Abstract
BACKGROUND
Cup malpositioning remains a common cause of dislocation, wear, osteolysis, and revision. The concept of a “Safe Zone” for acetabular component orientation was introduced more than 35 years ago. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation.
PURPOSE
We assessed the orientation of acetabular components revised due to recurrent instability and compared the results to a series of stable hip replacements.
METHODS
Cup orientation in 21 hips revised for recurrent instability was measured using CT. These hips were compared to a group of 115 stable hips measured using the same methods. Femoral anteversion in the stable hips was also measured. Images to assess femoral anteversion in the unstable group were not available.
RESULTS
Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p=.01). Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p<.001). Operative inclination was not significantly different between the control and dislocating groups. Adjusting for pelvic tilt in the supine position, all unstable hips had operative anteversion of less than 22.9 or more than 38.6 degrees or operative inclination of less than 28.9 or more than 55.9 degrees or both. The center of the “safe zone” is 30.7 ± 7.8 degrees of tilt-adjusted operative anteversion and 42.4 ± 13.5 degrees of operative inclination.
CONCLUSION
Using acetabular revision for recurrent instability as the end point, a safe zone for acetabular component orientation does exist. The range is narrower for anteversion than for inclination.
