Comparative study of normal condyle and temporomandibular joint prosthesis movement during mouth opening by dynamic magnetic resonance imaging and computed tomography

BACKGROUND: To analyze and compare the trajectory of condylar motion during mouth opening in normal volunteers and patients after total joint replacement (TJR) of the temporomandibular joint (TMJ). METHODS: Condylar movement during mouth opening was recorded by dynamic magnetic resonance imaging (MRI) for volunteers with normal TMJs and dynamic computed tomography (CT) for patients after TMJ TJR. Trajectories of the points selected every 5 mm from the superior point of the condyle (P(0)) along its axis to the mandibular angle (P(-25)) were recorded. The arc length and curvature radius of average trajectories for each point were calculated and compared between the normal joints and TJRs, especially P(-10) which is the corresponding point of the prosthesis apex without lateral pterygoid muscle (LPM) attachment at the normal joint with LPM attachment. The location of the point with the most similar trajectory was identified in the normal joints and compared with the condylar prosthesis. RESULTS: A total of 9 volunteers with 18 normal TMJs, and 5 patients with 6 prostheses were included in this study. For normal TMJs, the average condylar trajectories during mouth opening were a concave upward curve. Meanwhile, the trajectories of contralateral normal joints in patients with unilateral TJR and all condylar prostheses were significantly decreased. The arc length and curvature radius of average trajectories gradually decreased from P(0). In the normal joints, P(-20) had the most similar trajectories with the average arc lengths and a curvature radius of 13.0/4.2 mm. In P(-10), the average arc lengths and curvature radius of the normal cases, natural TMJ of the unilateral replacement patients, prosthetic TMJ of the unilateral replacement patients, and prosthetic TMJ of the bilateral replacement patient, were 15.6/6.6 mm, 13.1/4.9 mm, 4.7/4.4 mm, and 6.4/5.8 mm, respectively. CONCLUSIONS: P(-20) in the normal joint exhibited the most similar trajectory among individuals. The trajectory difference between the prosthesis apex without LPM attachment and the corresponding point at the normal joint with LPM attachment provides a reference for fossa prosthesis functional surface design.