Abstract
Introduction
Accurate and reproducible cup positioning is one the most important technical factors that affects outcomes of total hip arthroplasty (THA). Although Lewinnek's safe zone is the most accepted range for anteversion and abduction angles socket orientation, the effect of fixed lumbosacral spine on pelvic tilt and obliquity is not yet established.
Questions:
-
What is the change in anteversion and abduction angle from standing to sitting in a consecutive cohort of patients undergoing THA?
-
What is the effect of fixed and flexible spinal deformities on acetabular cup orientation after THA?
Material and Methods
Between July 2011 and October 2011, 68 consecutive unilateral THAs were implanted in 68 patients with a mean age of 71 ± 6 years old. Radiographic evaluation included standing anteroposterior (AP) and lateral pelvic radiographs, and sitting lateral pelvic radiograph, measuring lumbosacral angle (LSA), sacral angle (SA), and sagittal pelvic tilt angle (SPTA). Computer generated 3D pelvis models were used to analyze the correlation between different pelvic tilts and acetabular cup orientation in abduction and anteversion.
Results
The mean standing STPA was 3.7º of anterior tilt (range: −9º of anterior tilt to 25º of posterior tilt). The mean SPTA in sitting was −17.7º of posterior tilt (range: −38º of posterior tilt to 7º of anterior tilt). In a fixed spinal deformity (54%) the SPTA was significantly smaller compared to the flexible pelvis group (10º versus 30.9º, p=0.0001). Flexible pelvises (46%) had a posterior tilt from standing to sitting resulting in increased anteversion. Overall, mean change in LSA and SA from standing to sitting was 22.5º and 20.2º, respectively. The mean post-operative functional anteversion and abduction angles were 19.2º and 42.1º, respectively. In the virtual 3D pelvic models, when the abduction angle was between 40 and 45 degrees, anteversion changed of 0.75 degrees for 1 degree of change of pelvic tilt.
Discussion
There is a significant change in pelvic tilt from standing to sitting, especially in patients with flexible spines, where the functional anteversion increases with sitting. Patients with a fixed pelvis have significantly less change in sagittal tilt and therefore less change in anteversion from standing to sitting position. Care should be taken to adjust cup positioning in fixed spinal deformity.
