Abstract
The functional pelvic tilt when standing and sitting forward of 7402 cases on the OPS, Optimized Ortho, Australia Data Base were reviewed. All patients had undergone lateral radiographs when standing simulating extension of the hip, and sitting forward when the hip is near full flexion. Pelvic tilt was measured as the angle of the Anterior Pelvic Plane to the vertical Sagittal Plane, rotation anteriorly being given a positive value. Pelvises that had rotated more than 13 degrees anteriorly (+ve) when sitting forward or posteriorly (-ve) when standing were considered to place the hip at increased risk of dislocation or edge loading when flexed or extending respectively. This degree of rotation has the effect of changing the acetabular version by approximately100. Most safe zones that have been described have given a range of anteversion of 200 as safe. A change of 100 would potentially place the acetabular orientation outside this range. Further, clinical studies have supported this concept. All lateral radiographs were reviewed to confirm that 281 had undergone instrumented spinal fusion at some level between T12 and S1. There was a large variability in the number and the levels arthrodesed. The range of pelvic mobility in the non-arthrodesed group in extension was −370 to 310 (mean −0.90, Standard deviation 7.49) and in flexed position was −700 to 490 (mean −1.90, Standard deviation 14.01). For the group with any fusion the range of pelvic tilt in extension was −310 to 220 (mean −40, Standard deviation 8.21) and flexed −320 to 460 (mean 4.40, Standard deviation 13.79). Of the 7121 cases without instrumented fusion, 15.5% were considered to be at risk when in flexion and 6.1% when extended. The risk for those with any fusion was approximately doubled in both flexion and extension. Further, those with extensive arthrodesis from T12 to S1 had a range of pelvic tilts similar to the non-fused group, although they had a significantly higher percentage of cases in the ‘at risk’ zones. The proportion of the cases in the ‘at risk’ zones decreased progressively as the arthrodesed levels moved from L5/S1 to the upper lumbar spine, and with decreasing number of levels fused.
Conclusion
Spinal fusion is not just one group as there are many combinations of different levels fused. Patients with instrumented spinal fusions do have a proportionately high risk of failure of their THR than the majority of cases with no instrumentation, though the risk varies significantly with the number of levels and actual levels arthrodesed. Further approximately 21% of cases with no spinal fusion have functional pelvic movements that would potentially place them ‘at risk’ of edge loading or dislocation.
For any figures or tables, please contact authors directly.
