Abstract
Fractures of the navicular bone in foals are relatively rare. This case report describes a three-and-a-half-months old Quarter Horse filly suffering from an avulsion fracture of the navicular bone. The fracture was located at the insertion border of the impar ligament of the left hindlimb. The filly was presented to a private veterinary clinic with an acute and severe lameness of the left hindlimb. During the previous night, the filly and the mare were kept together in a large horse stall. The next morning, the filly displayed severe lameness with no obvious wounds, swellings or other visible changes to the left hindlimb. The filly elicited severe pain on the circle and the hoof testers also caused a pain response, especially in the area of the bulbs. A hoof bandage was applied, but the lameness remained unchanged. Ten days after the first consultation, given there was no improvement, a radiological examination of the affected hoof was performed at the barn. On the latero-medial view, a proximal dislocation of the navicular bone was clearly visible. All other bone structures seemed unaffected. Based on these X-ray images, a ruptured distal sesamoidean ligament was suspected. Due to the poor prognosis as a riding horse, the filly was euthanased and both hindlimbs were subjected to a thorough examination at the Vetsuisse Faculty of the University of Zurich. There, different imaging techniques, namely X-ray, computed tomography (CT) and magnetic resonance imaging (MRI), were used in order to understand the pathology. With the higher contrast of the modern methods and the possibility to create a 3D-model, three small pieces of bone tissue were identified at the distal border of the navicular bone, which were not visible on the first X-rays. Based on all images, the definitive diagnosis was an avulsion-fracture of the navicular bone at the Margo ligament, which is the insertion border of the impar ligament. Histological samples of both distal hindlimbs were prepared, where the bone tissue of the fragments was also clearly visible. Due to the configuration and based on histopathological findings, trauma and not degenerative bone-disease is considered to be the cause of the fracture. The impar ligament was normal. The trauma had occurred in a weak area of the Margo distalis of the navicular bone, which is the distally protruding Margo ligamenti. Moreover, histological findings evidenced a partial rotation of the small bone fragments from the distal border of the navicular bone, probably caused by pressure or tension in this area. In conclusion, in this article a rare pathology of an avulsion fracture of the navicular bone in a foal was described using modern imaging techniques, such as MRI and CT, and substantiated by histological findings.
Eichenberger, Sari