Conventional veterinary medicine views equine navicular disease as chronic forelimb lameness associated with pain originating from the distal sesamoid (navicular bone) and its closely related structures, including the distal impar ligament, collateral ligaments of the navicular bone, the navicular bursa, and the deep digital flexor tendon.
These collective structures are sometimes referred to as the navicular apparatus.
Equine Navicular disease is considered degenerative in nature, resulting in progressively worsening lameness.
Conventional veterinary medicine defines equine navicular disease as a single disease.
Given the variety of symptoms that manifest in lameness of the fore foot, however, it is likely that several different conditions, with different origins, are responsible for pain associated with the navicular region.
MRI evidence, as shown in the attached images, confirms that many other problems in horses cause the same clinical signs as those in horses diagnosed with navicular disease.
This has lead some researchers to question the term navicular disease, feeling that it no longer applies to many of the horses being evaluated for foot lameness problems.
Clearing Up the Confusion
Researchers have been unable to reproduce equine navicular disease in experiments, so they can only speculate about what causes it.
In fact, today’s veterinarian can subscribe to several theories on how the condition occurs and this will determine how your vet decides how to treat it.
One theory suggests vascular problems as the cause of equine navicular disease.
Researchers reportedly observed thrombosis (clotting) and arteriosclerosis (thickening arterial walls), leading to ischemia (insufficient blood supply) within the navicular bone in horses diagnosed with navicular disease.
This theory, however, has been largely rejected because of a failure to reproduce clinical signs or pathological changes when researchers reduce blood supply to the navicular bone of horses in clinical studies.
A second theory suggests bio-mechanical factors
This focuses on postmortem studies of horses with long-term chronic lameness and radiograph abnormalities, that suggest biomechanical factors may promote this degenerative disease.
Proponents of biomechanics as a cause define navicular disease as pathological changes of the soft tissue of the navicular apparatus; the navicular bursa and the articular cartilage of the joint. They propose that the pathological changes are the result of inflammation caused by vibration and friction.
In other words, this theory suggests that environmental influences can result in stress on the navicular area during movement.
Horses that work over hard surfaces, for example, experience excessive vibrations that result in changes to the mechanics of joint movement.
This leads to extreme compression of the navicular bone by the deep digital flexor tendon.
Toe first landing and foot imbalance are other examples of negative influences that can adversely affect the biomechanics of joint movement.