Paper # 40: Relationship Between the Presence of a Rim Fragment and Intra-Articular Pathology in Patients with Femoroacetabular Impingement

Abstract

This study evaluated the presence of a rim fragment in pre-operative radiographs of FAI patients, and found that hips with a rim fragment were 3.7 times less likely to have femoral head's chondral defects compared to the hips without a rim fragment. Femoroacetabular impingement (FAI) has been recognized as a common cause of hip pain. Plain radiograph has been used to identify cam and pincer type of FAI. An acetabular rim fragment is frequently found in radiographs of FAI patients. The purpose of this study was to analyze the relationship between the presence of a rim fragment and intra-articular pathology in patients with FAI. Preoperative radiographs, an anteroposterior view of the pelvis and a lateral cross-table view of the affected hip, of 86 hips of FAI patients who underwent primary hip arthroscopy were reviewed by a surgeon blinded to the patients' symptoms and intraoperative findings. An acetabular rim fragment was identified in 15 hips (17 %) with a median size of 2.5 mm (1.0 to 20.7 mm). There were no significant differences in radiographic measurements of minimum joint space, alpha angle, center edge angle, and Sharp angle between the patients with a rim fragment and those without a rim fragment. There was no association between the presence of an acetabular chondral defect and the presence of a rim fragment (p=0.579). Sixty-five percent of hips without a rim fragment had a chondral defect on the femoral head while 33% of hips with a rim fragment had it (p=0.025). Hips with no rim fragment were 3.7 (95CI 1.2 – 11.5) times more likely to have a chondral defect on the femoral head compared to those with a rim fragment. Hips with a rim fragment were less likely to have femoral head's chondral defects compared to the hips without a rim fragment. The presence of a rim fragment is an adaptive response to abnormal shearing forces in FAI, acting as a “natural rim trimming”.