Visual intraoperative estimation of cup and stem position is not reliable in minimally invasive hip arthroplasty

Abstract

Background and purpose - In hip arthroplasty, acetabular inclination and anteversion-and also femoral stem torsion-are generally assessed by eye intraoperatively. We assessed whether visual estimation of cup and stem position is reliable. Patients and methods - In the course of a subgroup analysis of a prospective clinical trial, 65 patients underwent cementless hip arthroplasty using a minimally invasive anterolateral approach in lateral decubitus position. Altogether, 4 experienced surgeons assessed cup position intraoperatively according to the operative definition by Murray in the anterior pelvic plane and stem torsion in relation to the femoral condylar plane. Inclination, anteversion, and stem torsion were measured blind postoperatively on 3D-CT and compared to intraoperative results. Results - The mean difference between the 3D-CT results and intraoperative estimations by eye was -4.9° (-18 to 8.7) for inclination, 9.7° (-16 to 41) for anteversion, and -7.3° (-34 to 15) for stem torsion. We found an overestimation of > 5° for cup inclination in 32 hips, an overestimation of > 5° for stem torsion in 40 hips, and an underestimation < 5° for cup anteversion in 42 hips. The level of professional experience and patient characteristics had no clinically relevant effect on the accuracy of estimation by eye. Altogether, 46 stems were located outside the native norm of 10-20° as defined by Tönnis, measured on 3D-CT. Interpretation - Even an experienced surgeon's intraoperative estimation of cup and stem position by eye is not reliable compared to 3D-CT in minimally invasive THA. The use of mechanical insertion jigs, intraoperative fluoroscopy, or imageless navigation is recommended for correct implant insertion.

Figure 1.

Operation setting with the patient in lateral decubitus position. The flexed insertion jig positions the cup in 45° inclination when its middle rod is brought parallel to the floor.

Figure 2.

3D-CT assessment of femoral stem torsion. The mechanical axis of the femur, defined by the center of the most caudal points of the femoral condyles, the center of the femoral head, and the vector, representing the neck of the prosthesis, formed a plane (a) from which a second vector (b) was calculated. With the condylar axis, it was projected onto a plane (c) that was orthogonal to the mechanical axis. The angle between these vectors minus 90° was the femoral stem torsion.

Figure 3.

Schematic depiction of the measurement of inclination and anteversion from a postoperative CT scan. An interactive image segmentation of the surface model was performed. Measurements were calculated by using the AP (anterior pelvic) plane.

Figure 4.

Distribution of femoral stem torsion measured by postoperative 3D-CT.

Figure 5.

Bland-Altman plots of the differences between CT measurements and surgeon’s estimations of acetabular inclination (A), acetabular anteversion (B), and femoral stem torsion (C). The continuous line represents the mean difference. Dashed lines show the 95% confidence intervals.