Robotic arm-assisted total hip arthroplasty enables accurate cup orientation and positioning in obese patients with osteoarthritis secondary to developmental dysplasia of the hip: a propensity score-matched comparative study

Introduction: This study evaluated the accuracy of cup placement by the robotic arm-assisted system (Mako) in obese patients with developmental dysplasia of the hip (DDH). Methods: This retrospective single-centre study included 923 consecutive hips with primary robotic arm-assisted total hip arthroplasty (THA). After propensity score-matching, 80 hips with DDH each were selected for the obese group and non-obese group. Postoperative cup orientation and positioning were assessed by superimposition of a 3-dimensional cup template, using postoperative computed tomography images with pelvic coordinates matching the preoperative planning. Results: The absolute error of cup inclination and anteversion was comparable between the obese and non-obese groups (1.2 ± 1.0° vs. 1.2 ± 1.0° and 1.4 ± 1.0° vs. 1.6 ± 1.2°, respectively). The absolute error of the transverse, sagittal, and longitudinal axes between the obese and non-obese groups were 1.0 ± 0.9 mm vs. 1.4 ± 1.0 mm, 1.5 ± 1.0 mm vs. 1.3 ± 1.8 mm, and 1.4 ± 1.2 mm vs. 1.3 ± 0.9 mm ( p = 0.02, p = 0.89, and p = 0.12, respectively). There was no significant difference in cup orientation or positioning in the obese group, regardless of body mass index. Conclusions: Robotic arm-assisted THA enables accurate cup placement even in obese patients with DDH.