The Preoperative Anterior Pelvic Plane Angle Predicts Cup Anteversion Changes at 1 Year after Total Hip Arthroplasty

We investigated global alignment changes following total hip arthroplasty (THA) and predictive alignment parameters for increased cup anteversion (CA) by retrospectively analyzing the primary THA data of 75 patients treated at our hospital (49 women, 26 men; age 65.1±5.7 years, BMI 28.3±3.4 kg/m2). Global alignment parameters, i.e., the anterior pelvic plane angle (APPa) and proximal femoral shaft angle (PFSa) and other alignment parameters were measured. CA was evaluated based on the patients' standing coronal radiographs. ΔCA was defined as the difference in CA from 2 weeks before to 1 year after each THA. We classified the cases as stable (S) (CA < 10°; n=63) and pelvic retroversion (R) (CA ≥ 10°; n=12) groups. Associations between ΔCA and alignment parameters were evaluated by linear regression and a receiver operating characteristic (ROC) analysis. A significant decrease in the PFSa occurred between the 2-week and 1-year post-THA timepoints (7.8±4.3° vs. 4.2±3.6°, p<0.001), with no notable change in other alignment parameters. At 1-year post-THA, the CA of 12 (16%) patients had increased to 4.5±4.4°. Only the preoperative APPa was positively associated with ΔCA (β=0.165, p=0.020). The ROC analysis revealed that the optimal cut-off value for increased CA in the APPa is 2.1° (area under the curve, 0.700; p=0.020; odds ratio, 4.80). The APPa change predicted increased CA, which emphasizes the importance of the use of preoperative standing radiography for identifying the optimal cup positioning for post-THA changes in CA.