The theoretical optimal center of rotation for a temporomandibular joint prosthesis: A three-dimensional kinematic study

A unilateral temporomandibular joint (TMJ) prosthesis may cause dysfunction of the contralateral, natural TMJ because of lack of translatorial movemen ts of the prosthetic side. The natural translatorial capacity of the mandib le can be restored in part by a TMJ prosthesis with a fixed center of rotat ion (CR), positioned inferiorly to the center of the natural mandibular con dyle. The aim of this study was to determine the optimal position for the f ixed CR of a unilateral TMJ prosthesis. A mathematical model was used to an alyze different positions of the CR. These positions were evaluated based o n the calculated rotation of the mandible in the frontal (theta f) and hori zontal (theta h) plane, and the mediolateral movement (MLM) of the contrala teral natural condyle. For current TMJ prostheses, with the CR positioned i n the center of the natural condyle, theta h exceeded the natural limits. W hen the CR was shifted inferiorly, all parameters improved, particularly th eta h. The addition of an anterior shift to an inferior shift slightly wors ened theta f, while the addition of a posterior shift to an inferior shift slightly improved theta f and worsened MLM. We concluded that the functioni ng of the contralateral TMJ improves by shifting the CR inferiorly. An ante rior shift may be added to remain within the contour of the mandibular ramu s. The proposed position of the CR is 15 mm inferior to the center of the n atural condyle, combined, if necessary, with an additional anterior shift o f 5 mm.