Instability after total hip arthroplasty: Analysis of combined anteversion and patient-related clinical parameters.

Introduction: While outcomes after total hip arthroplasty (THA) are generally excellent, prosthetic dislocation remains a multifactorial complication. This study hypothesized that differences in combined anteversion (CA) exist between patients with and without dislocation. The objectives were to (1) compare postoperative alignment parameters between dislocated and stable hips, (2) assess differences of alignement according to surgical approach, and (3) evaluate patient-related risk factors for dislocation.

Materials and methods: In this retrospective case-control study, 37 dislocated hips were matched to 74 stable hips by sex, age, body mass index, and surgical approach. Postoperative CT scans measured acetabular anteversion, femoral anteversion, CA, and cup inclination. Alignment was assessed relative to Lewinnek's safe zone (acetabular anteversion 15 ° ± 10 °, inclination 40 ° ± 10 °) and Jolles' target zone for CA (50 ° ± 10 °).

Results: Mean CA did not differ between dislocated and stable hips (45.9 ° vs 48.5 °, Δ = 2.6 °, p = 0.35). Target CA was achieved in 51% of dislocated and 54% of stable hips (p = 0.80). Cup inclination, acetabular anteversion, and femoral anteversion also showed no significant differences. Achievement of Lewinnek's safe zone was similar between groups, except for acetabular inclination (67.6% in dislocated vs 83.8% in stable hips, p = 0.04). Surgical approach (direct anterior vs posterior) was not associated with alignment differences. In multivariate analysis, ASA (American Society of Anesthesiologists) score ≥3 (OR = 2.5, p = 0.04) and degenerative lumbar spine symptoms (OR = 3.2, p < 0.01) were independently associated with dislocation risk.

Conclusion: CA did not differ between dislocated and stable hips, suggesting that implant orientation alone does not explain instability. Instead, acetabular inclination, high ASA score, and lumbar spine pathology emerged as significant risk factors, underscoring the multifactorial nature of dislocation after THA.

Level of evidence: III; case-control study.