In silico analysis of the patient-specific acetabular cup anteversion safe zone

Introduction: Various computer-assisted surgical systems claim to improve the accuracy of cup placement in total hip arthroplasties after assessing spinopelvic mobility to prevent prosthetic impingement. However, no study has yet analyzed the extent of the patient-specific cup anteversion safe zones. Hypothesis: We hypothesized that most patients have a safe zone >10 degrees, except those with abnormal spinopelvic mobility, who have a much narrower safe zone. Materials and methods: We simulated the risks of prosthetic impingement using the planned cup anteversion. The consecutive cohort included 341 patients who underwent total hip arthroplasty. Our primary endpoint was the patient-specific impingement-free zone for cup anteversion, which was then divided into four subgroups: 0 degrees, 1 degrees to 5 degrees, 6 degrees to 10 degrees, and >10 degrees. This data was then secondarily analyzed for abnormal spinopelvic mobility (the difference in the spinopelvic tilt [Delta SPT] from a standing to a flexed seated position >20 degrees). Results: The mean anteversion safe zone was 22.8 degrees with 82.4% (281/341) of patients with a zone strictly >10 degrees. The mean safe zone was 8.9 degrees (+/- 9 degrees) in patients with an Delta SPT >= 20 degrees (18.2%), with 37.1% of these patients having a zone of 0 degrees, 16.13% a zone between 1 degrees and 5 degrees, 8.06% a zone between 6 degrees and 10 degrees and 38.71% a zone >10 degrees. The mean safe zone was 25.9 degrees (+/- 9 degrees) in patients with an Delta SPT <20 degrees (81.8%), and the proportion of cases in each zone was 2.51%, 1.08%, 4.3%, and 92.11%, respectively (p < 0.001). Conclusion: The safe zone for anteversion appears to be fairly wide in most patients. However, identifying patients at risk of abnormal spinopelvic mobility seems necessary to identify the two-thirds of patients with a narrow safe zone. Level of evidence: IV; retrospective study