Introduction. 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 ago1. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation by comparing the orientation of acetabular components revised due to recurrent instability and to a series of stable hip replacements. Methods. Cup orientation in 50 hips revised for recurrent instability was measured using CT. These hips were compared to a group of 184 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. An application specific software modules was developed to measure cup orientation using CT (HipSextant Research Application 1.0.13 Surgical Planning Associates Inc., Boston, Massachusetts). The cup orientation was determined by first identifying Anterior Pelvic Plane Coordinate system landmarks on a 3D surface model. A multiplanar reconstruction module then allowed for the creation of a plane parallel with the opening plane of the acetabulum. The orientation of the cup opening plane in the AP Plane coordinate space was calculated according to Murray's definitions of operative anteversion and operative inclination2. Both absolute cup position relative to the APP and tilt-adjusted cup position3 were calculated. Results. Supine tilt-adjusted Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p< .0001). Supine tilt-adjusted Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p<.01). Alt 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 30.6 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 (Figure 1). Conclusions. The current study demonstrates that most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. 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. Improved methods of defining component positioning goals on a patient-specific basis and accurately placing the acetabular component may reduce the incidence of cup mal-position and its associated complications. For figures and tables, please contact authors directly

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

Introduction. 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 ago1. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation by comparing the orientation of acetabular components revised due to recurrent instability and 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. An application specific software modules was developed to measure cup orientation using CT (HipSextant Research Application 1.0.13 Surgical Planning Associates Inc., Boston, Massachusetts). The cup orientation was determined by first identifying Anterior Pelvic Plane Coordinate system landmarks on a 3D surface model. A multiplanar reconstruction module then allowed for the creation of a plane parallel with the opening plane of the acetabulum. The orientation of the cup opening plane in the AP Plane coordinate space was calculated according to Murray's definitions of operative anteversion and operative inclination2. Both absolute cup position relative to the APP and tilt-adjusted cup position3 were calculated. Results. Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p < .001). Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p=.01). 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. Conclusions. The current study demonstrates that most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. 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. Improved methods of defining component positioning goals on a patient-specific basis and accurately placing the acetabular component may reduce the incidence of cup malposition and its associated complications

INTRODUCTION. 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. 1. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation by comparing the orientation of acetabular components revised due to recurrent instability and to a series of stable hip replacements. METHODS. Cup orientation in 30 hips revisedin 27patients 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. An application specific software modules was developed to measure cup orientation using CT (HipSextant Research Application 1.0.13 Surgical Planning Associates Inc., Boston, Massachusetts). The cup orientation was determined by first identifying Anterior Pelvic Plane Coordinate system landmarks on a 3D surface model. A multiplanar reconstruction module then allowed for the creation of a plane parallel with the opening plane of the acetabulum. The orientation of the cup opening plane in the AP Plane coordinate space was calculated according to Murray's definitions of operative anteversion and operative inclination. 2. Both absolute cup position relative to the APP and tilt-adjusted cup position. 3. were calculated. RESULTS. Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p < 0.001). Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p < 0.01). Adjusting for pelvic tilt in the supine position, all unstable hips had operative anteversion of less than 21.8 or more than 42.6 degrees or operative inclination of less than 30.6 or more than 55.9 degrees or both. The center of the “safe zone” is 32.2 ± 10.4 degrees of tilt-adjusted operative anteversion and 45.3 ± 8.7 degrees of operative inclination (Figure 1). CONCLUSIONS. The current study demonstrates that most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. 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. Improved methods of accurately placing the acetabular component placement may reduce the incidence of cup malposition and its associated complications