Fracture of the Third Metacarpal Bone (Cannon Bone) in Horses

The most common site of major fractures of the third metacarpal bone (the cannon bone) is the distal articulation, or condyle. These condylar fractures occur in the sagittal plane and predominantly in young racehorses. Most condylar fractures are in the lateral condyle (see condylar fracture image).

Lateral condylar fracture, distal third metacarpal bone, radiograph,...

Image

Courtesy of Dr. Matthew T. Brokken.

In Thoroughbreds, condylar fractures of the third metacarpus are at least twice as common as third metatarsal fractures. In Standardbreds, the ratio of metacarpal to metatarsal fractures is nearly equal.

Nearly all lateral condylar fractures originate from the mid to midaxial portion of the lateral condyle and extend toward the lateral cortex; they are sometimes incomplete.

In contrast to medial condylar fractures, lateral condylar fractures rarely spiral into the diaphysis of the cannon bone. Medial condylar fractures nearly always appear incomplete and spiral toward the axial aspect of the cannon bone.

Clinical signs of condylar fractures are straightforward: acute lameness after exercise (or a race) with marked effusion in the fetlock joint. Radiographic examination should include a full series of the fetlock, as well as a flexed dorsopalmar view. For medial fractures, a radiographic series of the entire cannon bone should be performed, given the tendency for medial fractures to propagate proximally.

Lateral condylar fractures can be treated conservatively by casting, but the treatment of choice is lag screw fixation. Medial condylar fractures require plate fixation.

In contrast to most condylar fractures, fractures of the diaphyseal cannon bone are unstable and difficult to treat. Typically, these fractures result from trauma incurred while the horse is pastured with other horses.

Open reduction and internal fixation is the preferred treatment for diaphyseal metacarpal fractures, because the lack of soft tissue coverage over an unstable fracture typically causes this type of fracture to become open when treated simply with a cast or coaptation splint. Generally, treatment is pursued only in foals, because the prognosis for adult horses with diaphyseal metacarpal fractures is poor.