Magnetic Resonance Imaging of Cartilage—Detecting Early Degeneration and Monitoring Post-operative Improvement

Osteoarthritis (OA) affects millions of people and is the primary cause for disability in the US.¹ OA is a heterogeneous and multifactorial disease characterized primarily by the progressive loss of hyaline articular cartilage. Ultimately, the articular cartilage degenerates with fibrillation, fissures, ulceration, and full thickness loss of the joint surface. The clinical symptoms are characterized by joint pain, tenderness, limitation of movement, and functional disability. Our understanding of degenerative cartilage disease is currently confounded by the lack of ability to easily visualize the state of cartilage morphology and biochemistry directly. The current clinical evaluation of cartilage degeneration in OA relies primarily on plain radiography, which depicts only gross osseous changes that occur late in the disease. Early changes in the cartilage and other articular tissues are not directly visible using X-rays. Cartilage loss can only be indirectly inferred by the development of joint-space narrowing, which can be highly unreliable even with careful attention to proper technique. In addition, plain radiographs are insensitive to focal cartilage loss, and widening of the joint space despite significant cartilage loss can occur in one compartment of the knee simply as a result of narrowing in the other compartment, biomechanical instability, or joint laxity.

Imaging methods that allow visualization of structural, biochemical, and functional changes in cartilage are essential for detection of OA. Because of its ability to produce contrast in all tissues of a joint, magnetic resonance imaging (MRI) is ideally suited for this purpose: MR offers multiplanar capabilities, high spatial resolution without ionizing radiation, and superior contrast between joint tissues.

Standard MRI techniques have been used to detect gross morphological changes in cartilage from a qualitative perspective. Recht et al. have shown 96% sensitivity and 95% specificity for detecting cartilage abnormalities visible in cadaveric knees following pathological section. ² Several studies have recently been published that grade cartilage lesions in subjects with OA and compare the severity of the lesions with other findings such as meniscal defects, the presence of marrow lesions, and radiographic and clinical scores.^2–5 Quantification of articular cartilage volume and thickness has also been investigated using high-resolution, fat-suppressed MRI sequences.

While the standard clinical MRI techniques allow for high diagnostic accuracy of joint diseases compared with X-rays, these techniques are still limited to detecting cartilage morphological changes that occur at a relatively late stage of degeneration. Early changes in cartilage diseases involve changes in the collagen-proteoglycan (PG) matrix of the tissue. Therefore, imaging techniques that can probe early biochemical changes— specifically, changes in water content, proteoglycan content, and collagen structure and content—are desirable for detecting early cartilage degeneration. T1ρ and T2 relaxation times in MRI, besides distinguishing gross tissue morphology, can probe the changes in tissue biochemistry and macromolecular content. Below, quantitative cartilage volume and thickness measurements, T2 mapping, T1ρ mapping, and delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) imaging, as well as the role of MRI in post-operative evaluation of cartilage repair, will be discussed.

Quantitative Magnetic Resonance Imaging to Measure Knee Cartilage Thickness and Volume
High-resolution MR images can be used to create a multislice data set, which allows for an approximate three-dimensional (3-D) visualization of the joint and the measurement cartilage volume and thickness. Cartilage thickness maps, which illustrate the regional variations in cartilage thickness, are helpful for visualizing focal differences in cartilage thickness (see Figure 1). Such measures might aid in the understanding of the physical and physiological factors that contribute to cartilage degeneration, as well as the factors that might be protective of the joint. The inter- and intraobserver reproducibility of these techniques range from approximately 1 to 9% and has been summarized by Eckstein et al.⁶

Cartilage volume and thickness changes in OA have been evaluated in longitudinal studies, with variable results. Gandy et al. found no changes in cartilage thickness and volume in OA,⁷ while other studies⁶ have shown an approximate loss of 4–6% of cartilage annually. It is interesting to note a large standard deviation in percentage of cartilage loss, which is indicative of the heterogeneity of the disease.

MRI studies have evaluated the relationship between changes in cartilage volume and other knee joint tissues in OA. Lindsey et al. and Blumenkrantz et al. have found that cartilage loss on one side of the knee joint is related to trabecular bone loss on the opposite side of the knee joint. ^8,9 Cartilage degeneration is also associated with changes in subchondral bone architecture, ¹⁰ as well as changes in trabecular bone proximal to the joint line.⁸ Other factors associated with cartilage loss include mensical damage and edema-like lesions in the bone marrow. ^11,12 A recent longitudinal study by Hunter et al.¹³ has demonstrated that enlarging bone marrow lesions are associated with cartilage loss in OA. Therefore, the measurement of cartilage volume using MRI provides longitudinal quantification of cartilage loss in OA and establishes links between cartilage loss and degenerative changes in other tissues of the knee joint. An inverse relationship between pain, as measured by the Western Ontario and McMaster Universities scoring system (WOMAC), and cartilage volume has been demonstrated. ^9,14-16