Does relative anteversion or retroversion contribute to hip instability with Continuum cup system?

Hip dislocation is a common post-operative complication in total hip arthroplasty (THA) with the annual incidence of 1,9 % in Scotland. Several contributing factors have been identified including cup malpositioning. Optimal angles of cup inclination and anteversion have historically been guided by Lewinnek safe zone (30–50° for inclination and 5–25° for anteversion). Implant features and surgical approach are also considered as contributors to cup stability. The Continuum cup system raised concerns due to a notable rate of dislocation, particularly when used with a neutral liner. This configuration exacerbates the reduced jumping distance of the femoral head, further compromising joint stability. Patients who underwent THA in the April 2023–2024 period were identified from theatre lists. Basic demographic patient data and notes on any post-operative complications were obtained from electronic patient records whereas operative details were acquired from operative notes. Post-operative radiograph was used to obtain radiographic measurements. Univariable logistic regression was used to determine the association of baseline risk factors and hip dislocation. Data collected and analysed in this study is based on 251 primary THAs. The mean age of patients was 67,3 years (SD ± 9,73) and 58,2 % of the population were women. The average BMI of the patients was 28,4 (± 4,73). The overall incidence of dislocations was 4,00 % and on average occurred 9 days post-operatively (3h–23 days). Radiographic measurements obtained from dislocated and non-dislocated THA were found not to be significantly different (p > 0,05). However, 50 % of dislocated THA had radiographic angles outwit the ‘safe zone’. THAs performed using antero-lateral approach had 5,7 % rate of dislocation compared to 1,1 % of THAs done by posterior approach. This study found that 50 % of dislocations had radiographic angles outwith the recognised safe zones. Antero-lateral approach was found to be associated with higher rates of dislocation and should be used with extreme caution with the Continuum cup system.