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 defined1,2. 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:
We have performed CT-based navigation of hip arthroplasty and revision arthroplasty on a routine basis since 2003 and also use CT imaging to quantify periprosthetic osteolysis. In our image database from these, we have identified 98 hips and y patients who had a previously conventionally-placed cup on CT imaging. For each hip, cup orientation was determined in operative anteversion and operative inclination (according to the definitions of Murray) using an application specific software application (HipSextant Research Application 1.0.7, Surgical Planning Associates Inc., Boston, Massachusetts). This application allows for determination of the Anterior Pelvic Plane coordinates from a 3D surface model. A multiplanar reconstruction module allows for creation of a plane parallel with the opening plane of the acetabulum and subsequent calculation of plane orientation in the AP Plane coordinate space.
Results:
16 of 53 or 30.2% of hips in the control group were within 7.5 degrees of the safe zone center for both for both anteversion and inclination. 29 of 53 or 51.9% of hips in the control group were within 10 degrees of the safe zone center for both for 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.
