Congenital abnormalities of the navicular bone are not common but they do exist and can be quite important relative to the future soundness of a horse. Figures 87 and 88 demonstrate a small fragment of bone located along the distal border of the navicular bone. This fragment of bone could be an osteochondritis dissecans (OCD) type lesion or it could be a fracture from the distal border of the navicular bone. Damage to the impar ligament could also produce this type of calcification or bone fragment. These bone fragments have been called osseous bodies.
Figure 87. Figure 88. Figures 87 and 88 show small fragments of bone on the distal aspects of the navicular bones that could be OCD lesions, fractures or calcification resulting from damage to the impar ligament.
OCD type bone fragments are frequently bilateral and can be found in sound horses that remain sound. Figures 89 and 90 illustrate well rounded osseous bodies along the distal borders of both navicular bones in a sound and very successful jumper. There were identical lesions in both front feet. In my opinion these were OCD lesions that were not clinically significant. However, when these osseous bodies are found in the navicular bones of young horses, it is difficult if not impossible to determine whether this particular young horse will be lucky enough to stay sound. Therefore, I consider osseous bodies significant in weanlings, yearlings and two year olds. But in older, sound horses osseous bodies do not carry as much significance.
Figure 89. Figure 90. Figures 89 and 90 demonstrate osseous bodies along the distal border of both navicular bones of a successful jumper. While these lesions did not limit the success of this individual the same lesion in a weanling, yearling or two-year old could impact on the future performance of the individual and thus should be considered significant.
Figure 91 is a dorsal palmar view of the navicular bone demonstrating a separate center of ossification of the wing of the navicular bone. These separate centers of ossification can be confused radiographically with fractures of the navicular bone. However, if the radiolucent line in the wing of the navicular bone in Figure 91 was a fracture the horse should have exhibited an acute lameness at the time of the fracture. In this case this four year old horse had never exhibited any degree of lameness attributed to this foot and the owners of this horse had raised him from a foal. It has been my experience that a horse with a separate center of ossification of the navicular bone can stay sound up to and including their fifth year of age. However, after that age they will show increasing degrees of lameness. This is a result of the degenerative joint disease that develops in the distal interphalangeal (coffin joint ) because of the instability in the joint due to the unstable navicular bone.
Figure 91. Figure 91 shows the dorsal palmar view of a four-year old horse with a seperate center of ossification of the wing of the navicular bone. These lesions often produce lameness around the age of five years which would result from the degenerative joint disease associated with the lesion.
Figures 92 and 93 might be described as a bipartite navicular bone. This is another variation of separate centers of ossification of the navicular bone and as mentioned previously have to be considered radiographicly significant.
Figure 92. Figure 93. Figures 92 and 93 demonstrate bipartite navicular bones. Another example of separate centers of ossification these should be considered radiographically significant
Figures 94 and 95 demonstrate the degenerative changes affecting the navicular bone. In the dorsal palmar projection there are bone spurs present on the wings of the navicular bone and in the dorsal ventral projection there are significant osteolytic changes along the distal border of the navicular bone.
Figure 94. Figure 95. Figure 94 is a dorsal palmar view of bone spurs on the wings of the navicular bone and Figure 95 is the dorsal ventral projection demonstrating significant osteolytic changes along the distal border of the navicular bone. These are examples of degenerative changes.
Figures 96, 97, 98, 99, 100 and 101 demonstrate multiple separate centers of ossification in the navicular bone. In these cases both front feet were equally affected. In addition there is significant radiographic evidence of degenerative joint disease consisting of osteolytic and proliferative bone changes. These horses were clinically lame.
Figure 96. Figure 97. Figure 98. Figure 99. Figure 100. Figure 101. Figures 96, 97, 98, 99, 100 and 101 are radiographs from lame horses showing separate centers of ossification in both front feet accompanied by degenerative joint disease (osteolytic and proliferative changes).
There can be varying degrees of missing portions of the navicular bone. For example one quarter or even one half of the navicular bone could be missing. Figures 102 and 103 illustrate a rather unusual congenital abnormality. The central portion of the navicular bone is missing and only the two wings of the navicular bone are present. An unstable navicular bone such as this will inevitably lead to a significant lameness.
Figure 102. Figure 103. Figures 102 and 103 demonstrate the unusual congenital abnormality of missing parts of the navicular bone. Only the wings of the navicular bone are present and the center is missing. This will inevitably lead to lameness because of the inherent instability produced.
Figures 104, 105 and 106 demonstrate the most extreme congenital aberration, the complete absence of the navicular bone.
Figure 104. Figure 105. Figure 106. Figures 104, 105 and 106 are radiographs that demonstrate the most extreme congenital abnormality, a complete absence of the navicular bone.
Figure 107 demonstrates an osseous cyst-like lesion involving the navicular bone. This is an extremely large lesion and is radiographicly significant. Smaller cyst-like lesions in the navicular bone, Figure 108, should also be considered radiographicly significant in young horses that are sound. The probability of them going lame when they are put in training is quite high.
Figure 107. Figure 108. Figures 107 demonstrates a large, and radiographically significant, osseous cyst-like lesion in the navicular bone. Figure 108 shows a smaller cyst-like lesion that should be considered significant in a young horse that will be put into strenuous training.
Figures 109, 110, 111 and 112 demonstrate osseous cyst-like lesions in the center of the navicular bones in both front feet. Even if a horse with this type of lesion in the navicular bone is sound, in my opinion ,with any degree of work they will most likely go lame. These types of abnormalities have to be considered significant radiographic findings.
Figure 109. Figure 110. Figure 111. Figure 112. Figures 109, 110, 111 and 112 show the radiographically significant lesion of osseous-cyst-like lesions in the center of the navicular bone. It is likely that a sound horse with these lesions, when put into work, will become lame.
In summary, it is my opinion that separate centers of ossification of the navicular bone are radiographicly significant and will contribute to a future lameness problem even if the horse is sound at the time of obtaining the radiographs. There are various congenital aberrations that can effect the navicular bone. As already has been pointed out separate centers of ossification of the navicular bone is one of the more common aberrations. Lack of bone formation and osseuos cyst-like lesions are also significant and are consistent with a high probability of future lameness.