Paper #49 - ISHA Annual Scientific Meeting 2016

Acetabular Bone Density Decreases Following Surgical Correction of FAI Deformities

Tiffany Dang, BSc, Ottawa, Ontario CANADA
Paul Beaule, MD, FRCSC, Ottawa, Ontario CANADA
Andrew Speirs, PhD, Ottawa, Ontario CANADA

Ottawa Hospital, University of Ottawa, Ottawa, Ontario, CANADA

FDA Status Not Applicable

Summary: This study showed a decrease in BMD in the acetabular rim at two-year follow-up of patients who underwent corrective surgery for a symptomatic cam FAI deformity.

Femoroacetabular impingement (FAI) is caused by deformities of the hip joint which may result in repeated abnormal contact in the joint leading to osteoarthritis. Increased acetabular subchondral bone density was found symptomatic subjects with cam-type deformities as well as in asymptomatic subjects with cam deformities who may be in early stages of degeneration. Increased bone density due to abnormal hip contact and bone remodeling may play an important role in the pathogensis of hip OA. The cam deformity may be surgically removed using minimally invasive techniques, although the long-term clinical results are unknown. The purpose of this study was to examine the change in bone mineral density (BMD) following surgical removal of the cam deformity.
Twenty patients undergoing surgical correction of a symptomatic cam deformity were recruited. Prior to surgery and two years after surgery, bilateral quantitative CT scans of the hip were performed on all patients, including a CT calibration phantom. Image resolution was 0.8 to 0.9 mm in-plane with 0.625 mm slice thickness. Pre-operative CT images were segmented and divided into six wedge-shaped regions around the superior acetabulum. A tetrahedral mesh was created in each wedge to define sampling points in the CT image and the image intensity was converted to BMD using the calibration phantom. The average BMD was then calculated in each wedge in each of three levels based on the depth from the acetabular rim i.e. rim, mid-level and medial wall. For follow-up BMD the mesh was transformed based on a rigid registration to define the sampling points in the follow-up CT. The change in BMD was assessed in each region and significance was tested using a paired-sample t-test. The study was reviewed and approved by the hospital research ethics board.
To date, eleven patients have undergone follow-up CT scan. BMD decreased by 33 g/cc to 64 g/cc in the rim level of the superior acetabulum, or -5.0 to -9.5% (Figure 2). Significant changes were seen in wedges 1 (anterior), 4 and 5 (p<0.05). A decrease was also seen in wedges 2 and 3 but was not significant (p=0.13 and p=0.15). Changes in the middle level were smaller and were not significant (p=0.053 to 0.81).
This study showed a decrease in BMD in the acetabular rim at two-year follow-up of patients who underwent corrective surgery for a symptomatic cam FAI deformity. Subchondral sclerosis is a recognized feature of OA and the high pre-operative BMD may be a bone remodeling response from abnormal contact. The subsequent post-operative decrease in BMD suggests remodeling under more normal joint contact mechanics. This change towards normal bone tissue is encouraging and supports the joint preserving surgical treatment of cam FAI deformities.