Insufficient stem antetorsion is a crucial risk factor for posterior hip dislocation in patients undergoing THA for femoral neck fractures: A retrospective analysis

Background: The role of acetabular and femoral component positions with respect to the risk of post-operative instability and dislocation remains debated. In this study, we aimed to identify potential risk factors for early dislocation following primary total hip arthroplasty (THA) for displaced intracapsular femoral neck fractures (FNF) using radiological measurements. Methods We retrospectively analyzed data for patients who underwent cementless primary THA for FNF using a posterolateral approach between January 2018 and December 2021. Follow-up duration, age, sex, affected side, and mean time from THA to dislocation were recorded. Leg-length inequality, abductor lever arm, vertical and horizontal femoral offsets, vertical and horizontal hip centers of rotation, abduction, anteversion of the acetabulum and femoral prosthesis, and combined anteversion were measured. Results The study sample included 17 men and 34 women, with 21 and 30 patients undergoing left- and right-hip operations, respectively. The mean patient age was 70.18 ± 7.64 years, and the mean follow-up duration was 27.73 ± 13.52 months. The mean time between THA and dislocation was 1.58 ± 0.79 months. Seven patients (13.73%) sustained posterior dislocation of the hip. The abduction angle (36.05 ± 6.82° vs. 45.68 ± 8.78°) ( p  = 0.008) and anteversion of the femoral prosthesis (8.26 ± 4.47° vs. 19.47 ± 9.01°) ( p  = 0.002) were significantly lower in the dislocation group than in the control group. There were no significant differences in other parameters. Conclusions Insufficient stem antetorsion, lower abduction angle of the acetabular component, and joint hyperlaxity were associated with a high risk of dislocation. Insufficient stem antetorsion can shift the neck of the prosthesis in the posterosuperior direction, while joint hyperlaxity can make it unstable. When combined with a lower abduction angle of the acetabular component, especially in patients with deep flexion or internal rotation of the flexed hip joint and knees, impingement may then occur in the neck of the prosthesis and cup component, ultimately resulting in posterior dislocation. Based on these findings, surgeons should aim to restore the acetabular component and neck of the prosthesis to appropriate spatial locations. These results provide new insight into risk factors for hip dislocation in patients undergoing primary THA for FNF and may aid in reducing the risk of instability and dislocation. Level of evidence: Prospective comparative study Level II.