A Preliminary Study Into Meniscal Innervation in 2 Horses With Different Degrees of Chondropathy in the Femorotibial Joint

Osteoarthritis is a painful degenerative disease, sometimes responsible for the premature ending of the careers of sporting horses. In humans, meniscal neurogenesis has been suggested to contribute to pain in osteoarthritis. To the best of our knowledge, no study on this subject has been performed in equine species. The aim of this report was to compare the density of the nociceptive and sympathetic nerves in the menisci of two 11- and 13-year-old mares, one presenting a “low-chondropathy grade” in its stifle and the other a “high-chondropathy grade”. The cranial horns of the medial menisci, a commonly reported site of equine meniscal injuries, were harvested after a maximum of 48 hours postmortem. A macroscopic and microscopic grading system was established using a modified Pauli score. Nociceptive fibers were highlighted by immunohistochemical staining with an antibody against Substance P (SP) and postganglionic sympathetic fibers with an antibody against tyrosine hydroxylase (TH). Descriptive statistics were calculated, and a nonparametric statistical test was used to analyze the data. The meniscus from the “high-chondropathy horse (HCH)” showed a higher density and frequency of postganglionic sympathetic nerves compared with the meniscus of the “low-chondropathy horse (LCH)”. No difference was observed regarding nociceptive nerves. This report shows possible meniscal sympathetic efferent-mediated pain in a “highly chondropathic horse”. Although these results cannot be extended to the equine population as a whole because of the small sample size, they are nevertheless interesting as they provide new information regarding the possible etiology of the pain in equine stifle osteoarthritis.