Establishment and Clinical Verification of a Personalized Optimal Orientation-Based Safe Zone for Cup Placement for Total Hip Arthroplasty.

Abstract

Safe zones have been proposed to prevent dislocation after total hip arthroplasty. However, plenty of studies suggest that the current safe zones are not safe. In this study, we aimed to establish and verify a personalized optimal orientation-based safe zone for cup placement in total hip arthroplasty. Following the definition of the hip joint's functional range of motion, the kinematic boundary of the femoral neck was delineated. The cup's optimal orientation was calculated based on the femoral neck's cone-shaped kinematic boundary. We evaluated 50 patients who experienced at least one dislocation between 2013 and 2023 as the study group and 100 patients who had been functioning well postoperatively as the control group. The cup's optimal orientation was compared with its actual orientation, and the range with the highest differentiation ability between the two groups was designated the safe zone. The study and control groups showed significant differences in the anteversion (-13.1 versus 1.9°, P < 0.001) and abduction (5.8 versus 1.0°, P = 0.023) deviation from optimal orientation. The Shanghai Ninth People's Hospital cone-shaped safe zone is defined as -10 to 15° from the optimal anteversion and -10 to 10° in abduction. This safe zone provided good discrimination between patients who did and did not have dislocations (Chi-square = 50.0, P < 0.001). Subgroup analysis showed that patients who had anterior dislocation tended to have large abductions, whereas those who had posterior dislocation tended to have small anteversions and abductions. The cup's optimal orientation was recommended based on femoral anteversion and neck-shaft angle. The Shanghai Ninth People's Hospital cone-shaped safe zone was significantly more discerning for patients who had dislocation, relative to the Lewinnek safe zone and combined anteversion. Cup abduction is negatively correlated with anterior hip stability. III, Case-control study.