Abstract
Introduction
In total hip arthroplasty, a high radiographic inclination angle (RI) of the acetabular component has been linked to short- and long-term complications. There are several factors that lead to RI outliers including cup version, pelvic orientation and angle of the cup introducer relative to the floor. The primary aim of this study was to analyse what increases the risk of having a cup with an RI outside the target zone when controlling cup orientation with a digital inclinometer.
Methods
In this prospective study, we included 200 consecutive patients undergoing uncemented primary THA in the lateral decubitus position using a posterior approach. Preoperatively, the surgeon determined the target intraoperative inclination (IOItarget). The intra-operative inclination of the cup (IOIcup) was measured with the aid of a digital inclinometer after seating of the acetabular component. Anteroposterior pelvic radiographs were made to measure the RI of the acetabular component. The target zones were defined as 30°-45° and 35°-45° of RI. The operative inclination relative to the sagittal plane of the pelvis (OImath) was calculated based on the radiographic inclination and anteversion angle. The difference between two outcome measures was expressed as Δ.
Results
The mean RI was 37.9° SD 4.7, there were 12 cases with RI outside the 30°– 45° zone (6%) and 53 outliers (26.5%) with RI outside the 35°-45° zone. The mean absolute ΔIOIcup-IOItarget was 1.2° SD 1.0. The absolute ΔIOIcup-IOItarget was less than 1° in 108 patients (54%), less than 2° in 160 patients (80%), less than 3° in 186 patients (93%), and in 14 patients (7%) the difference was 3°-5°. The mean pelvic motion (ΔOImath-IOIcup) was 8.8° SD 3.9 (95% CI 8.2° to 9.3°). The absolute deviation from the mean ΔOImath-IOIcup, which corresponds with the amount of pelvic motion, was significantly higher in RI outliers compared with non-outliers for both the 30°-45° and 35°-45° inclination zone (7.4° SD 3.3 vs 2.8° SD 2.1 and 4.7° SD 2.8 vs 2.5° SD 2.0 respectively) (p<0.0001). A linear regression analysis demonstrated a strong correlation between ΔOImath-IOIcup and the RI of the cup (r2=0.70; P<0.0001). A multiple regression was run to predict ΔOImath-IOIcup from gender, BMI, side and hip circumference. These variables statistically significantly predicted ΔOImath-OIacup, F(4, 195) = 19,435, p<0.0001, R2 = 0.285, but only side (p=0.04) and hip circumference (p<0.0001) added statistically significantly to the prediction.
Discussion and Conclusion
When using a digital inclinometer 94% of cups had a RI within a 30°-45° zone and 73.5% of cups within a 35°-45° zone using a predefined IOItarget based on the patient's hip circumference. The difference between the IOItarget and the IOIcup of the acetabular component was less than 3° in 93% and less than 5° in all patients signifying that the surgeons were able to implant the cup close to their chosen intra-operative orientation. Deviation from the mean ΔOImath-IOIcup was significantly bigger in the RI outliers indicating that RI outliers were caused by more or less than deviation of the sagittal plane of the pelvis at time of cup impaction.
