Alignment restoration of the hip and the influence on outcomes following DAA THA in none-obese and obese patients: a retrospective comparative study

Abstract Background Accurate biomechanical alignment is crucial for the success of Total Hip Arthroplasty (THA). Obesity poses an increasing challenge in THA. This study compares hip restoration and clinical outcomes after direct anterior approach (DAA) THA between non-obese and obese patients. Methods Biomechanical alignments after DAA THA were evaluated on postoperative radiographs by global, acetabular and femoral offsets as well as leg length and abductor lever arm. Restoration was calculated as the difference of the operated hip to the contralateral native joint (Δ). Precision was assessed using ± 5 mm and ± 2,5 mm target zones. Significant alignment parameters influencing clinical outcomes were evaluated using one-year postoperative WOMAC scores and compared between obese (BMI ≥ 30 kg/cm 2 ) and non-obese (BMI < 30 kg/cm 2 ) patients. Results No significant differences in biomechanical restoration ( N = 494) were found between obese and non-obese patients. Precision within the ± 5 mm target zones ranged from 31,4% to 73,5%, while precision within the ± 2,5 mm target zone ranged from 12,1% to 35,8%. Precision was independent of BMI. The abductor lever arm (ALA) alignment was the only radiographic parameter that correlated with clinical outcome ( N = 344). In the non-obese, the one-year postoperative WOMAC score was significantly better when ΔALA was ≥ 0 mm ( p = 0.023). However, this correlation was not observed in obese patients ( p > 0.05). In obese patients, WOMAC scores were significantly better when ΔALA was ≥ 2 mm ( p = 0.003). Conclusion Accuracy of biomechanical restoration following DAA THA is comparable in non-obese and obese patients. To improve clinical outcomes, reduction of ΔALA should be avoided; obese patients may even benefit from a slight increase of ΔALA.