Abstract
Introduction
Cup malposition in hip arthroplasty and hip resurfacing is associated with instability, accelerated wear, and the need for revision. A recent study measuring cup orientation on conventional radiodiographs demonstrated an incidence of cup malpositioning of 50% according to the safe zone that they defined 1,2. A prior study of 105 conventionally placed cups using CT demonstrated a cup malpositioning incidence of 74%3. The current study similarly assesses the variation in cup position using conventional techniques as measured by CT.
Methods
CT studies of 123 hips in 119 patients with total hip arthroplasties performed using conventional techniques were used for this study. The indications for the CT studies were for CT-based surgical navigation of the contralateral side or for assessment of periprosthetic osteolysis. 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. Since these studies including images through the femoral condyles, femoral anteversion could be measured on these hips as well (Osirix v5.6, Pixmeo SARL, Bernex, Switzerland).
Results
Cup orientation for the 123 hips is shown in Figure 1. Operative anteversion averaged 29.7 degrees with a standard deviation of 12.2 and a range of −24.4 to 57.5. Operative inclination averaged 37.5 degrees with a standard deviation of 7.7 and a range of 18.4 to 68.2. Femoral anteversion averaged 21.1 degrees with a standard deviation of 14.0 and a range of −20.5 to 60.9. Using 25 degrees of operative anteversion and 45 degrees of operative inclination as the center of a safe zone for example, 78 of 123 (63%) were more than 10 degrees off in either anteversion and inclination and 23 of 123 (19%) were more than 10 degrees off in both anteversion and inclination.
Discussion and Conclusion
Most conventionally placed acetabular components are malpositioned. While the incidence of cup malorientation using conventional techniques is quite high, the incidence in our series appears to be lower than that reported by Saxler et al. It is curious that most experienced surgeons who perform total hip arthroplasty using conventional methods of cup alignment believe that their accuracy quite good. Yet, multiple objective studies of cup alignment demonstrate that accuracy is quite poor. Since cup malposition is so closely associated with instability, impingement, wear, bearing fracture, osteolysis and loosening, questions remain as to how conventional methods of cup alignment remain an acceptable standard of care in our field.
