Abstract
Introduction
A high inclination angle has been linked to an increased dislocation rate, liner fracture, and increased wear. The aim of this study was to compare the operative (OI) with the radiological inclination (RI) angle and determine the influence of patient morphology on pelvic tilt and cup inclination angle.
Methods
In the first cohort of 100 patients undergoing uncemented primary total hip arthroplasty, the cup was inserted freehand. In the second cohort of 100 patients, the OI was measured with the aid of a digital inclinometer. RI and pelvic tilt in lateral decubitus were measured.
Results
The mean RI in the freehand group was similar to the protractor group (38.5 SD 7.0 and 38.3 SD 4.7; p=0.83) with a significantly greater variance in the freehand group (range 22°-60° versus 27°-51°; p=0.0001) and more outliers for the inclination safe zone (24 versus 10; p=0.01). The mean difference between the RI and OI (ΔRI-OI) in the protractor group was 12.3° SD 4.2 (range 3.8°-19.8°). The mean pelvic tilt was 4.0° (SD 3.5) of adduction. Linear regression analysis demonstrated that RI was positively correlated with OI (r2=0.44, p<0.0001). Hip circumference was negatively correlated with pelvic tilt (r2=0.20, p=0.002) and ΔRI-OI (r2=0.37, p=0.0001). There was a significant reduction in the number of inclination outliers over time in the second cohort (6 versus 2 versus 1 versus 1; p=0.04).
Discussion
The mean ΔRI-OI was 12.3°. In patients with a larger hip circumference there was less pelvic tilt in the frontal plane and less ΔRI-OI. Surgeons using the posterior approach in lateral decubitus should aim for a lower OI in order to achieve an acceptable RI, especially in patients with a smaller hip circumference.
Conclusion
In our hands, taking into account patient morphology and using a digital protractor intraoperatively has significantly reduced the number of inclination outliers.
