Radiographic assessment of the navicular bone and its relation to prognosis

Navicular disease is lameness associated with pain arising from the navicular bone, the collateral sesamoidean and distal impar ligaments, and the navicular bursa. Concurrent degenerative change of the deep digital flexor tendon may be present. There are 4 basic forms of navicular disease based on the pathology and MRI appearance of lesions:

1. Degenerative disease of the palmar surface of the navicular bone results in degeneration and loss of fibrocartilage and cortical bone from the flexor surface. This has been called ‘classic navicular disease’.
2. Osseous fragmentation occurs at the lateral and/or medial angles of the distal border of the navicular bone. Distal border fragments may result in remodeling changes of the adjacent cortical bone and spongiosa at the distal border of the navicular bone.
3. Primary inflammation, contusion or necrosis of the medulla results in signs of oedema or haemorrhage in the spongiosa of the navicular bone. In rare cases, necrosis of the medullary adipose tissue may occur. These pathological changes may be the result of a traumatic rather than degenerative origin.
4. Primary desmitis or enthesopathy of the supporting ligaments of the navicular bone can be a cause of lameness and lead to remodeling changes of the proximal and distal borders of the navicular bone. Primary injury of these ligaments is rare and desmitis is usually secondary to chronic degenerative disease of the navicular bone or the deep digital flexor tendon, and associated with navicular bursitis.

Even though radiographs are too insensitive to detect early bone or any soft tissue or cartilage change, they remain the main diagnostic tool in practice to demonstrate morphologic changes in the navicular bone and adjacent structures. However, radiography only allows the recognition of advanced bone disease. A minimum of three high quality radiographic projections are required: a 60° dorsoproximal-palmarodistal oblique, a lateromedial, and a palmaroproximalpalmarodistal oblique (flexor) view. The diagnostic quality of the radiographs remains of primary importance and is frequently below par, even today when digital radiography has replaced the use of traditional film/screen combinations. Improvements have been suggested to increase the diagnostic yield of the flexor view (Bathe 2003; Johnsonet al.2018). The most reliable findings for diagnosing navicular disease are defects in the flexor cortex, medullary trabecular disruption, medullary pseudocyst formation, medullary sclerosis, poor flexor corticomedullary demarcation, distal border fragments, and enthesophyte formation of the proximal and/or distal borders. The size, shape and number of the synovial invaginations of the distal border are more ambiguous features. As there is a wide variation in the appearance of these synovial invaginations in horses without lameness, it is impossible to base a diagnosis of navicular disease or to predict the possibility of future navicular disease, on the appearance of the distal border of the bone. [...]