Stability analysis of stock alloplastic temporomandibular joint prostheses based on clinical cases of screw loosening

Screw loosening is one of the common complications that affect stability after stock alloplastic temporomandibular joint (TMJ) replacement. Based on clinically observed cases of screw loosening, this study aimed to evaluate how prosthesis installation patterns influence screw loading and to provide recommendations for installation safety. A three-dimensional finite element model of unilateral total TMJ replacement was constructed to analyze Biomet standard TMJ prosthesis stability under the following situations: 1) fossa component installed at different angles (0°–20°) relative to the bone surface; 2) mandibular component installed at different angles (0°–15°) from the posterior border of the ramus. Under maximum occlusal loading and mouth-opening muscle loading, the characteristics of stress and strain, force on the fixation screws and displacement were analysed. The ratio of the maximum screw pull-out strength from the bone tested in vitro to the force on the screws was calculated to predict the safe installation angle. As the angle between the fossa component and the bone surface increased, the stress, strain and force on the prosthesis and fixation screws gradually enlarged especially the fossa screws increased by 3.05 to 9.36 times. As the angle between the mandibular prosthesis and the posterior border of the ramus increased, the stress, strain and force on the prosthesis and screws enlarged especially the fossa screws increased by 2.47 times to 4.54 times. A reduction in fixation safety was already observed at an installation angle of 5°, with individual fossa screws showing FoS values below 4. As the installation angle further increased, screw loading increased and the number of screws with reduced safety margins increased. These findings suggest that increasing component malposition progressively compromises fixation safety. The fossa component should therefore be adapted as closely as possible to the bone surface, and the mandibular component should be positioned as parallel as possible to the posterior border of the ramus.