PROJECT SUMMARY Femoroacetabular impingement syndrome (FAIS) is a common cause of hip pain and disability and a known risk factor for hip osteoarthritis (OA). Patients with FAIS present with a loss of sphericity of the femoral head, reduction in femoral-neck offset, and/or prominent acetabular wall. Patient symptoms as well as patterns of soft tissue damage to hip cartilage and labrum (i.e., ‘chondrolabral’) tissues support the clinical theory that hip pathoanatomy creates pathomechanics. Still, the presence of impingement deformities does not reliably predict symptoms, soft tissue damage, or OA status. Earlier detection of structural and compositional changes to cartilage and labrum would enhance clinical diagnostics and decision making for FAIS. Small, physiologic changes in the water content of cartilage and associated changes in proteoglycan content and collagen density – factors involved in early joint OA – can be evaluated by T1ρ and T2 mapping with quantitative MRI (qMRI). Here, we propose an ancillary R21 study to interrogate relationships between hip anatomy, biomechanics, and image-based measurements of cartilage and labrum ultrastructure provided by qMRI. We will incorporate qMRI into our existing imaging workflow for the Parent R01. We will then leverage the statistical shape models (SSM) and finite element (FE) modeling data provided by the parent R01 to study form-function relationships in FAIS through two aims. Aim 1 will investigate links between hip morphology and chondrolabral tissue ultrastructure by quantifying associations between spatial T1ρ and T2 maps of cartilage and labrum with a local measure of the shape of proximal femur and acetabular bone shape as evaluated by a 3D statistical shape model. Aim 2 will then investigate links between hip mechanics and chondrolabral tissue ultrastructure by quantifying associations between spatial T1ρ and T2 maps with a local prediction of the stress and strain in the cartilage and labrum as evaluated by patient-specific finite element models. We will test the hypothesis that regions of greater bone deformity and elevated chondrolabral stress and strain as predicted by SSM and FE, respectively, will coincide with regions of decreased proteoglycan content and loss of collagen organization, as estimated by qMRI. The ancillary R21 complements and leverages the parent R01 project in ways that will help us to identify mechanisms underpinning chondrolabral damage and hip OA in patients with FAIS. To our knowledge, this will be the first study to demonstrate the spatial correlation of T1ρ and T2 maps with 3D hip shape and chondrolabral stresses/strains. This information will provide more direct evidence linking hip anatomy and biomechanics with biochemical changes associated with OA. In the future, the developed framework could be expanded to develop imaging biomarkers that predict hips at risk of degeneration and to evaluate the longitudinal effects of operative or non-operative treatments for FAIS.

PROJECT NARRATIVE The results of this research will yield an improved understanding of how a common condition termed “femoroacetabular impingement syndrome” alters the shape and biomechanics of the hip and will determine if these data are associated with image-based measurements of the structure and composition of hip cartilage and labrum health provided by quantitative magnetic resonance imaging. Our findings should offer more direct evidence linking hip anatomy and biomechanics with biochemical changes associated with hip osteoarthritis.