Navicular syndrome: An entire foot problem

Abstract

Most equine practitioners specializing in equine podiatry are familiar with the research of Robert M. Bowker, VMD, PhD, at Michigan State University. He is an anatomy professor who has come up with some revolutionary new ideas about the feet of horses based on his careful study of the feet of wild horses versus the feet of typical domestic horses. Last year Dr. Bowker explained his ideas on navicular syndrome at the Veterinary Surgical Forum in Chicago, IL, describing the navicular syndrome as an entire foot problem. This is because the foot and its tissues have continually adjusted to the stresses within the environment before the onset of clinical signs associated with the lameness condition he said. “In considering the actual source of pain in the navicular syndrome, little regard has ever been given to such areas as the distal phalanx, insertion sites of the deep digital flexor tendon (DDFT), the distal sesamoidean impar ligament (DSIL) and the soft tissues between the DDFT and the middle third of the frog,” he said. In his opinion, perineural and intraarticular anesthesia are too general in their coverage to provide specific diagnostic information within the equine foot. He suspects that the navicular syndrome, as it is known today, involves other structures in the foot besides the navicular bone. Thus, he refers to the navicular syndrome as “an entire foot problem.” In his research, as an anatomist, he has discovered that as the various tissues of the foot respond to the continual stress of the horse’s particular environment, the histologic changes can be documented with changes in staining post mortem. Dr. Bowker states, “These adaptive responses are considered to be normal and may not be pathological even though they are very different in terms of staining and tissue composition from that described previously in many published reports.” Although histologic staining is no help in diagnosing lameness in the live horse, it supports Dr. Bowker’s theory that navicular syndrome involves more than the navicular bone. He believes the so-called navicular syndrome may be caused by pathology in 1 area of the foot in 1 horse and quite another area in another horse. He has carefully studied the DSIL in normal and so-called navicular horses. He sees significant changes in the DSIL and in its attachment to the third phalanx at its intersection with the ttachment of the DDFT. He believes that in this area important initial changes take place in the pathogenesis of the navicular syndrome. “The microanatomy of this region appears to represent a complex anatomical ‘bottleneck’ as many sensory nerves and microvessels pass through this region to other foot regions including the dorsal hoof wall, the venous drainage of the lateral cartilages and the blood supply to the navicular bone,” he said. “The sensory nerves include the thinly myelinated peptides substance P, neurokinin A, calitonin gene-related peptide, to name a few, while many small vessels, including arteriovenous complexes are present within the depta of the DSIL and the dorsal half of the DDFT.” “Tachykinin receptors are present on the microvessels and the arteriovenous complexes of the intersection. These sensory peptides when released from the sensory nerve terminal have a vasodilation function to maintain vascular perfusion during high pressure changes during locomotion or to promote proper hydration status of the DSIL and DDFT,” explained Dr. Bowker. “Proper hydration of ligaments and tendons is necessary for them to function adequately.” In his continuing studies, Dr. Bowker examines horse feet with navicular syndrome and finds not only the previously described changes in the navicular bone but also changes in the intersection (distal attachment of the DSIL and DDFT). In this location, the microvasculature has been destroyed along with the tachykinin receptors. He postulates that these changes lead to other changes in the foot yet to be described. Copyright 2002, Elsevier Science (USA). All rights reserved. 0737-0806/02/2206-0010$35.00/0 doi:10.1053/jevs.2002.35604