A narrative overview of the current status of MRI of the hip and its relevance for osteoarthritis research – what we know, what has changed and where are we going?  M.D. Crema, G.J. Watts, A. Guermazi, Y.-J. Kim, R. Kijowski, F.W. Roemer  Osteoarthritis and Cartilage  Volume 25, Issue 1, Pages 1-13 (January 2017) DOI: 10.1016/j.joca.2016.08.015 Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 1 Flow chart of inclusion of relevant literature. Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 2 Evolution of MRI image quality over time. Comparison of image quality based on non-arthrographic acquisition on an old (A. and C. – acquired in 2005) and state-of-the-art 1.5 T MR system (B. and D. acquired in 2016). A. Coronal proton-density-weighted fat-suppressed image shows features of hip OA including acetabular and femoral BMLs (short white arrows) that are well depicted. In addition, there is a suggestion of focal cartilage damage at the central superior part of the joint (arrow) that remains not fully resolvable in regard to anatomical location (acetabular or femoral side affected) and extent (depth as well as area affected). B. Modern proton density-weighted fat suppressed image depicts multiple features of OA in a well delineable fashion. These include a large acetabular BML (arrowheads), focal cartilage damage of the superior-lateral acetabulum (black-filled arrow), large femoral osteophytes (white arrows) and joint effusion (asterisk). C. Old sagittal 3D image acquisition (multi-echo data image combination (MEDIC) sequence) of the same hip as in A. shows definite central superior cartilage damage (arrow). Extent and depth of lesion remains somewhat unclear. D. Modern 3D acquisition (true fast imaging with steady state precession (TrueFISP) sequence) clearly delineates a focal defect at the posterior aspect of the femoral head (arrow). Note that a definite differentiation of acetabular and femoral cartilage is possible. Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 3 Cartilage damage as depicted using modern 1.5 T MRI. A. Sagittal proton density-weighted image after intraarticular contrast administration shows focal cartilage defect at the weight bearing portions of the acetabulum and femoral head (arrow). B. The same defect is similarly depicted on this coronal fat suppressed T1-weighted image, but fat suppression enhances contrast between subchondral bone and intraarticular joint fluid that fills the cartilage defect (arrow). C. Sagittal 3D TrueFISP image shows diffuse cartilage thinning at the posterior aspects of the femoral head (arrows). D. Sagittal 3D TrueFISP shows diffuse cartilage loss with bone-to-bone contact at the anterior–superior aspects of the joint (long arrows). Note additional subchondral cystic lesions (short arrows), a feature of advanced OA. E. Sagittal 3D TrueFISP shows an example of focal partial thickness cartilage defect at the superior–posterior aspect of the femoral head is shown (arrow). F. Radial proton density-weighted image shows the same lesion is shown in this radial proton density-weighted image (arrow). Note that figures C–E are non-arthrographic images. Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 4 Acetabular labrum. A. Normal morphology of the acetabular labrum and physiologic attachment to the acetabulum as shown on this coronal arthrographic 1.5 T proton density-weighted fat suppressed image. The transition zone between the labrum and the hyaline cartilage is shown (long arrow). The labrum is surrounded by a small capsular recess (short arrow). Note the prominent femoral head-neck junction and delamination of the acetabular cartilage (arrowheads), findings that highly suggest underlying FAI, as well as the presence of neck synovial plica. B. At the posterior-inferior labral segment a small recess at the labral base may be observed as in this axial arthrographic 1.5T T1-weighted image, which is considered a normal variant (arrow). C. Coronal fat suppressed proton density-weighted image shows a small linear hyperintensity in the superior lateral acetabular labrum consistent with a tear (arrow). D. Another example of a labral tear is depicted on this coronal proton density-weighted image (arrows). Note additional labral hypertrophy and intralabral ganglion cyst. E. Proton density-weighted image (radial acquisition) shows anterior labral tear (arrow). Note large osteophytes at the femoral head-neck junction. Radial acquisitions increase sensitivity of tear detection in some cases. Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 5 Coronal 3D fat suppressed SPACE image shows a labral lesion superior-laterally (large arrow), focal cartilage damage at the superior femoral head (arrowhead) and subchondral BML consisting of diffuse, ill-defined (small arrows) and cystic portions. Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 6 MRI assessment of hip OA. A. Anterior–posterior radiograph of the left hip shows advanced OA. The X-ray image is only able to depict bony features such as osteophyte formation (arrows) and indirect signs of cartilage loss depicted as joint space narrowing (arrowhead). B. Corresponding proton-density-weighted fat suppressed MRI shows multiple features of the OA disease process that are occult to the radiographic evaluation such as joint effusion and synovitis (asterisk), acetabular and femoral BMLs (white arrows) and labral hypertrophy (black-filled arrow). All of these features are relevant for disease progression which makes MRI a much more powerful tool compared to radiography. C. Coronal fat-suppressed proton-density-weighted image illustrates subregional division in the HOAMS semiquantitative scoring instruments. The division into the different subregions is identical for BML, subchondral cyst and cartilage assessment with the addition that femoral regions are distinguished from acetabular regions for BML and cyst scoring. BMLs and subchondral cysts are assessed in 15 subregions and are evaluated by estimating the percentage involvement in the corresponding subregion (in the coronal plane the central-lateral (CLF), the central-superior (CSF), the central-central (CCF) the central-inferior (CIF) and the central-medial (CMF) femoral subregions are assessed. Three acetabular subregions are assessed in addition in the coronal plane: the central-superior (CSA), the central-central (CCA) and the central-inferior (CIA) subregions). Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 7 dGEMRIC performed in a 18-year-old athletic male with clinical features of FAI. A. Anteroposterior pelvic radiograph shows cam-type deformity at the superior and lateral femoral head-neck transition (arrow). Note the normal joint space width and the absence of marginal osteophytes at the right hip. B. Correspondent coronal proton density-weighted fat suppressed image of the right hip showing normal morphology of the articular cartilage and mild labral degeneration (arrow). C. Correspondent coronal dGEMRIC color map depicted a focal decreased dGEMRIC index represented by the focal dark line of the acetabular cartilage (arrow), indicating potential early cartilage degeneration. Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 8 Acetabular pseudodefect. The supraacetabular fossa is a normal variant at the 12 o'clock position of the acetabulum in 10% of individuals, particularly observed in young adults. It represents an approximately 5 mm large and up to 3 mm deep bony fossa, which may be entirely filled with cartilage or represent a fluid-filled groove of the joint surface. Frequently a sclerotic rim is present. The finding also has been termed “stellate lesion”. A. Coronal fat suppressed proton density-weighted image shows cartilage-filled supraacetabular fossa in the typical location (arrow). B. Another example of a cartilage-filled fossa is shown on this sagittal T1-weighted image (arrow). C. Proton density-weighted sagittal image shows fluid-filled acetabular fossa (arrow) that must not be mistaken as a traumatic or OA-related cartilage lesion. Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions

Fig. 9 FAI has been postulated to be one of the main risk factors for early hip OA. It is important to be aware that a cam-configuration of the femoral head-neck junction may be observed in healthy volunteers – particularly in men, and not all hips with this configuration will exhibit early degeneration. Which factors predict joint degeneration remains unknown, and prognosis on an individual patient level is challenging. A. Para-axial proton density-weighted fat suppressed image shows reduced offset of the femoral head-neck junction in a case of cam deformity (arrow). An increased alpha angle of 77° was determined on same mid-paraxial image. However, the relevance of angle measurements is a matter of controversy. B. Para-axial proton density-weighted image shows decreased asphericity of the anterior femoral head-neck junction characteristic of cam-deformity (arrow) in a patient with long-standing clinical signs of femoro-acetabular impingement (a former ice-hockey player). Note signs of definite OA with large femoral head-neck osteophyte posteriorly (arrowhead). C. Pincer impingement is defined by acetabular over-coverage of the femoral head. Coronal fat suppressed proton density-weighted image shows pincer configuration and signs of OA with subchondral BMLs (arrowhead) and medial femoral osteophyte (arrow). Osteoarthritis and Cartilage 2017 25, 1-13DOI: (10.1016/j.joca.2016.08.015) Copyright © 2016 Osteoarthritis Research Society International Terms and Conditions