What causes cup inclination outliers in primary total hip arthroplasty when using a digital inclinometer? A prospective cohort study

Background: A suboptimal radiographic inclination (RI) angle of the cup has been associated with worse short- and long-term outcomes after primary total hip arthroplasty (THA). There are several factors that lead to RI outliers including cup version, pelvic orientation at time of cup impaction and angle of the cup introducer relative to the floor. The primary aim of this study was to analyse if a digital inclinometer helps achieving a target RI of cups in primary THA and what factors increase the risk of having a cup inclination outside the target zone. Patients and methods: In this prospective study, we included 200 consecutive patients undergoing primary THA. Preoperatively, the surgeon determined the target intraoperative inclination (IOI). The IOI 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 zone for inclination was defined as 35–45° of RI. Results: The mean RI was 37.9° SD 4.7. There were 53 outliers (26.5%) for the 35–45° zone. The mean absolute ΔIOI cup -IOI target was 1.2° SD 1.0. The absolute ΔIOI cup -IOI target was <1° in 108 patients (54%), <2° in 160 patients (80%) and <3° in 186 patients (93%). The absolute deviation from the mean ΔOI math -IOI cup , which corresponds with pelvic motion, was significantly higher in RI outliers compared with non-outliers for the 35–45° inclination zone (4.7°, SD 2.8 vs. 2.5°, SD 2.0 respectively) ( p < 0.0001). Conclusions: Using a digital inclinometer allowed the surgeons to intraoperatively put the cup within 3° of the IOI target in 93% of cases. This led to 74% of the acetabular components being placed within an RI target zone of 35°- 45°. The remaining outliers were caused by more of less than average deviation of the pelvic position at time of impaction.