Accuracy and precision of radiostereometric analysis in the measurement of THR femoral component translations: human and canine in vitro models

Radiostereometric analysis (RSA) is used to measure translations of joint replacement components with respect to the host bone in vivo. We used two cadaveric models of hip arthroplasty, one human and one canine, to evaluate the accuracy and precision of RSA-based estimates of translations of the femoral component with respect to the femur under ideal conditions. The femoral components were attached rigidly to a micrometer stage that provided standard displacements in increments of 25 and 50 μm in the interval from zero to 500 μm along three orthogonal axes. Radiostereometric examinations were performed for each increment. Accuracy was calculated as the 95% prediction intervals from regression analyses between the measured and actual displacements. Precision was evaluated as the standard deviation of five repeated measurements of a 200 μm displacement along each axis. Both accuracy and precision were best along the longitudinal axis, with a prediction interval of ±47 μm in the human model and ±45 μm in the canine model and a standard deviation of 30 μm in the human model and 40 μm in the canine model. The use of only the prosthetic head as a landmark (as opposed to three markers placed on the femoral stem) led to a 3-fold larger prediction interval in the human model and a 2-fold greater prediction interval in the canine model.