Marker-based roentgen stereophotogrammetric analysis (RSA) is an accurate method for measuring in vivo implant migration,
which requires attachment of tantalum markers to the implant. Model-based RSA allows migration measurement without implant
markers; digital pose estimation, which can be thought of as casting a shadow of a surface model of the implant into the stereoradiographs,
is used instead. The number of surface models required in a given clinical study depends on the number of implanted sizes
and design variations of prostheses. Contour selection can be used to limit pose estimation to areas of the prosthesis that
do not vary with design, reducing the number of surface models required. The effect of contour reduction on the accuracy of
the model-based method was investigated using three different contour selection schemes on tibial components in 24 patients
at 3 and 6 month followup. The agreement interval (mean ± 2 standard deviations), which bounds the differences between the
marker-based and model-based methods with contour reduction was smaller than −0.028 ± 0.254 mm. The data suggest that contour
reduction does not result in unacceptable loss of model-based RSA accuracy, and that the model-based method can be used interchangeably
with the marker-based method for measuring tibial component migration.
Each author certifies that he or she has no commercial associations (eg, consultancies, stock ownership, equity interest,
patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.
Each author certifies that his or her institution has approved the human protocol for this investigation, that all investigations
were conducted in conformity with ethical principles of research, and that informed consent for participation in the study
was obtained.