Abstract
Background
The cementless acetabular component fixed with several screws is one of the most widely used approaches in THA. These screws rely on contact pressure and the resultant friction between the screw head and the cup to control translation and angulation of the prosthesis. However, intraoperative change of the acetabular component alignment during screw fixation should be hardly detected. Acetabular component alignment can be assessed using computer-assisted navigation systems with realtime adjustments for component position. The purpose of the current study was to evaluate intraoperative change of acetabular component alignment during screw fixation using navigation system.
Patients and Methods
Primary THAs were performed in 74 hips using CT based fluoroscopic matching navigation system (VectorVision, BrainLAB). The patients were 18 men and 56 women with a mean age of 64.4 years (range, 47–78 years) at operation. Intraoperative acetabular component inclination and anteversion were measured at the time of press-fit, and after screw fixation using the cup verification function in the system. Mean of the absolute difference between at the time of press-fit and after screw fixation was evaluated as intraoperative change of acetabular component. We measured the distance from the center of the femoral head to the inter-teardrop line as a horizontal and vertical reference on the postoperative radiograph. The number of screws was also investigated.
Results
Mean inclination and anteversion at the time of press-fit were 40.6° ± 3.6° and 14.6° ± 9.3°, respectively. Mean inclination and anteversion after screw fixation were 40.7° ± 4.4° for inclination and 14.5° ± 9.6° for anteversion. Mean intraoperative change of acetabular component was 1.9° ± 1.7° for inclination and 1.9° ± 1.9° for anteversion. The use number of the screw was an average of 1.46 (1 to 4). The intraoperative change of acetabular component anteversion correlated with number of screws (r = 0.381, p = 0.017), and vertical distance(r = 0.265, p = 0.05). The intraoperative change of acetabular component inclination also correlated with horizontal distance (r = 0.313, p = 0.02).
Discussion
Many studies have shown that multiple bone screws are very helpful aids for cementless acetabular cup fixation. The multiple-screw fixation could have an effect on micromotion of the acetabular component. However, the change of acetabular alignment during inserting screws has not been clearly reported. The current study showed mean intraoperative change of acetabular component was 1.9° for inclination and 1.9° for anteversion. However, maximum change of acetabular component was 5° for inclination and 13° for anteversion. These findings suggest that the alignment of acetabular component could change during screw fixation, and the change might cause malalignment of the acetabular component, and could increase the risks of impingement, dislocation, and accelerated wear. The change of acetabular alignment correlated with number of screws and radiographycal measurements in this series. Therefore, bone quality and bone stock could affect stability of acetabular component. In conclusion, navigation can show potential to help quantify intraoperative acetabular component alignment change during screw fixation and potentially reduce the risk for malpostion of acetabular components.
