The optimal centre of rotation for total hip arthroplasty in patients with unilateral Hartofilakidis type B developmental dysplasia of the hip: a cohort study with a mean follow-up of eight years

Aims: The position of the centre of rotation (COR) for the best function and recovery of gait after total hip arthroplasty (THA) in patients with Hartofilakidis type B developmental dysplasia of the hip (DDH) remains controversial. This study aimed to compare the functional outcomes between two methods of reconstruction and quantify the optimal COR for the recovery of gait. Methods: Out of 1,359 THAs which were identified, 223 patients (223 hips) with unilateral Hartofilakidis type B DDH and a normal contralateral hip who underwent anatomical mirroring reconstruction using autografts (Group A, 115 hips) or high hip centre (HHC) reconstruction (Group H, 108 hips) between January 2011 and June 2021 were reviewed. Functional and radiological results, including patient-reported outcome measures (PROMs), the height of the COR and offset, and recovery of gait, were compared between the groups. Factors associated with a limp were identified by Cox regression analysis and used to develop a nomogram. A power analysis indicated that 54 patients were required to detect a 25% difference in recovery of gait (α = 0.05, power = 0.80). Results: During mean lengths of follow-up of 9.09 years (SD 3.21) and 8.62 years (SD 2.74) in Groups A and H, respectively, the Harris and Oxford Hip Scores significantly improved in both groups, with significantly better improvements in Group A than in Group H (both p < 0.001). Significantly more patients in Group A achieved recovery of gait than in Group H (103 (89.6%) vs 72 (66.7%); p < 0.001) at final follow-up. Early recovery of gait was significantly more likely in patients with smaller differences in COR height bilaterally (p < 0.001), younger patients (p = 0.021), and those who had not previously undergone a shelf acetabuloplasty (p < 0.001). The nomogram showed good performance (area under the receiver operating characteristic curve = 83.9%). Conclusion: Significant functional improvement, with the COR placed < 1.18 cm above the normal contralateral hip, was achieved in both groups, but mirroring reconstruction provided earlier and better recovery of gait. We recommend that surgeons use this nomogram to assess the odds of there remaining a limp postoperatively before undertaking THA, in order to improve expectations and rehabilitation.