The influence of osteophyte depiction in CT for patient-specific guided hip resurfacing procedures.

PURPOSE: An accurate fit of a patient-specific instrument guide during an intervention is one of the critical factors affecting accuracy of the surgical procedure. In this study, we investigated how well osteophytes, which are abnormal bone growths that form along joints, are depicted in clinical preoperative CT scans and estimated the influence of such depiction errors on the intraoperative accuracy of the guide.
METHODS: In 34 hip resurfacing patients, 227 osteophyte surface points on the anterior aspect of the femoral neck were collected intraoperatively, using an optoelectronic navigation system. These points were registered to a preoperative CT scan of the patient, and distances between collected points and segmented virtual bone surface, as well as Hounsfield units for these points, were determined. We simulated the registration error of a patient-specific guide, using a modified registration algorithm, to test placement on the anterior aspect of the femoral neck without removing any osteophytes. This error was then applied to the surgical plan of the femoral central-pin position and orientation for evaluation.
RESULTS: The average distance between the collected points and the segmented surface was 2.6 mm. We estimated the average error for the entrance point of the central-pin to be 0.7 mm in the distal direction and 3.2 mm in the anterior direction. The average orientation error was 2.8° in anteversion.
CONCLUSIONS: The depiction of osteophytes in clinical preoperative CT scans for proximal femurs can be unreliable and can possibly result in significant intraoperative instrument alignment errors during image-guided surgeries.