Contemporary guidelines for acetabular positioning in hip arthroplasty may jeopardize hip dislocation for select ASD patients

BACKGROUND CONTEXT: Lewinnek et al previously defined a safe zone for cup inclination (40° ± 10°) and anteversion (15° ± 10°) to reduce dislocations after total hip arthroplasty (THA). Despite the Lewinnek safe zone being respected, higher rates of prosthetic dislocation are found in ASD patients undergoing THA. PURPOSE: This study aims to investigate if ASD patients with high PT need a specific safe zone for acetabular positioning. STUDY DESIGN/SETTING: Retrospective analysis of prospectively collected data. PATIENT SAMPLE: ASD patients with moderate to severe sagittal deformity and full-body xrays and a detailed 3D reconstruction of the pelvis and lower limbs in standing position preop. OUTCOME MEASURES: Radiographic spinal and acetabular parameters METHODS: The PI-adjusted PT was calculated using Vialle et al formula (adj.PT=0.37*PI-7°) and patients with PT> adj.PT were grouped as HighPT, otherwise as NormPT. Spino-pelvic, 3D acetabular parameters were compared between the groups. PT offset from adj.PT was calculated. The number of patients falling outside the Lewinnek safe zone between the 4 quartiles of PT offset was compared. A linear regression analysis was performed to assess the relationship between PT and acetabular parameters. RESULTS: A total of 132 patients were included (HighPT:86, NormPT:46) with no differences in age, sex, comorbidities, and hip OA grades between the groups. HighPT had higher PI-LL (32.1, 12.4°), SVA (79.7, 59.0 mm), T1PA (31.2, 19.5°), SFA (209.1, 199.3°), pelvic Shift (50.3, 17.2 mm), GSA (7.6, 4.5°), and L4-S1 lordosis (26.2, 34.5°; all p<0.05). HighPT had higher acetabular abduction (60.5, 58.4°), acetabular anteversion (23.4, 20.1°), acetabular tilt (36.5, 29.8°) and posterior coverage (102.6, 98.6°, all p<0.05).The % of patients falling outside of the anteversion safe zone increased with PT offset (Q1:32%, Q2:47, Q3:57, Q4:75%, p=0.003) without being significant for abduction. The equation to determine acetabular anteversion in function of PT was derived: 16.582 + 0.215*PT (SE:6) (Figure). Anteversion of 21, 23, 25, 27 was associated with PT of 20, 30, 40 and 50°. CONCLUSIONS: This is the first study to investigate impact of severe spinal deformity on pelvic tilt and acetabular anteversion in native hips. It also proposes an equation to calculate safe zone for acetabular component position and establishes thresholds of PT adjusted acetabular anteversion. FDA Device/Drug Status: This abstract does not discuss or include any applicable devices or drugs.

Duke Scholars

Author Christopher Ignatius Shaffrey Orthopaedic Surgery

Author Peter Passias Orthopaedic Surgery