Synovitis of the knee

Etymologically, the word synovitis generally refers to acute or chronic inflammation of the synovial membrane. Today, however, the term is used to refer to a disease process (not necessarily inflammatory) that may originate from the synovial membrane and involve the structures of the joint. Synovitis can have different causes: traumatic, autoimmune, infectious, dysmetabolic or neoplastic, and it is typically characterised by the presence of varying degrees of swelling, pain, redness, heat and functional impairment. The knee is a frequent target of this disease, and it is the joint most likely to be affected by special forms of synovitis such as pigmented villonodular synovitis and synovial chondromatosis. Inflammatory and degenerative joint diseases are usually diagnosed on the basis of clinical and imaging data. However, these data are not always sufficient, especially in the event of a monoarticular onset; in such cases, the use of histopathology and a synovitis score will increase the diagnostic accuracy. There exist various histological scoring systems for synovitis, and the one proposed by Krenn, which is essentially applicable to all forms of synovitis, is the most commonly used. It is a numerical scoring system based on semi-quantitative grading of three key features of synovitis: enlargement of the lining cell layer, activation of stromal cells (as shown by the cellular density of the synovial stroma) and leukocytic infiltration. Each of these three components is assigned a score from 0 to 3, resulting in an overall score of between 0 and 9. Through the analysis of numerous large samples, mean Krenn scores have been established for normality (1.0), post-traumatic arthritis (2.0), osteoarthritis (2.0), psoriatic arthritis (3.5), reactive arthritis (5.0) and rheumatoid arthritis (5.0) [15]. A strong correlation has since emerged between the synovitis score, immunohistochemistry findings (Ki-67, CD68) and the clinical severity of the disease [7, 26]. The use of the synovitis score has proved to be useful in defining forms lacking the typical histological features, allowing them to be distinguished on the basis of the intensity and severity of the synovial inflammation.