Bone Marrow Edema: Bone marrow edema refers to a MRI appearance and not to a pan-arthritis, specific pathology. In OA, there is relatively little information on what areas of bone marrow edema refer to, and often they contain areas of bone cysts. From the limited information available, these may represent areas of necrosis and trabecular remodeling. They are more frequent in areas with increased Kellgren-Lawrence (KL) grade. MRI also shows bone attrition, and osteophytes are seen with much greater ease and frequency with MRI than with radiographs.
Meniscal Pathology: Large cohort studies employing MRI have demonstrated a vast amount of meniscal problems.
Soft Tissue Pathology: Synovitis is frequently seen using MRI. Ligament abnormalities, especially of the medial and lateral ligaments, are also commonly present and—surprisingly—abnormalities of the anterior cruciate ligament are quite common in large cohorts despite no history of significant knee injury. MRI can also demonstrate foreign bodies (although these are rarely of clinical significance).
MRI in OA Research
The wonderful sensitivity of MRI means we are able to phenotype OA better than before. We now need to understand the relative importance of the different pathological processes within the OA joint; this information is accruing from large clinical cohorts such as the NIH Osteoarthritis Initiative. Structure-pain studies will help us understand which features are most related to pain, allowing subsequent targeting of novel and existing therapies.
To date, the most work quantifying pathology has been on evaluating cartilage. Most cartilage measurement work employs image analysis with some element of manual alignment before automatic segmentation can occur. These systems now demonstrate good precision. In general, cartilage volume measurements are easier to obtain in flat areas such as the tibial plateau or central region of the femur, rather than in curved surfaces such as at the hip. Measurable cartilage parameters include total volume of cartilage, cartilage thickness, and cartilage signal intensity. The rate of cartilage loss is very important for studies that will examine the chondroprotective nature of drugs. Early studies suggest that the rate of cartilage loss is approximately 4% per year, but rates of loss may be lower than this in, for example, normally aligned knees.8 Higher rates may be obtained in studies by including subjects who are at risk of more rapid progression, such as those with higher KL grade or obesity. Some studies suggest that cartilage thickness may be a more important measure than cartilage volume over time.
