Structure and Innervation of the Equine Supraspinous and Interspinous Ligaments

Abstract

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/mm ) and ISL (mean = 0.75 fibres/mm ), 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.

Long‐term outcome and effect of diagnostic analgesia in horses undergoing interspinous ligament desmotomy for overriding dorsal spinous processes

“…Therefore, improvement after infiltration of local anesthetic in the region of the dorsal ramus would logically predict results of surgical transection. 21,32,35 Positive response to diagnostic analgesia or local medication did not improve long-term outcomes postsurgically in a study by Coomer et al 24 In that Coomer et al 24 study, local medication and diagnostic analgesia were combined into one independent variable, potentially mitigating the long-term effects of diagnostic analgesia on long-term outcome. Although types of medications used for local injection were not enumerated, it is likely that a large proportion involved local injection of corticosteroids, with injection into or diffusion to the epaxial musculature.…”

“…Diagnostic analgesia of the DSP region seeks to anesthetize the nerves innervating the bony and soft tissue structures of this region, therefore eliminating pain and, subsequently, clinical signs associated with pain. Although the origin of pain attributable to ORDSP is currently unknown, there is evidence that pain originates from inflammation and damage to the interspinous ligament itself rather than from associated bony pathology . A recent study in which histologic changes in the interspinous ligament in horses with ORDSP was evaluated provided evidence of loss of integrity of the interspinous ligament along with fibrous metaplasia .…”

“…Although no postmortem studies have been performed documenting the anatomical or pathophysiological consequences of ISLD, one potential consequence of this procedure is neurectomy of the branches of the dorsal ramus of the spinal nerve root for management of pain. Therefore, improvement after infiltration of local anesthetic in the region of the dorsal ramus would logically predict results of surgical transection …”

“…Although the origin of pain attributable to ORDSP is currently unknown, there is evidence that pain originates from inflammation and damage to the interspinous ligament itself rather than from associated bony pathology. 32,33 A recent study in which histologic changes in the interspinous ligament in horses with ORDSP was evaluated provided evidence of loss of integrity of the interspinous ligament along with fibrous metaplasia. 33 In addition, there was a significant increase in small nerves within the interspinous ligaments of horses with ORDSP compared with normal horses.…”

Novel dissection approach of equine back muscles: new advances in anatomy and topography - and comparison to present literature

“…Knowledge of the anatomy and topography of the equine back is essential for optimal examination and treatment of back problems in horses. The literature comprises of divergent information about the anatomy of muscles, especially m. iliocostalis (IL), which is positioned and outlined markedly differently by a number of authors [1][2][3][4][5][6][7][8][9][10][11].…”