Method to reduce variations of inclination angle of the acetabular component during mini-incision hip arthroplasty.

Background: To reduce variations of cup inclination after total hip arthroplasty using the mini-incision posterior approach, we introduced two techniques, one at a time. The first technique is measuring a pelvic tilt angle in the frontal plane in the initial lateral position in the operating room. The second technique is using a tilt-meter to adjust the direction of a cup holder. The purpose of this study was to evaluate the usefulness of these techniques.

Methods: For 106 hips operated on, the cementless acetabular component was impacted using a cup holder targeting 45 degrees in inclination and 20 degrees in anteversion. These hips were divided into three consecutive groups. For the first group (30 hips), no radiograph in the lateral position was obtained, and the alignment frame of the cup holder was aligned parallel to the floor by eye measurement. For the second group (56 hips), we measured the pelvic tilt angle, and tilted the alignment frame by eye measurement. For the third group (20 hips), we measured the pelvic tilt angle and tilted the alignment frame using the tilt meter. Inclination and anteversion angles were measured on postoperative radiographs. The absolute value of the difference between the measured angle and the target angle was defined as the inclination error or anteversion error, respectively.

Results: The inclination error was more than 5 degrees for 33% in the first group, 20% in the second group, and 0% in the third group (P = 0.015, chi-squared test). There was a significant difference between the first group and the third group (P = 0.0039). For the anteversion error, there were no significant differences among the three groups.

Conclusions: Adjustment using a tilt-meter after measuring a pelvic tilt angle is a useful method to reduce the rate of large inclination error.