Histological examination of the interspinous ligament in horses with overriding spinous processes.

The equine interspinous ligament (ISL) consists of an oblique crossing arrangement of collagenous bundles which are thought to counteract the tensile and rotational forces of distraction between the spinous processes (SPs) in the caudal thoracic and cranial lumbar spine. The aim of this controlled histological study was to assess the structural anatomy and innervation of the ISL in horses with clinically significant overriding (dorsal) SPs (ORSPs) and to compare the findings with the ISL of normal horses. Samples of the ISL were obtained from 10 horses that underwent subtotal ostectomy for treatment of ORSPs. Control samples were obtained from horses without spinal pathology. Histological staining of ISL sections with haematoxylin and eosin was performed to assess the morphology of the ligaments and with Alcian blue-periodic acid-Schiff to determine the proteoglycan and glycosaminoglycan content. Immunohistochemistry for S100 was performed for quantitative evaluation of nerves within the ISL. The ISL in horses with ORSPs had an altered collagen fibre alignment and arrangement of the ligamentous layers when compared to healthy controls. A significant increase in fibrocartilaginous tissue with evidence of fibrocartilaginous metaplasia was detected (P=0.001). The number of nerves in the ISL samples was significantly higher in horses with ORSPs than in controls (P=0.017). Structural alterations of the ISL, including loss of fibre alignment and fibrocartilaginous metaplasia, are associated with ORSPs in the equine thoracolumbar spine. In addition, an increase in innervation of the ISL in horses with ORSPs, compared to normal, may explain the thoracolumbar pain experienced by some horses with ORSPs.

Structure and Innervation of the Equine Supraspinous and Interspinous Ligaments.

Pain related to the osseous thoracolumbar spine is common in the equine athlete, with minimal information available regarding soft tissue pathology. The aims of this study were to describe the anatomy of the equine SSL and ISL (supraspinous and interspinous ligaments) in detail and to assess the innervation of the ligaments and their myofascial attachments including the thoracolumbar fascia. Ten equine thoracolumbar spines (T15-L1) were dissected to define structure and anatomy of the SSL, ISL and adjacent myofascial attachments. Morphological evaluation included histology, electron microscopy and immunohistochemistry (S100 and Substance P) of the SSL, ISL, adjacent fascial attachments, connective tissue and musculature. The anatomical study demonstrated that the SSL and ISL tissues merge with the adjacent myofascia. The ISL has a crossing fibre arrangement consisting of four ligamentous layers with adipose tissue axially. A high proportion of single nerve fibres were detected in the SSL (mean = 2.08 fibres/mm2 ) and ISL (mean = 0.75 fibres/mm2 ), with the larger nerves located between the ligamentous and muscular tissue. The oblique crossing arrangement of the fibres of the ISL likely functions to resist distractive and rotational forces, therefore stabilizing the equine thoracolumbar spine. The dense sensory innervation within the SSL and ISL could explain the severe pain experienced by some horses with impinging dorsal spinous processes. Documentation of the nervous supply of the soft tissues associated with the dorsal spinous processes is a key step towards improving our understanding of equine back pain.