Hip replacement improves lumbar flexibility and intervertebral disc height — a prospective observational investigation with standing and sitting assessment of patients undergoing total hip arthroplasty

PURPOSE: The pathogenic mechanism of the hip–spine syndrome is still poorly elucidated. Some studies have reported a reduction in low back pain after total hip arthroplasty (THA). However, the biomechanical mechanisms of THA acting on the lumbar spine are not well understood. The aim of the study is to evaluate the influence of THA on (1) the lumbar lordosis and the lumbar flexibility and (2) the lumbar intervertebral disc height. METHODS: A total of 197 primary THA patients were prospectively enrolled. Pre- and post-operative biplanar stereoradiography was performed in standing and sitting positions. Spinopelvic parameters (lumbar lordosis (LL), pelvic tilt, sacral slope, pelvic incidence), sagittal spinal alignment (sagittal vertical axis, PI-LL mismatch (PI-LL)) and lumbar disc height index (DHI) for each segment (L1/2 to L5/S1) were evaluated. The difference between standing and sitting LL (∆LL = LL(standing) − LL(sitting)) was determined as lumbar flexibility. Osteochondrosis intervertebralis was graded according to Kellgren and Lawrence (0–4), and patients were assigned to subgroups (mild: 0–2; severe: 3–4). RESULTS: Lumbar flexibility increased significantly after THA (pre: 22.04 ± 12.26°; post: 25.87 ± 12.26°; p < 0.001), due to significant alterations in LL in standing (pre: 51.3 ± 14.3°; post: 52.4 ± 13.8°; p < 0.001) and sitting (pre: 29.4 ± 15.4°; post: 26.7 ± 15.4°; p = 0.01). ∆LL increased significantly in both subgroups stratified by osteochondrosis (pre/post: ΔLL(mild): 25.4 (± 11.8)/29.4 ± 12.0°; p < 0.001; ΔLL(severe): 17.5 (± 11.4)/21.0 ± 10.9°; p = 0.003). The DHI increased significantly from pre-operatively to post-operatively in each lumbar segment. PI-LL mismatch decreased significantly after THA (pre: 3.5°; post: 1.4°; p < 0.001). CONCLUSION: The impact of THA on the spinopelvic complex was demonstrated by significantly improved lumbar flexibility and a gain in post-operative disc height. These results illustrate the close interaction between the pelvis and the vertebral column. The investigation provides new insights into the biomechanical patterns influencing the hip–spine syndrome. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00264-022-05497-9.