Skull biomechanics and simplified cephalometric lines for the estimation of muscular lines of action

Aim: The aim of the study is to propose a new cephalometric analysis model to facilitate biomechanical simulations and clarify the relationship between craniofacial morphology and the size and inclination of the masseter muscle (MM). The goal is to reduce methodological inconsistencies in the literature and provide a reproducible clinical tool for biomechanical analysis and orthodontic planning by standardizing measurements of the masseter muscle lines of action, promoting the achievement of neuromuscular balance and stable occlusion. Materials and methods: 510 pre-treatment lateral cephalometric tracings (217 males and 293 females; ages 6-50 years), obtained by CBCT and compared with Bolton analysis, were analyzed. The parameters considered included: determination of the skeletal- cutaneous class (linear distance between A' and B' projected onto the occlusal plane); the angle between the (Go-Or and the perpendicular to the occlusal plane passing through the molar occlusal point; the angle between the Go-Or and the Frankfurt plane. Results and Conclusion: The skeletal-cutaneous class showed wide individual variability (-14.5 mm to +15.5 mm), with no significant differences by sex or age. The Go-Or/occlusal plane angle showed a mean of 39.3° with a slight reduction in adulthood. The Go-Or/Frankfurt plane angle (mean 41.9°) showed a mean increase of approximately 4° from 6 to 50 years of age, significantly correlating with age (r = 0.449). A weak relationship was found between sagittal discrepancies and the Go-Or/Frankfurt plane angle, with approximately 20% of the variation due to the anteroposterior relationship between the maxilla and mandible. The proposed cephalometric model therefore allows for a simple and reproducible estimate of the masseter's line of action and its relationship with the main craniofacial landmarks. The influence of age on the angles analyzed suggests its importance in diagnostic processes and orthodontic planning. This approach offers objective support for biomechanical analysis and provides a useful basis for the adoption of personalized treatment strategies.