What makes osteoarthritis painful? The evidence for local and central pain processing

OA is a chronic arthritic disease characterized by pain, local tissue damage and attempts at tissue repair. Historically, cartilage damage was believed to be the hallmark of OA. However, since cartilage is an avascular, aneural tissue, the mechanisms of pain are likely to be complex and influenced by non-cartilaginous structures in the joint including the synovium, bone and soft tissue. Imaging studies reveal the presence of synovitis and bone marrow lesions that may mediate pain. The presence of local joint inflammation and altered cartilage and bone turnover in OA implicates a potential role for a range of molecular mediators in OA pain. Mechanisms of pain perception may include the activation and release of local pro-inflammatory mediators such as prostaglandins and cytokines accompanied by the destruction of tissue, which is mediated by proteases. However, clinically, there is often disparity between the degree of pain perception and the extent of joint changes in subjects with OA. Such observations have prompted work to investigate the mechanisms of central pain perception in OA. Functional MRI has identified multiple areas of the brain that are involved in OA pain processing. These data demonstrate that pain perception in OA is complex in being influenced by local factors and activation of central pain-processing pathways. In this review, we will discuss current concepts underlying the pathophysiology of pain perception in OA and suggest possible directions for the future management of pain in this condition based on recent clinical studies.