Assessment of native hip offset on corresponding radiographs and CT: Implications for THA planning

INTRODUCTION: Restoration of hip offset (HO), which incorporates both acetabular (AO) and femoral offset (FO), determines abductor muscle function and is essential to prevent adverse outcomes such as impingement, dislocation and wear after total hip arthroplasty (THA). Accurate and reliable assessment of femoral and acetabular offset in preoperative planning is crucial as it suggests appropriate design, size and positioning of prosthetic components.
OBJECTIVES: The aim of the present study was to evaluate the accuracy and reliability of native femoral and acetabular offset measurements performed on conventional radiographs with reference to three-dimensional measurements performed on computed tomography (CT) scans in patients with primary end-stage hip osteoarthritis (OA).
METHODS: In a retrospective cohort study, pre-operative anterior-posterior (AP) hip radiographs, AP pelvis radiographs and CT scans of a consecutive series of 131 patients (57 males, 74 females, mean age 60 (range: 42- 79) years, mean body-mass-index (BMI) 27 (range: 19-45) kg/m²) with primary end-stage hip OA were measured. To achieve reproducible radiographic projection, patients were positioned according to a standardized protocol and all images were calibrated. Using validated software programs, FO and AO were measured on corresponding radiographs and on CT scans. Interand intra-observer reliability was evaluated using intra-class correlation coefficients (ICC). Distributions of offset measurements on corresponding images were compared using paired-samples t-tests. Measurement agreement between corresponding radiographic images and CT was assessed using Pearson’s coefficient of correlation (r) and Bland-Altman plots.
RESULTS: ICCs demonstrated a high inter- and intra-observer reliability for all measured parameters (ICC-range: 0.89- 0.99). In the entire cohort, AP pelvis based FO (40.1 mm, 95%CI: 38.9-41.2 mm) was significantly underestimated compared to CT based FO (45.3 mm, 95%CI: 44.3-46.2 mm, p<0.001). There was no difference in mean FO between AP hip radiographs (44.9 mm, 95%CI: 43.8-46.1 mm) and CT scans (p=0.431). No difference in acetabular offset measurements was seen on corresponding AP pelvis radiographs (34.1 mm, 95%CI: 33.0-35.3 mm) and CT scans (33.7 mm, 95%CI: 32.9-34.6 mm, p=0.329). We observed good agreement between FO measurements on AP hip radiographs and CT (r=0.71, p<0.001), and AO measurements on AP pelvis radiographs and CT (r=0.62, p<0.001), respectively.
CONCLUSION: Native hip offset of patients with primary end-stage hip osteoarthritis can be accurately and reliably determined on conventional radiographs when assessing femoral offset on AP hip views and acetabular offset on AP pelvis views, respectively. The present findings support the surgeon in pre-operative templating and may improve individual restoration of hip offset in THA without the routine performance of CT. We recommend to routinely obtain corresponding AP pelvis and AP hip radiographs for pre-operative planning in THA.