofacial morphology. Two major objectives of the study were (1) to determine, in a group of control subjects, whether the EMG-force function characteristic of slope was a reliable parameter for as- sessing masticatory muscle function quantitatively and (2) to use that technique as a means of studying muscle function in a group of untreated, adult orthodontic pa- tients.

EMG-force function characteristics were derived from masseter muscle activity recordings made during

controlled biting exercises for each of the 14 control subjects and were compared for consistency by re- peating the procedure 1 week later. The EMG-force function characteristic of slope (~V/kg) was shown to be significantly reproducible between trials (p < 0.001).

In the second part of the investigation, the slopes of EMG-force function curves were used to characterize and compare muscle function patterns of a heteroge- neous group of 16 adult preorthodontic patients. Ac- tivity of the masseter and the anterior, middle, and posterior temporalis muscles was recorded and was plotted against bite force.

For the experimental group as a whole, consistent increases in EMG activity as a function of bite force were found for the masseter and the anterior and middle temporalis muscles, with the masseter muscle having the greatest increments (slope). Considerable variation in EMG-force function slope values was noted between subjects. Differences in masseter muscle activity in- crements as a function of bite force were found to be related to mandibular morphology (p < 0.02), with greater increments being weakly associated with short and steeply oriented mandibles. Substantial individual exceptions were noted, however, suggesting that other factors probably play a determining role in establishing muscle function characteristics. Such factors might in- clude physiologic, developmental, or neural control pat- tern differences.

Correction In the April 1991 issue of the JOURNAL, two of the

thesis abstracts were incorrectly credited. The abstracts by Heon-Jae Cho and Douglas Barkett should cite their university as the University of the Pacific, Stockton, Calif., not Temple University, Philadelphia, Pennsylvania.