AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 80:429-445 (1989) Relationships Between the Size and Spatial Morphology of Human Masseter and Medial Pterygoid Muscles, the Craniofacial Skeleton, and Jaw Biomechanics A.G. HANNAM AND W.W. WOOD Faculty of Dentistry, The Uniuersity of British Columbia, Vancouuer, British Columbia, Canada V6T 127 KEY WORDS Jaw muscles, Cross-sectional area, Muscle angula- tion, Bite force ABSTRACT The relationship between human craniofacial morphology and the biomechanical efficiency of bite force generation in widely varying muscular and skeletal types is unknown. To address this problem, we selected 22 subjects with different facial morphologies and used magnetic resonance imaging, cephalometric radiography, and data from dental casts to reconstruct their craniofacial tissues in three dimensions. Conventional cephalometric analyses were carried out, and the cross-sectional sizes of the masseter and medial pterygoid muscles were measured from reconstituted sections. The potential abilities of the muscles to generate bite forces at the molar teeth and mandibular condyles were calculated according to static equilibrium theory using muscle, first molar, and condylar moment arms. On average, the masseter muscle was about 66% larger in cross section than the medial pterygoid and was inclined more anteriorly relative to the functional occlusal plane. There was a significant positive correlation (P < 0.01) between the cross-sectional areas of the masseter and medial pterygoid muscles (r = 0.75) and between the bizygomatic arch width and masseter cross-sectional area (r = 0.56) and medial pterygoid cross-sectional area (r = 0.69). The masseter muscle was always a more efficient producer of vertically oriented bite force than the medial pterygoid. Putative bite force from the medial pterygoid muscle alone correlated positively with mandibular length and inversely with upper face height. When muscle and tooth moment arms were considered together, a system efficient at producing force on the first molar was statisti- cally associated with a face having a large intergonial width, small intercondy- lar width, narrow dental arch, forward maxilla, and forward mandible. There was no significant correlation between muscle cross-sectional areas and their respective putative bite forces. This suggests that there is no simple relation- ship between the tension-generating capacity of the muscles and their me- chanical efficiency as described by their spatial arrangement. The study shows that in a modern human population so many combinations of biomechanically relevant variables are possible that subjects cannot easily be placed into ideal or nonideal categories for producing molar force. Our findings also confirm the impression that similar bite-force efficiencies can be found in subjects with disparate facial features. Received June 27,1988; accepted November 16,1988. @ 1989 ALAN R. LISS, INC. 430 A.G. HA"AM AND W.W. WOOD During chewing and tooth clenching in the force is greatest in the same facial types primate, muscle forces are resisted both at (Ringqvist, 1973; Proffit and Fields, 1983). the bite point and at the temporomandibular However, the degree of activity recorded articulations (Hylander, 1975, 1985a). Re- from a muscle may reflect both its absolute cent observations from mathematical mod- size and its mechanical efficiency for the els simulating jaw biomechanics indicate task at hand. Whereas a high level of activity that muscle cross-sectional sizes and pat- might be expected from a large muscle mass, terns of activation, as well as the moment a low recorded level is more ambiguous in arms ofmuscles, bite points, and mandibular that it may be generated either by a small condyles, are the major determinants of bite muscle mass or by a lessened need to activate and articular forces (Nelson and Hannam, a large one. Both factors may vary from 1982, 1983, 1986; Osborn and Baragar, subject to subject since they are related to 1985; Smith et al., 1986; Hatcheret al., 1986; morphology. Throckmorton and Throckmorton, 1985; Muscle moment arms almost certainly dif- Throckmorton, 1985; Nelson, 1986; Iwasaki, fer between subjects, but data describing 1987; Baragar and Osborn, 1987; Faulkner human jaw muscle locations and angula- et al., 1987; Koolstra et al., 1988). Since tions are sparse and at best inferential. modern human populations show consider- When individuals with hyperdivergent jaws able diversity in craniofacial form (Solow, are viewed laterally, their masseter muscles 1966; Brown et al., 1965; Bjork, 1963; Cleall appear more vertically inclined to the man- et al., 1979; Anderson and Popovitch, 19831, dibular plane than normal; and, as the man- corresponding variations in some or all of