Fractures and injuries of the middle phalanx

Degenerative disease of the proximal interphalangeal joint as well as fractures of the middle phalanx are seen in most breeds of horses. However horses engaged in western performance activities and Arabian breeds seem predisposed to injuries involving the proximal interphalangeal joint, especially fractures of the middle phalanx, likely due to the bending and torsional forces generated during abrupt stops and changes in direction.

The most common indication for surgical intervention of the pastern region is the management of degenerative joint disease of the proximal interphalangeal joint, commonly referred to as high ringbone. There are a variety of conditions which can lead to degeneration of the proximal interphalangeal joint, including developmental disorders such as osteochondritis dessicans and subchondral bone cysts as well as joint infection and external trauma to the periarticular tissues. Cumulative damage to the articular structures during repetitive use is considered a common cause of high ringbone as well.

Fractures of the middle phalanx are most often associated with acute injury during intense performance activities. However, fractures are not always associated with performance and have been noted to occur with minimal activity, indicating that pre-existing disease may play a role in some patients. The most common configurations are uniaxial and biaxial fractures of the palmar / plantar eminences and comminuted fractures of the middle phalanx. Occasionally, the palmar / plantar soft-tissue support will be disrupted and result in complete luxation.

The most common injury following acute, disruptive trauma, is a multi-fragmented middle phalanx fracture. These fractures are characteristically comminuted on the proximal articular surface of the middle phalanx and often have a single oblique fracture line extending into the distal interphalangeal joint. Such injuries are highly unstable, especially in the palmar / plantar direction.
Other injuries that result in palmar / plantar instability include biaxial eminence fracture and complete disruption of the palmar / plantar soft tissue structures. It is important to recognize when the injury has resulted in palmar / plantar instability as the technique of choice for repair is double-plate fixation.

Other types of injury involving the proximal interphalangeal joint, which do not result in palmar / plantar instability, are preferentially managed by arthrodesis using a single plate positioned axially in conjunction with 2, 5.5mm cortex screws placed in lag fashion abaxial to the plate. Although simple fractures of the middle phalanx with fragments of sufficient size, may appear to be candidates for fragment fixation using lag screw technique, with the objective of preserving pastern joint function, it has been the authors’ experience that primary arthrodesis provides a more assured outcome.

Clinical signs can vary from mild, performance limiting lameness to non-weight bearing lameness accompanied by instability, depending on the severity of the insult to the articular and periarticular structures. In cases of mild lameness without additional localizing signs, diagnostic anesthesia will be necessary to confirm the pastern joint as the cause of disability. More severe injury or long standing degenerative disease will often have additional localizing signs, such as instability or enlargement of the pastern region.

Radiography confirms the diagnosis in most cases. However, occasionally it is difficult to detect a fracture line acutely because of minimal displacement. If a fracture is suspected but not evident on radiographs, and the injury occurred within 2 weeks of presentation, the patient should be placed in a box stall and the radiographs repeated in 10 to 14 days. In most cases, this amount of time is adequate for demineralilzation along the fracture line to occur and for the fracture to become evident.

Scintigraphy, computed tomography (CT), and magnetic resonance (MR) imaging are useful adjuncts for delineating an obscure fracture as well as other types of injury to the pastern region and should be considered when standard techniques fail to render a diagnosis.