Fracture of the navicular bone in horses is much less common than fracture of the distal phalanx. It can occur in forefeet or hindfeet and is thought to be secondary to concussive injury. It can also occur in horses that have navicular bone degeneration (navicular syndrome).
Bipartite or tripartite navicular bones are congenital abnormalities in which the bone has failed to fully fuse during fetal development, and they can be easily confused with fractures (see bipartite navicular bone image).
Courtesy of Dr. Valerie J. Moorman.
Horses with navicular bone fractures may have mild to severe lameness, and often they exhibit pain to hoof testers in the region of the heels or frog. Affected horses also tend to improve with a palmar/plantar digital nerve block.
Radiographic evaluation is the most common means of diagnosis navicular bone fractures, and a 55°–65° dorsoproximal-palmarodistal oblique radiograph usually shows the fracture line. As always when the foot is being radiographed, the sulci of the frog must be packed with modeling compound to eliminate gas artifacts, which can be confused with fracture lines.
Conservative treatment with shoeing support, such as a bar shoe with heel elevation, can be attempted; however, many horses do not improve with this treatment. A fibrous union is a common outcome, and the deep digital flexor tendon can also sustain damage during the convalescence period.
Navicular bone fractures can be repaired surgically using a lag screw technique. However, surgical planning using three-dimensional imaging, such as CT or MRI, may be needed. These imaging techniques can also be helpful in assessing additional soft tissue injuries that may have occurred with the fracture.
The overall prognosis for soundness after navicular bone fractures is guarded to poor.
See Regional Anesthesia in Horses for discussion of specific anesthetic techniques used to localize lameness.
