Navicular. Just saying the word during a lameness exam can strike fear in a horse owner’s heart. Although navicular syndrome is a progressive degenerative disease with no cure, increased knowledge about the navicular bone and its related structures has led to improved management options and outcomes for horses. Here are 12 important things to know about this hoof lameness.
1. Its anatomy is complex.
The navicular bone is a small but complex structure located behind the third phalanx (also known as the P3, or coffin bone) and under the second phalanx (P2, or short pastern bone). It provides a fulcrum for the deep digital flexor tendon (DDFT) that runs down the back of the leg and into the back of the foot. Several ligaments, such as the impar (distal sesamoidean) and collateral sesamoidean, help support the navicular bone.
2. It goes by many names.
Veterinarians and horse owners have referred to navicular-associated lameness by several names over the years. Once called navicular disease, veterinarians have most recently characterized it as a syndrome. It’s also referred to as podotrochleitis, podotrochlosis, or podotrochlear apparatus disease (podo- refers to the foot, and the podotrochlear apparatus includes the navicular bone, impar ligament, collateral sesamoidean ligament, navicular bursa, and DDFT).
3. The early signs can be subtle.
Most horses with navicular syndrome first show subtle signs of lameness that progress with time, although acute lameness can occur. Lameness develops in the front feet and is typically bilateral (involving both feet). Horses with navicular syndrome try to avoid excessive weight-bearing on their heels, often display a short, choppy stride, and have difficulty turning in a circle or trotting on hard surfaces. Veterinarians consider toe-pointing or alternating front limb-pointing to be classic signs of navicular syndrome.
4. The navicular bone isn’t the only culprit.
In a 2015 study researchers reviewed medical records of 702 horses that presented with foot pain and found that only 8.8% of the horses ultimately were diagnosed with navicular bone injuries; 25.6% had injuries to the distal sesamoidean impar ligament and/or collateral sesamoidean ligament; 9.8% had damage to the deep digital flexor tendon; and 13.1% had navicular bone injuries in concert with damage to the podotrochlear apparatus with or without DDFT involvement.
5. MRI is an invaluable diagnostic tool.
Magnetic resonance imaging has given veterinarians a “window” into the foot and a better understanding of pathologies within it. Previously, practitioners believed that diseased changes in the navicular bone caused the majority of foot pain described as “navicular disease.” The use of MRI in lameness diagnostics has led to a better understanding of how related structures of the foot, including soft tissue, cause unsoundness.
6. Its exact cause is unknown.
The etiopathogenesis (the cause and development of a disease or abnormal condition) of navicular syndrome is unknown, and experts have differing opinions on the subject. Two theories include the vascular or ischemic theory and the biomechanical stress theory. The former focuses on a response to thrombosis or arteriosclerosis of the distal navicular artery. The latter theory attributes navicular syndrome to repetitive stress or abnormal biomechanical forces causing abnormal wear and tear to the navicular apparatus.
7. Certain breeds are more predisposed than others.
Veterinarians typically diagnose navicular syndrome in mature horses 7 to 14 years old. The condition is mostly commonly associated with Quarter Horses, Thoroughbreds, Standardbreds, and Warmbloods. The syndrome is rarely seen in Arabians, Friesians, and ponies.
8. Some conformation traits put horses at risk.
Horses with small feet (relative to their body size), upright pasterns, or long toes/low heels might be more likely to develop navicular syndrome. Veterinarians also see mediolateral imbalances (e.g., sheared heels) in afflicted horses.
9. Genetics might be involved.
Researchers believe navicular syndrome might involve a genetic component, which could be linked to heritable conformation traits (e.g., small feet) that predispose horses to the condition.
10. Lameness is gradual and progressive.
Horses diagnosed with navicular-related conditions often have a history of gradually reduced performance or intermittent lameness. Lameness is usually more pronounced after heavy work and improves temporarily after rest. Horses with navicular syndrome are typically more lame on hard surfaces.
11. Treatment is multifaceted.
Treatment for navicular disease is multimodal and can include specifically prescribed hoof trimming and/or shoeing, non-steroidal anti-inflammatory drugs, steroid and/or hyaluronic acid injections, shock wave therapy, and more.
12. Medications might help.
In 2014, the U.S. Food and Drug Administration (FDA) approved two bisphosphonate drugs to treat navicular syndrome in horses: clodronate disodium and tiludronate disodium. According to the FDA, bisphosphonates inhibit bone resorption by encouraging osteoclasts (cells responsible for bone turnover) to undergo cell death, leading to a decrease in bone breakdown.
Related Reading:
- Recognizing the Signs Your Horse Has a Hoof Problem
- Identifying Lameness in Horses: Which Leg Hurts?
- Why Is My Horse Limping? Causes of Equine Lameness
This article originally ran on TheHorse.com.
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