Ask a vet – Pedal bone fractures in the horse

[dropcap]T[/dropcap]he pedal bone, also known as the coffin bone or distal phalanx, is a small half-moon shaped bone within the horse’s hoof capsule surrounded by delicate tissue called laminae. The pedal bone articulates with the short pastern bone via the distal inter-phalangeal or coffin joint. As the pedal bone is close to the sole of the horse’s hoof, fractures can occur if the horse slips or steps heavily on a solid object. Fractures of hind limb pedal bones can also occur if a horse kicks a rigid object like a wall. Some pedal bone fractures don’t involve a single impact, but are thought to result from cumulative stress, as seen in racehorses. In other horses, these injuries may be related to a genetic weakness. Fractures of the pedal bone are fairly common and can occur in horses and ponies of all ages, types and breeds.

Signs of a pedal bone fracture

• Sudden lameness
• Heat within the hoof capsule
• Increased digital pulses
• Swelling of the coffin joint
• Positive response to hoof tester application

As these clinical signs also occur with abscesses and bruising, pedal bone fractures can initially be undiagnosed.

Diagnosis

Radiographs (x-rays) can be used to diagnose a fracture, though some fractures may not show on an x-ray until 10 to 14 days after the injury, and multiple x-rays may need to be taken to identify the fracture plane. Other techniques such as nuclear scintigraphy, MRI and CT can also be used to diagnose pedal bone fractures.

Types of fracture

There are seven types of pedal bone fracture that are classified according to the part of the bone involved. Fractures can involve the tip of the bone, the outer edge of the bone or the centre of the bone. Chip fractures are small fractures of the toe and comminuted fractures occur when the bone is fractured into many pieces.

• Type 1: The medial (inside) or lateral (outside) wing of the pedal bone is fractured.
• Type 2: The fracture involves a larger part of the bone including the wing and communicates with the coffin joint.
• Type 3: The fracture is in the centre of the bone.
• Type 4: The extensor process of the pedal bone is involved in the fracture.
• Type 5: Comminuted fracture of the pedal bone.
• Type 6: Chip fracture of the toe.
• Type 7: Similar to foals but specific to foals.

Treatment of pedal bone fractures

The mainstay of treatment is immobilising the fractured bone and decreasing pain associated with the fracture. For most pedal bone fractures, keeping the horse on box rest and immobilising the foot with a bar shoe or foot cast stabilises the structures inside the hoof enough to allow the fracture to heal.

If there are multiple fractures, as in a comminuted fracture, the prognosis is not as good and the healing process will take longer. Additionally, horses with complicated fractures that involve the coffin joint often require surgical repair with a screw or removal of bone fragments. Anti-inflammatory medication will also help to control pain in horses with a pedal bone fracture.

It is important that the convalescence period isn’t hurried, as the horse may appear sound before the fracture is completely repaired, and returning the horse to work too soon greatly increases the risk for re-injury. As a rough guide, horses with uncomplicated pedal bone fractures should have a minimum of two months of box rest before exercise and turnout are recommenced. Assessment of the fracture by repeat x-rays can be a useful guide to the progress of the repair, but a fracture line can sometimes remain after the horse has become sound, so a combination of clinical and radiographic assessments is important.

By: Dr Luke Poore

The full article appears in the Winter Guide issue of HQ (June 123) > Shop now