Equine myodural bridges - An Anatomical and integrative/functional description of myodural bridges along the spine of horses: Special focus on the atlanto-occipital and atlanto-axial regions.

Abstract

Introduction: Socalled myodural bridges (MDB) linking the suboccipital muscles to the dura mater have been described in the human, canine, small ruminants, monkeys, rodents, porpoises, crocodiles, sperm whales, chickens and lately in equines. They are believed to have biomechanical functions and might also play a role in head/neck pathology and the pumping function of the cerebrospinal fluid. Up to now these bridges have only been described briefly in the horse in terms of their anatomy, and then only in relation to Ehlers-Danlos syndrome. The aim of this study was therefore to investigate and thoroughly describe the anatomy, biomechanics and integration of this complex throughout the equine spine, with special focus on the upper neck, the cervicothoracic- and the lumbosacral transitions. Pathology in these regions is well recognized in horses.

Material and methods: Horses were dissected, the heads and spine prepared in several anatomical planes, the heads MR-scanned and histology performed on the AO and AA MDBs.

Results: Gross anatomical observations showed that muscle-membrane-spinal dura mater connections (MDB) were evident in the full equine columna vertebralis, and were specifically developed in the upper cervical, the cervicothoracic and the lumbosacral transitions. In the upper cervical region, the m. rectus capitis minor and major and the m. obl. cap caudalis attached tightly to the dorsal intervertebral AO and AA membranes. On the ventral membrane surface there was a trabecular connection to dura mater. The two membranes differed markedly in the amount of elastic fibers giving them different biomechanical function. The structures of the AO MDB were evident on the MRI scans.

Conclusions: Horses, like humans, other mammals, a reptile and a bird to date, have myodural bridges, which are tightly integrated with surrounding structures as well as the biomechanics of the upper neck. In addition, similar structures are present throughout the whole spine.