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Roentgen stereophotogrammetry is a highly accurate technique for the assessment of three-dimensional migration and micromotion of a joint replacement prosthesis relative to the bone it is attached to. It was introduced in 1974 by Goran Selvic PMID 2686344.
Several studies have found implant migration to be predictive of long-term implant survival and, for most devices, measurement over 2 years might therefore provide a surrogate outcome measure with relatively low numbers of subjects, e.g. from 15 to 25 patients in each group in randomized studies (Grewal, Rimmer, and Freeman 1992 PMID 1544960), (Kärrholm et al. 1994 PMID 7983118), (Ryd et al. 1995 PMID 7744919). A smaller number of subjects can be used in these studies as a consequence of the high accuracy of the measurement technique. Because of this, RSA is an important technique in early clinical trials for screening new joint replacement prostheses. ))
To achieve the high accuracy, the following steps are carried out. Small roentgen opaque markers have to be introduced in the bone and attached to the prosthesis to serve as well-defined artificial landmarks. Two synchronised roentgen foci are used to obtain a stereo image of the bone and the prosthesis. Using a calibration object that holds tantalum markers at accurately known positions, the positions of the roentgen foci are assessed. The coordinates of the bone and prosthesis markers are accurately measured. After the coordinates have been measured, the three-dimensional position of the markers is reconstructed with special RSA software. Finally, the change in the position of the prosthesis markers relative to the bone markers is determined and the translation and rotations of the prosthesis can be calculated. The reported accuracy of RSA ranges between 0.05 and 0.5 mm for translations and between 0.15˚ and 1.15˚ for rotations (95% confidence interval; Kärrholm, 1989 PMID 2683567). New RSA techniques that avoid the need for attaching markers to the prosthesis have been introduced (Kaptein 2003 PMID 12742455).
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