Acetabular Bone Density Decreases Following Surgical Correction of FAI Deformities

Abstract

INTRODUCTION: Femoroacetabular impingement (FAI) is caused by deformities of the hip joint which may result in repeated abnormal contact between the femur and acetabular rim and labrum. FAI is thought to be the leading cause of adult idiopathic hip osteoarthritis (OA). Increased acetabular subchondral bone density was found not only in symptomatic subjects with the more common cam-type deformity, but also in subjects with cam deformities but no symptoms[l]. These asymptomatic subjects may be in early stages of degeneration, and increased bone density due to abnormal hip contact and bone remodeling may play an important role in the pathogensis of hip OA. SPECT scans previously showed radiotracer uptake in the acetabular rim of subjects with cam deformities[2]. 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. We hypothesized that BMD would decrease in the superior acetabular rim where impingement typically occurs. METHOD(S): Twenty patients undergoing surgical correction of a symptomatic cam deformity were recruited. Prior to surgery and two years after surgery, bilateral CT scans were performed on patients from the iliac crest to the lesser trochanter. Each patient was scanned with a CT calibration phantom that allowed conversion of greyscale voxel values to bone mineral density. Image slice thickness was 0.625 mm and in-plane resolution was 0.8 to 0.9 mm depending on the size of the patient. Three dimensional image volumes were reconstructed using the manufacturer's bone window algorithm. Pre-operative CT images were automatically segmented[3] to create surface models of the acetabular region. These models were divided into 6 pie-shaped wedges around the superior acetabulum based on pelvis landmarks, and included a depth of 10 mm of bone from the acetabular subchondral surface (Figure 1). A tetrahedral mesh was then created in each wedge to define sampling points in the CT image. The CT intensity was measured at these points and 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, a rigid registration was performed between the two CT scans of each patient to determine the 3D transform. This was used to transform the mesh to align with the follow-up CT scan and the sampling and BMD calculation repeated. 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. RESULTS SECTION: To date, eleven patients have undergone follow-up CT scan and were included in the current study. BMD decreased from -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, 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, ranging from -2.0 to -36 g/cc but were not significant (p=0.053 to 0.81). DISCUSSION: 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. The higher pre-operative BMD, especially in the superior rim, in these subjects may be due to a strain-adaptive bone remodeling response from abnormal contact, and the subsequent post-operative BMD decrease results from remodeling under more normal joint contact mechanics. The BMD changes found in this study were lower than the 20-40% differences between the same subjects and controls [1]. It is not known whether further changes will occur in the future or whether a new baseline BMD has been established. However the decrease in BMD is encouraging and suggests the joint may recover. Longer-term clinical follow-up is required to establish the efficacy of surgical treatment of FAI. SIGNIFICANCE: Subchondral sclerosis is a recognized feature of OA. A decrease in BMD post-operatively suggests that the subchondral bone is remodeling towards normal tissue and that the hip joint may be recovering. This study supports the joint preserving surgical treatment of cam FAI deformities.