Injuries to the navicular bone and its ligaments are a common problem for racehorses, especially those competing at the top level. They are referred to by a variety of names, from caudal heel pain and palmar foot pain to podotrochlosis and navicular syndrome, but whatever their specific name, these injuries are usually associated with poor performance and an unfavorable prognosis.
Historically, veterinarians have treated these injuries with rest, pharmacotherapy, localized external support and restriction/redirection of movement, as well as surgery to relieve pressure on the navicular bone or its ligaments. These strategies have been successful in treating chronic work and athletic injuries in humans, but they have not yet been systematically adapted for horses. Frustration with the limitations of these approaches, combined with major advances in human medicine and biomechanical analysis, has led to increased emphasis on examining the whole-body mechanics that lead to tissue lesions.
The navicular is a premier enthesis organ, a highly specialized structure that helps to transfer forces from the fetlock to the ground in order to allow for movement and exercise. Like all enthesis organs, the navicular apparatus is designed to adapt to mechanical forces that impact it. However, when the system fails to adapt, as is seen in navicular syndrome, the navicular bone and its ligaments can become damaged.
Navicular syndrome is characterized by fractures of the navicular bone and a variety of other abnormalities that affect the soft tissues surrounding it. The overall clinical picture is one of lameness and poor response to exercise and training. When the navicular bones fracture, they can do so in one of three ways-apical (the top third), mid-body or basal (at the bottom). The latter two types of fractures are most likely to cause the horse to have problems at high levels of competition and generally have the poorest prognoses.
Veterinarians have found that a combination of factors can predispose a horse to navicular syndrome, including the horse’s conformation and training regimen. Small feet, upright leg conformation and galloping speeds can also increase the stress placed on the navicular apparatus.
Peloso says MRI can help to catch early signs of injury to the navicular bones before they become brittle. He also cites studies that suggest that horses with navicular syndrome have hypertrophy of the bone and/or ligaments, which can be detected on radiographs. He suggests that monitoring the contralateral fetlock for signs of injury can also help to identify early changes in a navicular’s density, potentially catching a fracture before it occurs. However, he stresses that there is no single test that can predict when a navicular will fracture and that even the best diagnostic tools do not always prevent these injuries. The only way to prevent them is to use good management techniques, keep the fetlock in good shape and watch for any signs of lameness in the navicular limb.