Running title: Association of femoroacetabular impingement with dGEMRIC THE ASSOCIATION OF FEMOROACETABULAR IMPINGEMENT AND DELAYED GADOLINIUM ENHANCED MRI OF CARTILAGE (DGEMRIC): A POPULATION-BASED STUDY

Objective: 1) To assess the association of FAI and dGEMRIC T1 relaxation values (RV). 2) To evaluate whether subtypes of FAI (cam, pincer, mixed) are associated with region-specific dGEMRIC T1 RVs. Methods: A population-based sample of Caucasian subjects with and without hip pain, aged 2049, was selected through random digit dialing. A sample of 128 subjects underwent hip joint 3T dGEMRIC scans. Radiographic cam FAI was defined as an alpha angle >55°, while pincer FAI was defined by a lateral center edge angle >40° or a positive cross-over sign. Mixed impingement was defined by the presence of both cam and pincer impingement. Overall and region-specific T1 RVs were compared between all FAI subtypes using weighted linear regression analysis to account for sampling design of the study. Results: Subjects had mean age of 38 years and 51% were female. We did not find an association of FAI with overall hip T1 RV (mean difference =-15.5, 95% CI -77.23, 47.14). Significant associations of cartilage degeneration in anterior superior and central superior regions were found in subjects with mixed FAI compared to other FAI subtypes and non-FAI subjects. Conclusion: Subjects with mixed FAI had reduced T1 RVs compared other FAI subtypes. No substantial cartilage degeneration was found in pure cam or pincer FAI compared to non-FAI hips. These results indicate that the presence of cam or pincer impingements alone does not suggest the beginning of cartilage degeneration. In contrast, the presence of both FAI subtypes is a risk factor for early cartilage damage. A cc ep te d A rt ic le This article is protected by copyright. All rights reserved. Significance and Innovations The presence of any FAI (cam, pincer or mixed) was not associated with T1 relaxation values on dGEMRIC for the hip as a whole in this young population-based cohort. In region-specific analyses, subjects with mixed FAI had reduced T1 relaxation values, i.e. worse cartilage scores, compared to those with no FAI and those with cam or pincer FAI alone in the anterosuperior and central superior regions, whereas pure cam and pure pincer FAI, compared to non-FAI, was not significantly associated with cartilage degeneration. These findings indicate a synergistic effect of cam and pincer FAI on cartilage degeneration in early hip disease. Femoroacetabular impingement (FAI) is a condition in which the proximal femoral head abuts against the acetabular rim, leading to damage to surrounding tissues and underlying cartilage (1). There is evidence that the presence of FAI strongly associates with early development of hip osteoarthritis (OA) (2-6). Cam and pincer impingement are the two main variations of FAI. Cam usually occurs in young active men (1, 7, 8) and is caused by an enlarged femoral head-neck junction, which abuts against the acetabular rim (9). This abnormal contact leads to delamination and damage of the labrum (10). Pincer impingement is more common in middleaged active women (1) and is caused by the over-coverage of the femoral head by the acetabulum. The continuous loading of the acetabular rim may result in chronic degenerative changes such as labral degeneration, intra-substance ganglion formation, ossification of acetabular rim, and deepening of the acetabulum (1, 11, 12). Cam and pincer morphology can occur as separate conditions, but, more commonly, occur as mixed cam and pincer impingement (1).