Hip dislocation remains one of the most frequent complications of total hip arthroplasty (THA). To minimize the risk of dislocation, cup placement has traditionally been guided by a defined "safe zone". However, dislocations still occur even when the implant components are positioned within this zone, which may be due to the influence of spinopelvic motion on THA stability. This study aimed to (1) compare spinopelvic risk factors for dislocation between patients who experienced dislocations and those who did not after anterior-approach surgery and (2) analyze the risk factors associated with anterior versus posterior dislocations.

Our hypothesis was that patients with dislocation of a total hip arthroplasty after the anterior approach had a higher rate of risk factors for adverse spinopelvic mobility and that implant versions, as well as hip lengths and offsets, play an important role in anterior and posterior dislocations.

Patient with dislocation were prospectively collected from August 2018 to August 2022. Out of a total of 6,166 THAs, 35 dislocations were recorded, and 7 patients were excluded. This single-center study included a prospective cohort of 28patients who experienced dislocations (19 anterior, 9 posterior) compared with a consecutive control cohort of 278 patients who did not, all of whom underwent primary THA via the anterior approach. Preoperative spinopelvic parameters such as lumbar flexion (LF), spinopelvic tilt (SPT), pelvic incidence (PI), and pelvic mobility (change in SPT [ΔSPT]) were analyzed in the control group using pelvic-femoral computed tomography and lateral X-rays. Patients who experienced dislocation underwent advanced postoperative functional analysis, in which spinopelvic parameters, implant version, hip length discrepancy, and femoral offset were assessed.

The prevalence of spinopelvic risk factors was greater in the dislocation cohort than in the control cohort [SPT≤−10°: 42.5% vs. 10.5% (p < 0.001); LF ≤ 35°: 46.1% vs. 11.9% (p < 0.001); PI–lumbar lordosis (LL) ≥ 10°: 33.9% vs. 14.8% (p = 0.003); ΔSPT ≥ 20 ° from standing to seated: 50% vs. 8.3% (p < 0.001); and ΔSPT ≤−13 ° from supine to standing: 21.4% vs. 6.7% (p = 0.012)]. The mean combined anteversion (CA) was 35 ° (7°–53 °) in the anterior dislocation group and 24 ° (15°–30 °) in the posterior dislocation group.

Patients with dislocations presented a high prevalence of spinopelvic risk factors. Anterior dislocations were linked to spinopelvic abnormalities rather than excessive CA. In contrast, posterior dislocations occurred in patients with low CA, especially at the expense of stem version and spinopelvic risk factors. Therefore, in patients undergoing anterior-approach THA, restricting implant anteversion may not be the primary factor in reducing the risk of anterior dislocation but may increase the risk of posterior dislocation in patients with adverse spinopelvic mobility.

III; Case-control study.