Acetabular orientation measured in the Lewinnek plane is not adequate for adult spinal deformity patients with high pelvic retroversion

Patients with adult spinal deformity (ASD) are known to compensate by retroverting their pelvis and flexing their knees in order to maintain postural stability [1]. Increased pelvic retroversion in patients with ASD is associated with alteration of acetabular orientation both in standing and during walking, increasing the risk of hip osteoarthritis usually treated by total hip replacement [2,3]. A safe zone is targeted during cup positioning where acetabular orientation is calculated relatively to the invariant morphological Lewinnek plane, unruled by the patient’s position. Changes in hip positioning encountered in daily life activities were associated with higher rates of prosthesis instability in ASD patients. To evaluate the mismatch between Lewinnek and positional acetabular measurements in variable patient’s postures. 121 primary ASD and 32 controls (age and sex matched: 54 years, 73% F) underwent biplanar X-rays in both standing and sitting positions. 3D acetabular parameters (anteversion, abduction, anterior coverage, posterior coverage) were calculated in both the Lewinnek and radiological positional planes (frontal, sagittal and horizontal). The mismatch between Lewinnek and positional acetabular measurements (Δ=Lewinnek-Positional) was evaluated. Radiographic pelvic tilt (PT) adjusted to pelvic incidence (PI) was calculated (adj.PT=0.37*PI-7°). Patients having a high adjusted PT (>2 SD in controls) were grouped as ASD-HighPT, otherwise as ASD-NormPT. 42 ASD had a high PT and 79 a normal PT. Although all 3 groups had similar PI (average: 52°), ASD-HighPT had a decreased lumbar lordosis (L1S1=33°, PT=31°) and decompensated sagittal malalignment (SVA=76 mm). In standing position, ASD-HighPT showed an increased planes mismatch of their acetabular parameters (Δanteversion=-12 vs 2°, Δabduction=-8 vs 0°, ∆anterior coverage=13 vs 0°, Δposterior coverage=-8 vs -1°, all p<0.001), compared to other groups. In the sitting position, ASD-HighPT showed an increased planes mismatch of their acetabular parameters (Δanteversion=-16 vs -10°, Δabduction=-12 vs -8°, ∆anterior coverage=16 vs 11°, Δposterior coverage=-12 vs -8°, all p<0.001), but to a lesser extent than the standing position. PT was strongly correlated to Δanteversion (r=-0.74) and Δanterior coverage (r=0.67, Fig. 1) in the standing position, and moderately correlated in the sitting position (r=-0.40 & 0.28 resp., all p<0.001). This study showed that the Lewinnek plane is not representative of the positional acetabular orientation in the presence of sagittal malalignment. This emphasizes the importance to consider the variation of the acetabular orientation between different postures. It is then necessary to determine a patient-specific functional safe zone in the preoperative planning of total hip replacement to avoid cup instability. Fig. 1: Correlation between pelvic tilt and planes mismatch of acetabular orientation.Download : Download high-res image (91KB)Download : Download full-size image