Fracture of Distal Phalanx in Horses
(Fracture of third phalanx, os pedis, or coffin bone)
Fracture of the distal phalanx is a fairly common injury that occurs most commonly at high speed (ie, during a race) or less commonly from kicking a firm object (eg, a stall wall). The fracture is caused by concussion and produces a sudden onset of lameness. The lameness is severe if the fracture is intra-articular but may be less severe if only a wing (or solar margin of the distal phalanx) is fractured with no articular component. Distal phalangeal fractures occur more frequently in the forelimb but are also common in the hindlimb. Intra-articular fractures may be easily isolated to the foot; lameness is commonly associated with joint effusion. Nonarticular fractures may require compression of the foot with hoof testers and possibly unilateral palmar digital nerve anesthesia for localization. Lameness is exacerbated by turning the horse or making it pivot on the affected leg. If the fracture does not extend into the joint, the lameness may improve considerably after 48 hr of stall rest.
The clinical signs may be suggestive, but the diagnosis is confirmed by palmar digital nerve block and radiographic imaging. Radiographic confirmation may be difficult immediately after the injury, because the fracture is only a hairline at this stage. Often, more than two views are required before the fracture line is evident. Repeating the radiography several days or weeks later (to allow bone resorption) and using oblique views may be necessary to confirm the presence and exact site of the fracture. Additionally, if the suspected fracture is in a wing of the distal phalanx, unilateral palmar digital nerve anesthesia may be performed to localize the lameness to that side. Determining whether the fracture extends into the distal phalangeal joint is important. Scintigraphy and MRI are other imaging options if a definitive diagnosis is not possible by radiographs.
Conservative treatment of 6–9 mo rest is usually all that is required for fractures not involving the joint. Fractures often heal with a fibrous union, so that, even though the horse returns to soundness, radiographic evidence of the fracture remains. A straight bar shoe with a clip well back on each quarter can be applied to limit expansion and contraction of the heels. In young horses (<3 yr old), fractures into the joint may heal satisfactorily, provided a 12-mo rest period is given. Older horses (>3 yr old) have a much less favorable prognosis, and insertion of a cortical bone screw using interfragmentary compression across the fracture site is indicated; however, infection is a frequent complication, because an extracapsular approach is required. Many fractures heal in the presence of infection, but the screw must be removed at a second surgery to restore the horse to complete working soundness. Unilateral palmar digital neurectomy of racehorses with nonarticular wing fractures has been used to allow return to competition without the delay for complete healing.