Model-based Roentgen stereophotogrammetry of orthopaedic implants

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 0.52°. 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 0.22°. 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.

Introduction

Roentgen stereophotogrammetric analysis (RSA) is an accurate measurement technique to assess micromotion 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 documentation. 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 Kärrholm, 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 (Önsten et al., 1995; Kärrholm, 1989, Kärrholm et al., 1997). For polyethylene cups with a metal ring, the ring has been used to obtain the cup's position (Snorrason and Kärrholm, 1990). Furthermore, for hemispherical metal-backed cups the position as well as the orientation could be assessed by using the projection of the hemispherical part and the projection of the base circle (Valstar et al., 1997). Others have used the shape of hip stems to obtain the position and the orientation of the stem (Turner-Smith and Bulstrode, 1993; Valstar (1996), Valstar (2001)). All of these techniques used basic geometrical shapes—circles, spheres, and straight lines or single well defined landmarks to define the position and orientation of the implant. These techniques cannot be used for total knee prostheses and other more complex shaped implants since the shape of these implants cannot be described by these basic geometrical shapes.

A model-based RSA method has been developed to overcome the above mentioned problems. With this method the three-dimensional position and orientation of complex shaped prosthesis components is assessed without the use of markers. The method is based on matching of the detected contour of an implant (that is inserted in a patient) with the calculated projected contour of a three-dimensional model of the same implant (De Jong, 1997). Similar techniques have been used for other applications: the determination of the position of vertebrae (Lavalée and Szeliski, 1997) and the assessment of the position and orientation of total knee prostheses from single focus fluoroscopic images (Banks and Hodge, 1996; Walker et al., 1996; Zuffi et al., 1999).

In this study, the model-based RSA method is presented and the accuracy of the method is tested by an in vitro experiment with a phantom.