Virtual range of motion is a safe and acceptable alternative to intra-operative manual range of motion testing in direct anterior robotic total hip arthroplasty.

Abstract

In robotic total hip arthroplasty (THA), virtual range of motion (VROM) modeling allows the surgeon to account for spinopelvic motion and create an impingement-free range of motion that is patient-specific. The primary purpose of this study was to evaluate the risk of dislocation in patients undergoing direct anterior THA using VROM rather than manual ROM trialing. Prospectively collected data was reviewed retrospectively of all consecutive anterior THAs performed by a single fellowship-trained surgeon. No patients were excluded from the study. VROM identified bone and implant impingement, which was recorded in degrees of hip external rotation(ER) at 0° of hip extension (standing), and in degrees of hip internal rotation(IR) at 90° of hip flexion (sitting). No patients had manual ROM trialing performed. Dislocation events were recorded during the first 3 months of follow-up. 362 patients, with a mean age of 67 and mean BMI of 28.8, were included. This cohort, including 154 patients (42.5%) with abnormal spinopelvic motion, demonstrated zero dislocations using VROM. The average ER impingement occurred at 60.9° ER (range 20-90°), and 50.8° IR (range 25-90°). Patients with ER impingement <55° had significantly less acetabular anteversion (16.4° ±3.3°, p < 0.001). Compared to the entire cohort, anteversion of the acetabulum was also decreased in the stuck-sitting subgroup (17.4° ±3.3°, p < 0.001) and increased in the stuck standing subgroup (20.5° ±3.6°, p < 0.001). ER impingement was a stronger predictor of acetabular anteversion than spinopelvic motion category (r = 0.458). Patients with ER impingement <45° (6.4%) or IR impingement <35° (6.6%) were "early impingers". In this cohort of anterior THA patients with a high proportion of abnormal spinopelvic motion, a technique utilizing only VROM produced no dislocations. An impingement-free zone of 45° ER standing and 35° IR sitting is recommended.