ARCHITECTURE OF THE HUMAN JAW-CLOSING AND JAW-OPENING MUSCLES

Background: The human jaw-closing and jaw-opening muscles produce forc es leading to the development of three-dimensional bite and chewing fo rces and to three-dimensional movements of the jaw. The length of the sarcomeres is a major determinant for both force and velocity, and the maximal work, force, and shortening range each muscle is capable of p roducing are proportional to the architectural parameter volume, physi ological cross-sectional area, and fiber length, respectively. In addi tion, the mechanical role the muscles play is strongly related to thei r three-dimensional position and orientation in the muscle-bone-joint system. The objective of this study was to compare relevant architectu ral characteristics for the jaw-closing and jaw-opening muscles and to provide a set of data that can be used in biomechanical modeling of t he masticatory system. Methods: In eight cadavers, sarcomere lengths, muscle masses, fiber lengths, pennation angles, and physiological cros s-sectional areas were determined for the following muscles: superfici al and deep masseter, anterior and posterior temporalis, anterior and posterior medial pterygoid, inferior and superior lateral pterygoid, p osterior and anterior digastric, geniohyoid, posterior and anterior my lohyoid, and stylohyoid. To determine the spatial position of their ac tion lines, the three-dimensional coordinates of the attachment sites were registered. Results: Compared with the jaw openers, the jaw doser s were characterized by shorter sarcomere lengths at the closed jaw, l arger masses of contractile and tendinous tissue, larger physiological cross-sectional areas, larger pennation angles, shorter fiber lengths , shorter moment arms, and lower fiber-length-to-muscle-length ratios. In addition, architectural features differed across the muscles of th e same functional group. Sarcomere length did not differ significantly among the regions of the same muscle. In contrast, in some muscles, s ignificant intramuscular differences were found with respect to, e.g., physiological cross-sectional area, fiber length, pennation angle, an d moment arm length. Conclusions: The results suggest that the jaw-clo sing muscles have architectural features that suit them for force prod uction. Conversely, the jaw-opening muscles are better designed to pro duce velocity and displacement. (C) 1997 Wiley-Liss, Inc.