Simplified cephalometric lines for the estimation of muscular lines of action

In the study of masticatory muscle performance, one of the biomechanical variables that can be estimated is the mechanical advantage of the masticatory muscles, namely, the ratio between the muscular moment arm and the bite force moment arm. In the present study, the position of the estimated line of action of the masseter muscle, drawn between gonion and orbitale (Go-Or) relative to dental (occlusal plane) and skeletal (Frankfort plane) references was analyzed in 431 pretreatment lateral cephalograms of orthodontic patients (195 males, 236 females) aged 6 to 50 years, and in the lateral tracings of the Bolton standards (6 to 18 years of age). The following measurements were evaluated: (1) skeletocutaneous class (soft tissue equivalent of Wits appraisal, linear distance in millimeters between the projections of points A' and B' on the occlusal plane); (2) angle between the Go-Or line and the perpendicular to the occlusal plane at the molar occlusal point; and (3) angle between the Go-Or line and the Frankfort plane. In the patients, the skeletocutaneous class ranged between--14.5 and 15.5 mm, without any sex- or age-related differences. The angle between the Go-Or line and the perpendicular to the occlusal plane was, on average, 39 degrees (range 15 to 53 degrees), and it decreased with advancing age; while the average angle between Go-Or and Frankfort plane was 42 degrees (range 30 to 54 degrees), and it increased in older patients. No effects of sex were found. The two angles were significantly correlated to each other, while no correlations were found with the sagittal jaw discrepancy. Similar results were obtained on the Bolton tracings. Overall, the present cephalometric analogue could be useful in biomechanical simulations.