Abstract
Introduction
Computer navigation systems are quite sophisticated intra-operative support systems for the precise placement of acetabular or femoral components in THA. However, few studies have addressed the clinical benefits derived from using a navigation system to achieve precise placement of the implants. The purpose of this study is to investigate the early dislocation rate of navigation-assisted primary THA through a posterior approach in order to clarify the short-term benefit of using a computer navigation system.
Methods
We retrospectively reviewed the early dislocation rate (within 12 months after surgery) of 475 consecutive primary cementless or hybrid THAs with femoral head sizes ≦32mm performed via posterior approach. There were 85 men and 390 women, with a mean age of 60 years (17 to 88) at operation. Preoperative diagnoses included osteoarthritis in 384 hips, osteonecrosis in 45 hips, and others in 46 hips (ex. RA, trauma, infection, congenital disease). All THAs were planned using a 3D templating system based on the combined anteversion theory, performed by single surgeon through a posterior approach with repair of the posterior capsule, assisted by a CT-based surface matching type computer navigation system for cup implantation. All patients were directly followed up at least 1 year after surgery. We classified all 475 joints into four groups: normal or mildly deformed hips (Group A; 308 joints, ex. primary OA, Crowe group 1, osteonecrosis), moderately deformed hips (Group B; 97 joints, ex. Crowe group 2, protrusio acetabuli, Perthes like deformity), severely deformed hips (Group C; 53 joints, ex. Crowe group 3 or 4, ankylosis, fused hip), and neuromuscular and cognitive disorders (Group D; 17 joints), and examined the dislocation rate for each group.
Results
We had eleven early dislocations, for an overall dislocation rate of 2.3% (11/475). All dislocations occurred posteriorly within 6 weeks after surgery. Three joints were Crowe group 4 dislocated hips, three were Charcot joints, two were Girdlestone hips after pyogenic arthritis, two was a Crowe group 1 hip, and one was osteonecrosis. All 11 cups were implanted within 5 degree of error from the preoperative planning, and all were placed within the Lewinnek safe zone. The dislocation rates according to group were 0.6% for group A (2/308), 0% for B (0/97), 9.4% for C (5/53), and 23.5% for D (4/17).
Discussion
The use of computer navigation system in patients undergoing THA improves the precision of acetabular cup placement by decreasing the number of outliers, which may result in reducing the risk of dislocation. In this series, most dislocations occurred in the highly risky or rare condition cases in groups C or D. In such cases, precise and appropriate cup implantation assisted by the navigation system could not completely prevent dislocation because of the patients' specific special backgrounds. On the other hand, early dislocation was prevented for the normal/mild to moderately deformed joints such as those in groups A or B. Computer navigation system was effective for prevent early dislocation in the normal or mild to moderately deformed joints.
