The purpose of this study was to determine if a temporomandibular joint (TMJ) replaced by a custom joint prosthesis could have been replaced by a stock joint prosthesis. Stereolithic models of patients treated with TMJ Concepts ® patient fitted joint prosthesis (e.g. custom) were obtained. Biomet Microfixation ® TMJ prostheses (e.g. stock) were adapted to these models. Intra-operative insertion of prosthesis, fit and size of stock joints were simulated and evaluated. Adaptability and stability of condyle and fossa as well as their articulation were recorded. 20 models consisting of 34 joints were examined by two oral and maxillofacial surgeons who were blinded to the patient’s diagnosis. Overall, 77% of the stock TMJ system fit the stereolithic models. 3 mm or less of bone modification was necessary to achieve an acceptable fit. In the majority of the cases examined, a stock TMJ prosthesis had good anatomical adaptation to stereolithic models of patients previously treated with custom TMJ prosthesis, so a stock TMJ prosthesis could have been an acceptable option for these patients. Further prospective clinical studies to compare both systems are necessary.
Indications for temporomandibular joint (TMJ) reconstruction in the skeletally mature person include: failed autogenous grafts; failed alloplastic reconstruction; severe inflammatory and degenerative joint disease; ankylosis; fracture; benign neoplasms; and skeletal-developmental abnormality . The rationale for these indications has been discussed at length . When a component of the joint is extensively damaged, degenerated or lost, replacement with an alloplastic implant is the ideal approach to achieve functional and symptomatic improvement in the adult patient . Total TMJ replacement as a unit has been shown to be superior to partial TMJ replacement . The two systems used most often are the TMJ Concepts ® prosthesis (now, TMJ Concepts Patient-Fitted Total Temporomandibular Joint Reconstruction System, Ventura, CA; previously referred to as CAD/CAM Patient-Fitted Total Temporomandibular Joint Reconstruction System, Techmedica, Camarillo, CA, USA) and Biomet TMJ Replacement System ® (Biomet Microfixation, Jacksonville, FL, USA).
The TMJ Concepts ® prosthesis is custom fitted to each patient’s anatomy. Literature has shown that the accuracy of component fit facilitates the surgery by decreasing operating time and improving long-term functional stability . In certain situations, a patient-fitted prosthesis is not feasible. In those cases, a stock TMJ prosthesis system is a good option. The purpose of this study was to determine if a TMJ replaced by a custom joint prosthesis could have been replaced by a stock joint prosthesis.
Materials and methods
This was a retrospective study of stereolithic models (ProtoMED, Arava, CO, USA) routinely used in the production of TMJ Concepts ® . These models were obtained according to a protocol-specific computed tomography (CT) scan of patients whose TMJs were replaced with TMJ Concepts ® between 2004 and 2007. The fossas and condyles of Biomet ® TMJ Replacement System were adapted to these TMJ Concepts ® stereolithic models. Available options for standard condyle and fossa components were small (45 mm), medium (50 mm) and large (55 mm). The narrow and offset mandibular components were not used.
Two oral and maxillofacial surgeons, blinded to preoperative diagnosis and previous surgical interventions, analysed models. They determined: adaptability; articulation; amount of model reduction for fit (small; ≤3 mm or large; >3 mm); and risk of nerve damage by prosthesis insertion. Data were collected using a standardized collection form, entered into Excel, and analysed for descriptive information.
There were 22 models consisting of 34 joints (12 bilateral, 7 right, 3 left). 82% (28/34) of fossas, sizes small ( N = 13), medium ( N = 14), and large ( N = 1), fit with minimal or no modifications. Of them, 92% (26/28) had ≤3 mm of bone reduction and two models needed more than 3 mm bone reduction to allow fit. 76% (26/34) of condyles, sizes small ( N = 20), medium ( N = 5), and large ( N = 1) fit with minimal or no modifications. 23 condyles (82%) required less than 3 mm of bone reduction and 3 condyles required more than 3 mm bone reduction to achieve good fit.
Six fossas (17%) and 8 condyles (23%), (or six condyle-fossa pairs and two condyle only replacements) were not amendable to treatment with the stock prosthesis. Since total TMJ replacement has proven to be superior to partial replacement, lack of fit in one component was considered a failure of the system. 23% joints (8/34), consisting of the above mentioned, were deemed to have a poor fit. Four of the failed joints required large amounts of model modification, which were located in multiple areas. This would destroy the patient’s anatomy and would not be realistic to perform intra-operatively. In the other four cases, placement of the stock joints would jeopardize the inferior alveolar nerve ( Table 1 ).