Evaluation of mandibular motions in patients with anterior disc displacement during mouth opening and closing using finite helical axis

Understanding temporomandibular joint (TMJ) kinematics is essential for the clinical diagnosis and treatment of TMJ disorders. Yet, a comprehensive description of mandibular motion information in patients with anterior disc displacement (ADD) is lacking. The finite helical axis (FHA) is a mathematical model describing the motion of a rigid body in space. This model quantifies mandibular motion patterns by differentiating between rotation around the FHA and translation along it. This study aimed to compare the mandibular motion patterns between patients with ADD and asymptomatic subjects during mouth opening and closing utilizing the FHA. Ten asymptomatic subjects (2 females and 8 males, aged 19–22) and ten patients with ADD (8 females and 2 males, aged 19–57) were tracked using an optical motion tracking system for mouth opening and closing. The FHA during mouth opening and closing was determined from motion trajectory. The distance from the condylar center to the FHA (d_CP), the angles between the FHA and the head coordinate system (θ_x, θ_y, θ_z), and the global fluctuation of the FHA spatial orientation (θ_f) were further calculated. In addition, the helical axis of each frame relative to the initial frame was computed to determine the maximum rotation angle (Θ_max) and maximum offset (T_max) of mandibular motion during mouth opening and closing. It was found that Θ_max, T_max, d_LCPmean, d_LCPmin, θ_x, and θ_f for patients with ADD differed significantly from those of asymptomatic subjects. These findings imply that the FHA effectively describes the disparities between patients with ADD and asymptomatic subjects.