Spinal Fusion Is Associated With Changes in Acetabular Orientation and Reductions in Pelvic Mobility

The importance of spinopelvic motion and its influence on THA stability are well recognized but poorly defined. With dynamic motion, compensatory changes in spine and pelvic positions are required to keep the necessary balance between the axial skeleton and lower extremity to maintain an erect posture. Although prior studies have shown spinal fusions to be an independent risk factor for hip dislocations after primary THA, the direct impact of fusion levels on spinopelvic motion remains unknown.

The purposes of this study were (1) to determine if acetabular orientation changes with flexion and extension of the lumbar spine; (2) to determine if the amount of change is different in patients who have undergone spinal fusion at the L5-S1 level; and (3) to identify if the amount of change in acetabular motion is increased in patients who have undergone fusion at additional or other spinal levels.

We reviewed 100 flexion-extension spine films of patients older than 18 years of age with a history of back pain who had not undergone spinal or hip surgery and compared them with 50 flexion-extension spine films of patients who had undergone lumbar fusion at various levels. These radiographs were acquired between 2012 and 2017 and stored in our institutional radiology database. Only patients with flexion and extension films able to visualize the greater trochanter of the femur were included. For each film, measurements of acetabular version, acetabular version relative to the femoral shaft, lumbar lordosis angle, and sacral slope were digitally performed by two independent observers. Intra- and interrater variability was assessed using Lin's concordance correlation (Rho_c) ranging from 0.59 to 0.91. The change in acetabular version for each patient when going from spinal flexion to extension was compared between patients with no prior spinal or hip surgery and those with prior spinal fusions using a two-tailed t-test.

Acetabular version changed -21° as the lumbar spine changed position from flexion to extension in patients without spine surgery (95% confidence interval [CI], -24° to -18°). Acetabular version changed 15° as the lumbar spine changed position from flexion to extension in patients who had undergone prior lumbar spine fusion at all levels (95% CI, -18° to -12°). There was a difference in the change in acetabular version between these two groups of -6° (95% CI, -11° to -1°; p = 0.01). In patients with prior L5-S1 fusion, the change in acetabular version was decreased when compared with patients without prior spine surgery. The change was -10° (95% CI, -15° to -6°), which is less than the change of acetabular version of -21° that we saw in patients without prior spinal fusion (p < 0.01). The difference between these groups was -10° (95% CI, -18° to -3°). Fusion levels above L5 that did not cross the L5-S1 joint did not have a difference in change in acetabular version when compared with patients without surgery with a mean difference of -4° (95% CI, -9° to 2°).

Spinal fusion, specifically at the L5-S1 level, reduces pelvic mobility as the spine moves from flexion to extension. This reduction in motion can reduce the distance to impingement and place patients undergoing THA at risk for dislocation. Further research utilizing three-dimensional imaging modalities and motion analysis can further help define the best hip implant position in these patients.

Level III, prognostic study.