The trials and frustrations of navicular disease
Podotrochlosis. Navicular disease. Caudal heel pain. A rose by any other name would be as … frustrating? Though it goes by several names, the condition is common among horses and frustrating for owners, farriers, and veterinarians alike. It takes a team to truly address and treat the underlying cause of a horse’s navicular-related lameness.
Navicular Apparatus Anatomy
Thirty years ago veterinarians believed any horse with navicular disease must have degeneration of the navicular bone. Decades of research and education later, we know the lameness associated with the condition might not be solely the navicular bone’s fault. Rather, many structures work in sync as the navicular apparatus. This includes the navicular bone, the navicular bursa, the coffin joint, the impar ligament, the suspensory ligaments of the navicular bone, and the deep digital flexor tendon (DDFT). One side of the navicular bone (the flexor surface) borders the DDFT. The navicular bursa “buffers” the contact between the two structures to prevent adhesions from forming between the navicular bone and the DDFT.
“The role of the soft tissue structures surrounding the navicular bone is to support the bone while the animal is standing and moving,” says Tammy Muirhead, DVM, professor of anatomy at the Atlantic Veterinary College, on Canada’s Prince Edward Island. “If one or more of these structures are affected, excessive strain can be placed on the (navicular) bone, leading to degeneration or remodeling.”
Equine practitioners classify the resulting lameness from the bone or any of the soft tissue structures as podotrochlosis, because the condition can involve any part of the podotrochlear apparatus: the navicular bone, DDFT, navicular bursa, and supporting ligaments.
The manifestations of caudal heel pain aren’t black and white. Horses with podotrochlosis display any number of clinical signs, which can make coming up with a diagnosis and treatment plan a long, challenging battle.
The lameness most often affects horses around the ages of 7 to 9, though it can appear in equids of any age. The horse often starts showing signs of lameness after a period of rest and reintroduction to work. Early in the disease, some horses “work out” of the lameness after a proper warmup. The degree of lameness might vary from day to day, which can be incredibly frustrating for owners and vets.
Horses with navicular-related lameness often display a very short-strided or “choppy” trot. They tend to stumble at a trot or even a walk and have a visible head bob correlating with one of the forelimbs. The lameness usually worsens if the horse is forced to circle or trot on a hard surface (concrete or driveways are ideal for lameness workups).
The disease usually affects both forelimbs, though one forelimb might be worse than the other. Nerve blocks (by which veterinarians desensitize the navicular apparatus) of the lame foot often reveal lameness in the seemingly “good” leg.
Veterinarians can use several techniques to definitively diagnose podotrochlosis in horses. However, many “navicular disease” diagnoses are based on clinical signs alone due to lack of practitioner experience, availability of diagnostic equipment, or owner finances.
The most common imaging modality used to diagnose the condition is radiography. The veterinarian takes a series of X rays to get a detailed look at the affected areas of the navicular bone. He or she scrutinizes the images for bone surface abnormalities, the distinction between the outer cortex and inner medulla, and bony remodeling and lysis (bone loss). The vet also carefully analyzes the interface between the DDFT and the flexor surface of the navicular bone for lesions.
If you’re thinking, “I thought ultrasound was the best way to visualize tendons,” you’re right! However, due to the tendon’s close association with the navicular bursa, lesions and calcifications on the tendon can be visible on radiographs.
Some horses with caudal heel pain have few to no visible changes on radiographs. These horses might warrant a more intensive diagnostic plan. While ultrasound is a great tool for looking at tendon and ligament injuries, it can be difficult to see the tissues of the navicular apparatus due to the horse’s lower limb anatomy and ultrasound’s limited ability to penetrate the hoof wall. Researchers have found that MRI is incredibly helpful for definitively diagnosing podotrochlosis in horses. It provides the best and most complete imaging method for the equine hoof.
This article continues in the January 2020 issue of The Horse: Your Guide to Equine Health Care. Subscribe now and get an immediate download of this issue, including this in-depth article on the trials and frustrations of navicular disease in horses.