FEASIBILITY OF MODEL-BASED RSA IN TMC JOINT REPLACEMENT

Abstract

Aseptic loosening of the total TMC joint prosthesis occurs frequently and may depend on the design of the prosthesis. Numerous TMC prosthesis designs are available, and new designs are being developed and tested. One of the problems in the clinical studies of TMC prostheses is identifying and predicting prosthetic loosening at an early stage. Roentgen Stereophotogrammetric Analysis (RSA). allows assessment of three-dimensional micromotion of orthopaedic implants with high accuracy. Early micromotion (in the first two postoperative years) of most prostheses is strongly correlated with the development of aseptic loosening. We studied if RSA assessment was possible after total TMC joint arthroplasty.

In five cadaveric hands the TMC joint was replaced by the SR-TMC prosthesis. Tantalum beads of 0.8 mm were implanted in the trapezium and first metacarpal bone without extending the standard surgical exposure. The metacarpal prosthesis component was provided with 0.5 mm beads. A three-dimensional surface model of the trapezium component of the SR-TMC prosthesis was prepared to facilitate model-based RSA. After the surgical procedure, RSA radiographs were made of all hands in two commonly used positions for imaging of the TMC joint. The number of visually detected markers for each bone/implant was recorded. Of one cadaver hand, RSA radiographs were made in ten different positions to calculate the measurement error of the performed technique.

For the metacarpal bone, all beads were visible in all positions and both (L+R) RSA radiographs. For beads in the polyethylene metacarpal prosthesis component three beads seem sufficient, however in exceptional cases the most proximal placed bead might be invisible due to overprojection by the metal trapezium prosthesis component. Therefore the X-rays should be carefully checked at the radiology department before the patient leaves the ward. Alternatively, an extra bead can be placed in the prosthesis, although this is a lesser option due to possible weakening of the component caused by the placement of the beads. The use of different sizes of beads (0.5/0.8 mm) in the metacarpal bone and metacarpal prosthesis made the interpretation for the analyser easier

The accuracy analysis is currently carried out. First results of these measurements are promising and placement of tantalum beads for RSA analysis during TMC-joint replacement seems feasible.