Dislocation is the most relevant early complication after primary total hip replacement (THR) in literature. Many factors have been advocated for dislocation, either surgeon-related either patient-related. Component positioning seems to be of major importance in determining dislocation. We evaluated 152 randomised THR with a CT study between 985 THR done at our Institute since november 2004 to november 2006. 152 randomised primary THR on a total of 985. The same prosthetic pattern (head size, stem, cup). Lateral approach with total capsulectomy and external rotator tendon resection. All 152 patients underwent a post operative standardized CT study assessing cup antiverion and inclination angles and stem antiversion angle. Dislocated patients where furthermore analysed for any detail concerning their procedure and follow-up. A safe zone was then deduced for safer positioning. During the follow-up period dislocation occurred in 5 hips (only one in the randomised group) assessing our rate of dislocation at 0.5%. All dislocation were managed with closed reduction and an articulated hip brace. No open reduction or revision surgery were further needed. The mean cup abduction was 47° in the dislocated hips and 49° in the control group. Mean cup anteversion was 29° in both groups. The mean stem anteversion was 8.2° in the dislocated group and 3.1° in the control group. No statistical difference could be reached between dislocation and cup positioning. A correlation between hip dyspalsia (Crowe II) as primitive diagnosis and dislocation could be reached considering all the THR procedures. In THR inappropriate cup and stem positioning is considered an important risk factor of postoperative dislocation. Accurate and reproducible measurement is mandatory for implant positioning evaluation. Conventional radiographs cannot provide accurate and reproducible measurement. CT can provide a precise measurement of prosthetic components. Several studies failed to demonstrate a correlation between component positioning and dislocation often because of small number of patients and many bias. We tried to reduce bias using the same prosthetic pattern and the same surgical approach. Notwithstanding we could not reach a statistical difference in term of prosthetic positioning between dislocated and control group. Perhaps the dislocated group was too small to have a statistical meaning. We could determine a Safe Zone of cup and stem positioning for our patients: cup anteversion between 24° and 33°, cup inclination between 42° and 50°, stem anteversion between −3° and 10°. Dislocation is the main early complication after THR. Its etiology depends on many factors. Sometimes the cause can’t be identified. Orientation of prosthetic components may be responsable for dislocation but its truly correlation can be hard to be assessed. In this study we found no correlation between implant positioning and occurrence of dislocation, but we defined a tighter Safe Zone than previous reported, in which the risk of dislocation is nought. A correlation between hip dyspalsia (Crowe II) as primitive diagnosis and dislocation could be reached.
