Structure and Innervation of the Equine Supraspinous and Interspinous Ligaments.

This research investigated the detailed anatomy and nerve supply of the equine supraspinous and interspinous ligaments (SSL and ISL), as well as their myofascial attachments and nearby tissues. The study found a complex structure in these ligaments and a substantial nerve supply which could contribute to the severe backpain experienced by horses.

Anatomy of the SSL and ISL

• The researchers dissected ten equine thoracolumbar spines and assessed the structure and anatomy of the SSL, ISL, adjacent myofascial attachments, connective tissue and musculature.
• The findings showed that the SSL and ISL tissues merge with the adjacent myofascia, a kind of connective tissue that surrounds muscles.
• The ISL was found to consist of four ligamentous layers, with an arrangement of crossing fibres and a central portion of adipose tissue, or body fat.

Innervation of the Ligaments and Myofascial Attachments

• Morphological assessment was carried out through histology (the study of microscopic anatomy of cells and tissues), electron microscopy (a method for obtaining high resolution images) and immunohistochemistry (a process for identifying cells in tissues).
• A substantial number of single nerve fibres were detected in SSL (mean = 2.08 fibres/mm) and ISL (mean = 0.75 fibres/mm), with the larger nerves located between the ligamentous and muscular tissue.

Implications of Findings

• The complex and criss-cross fibre organization in the ISL is thought to function in resisting distractive and rotating forces, thus stabilizing the equine thoracolumbar spine.
• The dense sensory innervation found within the SSL and ISL provides an explanation for the severe pain experienced by some horses with impinging dorsal spinous processes, a common veterinary concern where the bony projections along a horse’s spine come too close together or touch.
• The detailed documentation of the nerve supply of the soft tissues connected to the dorsal spinous processes contributes to our understanding of back pain in horses and could guide future strategies for treatment and prevention.