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

The innervation of the navicular bone (os sesamoideum distale) and its suspensory ligaments (ligamenta sesamoidea collateralia) (CSL) or proximal suspensory ligament and the ligamentum sesamoideum distale impar or the distal sesamoidean impar ligament (DS-impar ligament) was examined using combined anatomical techniques of silver impregnation and immunocytochemistry. Silver impregnation studies revealed an abundance of nerve fibres present in both the CSL and DS-impar ligament with the latter having relatively more nerve fibres. These silver-impregnated nerves coursed parallel to and were associated with the vasculature rather than appearing to innervate the vessels. Immunocytochemistry identified several sensory-related neuropeptides (calcitonin gene-related peptide (CGRP), substance P (SP) and neurokinin A (NKA)) in the nerves of the navicular bone and suspensory ligaments. More peptidergic nerves were evident within the synovial membrane and loose connective tissue in the dorsal part than in the palmar aspect of the CSL. In the CSL along the synovial membrane bordering the distal interphalangeal joint, the CGRP, SP and NKA were present in the nerves of vessels as well as the intimal layer of the distal interphalangeal joint. In the DS-impar ligament, there were many more nerves innervating vessels and the synovial membrane between the navicular bone and the third phalanx than were present in these structures in the CSL. Nerves with all 3 peptides entered the navicular bone via the proximal border and the distal groove to innervate the perichondrium, trabeculae and osteons. SP-like nerves also innervated the cortical bone underlying the articular cartilage. We suggest that these sensory nerve peptides contribute to the pathology of the navicular syndrome. The distribution of the nerves in the CSL and the DS-impar ligament could explain the clinical effects of local anaesthetics injected into the distal interphalangeal joint.