Differences in the axon composition of nerves supplying the rat knee joint following intra-articular injection of capsaicin.

This study was used to examine the ultrastructure of articular nerves supplying the medial aspect of the rat knee joint following injection of capsaicin into the synovial cavity. One week after the intra-articular injection of 0.2 ml of 1% capsaicin solution there was significant reduction in the number of unmyelinated fibres. Two weeks post-injection this appeared to recover and by 4 weeks post-injection no further changes were observed. Myelinated fibres were unaffected by capsaicin injection. This procedure provides an effective means of producing short-term degeneration of unmyelinated fibres innervating the knee joint.