Background: Spinopelvic mobility gained increased attention as a contributing factor for total hip arthroplasty (THA) instability. However, it is unknown how a pre-existing THA affects spinopelvic mobility. Therefore, a propensity-score-matched analysis of primary THA patients comparing the individual segments of spinopelvic mobility between patients with pre-existing THA and no-existing THA was conducted. Consequently, the study aimed to discuss (1) whether patients with a pre-existing THA have altered spinopelvic mobility compared to the control group and (2) if spinopelvic mobility changes after THA.
Methods: A prospective observational study enrolled 197 elective primary THA patients, including N = 44 patients with a pre-existing unilateral THA. Using propensity-score matching adapted for age, sex, and BMI, N = 44 patients without a pre-existing THA were determined. The patients received stereoradiography in standing and relaxed sitting position pre- and postoperatively. Assessed parameters were lumbar lordosis (LL), pelvic tilt (PT), and pelvic femoral angle (PFA). Key parameters of the spinopelvic mobility were defined as lumbar flexibility ( increment LL = LLstanding - LLsitting), pelvic mobility ( increment PT = PTstanding - PTsitting) and hip motion ( increment PFA = PFA(standing) - PFA(sitting)). Pelvic mobility was classified as stiff ( increment PT < 10 degrees), normal ( increment PT >= 10 degrees-30 degrees) and hypermobile ( increment PT > 30 degrees). The Wilcoxon rank sum test for dependent samples was used.
Results: Pelvic mobility was significantly increased in the pre-existing THA group ( increment PT 18.2 degrees +/- 10.7) compared to the control group ( increment PT 7.7 degrees +/- 8.0; p < 0.001) preoperatively and postoperatively (pre-existing: 22.2 degrees +/- 9.3; control: 17.0 degrees +/- 9.2, p = 0.022). Lumbar flexibility was significantly increased in the pre-existing THA group ( increment LL 21.6 degrees +/- 11.8) compared to the control group ( increment LL 12.4 degrees +/- 7.8; p < 0.001) preoperatively and postoperatively (pre-existing: 25.7 degrees +/- 11.0; control: 19.0 degrees +/- 10.2; p = 0.011). The contribution of stiff pelvic mobility is distinctly smaller in the pre-existing THA group (25%) than in the control group (75%) preoperatively.
Conclusions: Pre-existing THA is associated with significantly enhanced pelvic mobility and lumbar flexibility. Accordingly, we identified the patients without a pre-existing THA as risk candidates with higher likelihood for pathological spinopelvic mobility. This information will assist arthroplasty surgeons in deciding which THA candidates require preoperative radiological screening for pathologic spinopelvic mobility.
