What do we see in Imaging after different Cartilage Procedures?
What do we see in Imaging after different Cartilage Procedures?
Ali Guermazi, MD, PhD
Boston University School of Medicine, Boston/MA, USA
Since 1994, when the first autologous chondrocyte transplantation surgery was described by Brittberg and colleagues, knee cartilage repair surgery has evolved rapidly thanks to many factors including improvement of pre-surgical assessment, imaging techniques, increased availability of matrix products including both fresh and frozen allografts, and focused research on the clinical outcomes. Cartilage repair surgery is aimed to alleviate patient symptoms, to promote cartilage healing, and to prevent or delay the onset of osteoarthritis. There are still a variety of barriers (including cost, regulatory, insurance, and logistical issues) between new cartilage repair products/techniques and their routine clinical applications. However, over the recent years there have been significant advances in our scientific knowledge in regards to cartilage repair techniques and imaging methods for evaluating post-operative repair status. Different MRI techniques are available to assess post-operative cartilage. Conventional, morphological MRI sequences include 2-dimensional (2D) and 3-dimensional (3D) fast spin echo (FSE) sequences provide excellent signal to noise ratio, contrast between tissues, and faster acquisition times. 2D-FSE is the core imaging technique and is part of the cartilage imaging protocol recommended by International Cartilage Repair Society. Isotropic, or near isotropic, 3D sequences can produce higher spatial resolution and high quality reformatted images in any plane, and are thus advantageous over 2D-FSE for shorter image acquisition time. Gradient echo type sequences such as SPGR, DESS are excellent for cartilage segmentation and quantification of cartilage volume and thickness due to the good image contrast between cartilage and subchondral bone, but not ideal for focal cartilage defect evaluation. There are semiquantitative MRI scoring tools for assessment of post-operative cartilage after repair surgery. One is called MRI Observation of Cartilage Repair Tissue (MOCART) and its usefulness in randomized controlled clinical trials of autologous cartilage tissue implants has been demonstrated. MOCART is comprehensive for assessment of the repair site itself; however, assessment of the other structures of the joint is paramount to assess longitudinal outcomes and development of osteoarthritis. The cartilage repair OA knee score (CROAKS) combines features of MOCART and the MRI osteoarthritis knee score (MOAKS), which is an established semi-quantitative scoring system for whole organ assessment of the knee, to provide a comprehensive, reproducible tool for longitudinal postoperative assessment after surgical cartilage. Compositional MRI acquisitions provide a way to detect biochemical and microstructural changes in the cartilage extracellular matrix even before gross morphological changes occur. Although not in routine clinical use, these techniques have been used extensively in cartilage research. Compositional MRI can supplement morphologic imaging, by potentially defining the biomechanical quality of cartilage repair tissue. Available surgical cartilage repair techniques include microfracture/marrow stimulation, osteochondral autograft/allograft (OATS) transplantation, particulate cartilage allograft, autologous chondrocyte implantation (ACI), open reduction and internal fixation of a large osteochondral lesion, and femoral condyle transplantation. All cartilage repair techniques have the same primary goal; to decrease pain symptoms, improve mobility and function, and to prevent the progression of osteoarthritis. These cartilage repair surgery techniques have shown to improve functional outcomes, however, there is urgent need to define outcomes clinically and by MRI measurements including local assessment and in regard to long-term osteoarthritis development/progression. In clinical practice, the MRI assessment of repair tissue relies heavily on morphologic imaging. Compositional MRI provides the opportunity to measure the biochemical and microstructural time-dependent processes of maturation occurring within the cartilage repair tissue. Compositional MRI techniques hold great promise for the clinical determination of surgical success, although such techniques are still limited for research use. Before they can become routinely used in clinical practice, however, compositional MRI techniques needs to be standardized and validated for post-operative cartilage repair tissue evaluation and made time efficient. The combination of MRI–based morphologic and compositional imaging plays a key role in post-operative assessment of cartilage repair tissue and its integration to native tissues.