A silver-impregnation and immunocytochemical study of innervation of the distal sesamoid bone and its suspensory ligaments in the horse.

The research investigates the arrangement and structure of nerve fibers in the navicular bone and its supporting ligaments of a horse, using two scientific methods. The study narrows down on the implication of these nerves in the manifestation of a common horse hoof ailment known as navicular syndrome.

Methods and Observations

• The study employed a blend of silver impregnation and immunocytochemistry as the investigation techniques. This combined method was geared toward determining the network, arrangement, and reach of nerve fibers in the particular bone and ligaments of the horse’s hoof.
• An intriguing finding was the existence of abundant nerve fibers in both the Collateral Sesamoidean Ligaments (CSL) and the distal sesamoidean impar ligament (DS-impar ligament), with the latter having relatively more nerve fibers.
• The nerve fibers stained with silver were not directly innervating the blood vessels but ran parallel to them.
• Immunocytochemistry was instrumental in pointing out several sensory-related neuropeptides – mainly calcitonin gene-related peptide (CGRP), substance P (SP), and neurokinin A (NKA) – within the nerves of these anatomical structures.

Findings and Implications

• The study demonstrated a disproportionately high presence of peptidergic nerves within the synovial membrane and loose connective tissue on the dorsal aspect compared to the palmar aspect of the CSL.
• Nerve fibers possessing all three types of peptides extended into the navicular bone and were observed in areas such as the perichondrium, trabeculae, osteons, and even the cortical bone underlying the articular cartilage.
• In the context of the distal sesamoidean ligament, more nerve fibers could be observed to have innervated the blood vessels and synovial membrane placed between the navicular bone and the third phalanx than those present in counterpart structures within the CSL.
• The researchers suggest that these sensory nerve peptides may play a significant role in the onset and advancement of navicular syndrome, a painful condition affecting the horse’s hoof.
• The study also proposes a possible explanation for the localized numbness effect experienced when local anesthetics are administered into the distal interphalangeal joint. This could be due to the specific distribution of nerves in the CSL and the DS-impar ligament that the study outlines.