Prospective Evaluation of Hip-Spine Dynamics in Patients Who Have a Primary Total Hip Arthroplasty Dislocation: A Matched Case-Control Study

BACKGROUND: Understanding of the comprehensive hip-spine relationship may reduce total hip arthroplasty (THA) dislocation. However, the impact of sagittal spino-pelvic dynamics on three-dimensional implant orientation has not been investigated prospectively. This study aimed to assess hip-spine dynamics and changes in three-dimensional implant orientation in patients who have stable and unstable primary THAs.

METHODS: In this prospective, case-control study, 23 adults who had a THA dislocation were matched to 23 patients who had a stable implant. Of the 23 dislocations, 17 sustained a posterior, and five sustained an anterior dislocation; one was unknown. Standing anterior-posterior and standing and sitting lateral pelvic radiographs were obtained. Sagittal spinopelvic morphology and orientation parameters, coronal and sagittal acetabular cup, and femoral component orientation parameters were measured. Transverse component orientation parameters were computed. Logistic regressions were conducted to determine the impact of demographics and hip-spine parameters on the likelihood for dislocation.

RESULTS: The unstable THAs had significantly higher pelvic incidence (PI; 60 ± 13° versus 52 ± 10°, P = 0.015) and transverse version of the acetabular component (TVCup; 38 ± 11° versus 32 ± 8°, P = 0.042) compared to the stable THAs. Patients who have anterior dislocations had higher sagittal ante-inclination of the acetabular component (SAICup) than posterior dislocations (48 ± 5° versus 34 ± 10°, P = 0.012). Based on logistic regression analyses, PI, TVCup, and the approach were significant predictors of THA dislocation.

CONCLUSIONS: By assessment of spino-pelvic characteristics, surgeons could identify patients at increased risk for THA dislocation preoperatively based on a high PI. Posterior dislocations seem to occur more in patients who have more TVCup and a postero-lateral approach, anterior dislocations seem to occur in patients who have more SAICup, TVCup, and a direct anterior approach. This suggests implementing a patient-specific functional safe zone of the acetabular component may further reduce THA dislocation rates.