Although navicular disease has been recognized for a long time, there’s still no consensus as to what actually causes it. This has led to a wide variety of treatments being suggested, from blood thinners to drugs which inhibit the resorption of bone, none of which has really been adequately studied except for the pain-relieving effects of nonsteroidal anti-inflammatory drugs.
It’s clear from both new MRI information and histological studies of the structures in the hind foot that pathology of the navicular bone itself isn’t the only possible cause of pain.
Changes involving the deep digital flexor tendon, navicular bursa or joint cartilage on the navicular bone have all been found and are all potentially painful. However, the bone itself is the focus of anticoagulants/circulation enhancers and drugs used in people for osteoporosis.
A study from University of Wisconsin-Madison in the Journal of Anatomy looks at the navicular bones from young unshod ponies, young racing Thoroughbreds and older horses with an established diagnosis of navicular disease/syndrome based on bone remodeling as would be seen on X-rays.
The major difference between the young ponies and the horses in training was changes in the orientation of the collagen and density of the bone, which reflected greater strength developing from medial to lateral.
In navicular disease, they found cysts within the bone, irregularity at the joint surface, microcracks, lower numbers of osteocytes (cells that both build and resorb bone) but no increase in resorption spaces.
The cyst formation correlates with with the enhanced ”fluid” signal seen on MRI scans. They also noted that horses with these navicular changes that were actually lame were likely to have changes involving the impar ligament (connects the bottom of the navicular bone to the coffin bone) and/or the navicular bursa.
We still have a lot to learn about navicular disease, but it’s becoming increasingly clear that X-rays alone don’t tell the whole story, especially in terms of pain, and future treatment plans are going to have to take into account all the structures in this complicated area and how they interact. A miracle cure in the form of a pill or shot isn’t likely for navicular. The best advice for prevention is still careful hoof balance and trimming.