Metacarpals/-tarsals II and IV: Distal fractures

Fractures of the distal third of the splint bone are usually simple and occur most often at the narrowest part of the bone or immediately distal to the attachment of the interosseus ligament. Distal fractures are very common and can result from external or internal trauma. Internal trauma, especially in the forelimb, occurs secondary to excessive stress with extension of the carpus during exercise or after internal trauma attributable to concurrent suspensory desmitis and loss of pliability of the suspensory ligament.

Distal fractures are often associated with suspensory desmitis.

Standard radiographic views of the splint bones, including proximal sesamoid bones and ultrasonography of the suspensory ligament are recommended.

Pitfall:
The nutrient foramen of the third metacarpal/metatarsal bone may be misdiagnosed as a fracture of the distal splint bone. This can be avoided through additional radiographic views at different angles.

Fracture history
Fractures of the vestigial metacarpal and metatarsal bones, which are commonly referred to as splint bones, can occur anywhere along the bones and are very common in horses of all ages (Jackson et al., 2007). These bones are predisposed to injury because of their anatomic location, the nature of the horse and equine management practices. Kicks from other horses are probably the most important cause of splint bone fractures (Derungs et al., 2004), but fractures may also occur spontaneously, such as cyclic loading of the most distal aspect of the bone during exercise.

Fracture characteristics
Splint bone fractures can be open or closed, simple or comminuted, and localized in the proximal, middle or distal part of the bone.

Physical exam
Once a splint bone fracture has been diagnosed, the structures close to the bone including the suspensory ligament, proximal sesamoid bones as well as the third metacarpal/metatarsal bone must be examined carefully. The lameness varies from moderate to severe; it is usually severe in horses with open proximal fractures and moderate in those with distal fractures. The interval between the injury and the time of presentation also profoundly affects the severity of lameness.

Open fractures
In addition to lameness, affected horses often also have an open wound near the splint bone. The degree of swelling, pain and heat is directly related to the extent of soft-tissue damage. The size of the wound caused by external trauma varies and is no indication of the severity of the fracture.

Imaging
Radiographs are required to confirm the diagnosis and to rule out other complications. It is very important to take several views. The proximal articulation should always be included.

Ultrasonography is essential for the assessment of the suspensory ligament.
Computed tomography, if available, may be valuable for a definitive diagnosis in selected cases.

Complications
Complications, including nonunion (left), osteomyelitis and the formation of a sequestrum or excessive callus are common when the fracture is not treated properly. Involvement of the third metacarpal/metatarsal bone affects the prognosis negatively.

Prognosis
The prognosis of splint bone fractures depends primarily on the location, type and age of the fracture, but also on the involvement of tendons, soft tissues and the third metacarpal/metatarsal bone. If treated properly, the prognosis is good. It is critical that the clinician be alert and quickly identify fractures and infection, so that appropriate therapeutic measures can be implemented to improve the chances of the patient returning to its intended use.

Anatomical considerations
Strong collateral ligaments attach to the proximal part of the splint bones. The metacarpal/metatarsal interosseus ligament differs substantially among horses and may start to ossify at an early age. A firm fascia (A) covers the tendons in the proximal region of the metacarpus and is attached to the medial and lateral splint bone. A band-like structure (B) extends distally from the distal end of the splint bone toward the proximal sesamoid bone (Jackson et al., 2005).