Is Model-based Radiostereometric Analysis Suitable for Clinical Trials of a Cementless Tapered Wedge Femoral Stem?

Abstract

Background: In clinical trials of THA, model-based radiostereometric analysis (RSA) techniques may be less precise than conventional marker-based RSA for measurement of femoral stem rotation. We verified the accuracy and clinical precision of RSA based on computer-aided design models of a cementless tapered wedge femoral stem.

Questions: We asked: (1) Is the accuracy of model-based RSA comparable to that of marker-based RSA? (2) What is the clinical precision of model-based RSA?

Methods: Model-based RSA was performed using combined three-dimensional computer-aided design models of the stem and head provided by the implant manufacturer. The accuracy of model-based RSA was compared with that of marker-based RSA in a phantom model using micromanipulators for controlled translation in three axes (x, y, z) and rotation around the y axis. The clinical precision of model-based RSA was evaluated by double examinations of patients who had arthroplasties (n = 24) in an ongoing trial. The clinical precision was defined as being at an acceptable level if the number of patients needed for a randomized trial would not differ from a trial done with conventional marker-based RSA (15-25 patients per group).

Results: The accuracy of model-based RSA was 0.03 mm for subsidence (translation along the y axis) (95% CI for the difference between RSA measurements and actual displacement measured with micrometers, -0.03-0.00) and 0.39° for rotation around the y axis (95% CI, -0.41 to -0.06). The accuracy of marker-based RSA was 0.06 mm for subsidence (95% CI, -0.04-0.01; p = 0.728 compared with model-based RSA) and 0.18° for the y axis rotation (95% CI, -0.23 to -0.07; p = 0.358). The clinical precision of model-based RSA was 0.14 mm for subsidence (95% CI for the difference between double examinations, -0.02-0.04) and 0.79° for the y axis rotation (95% CI, -0.16-0.18).

Conclusions: The accuracy of model-based RSA for measurement of the y axis rotation was not quite as high as that of marker-based RSA, but its clinical precision is at an acceptable level.

Clinical relevance: Model-based RSA may be suitable for clinical trials of cementless tapered wedge femoral stem designs.

Figures

Fig. 1A–C

(A) The original phantom model consisted of a total hip prosthesis and a plastic model of the human proximal femur with six tantalum RSA bone-markers. (B) To achieve unrestricted motion of the stem, the femur model was replaced by a plastic tube with tantalum markers (open circles). Only the plastic tube model was applied in the current phantom study. For marker-based RSA, three implant markers (arrows) were attached by plastic studs in a configuration similar to that in clinical RSA-marked stems. (C) The implant construct was rigidly fixed to the base plate, forming the nonmovable portion of the phantom. The plastic femur/tube was distally attached to an x, y, z translation stage and a y rotation stage with micrometers (arrows) for highly controlled simulation of stem migration in relation to the surrounding bone.

Fig. 2A–D

Bland–Altman plots comparing model-based and marker-based RSA (MBRSA software) for measurement of micromanipulator-induced stem translation and rotation in the phantom model are shown. The difference between the model-based RSA and marker-based RSA measurements (y axis) was calculated for each increment of micromovement and plotted against the mean of the two measurements (x axis). The two methods showed good global agreement for measurement of (A) x axis translation, (B) y axis translation, (C) z axis translation, and (D) y axis rotation. The dashed line represents the mean of the differences, which was close to zero in each comparison, suggesting that model-based RSA measurements were not biased. The two solid lines represent the 95% limits of agreement.