Relationships Between the Size, Position, and Angulation of Human Jaw Muscles and Unilateral First Molar Bite Force

Abstract

Human subjects commonly show large variations in bite force produced at the first molar teeth. To evaluate the role of muscle cross-sectional sizes and lever arms in bite-force production, we correlated these variables in 11 healthy adults. Axial and coronal images obtained by magnetic resonance were combined with conventional lateral cephalograms and dental cast data to reconstruct the craniomandibular morphology in each subject. The cross-sectional sizes of the right masseter and medial pterygoid muscles, their lever arms, and the bite-point lever arms were measured directly from these reconstructions. Physiological recordings of bite force were made in the region of the right first molar by means of a customized transducer aligned perpendicular to the functional occlusal plane. The average bite force for the sample as a whole was 189 +/- 78 N. The coefficients of variance were greater for bite forces, and for the cross-sectional sizes of the two muscles, than for their respective lever arms. Highly significant Pearson Product Moment correlation coefficients (p less than 0.005) were found between masseter and medial pterygoid cross-sectional size, and between the cross-sectional size of each muscle and bite force. No significant correlations (p greater than 0.1) were found between muscle or bite-point lever arms and bite force. Despite the fact that craniofacial spatial morphology may differ among subjects, jaw muscle size alone seems to explain most of the variation in bite force reported by ourselves and others.

Morphological Characteristics of Vertical Sections of the Mandible Obtained by CT Scanning.

“…Recently, the cross-sectional areas of the jaw muscles have been imaged by Computer Tomography (CT) and Magnetic Reso nanceImaging (MRI), and the muscles have been show to be different in shape between groups with different skeletal forms (Weijs and Hillen, 1984a, 1984bHannam and Wood, 1989;Gionhaku and Lowe, 1989;Van Spronsen et al, 1989, 1991, 1992Sasaki et al, 1989). The jaw-closing muscle activity is said to be the greatest in subjects with a large posterior facial height, a small anterior facial height, a long mandible, a flat mandibular plane, and a small gonial angle (Moller, 1966;Ingervell and Thilander, 1974;Tabe, 1976).…”

Neuromuscular System and Jaw Deformities.

“…The muscles are attached at odd angles and directions to the mandible so that many different combinations can occur to move the mandible in one direction (Hannam and Wood, 1989 ;Hannam and McMillan, 1994 ;Koolstra and van Eijden, 1999 ;Fig. 3). There is a redundancy in the combinations of muscles that can be recruited (Weijs and van Ruijven, 1990).…”

“…There is a redundancy in the combinations of muscles that can be recruited (Weijs and van Ruijven, 1990). The angulation and position of the muscles to the attached mandible provide the central nervous system with multiple ways to achieve the same response (e.g., protrusion; Mao and Osborn, 1994 ;Hannam and Wood, 1989). The jaw muscles are always recruited in groups defined as "coactivation patterns" .…”

“…The maximum tension developed by a skeletal muscle directly relates to the cross-sectional area of that muscle and is defined as the "physiological cross-sectional area" (Schantz et al, 1983 ;Hillen, 1984a, 1984b ;Sasaki et al, 1989 ; Table 2). …”

“…(Hannam and Wood, 1989). Secondly, the length of the moment arms of the muscles can have some subtle effect.…”