Radiological Acetabular Cup Position and its Impact on Hip Dislocation: A Study of Primary Total Hip Arthroplasty at Muhimbili Orthopedic Institute, Dar es Salaam, Tanzania
Authors: Shedura Vulstan James , Ngomo Geofrey John , Lupondo Violet , Bwemelo Joel , Mnkungwe Rajabu Hussein , Kunambi Peter 
Published: December 2024

Background: Total hip arthroplasty (THA) remains the mainstay of orthopedic management for patients with advanced hip osteoarthritis and post-traumatic arthritis. Despite its reported success, some patients suffer post-operative complications including hip dislocation. Among other reasons, failure to achieve the Lewinnek safe zone during surgery is associated with an early hip dislocation. Studies to assess acetabular cup position and its association with hip dislocation post-operatively are limited in Sub-Saharan Africa including Tanzania. We aimed to determine the radiological acetabular cup position in relation to hip dislocation in patients who underwent total hip arthroplasty at Muhimbili Orthopedic Institute from July 15, 2020 to January 25, 2021.
Methodology: This was a cohort hospital-based study that involved follow-up of patients for 3 months after THA. The study was conducted from July 15, 2020 to January 25, 2021. Information such as age, sex, surgical approach, position of the patient during surgery, indication of the surgery, and type of implant used were collected from both patients and post-operative notes. Both inclination and anteversion angles of the cup were measured from anteroposterior (AP) pelvic digital radiography following THA to determine the Lewinnek safe zone. The RadiAnt DICOM (Digital Imaging and Communications in Medicine) Viewer analysis software tool was used to measure both the inclination and anteversion angles. The hip dislocation was assessed within 3 months post THA.
Results: A total of 51 patients were enrolled and were followed for 3 months after THA, 4 patients were lost before the period of 3 months and only 47 patients completed the study. Of the 47 patients, 38 (81.0%, 38/47) had the acetabular cup within the Lewinnek safe zone for both inclination and anteversion angles while 9 patients (19.0%, 9/47) had their acetabular cup outside the Lewinnek safe zone. Among those patients who had an acetabular cup outside the safe zone, 6 patients (66.7%, 6/9) had their cup abducted and 3 patients (33.3%, 3/9) the angles were more anteverted. At 3 months post-THA, only 3 out of 47 patients (6.4%) had hip dislocation.
There was a significant difference in hip dislocation post-THA between patients who achieved the safe zone (2.6%, 1/38) and those who did not achieve the safe zone (22.2%, 2/9) (p = 0.03). Although not statistically significant, the majority of the laterally positioned patients during THA were found to have their acetabular cup position within the Lewinnek safe zone (90%) compared to those who were supine positioned (74.1%) (p = 0.26).
Conclusion: The majority of patients in this study had their acetabular cups positioned within the Lewinnek safe zone. However, a slightly higher dislocation rate of 6.4% was observed. Furthermore, the study revealed a significantly elevated dislocation rate of 22.2% among patients whose cups were outside the Lewinnek safe zone compared to 2.6% among those within the safe zone. Malpositioning the acetabular cup outside the Lewinnek safe zone was associated with early hip dislocation. Lateral positioning of patients during total hip arthroplasty (THA) may lead to improved clinical outcomes and a reduced risk of dislocation.