Temporomandibular condylar articulation and finite helical axis determination using a motion tracking system

Kinematics play an important role in assessing the recovery of the patients' temporomandibular joint (TMJ) and occlusal functions. The finite helical axis (FHA), which simplifies three parameters in Euler-angle descriptions, provides a comprehensive insight into TMJ kinematics. Additionally, the FHA is one of the potential indicators used in the diagnosis and treatment of TMDs and the design and use of the TMJ replacement. This study aimed to illustrate the changes in the FHA of the TMJs during basic mandibular motions. Visible markers were rigidly affixed to the mandibular dentition and a helmet. Four active motions were registered: mouth opening, mandibular protrusion, and left and right lateral protrusions. According to the models reconstructed from the computed tomography of the same subject and the relative distance of the markers, subject-specific condylar tracking was achieved, and the FHAs for the four motions were determined. In addition to the irregular distribution in the initial opening, the FHA of the opening formed an "L-shaped" curve. Mandibular protrusion is a translational motion with little rotation. Additionally, the FHA crossed the ipsilateral TMJ during lateral protrusions, from initially vertical directions generally to horizontal directions at the front view. The proposed method provides a feasible way for measuring the FHA.