Reducing variability in apparent operative inclination (AOI) during total hip arthroplasty: comparing the ball in cage mechanical alignment device to a digital inclinometer.

Abstract

When operating in lateral decubitus apparent operative inclination (AOI) is the angle between the handle of the acetabular cup introducer and the theatre floor. A digital inclinometer has demonstrated optimal control of AOI. This study aims to compare the deviation from a target AOI with a novel autoclavable ball-in-cage mechanical alignment guide (MAG) as compared to a digital inclinometer. 90 primary total hip arthroplasty cases performed in lateral decubitus using a posterior approach were included. The ball-in-cage mechanical alignment guide on a standard Pinnacle cup introducer handle was used to achieve the target AOI of 35° ± 2.5°. Final AOI was measured with a digital inclinometer. These readings were compared to a published historical series of 90 patients using a digital inclinometer. The ball-in-cage MAG mean inclination was 35.0° (range 32.5-37.4°), with 100% falling within target. This compared to a mean inclination of 34.0° (range 27.5-37.5°), with the digital inclinometer where 87.8% fell within target. However, the range of postoperative radiographic inclination for the ball-in-cage MAG was 23.4-53.9° with 31.1% being outside a ±5° target. These results indicate that this simple autoclavable ball-in-cage MAG consistently achieves an intraoperative AOI within target range. In isolation this does not guarantee a postoperative radiographic inclination within target range.