Model-based Roentgen stereophotogrammetry of orthopaedic implants

Attaching tantalum markers to prostheses for Roentgen stereophotogrammetry (RSA) may be difficult and is sometimes even impossible. In this study, a m odel-based RSA method that avoids the attachment of markers to prostheses i s presented and validated. This model-based RSA method uses a triangulated surface model of the implant. A projected contour of this model is calculat ed and this calculated model contour is matched onto the detected contour o f the actual implant in the RSA radiograph. The difference between the two contours is minimized by variation of the position and orientation of the m odel. When a minimal difference between the contours is found, an optimal p osition and orientation of the model has been obtained. The method was vali dated by means of a phantom experiment. Three prosthesis components were us ed in this experiment: the femoral and tibial component of an Interax total knee prosthesis (Stryker Howmedica Osteonics Corp., Rutherfort, USA) and t he femoral component of a Prefix total knee prosthesis (Smith & Nephew, Mem phis, USA). For the prosthesis components used in this study, the accuracy of the model-based method is lower than the accuracy of traditional RSA. Fo r the Interax?; femoral and tibial components, significant dimensional tole rances were found that were probably caused by the casting process and manu al polishing of the components surf;ices. The largest standard deviation fo r any translation was 0.19 mm and for any rotation it was 0.52 degrees. For the Prefix femoral component that had no large dimensional tolerances, the largest standard deviation for any translation was 0.22 mm and for any rot ation it was 0.22 degrees. From this study we may conclude that the accurac y of the current model-based RSA method is sensitive to dimensional toleran ces of the implant. Research is now being conducted to make model-based RSA less sensitive to dimensional tolerances and thereby improving its accurac y. (C) 2001 Elsevier Science Ltd. All rights reserved.