Abstract
Purpose
Femoroacetabular impingement (FAI) may contribute to the development of early onset hip osteoarthritis (OA). A cam lesion (or pistol grip deformity) of the proximal femur reduces head-neck offset resulting in cam type FAI. The alpha angle is a radiographic measurement recommended for diagnosis of cam type FAI. The purpose of this study was to determine if patients that develop end stage hip OA prior to 55 years of age have radiographic evidence of cam type FAI.
Method
The anteroposterior (AP) pelvis and lateral hip radiographs of 244 patients (261 hips) who presented to our institution for hip arthroplasty or hip fracture fixation between 2006 and 2008 were retrospectively reviewed. Three cohorts were compared: 1) patients with end stage hip OA < 55 years old (N=76); 2) patients with end stage hip OA > 55 years old (N=84); 3) hip fracture patients > 65 years old without radiographic evidence of hip arthritis were used as controls (N=101). Patients with inflammatory arthritis, avascular necrosis and post-traumatic hip OA were excluded. Alpha angles were measured on the AP pelvis and lateral radiographs by three coauthors using ImageJ 1.43 software (National Institutes of Health, USA). For patients with end stage hip OA, AP alpha angles were measured on both the hip with OA and the contralateral hip. Lateral alpha angles were measured only on the hip with OA. For patients with hip fracture, AP alpha angles were measured on the non-fractured hip and lateral alpha angles were measured on the fractured hip. A one-way ANOVA with post hoc Tukeys HSD test was used to compare the AP and lateral alpha angles for the three cohorts.
Results
The intraclass correlation coefficient (ICC) for the three coauthors measuring AP and lateral alpha angles was 0.85 and 0.86 respectively, indicating excellent inter-rater agreement. Patients < 55 years old with end stage hip OA had the largest AP and lateral alpha angles (82.711.6 degrees AP and 63.918.5 degrees lateral). These angles were significantly larger (p<0.01 for both comparisons) than patients > 55 years old with end stage hip OA (71.717.8 degrees AP and 55.518.0 degrees lateral) and hip fracture patients without hip OA (52.710.9 degrees AP and 44.411.4 degrees lateral). Comparing AP alpha angles of the contralateral hips, the mean AP alpha angle for patients < 55 years old with hip OA (70.813.2 degrees) was significantly larger (p=0.04) than patients > 55 years old with hip OA (64.516.2 degrees) which in turn was significantly larger (p<0.01) than the hip fracture patients (52.710.9 degrees).
Conclusion
Patients < 55 years old with hip OA had the largest mean AP and lateral alpha angles, significantly larger than patients > 55 years old with hip OA and hip fracture patients without hip OA. Thus young patients with end stage hip OA do have radiographic evidence of cam type FAI. Furthermore, this case-controlled study suggests that cam type FAI may contribute to the development of early onset hip OA.
