Journal of Biomechanics 34 (2001) 715–722 Model-based Roentgen stereophotogrammetry of orthopaedic implants E.R. Valstar a, *, F.W. de Jong a , H.A. Vrooman b , P.M. Rozing a , J.H.C. Reiber b a Department of Orthopaedics, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands b Division of Image Processing, Department of Radiology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands Accepted 14 February 2001 Abstract Attaching tantalum markers to prostheses for Roentgen stereophotogrammetry (RSA) may be difficult and is sometimes even impossible. In this study, a model-based RSA method that avoids the attachment of markers to prostheses is presented and validated. This model-based RSA method uses a triangulated surface model of the implant. A projected contour of this model is calculated and this calculated model contour is matched onto the detected contour of the actual implant in the RSA radiograph. The difference between the two contours is minimized by variation of the position and orientation of the model. When a minimal difference between the contours is found, an optimal position and orientation of the model has been obtained. The method was validated by means of a phantom experiment. Three prosthesis components were used in this experiment: the femoral and tibial component of an Interax total knee prosthesis (Stryker Howmedica Osteonics Corp., Rutherfort, USA) and the femoral component of a Profix total knee prosthesis (Smith & Nephew, Memphis, USA). For the prosthesis components used in this study, the accuracy of the model-based method is lower than the accuracy of traditional RSA. For the Interax femoral and tibial components, significant dimensional tolerances were found that were probably caused by the casting process and manual polishing of the components surfaces. The largest standard deviation for any translation was 0.19 mm and for any rotation it was 0.528. For the Profix femoral component that had no large dimensional tolerances, the largest standard deviation for any translation was 0.22 mm and for any rotation it was 0.228. From this study we may conclude that the accuracy of the current model-based RSA method is sensitive to dimensional tolerances of the implant. Research is now being conducted to make model-based RSA less sensitive to dimensional tolerances and thereby improving its accuracy. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Roentgen stereophotogrammetry; Three-dimensional pose estimation; Surface models; Micromotion 1. Introduction Roentgen stereophotogrammetric analysis (RSA) is an accurate measurement technique to assess micromo- tion of implants with respect to the surrounding bone (Selvik, 1989). In RSA, the three-dimensional position and orientation of objects is determined by the reconstruction of the three-dimensional position of well-defined markers. For this purpose, tantalum markers are used that are inserted into the bone and are either attached to or inserted into the implant. However, marking of implants may be difficult and is sometimes even impossible. Furthermore, marking of implants is an expensive procedure and in some countries it is only allowed by the regulatory bodies after extensive testing and comprehensive documenta- tion. For some implants, like metal-backed cups in total hip arthroplasty and femoral components in total knee arthroplasty, the metal of the implant often obscures the attached markers when RSA radiographs are taken. RSA studies of these implants with attached markers are only possible when care is taken in positioning the patient during radiography. Because of these difficulties, only one clinical RSA study of femoral components in total knee arthroplasty has been performed (Nilsson et al., 1995). In contrast, many clinical RSA studies of tibial components in total knee arthroplasty have been conducted (Overview in K . arrholm, 1989). Several attempts have been made to perform RSA studies without attaching markers. For several clinical RSA studies of hip stems, the head of the prosthesis has been used as a marker ( . Onsten et al., 1995; K . arrholm, *Corresponding author. Tel.:+31-71-5262975; fax:+31-71- 5266743. E-mail address: e.r.valstar@lumc.nl (E.R. Valstar). 0021-9290/01/$-see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII:S0021-9290(01)00028-8