Neurogenic inflammation in osteoarthritis of the knee

Abstract The synovium in a knee joint has an extensive neural network in the somatic and autonomic nervous systems. In medial compartmental osteoarthritis of the knee, neuropeptides were most abundant, with an especially large number of substance P and calcitonin gene-related peptide-immunoreactive free nerve endings. Some of the substance P-positive nerve endings were surrounded by monocytes. Substance P and calcitonin gene-related peptide were found more frequently in the medial than in the lateral or suprapatellar areas. Substance P-positive free nerve endings showed more dendritic morphologic features in the medial region than those in the lateral and suprapatellar regions, and small nerves were accompanied by newly developed vessels in synovial villi. In the medial region, the synovitis was more remarkable than in the lateral region. Patients suffering from medial compartmental osteoarthritis of the knee complain of pain on the anteromedial portion of the knee joint when walking or standing. Therefore, these findings suggest that free nerve endings containing substance P may be implicated in the development and persistence of inflammatory synovitis and the pain pathway in osteoarthritis of the knee.