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UF Health University of Florida
Veterinary Diagnostic Imaging College of Veterinary Medicine
LA Radiology Overview

Case reveal

Left thoracic limb:

  • Within the medial and lateral parasagittal aspects of the navicular bone, there are 2 well-defined and smoothly smoothly marginated linear lucent defects extending through the distal and proximal margins of the navicular bone, separating the navicular bone into three parts (yellow outline). The lateral segment of the navicular bone is mildly proximally displaced relative to the middle segment. A marked increase in mineral opacity is present within the medullary cavity, resulting in decreased distinction between the trabecular and cortical bone.
  • The medial and lateral collateral cartilages are mildly mineralized.

Right thoracic limb:

  • Within the lateral parasagittal aspect of the navicular bone, a well-defined and smoothly smoothly marginated linear Lucent defect is seen extending through the distal and proximal margins of the navicular bone, separating the navicular bone into two parts (green outline). A marked increase in mineral opacity is present within the medullary cavity, resulting in decreased distinction between the trabecular and cortical bone.

Conclusions:

  • Tripartite left and bipartite right navicular bones with associated sclerosis. Chronic navicular bone fractures are thought much less likely.
  • Mineralization of the ungual cartilages, left forelimb.

Follow-up:

  • The patient responded positively to a PD block of the left thoracic limb.
  • Due to the limited management options, the patient went home to be kept as a pasture pet.

A little bit more…

  • Multipartite navicular bones in horses are congenital abnormalities where the navicular bone fails to fully fuse during fetal development, resulting in a bone that is divided into two or more parts. This condition is not reversible and cannot be surgically treated to restore the bone to a normal state (Thrall et al., van der Zaag et al.).
  • Radiography is most commonly used to detect any abnormalities of the navicular bones and is most often sufficient to support a diagnosis (Dyson et al.). In some instance, computed tomography may be elected to visualized more subtle structural abnormalities to the navicular bone (van der Zaag et al.).
  • Differentiating between a congenitally multipartite navicular bone and a chronic fracture using radiographs is difficult. Multipartite sesamoid bones, which can be bipartite or tripartite, are often bilateral. The fragments typically have smooth, rounded edges and wide radiolucent gaps between them. The distribution of the partition sites tend to be uniform (van der Zaag et al.). Initially, multiple ossification centers cause little to no lameness. However, if instability is present, degenerative changes can occur, leading to lameness (Thrall et al.). Occasionally, multipartite navicular bones are a coincidental finding on lameness or prepurchase examination (van der Zaag et al).  
Note the presence of degenerative changes within the navicular bone, as characterized by the presence of sclerosis and well defined lobular lucencies along the opposing partition margins of the navicular bone. Theses lucencies may represent “subchondral cysts (poss. necrotic cartilaginous fragments retained in the subchondral bone), focal necrosis of the subchondral bone itself or as a result of synovial fluid being forced into the subchondral bone via defects in the articular cartilage”.
(van der Zaag et al.)
  • Certain treatments, such as steroid and hyaluronic acid injections into the navicular bursa, may help reduce inflammation and provide temporary relief from discomfort. However, these treatments do not address the underlying condition. Another option, performing a palmar digital neurectomy to eliminate sensation from the heels, can also offer temporary comfort. However, this procedure carries significant risks, including the potential for catastrophic rupture of the deep digital flexor tendon and other complications
  • Advanced imaging techniques, such as MRI, can provide detailed information about the condition of the navicular bursa and the deep digital flexor tendon. This can help in making a more accurate assessment of these soft tissue structures and determining the horse’s long-term prognosis (Harcourt et al.).

References:

  1. Morandi F, Thrall DE. The equine navicular bone. Textbook of veterinary diagnostic radiology. St. Louis: Saunders Elsevier. 2012 Jun 8:457-73.
  2. van der Zaag EJ, Weerts EA, van den Belt AJ, Back W. Clinicopathological findings in horses with a bi-or tripartite navicular bone. BMC veterinary research. 2016 Dec;12:1-0.
  3. Harcourt M, Smith C, Bell R, Young A. Magnetic resonance and radiographic imaging of a case of bilateral bipartite navicular bones in a horse. Australian veterinary journal. 2018 Nov;96(11):464-9.
  4. Dyson S. Radiological interpretation of the navicular bone. Equine Veterinary Education. 2008 May;20(5):268-80.