dibular plane becomes steeper, the muscles' these determinants of tooth and articular angulations relative to the porion-orbitale forces are likely. It also seems theoretically (Frankfort horizontal) plane and sella-na- possible for subjects with apparently differ- sion line become more acute (Proctor and De ent craniofacial morphologies to function Vincenzo, 1970; Tetz, 1983). The masseter with equal biomechanical efficiency pro- and medial pterygoid muscles insert more vided the correct combinations of determi- posteriorly on the mandibular corpus and nant variables exist. Here, a recent demon- more superiorly on the ramus in these sub- stration of comparable jaw mechanics in jects (Horaist, 1974). Radiographic land- dolichocephalic and brachycephalic subjects marks assumed to represent muscle attach- (Iwasaki, 1987) is pertinent. ments in subjects with short posterior facial Jaw muscle cross-sectional sizes have heights, steep mandibular planes, and large been shown to alter with skeletal shape gonial angles suggest that they have mas- (Weijs and Hillen, 198413, 1986). Temporal seter muscles with a more superior insertion and masseter cross-sectional areas increase on the mandibular ramus than normal with facial width; those of the masseter and (Takada et al., 1984). Whether masseter pterygoid muscles increase with mandibular muscle angulation is constant relative to the length (Weijs and Hillen, 1986). These rela- occlusal plane is controversial. Some con- tionships are independent of a general size sider this to be so (Proctor and De Vincenzo, effect, and their specificity argues for differ- 1970), but others suggest that the masseter ences in the tension-generating capacities of muscle is more anteriorly inclined relative to muscles according to facial type. the occlusal plane in subjects with short Muscle use may differ between subjects posterior facial heights, steep mandibular apparently performing similar chewing and planes, and large gonial angles (Takada et clenching tasks (Moller, 1966; Hannam and al., 1984). Wood, 1981; MacDonald and Hannam, It is difficult to explain how the major 1984a,b;Wood et al., 1986; Wood, 1986; Ma- determinants of tooth and articular forces han et al., 19831, and distinct patterns of interact in living subjects with different fa- muscle contraction have been correlated cial morphologies. Many combinations of with facial morphology (Moller, 1966; Inger- muscle cross-sectional sizes and muscle, Val1 and Thilander, 1974; Lowe and Takada, tooth, and articular moment arms are possi- 1984). For example, jaw closing muscle activ- ble, especially when these are considered in ity is greatest in subjects with long posterior three dimensions of space. It is equally un- facial heights, short anterior facial heights, certain whether muscle cross-sectional sizes long mandibles, flat mandibular planes, and vary predictably with given combinations of small gonial angles (Moller, 1966; Ingervall moment arms and whether these biome- and Thilander, 1974; Tabe, 19761, and bite chanical variables correlate in any way with RELATIONSHIPS IN JAW BIOMECHANICS 43 1 conventional measures of craniofacial form. In each subject, MR images were obtained To address these problems, we combined by means of a 0.15T superconducting scan- data from magnetic resonance (MR) images, ner (Picker Vista MR1100). Spin-echo se- dental casts, and cephalometric radiographs quences were used with an echo time of 40 in living subjects with different craniofacial msec and repetition times ranging from 1.5 morphologies. We used three-dimensional to 3 sec. Sixteen contiguous sections spaced reconstructions to measure masseter and approximately 9 mm apart were sampled in medial pterygoid muscle cross-sectional ar- both the axial and coronal planes. In the eas and relevant moment arms and carried axial plane, four sections lay rostral to the out a biomechanical assessment of the tooth porion-orbitale (Frankfort horizontal) plane, and articular force-generating capacities of one passed throughit, and 11 lay caudal to it. the two muscles. In the coronal plane, four sections lay dorsal to an orthogonal plane passing through the MATERIALS AND METHODS centers of the mandibular condyles, one Our sample consisted of 22 fully dentate passed through it, and 11 lay ventral to it. adults between 22 and 48 years of age. It Figure 1 shows typical axial and coronal included 16 males and six females, with sections depicting the masseter and medial different facial characteristics. Two females pterygoid muscles, mandibular condyles, were identical twins. and maxillary teeth. Differences in head size Fig. 1. MR images representative of those used to reveal the craniofacial tissues, with arrows indicating the relevant structures. The axial section at