Issue 22   -   February 2016

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Equine Nuclear Medicine
Animal Imaging's use of nuclear medicine is particularly well suited for the diagnosis of equine lameness and pathologies involving the pelvis, back, cervical spine and head.
In diagnostic nuclear medicine, low dosage radioactive substances are administered to patients. These radiopharmaceutical "tracers" settle in particular organs or bones, where they can be detected by a gamma camera and provide diagnostic information about those areas. Importantly, equine patients do not experience pain or short-term side effects from these modalities. Equine scintigraphy is often the best diagnostic choice for subtle lameness or performance limiting conditions that are difficult to diagnose with conventional diagnostic modalities. 
Magnetic Resonance Imaging of the Equine Navicular Bone
Patient History:
A 7-year-old Quarter horse mare was presented to Animal Imaging for an MRI of both front feet. She had a history of chronic heel pain and had an intermittent bilateral forelimb lameness that blocked to a palmar digital nerve block. Injections of the both front distal interphalangeal joints helped, but only for a short while.
When jogged in hand, the mare was lame on both forelimbs, but had a primary grade 2/5 right forelimb lameness. When trotting a circle to the right, the right fore limb lameness was most noticeable, and when trotting a circle to the left, the left forelimb lameness was most noticeable. She was mildly positive to hoof tester application across both front heels. She was negative to lower and upper limb flexion of both fore limbs. The front flexor tendons and suspensory ligaments palpated within normal limits.
On routine radiographs obtained prior to MRI, a sagittal lucency of the medial wing of each front navicular bone was evident. The margins of the bone on each side of the lucency was smooth. There was sclerosis and multiple areas of lucency around the margins of the defect. Differentials included bipartite navicular bones or sagittal fractures.

The most significant pathology, and the most likely cause of lameness, identified on the MRI were the abnormalities of the navicular bone and its associated structures.
Axial STIR images of the left front foot (above) and right front foot (below) showing increased fluid signal within the medullary cavity of the navicular bones, along the sagittal defect.

There were sagittal defects in the medial body of both navicular bones. These defects were likely fractures, though bipartite navicular bones could not be excluded. Even if these were bilateral bipartite navicular bones, the areas of hyperintensity evident on the STIR sequences in the cortical bone on each side of the defect suggest that there may be some motion and subsequent bony change/remodeling, with resultant effects on the soft tissue in this area.

The sagittal VIBE image of the left front foot on the left shows multiple large navicular cortex erosions. In the axial VIBE image of the left front foot, cystic change (green arrow) along the lateral aspect of defect (orange arrow) can be seen.
There were mild dorsal border changes of both deep digital flexor tendons with adhesions to the collateral ligament of the navicular bone bilaterally. The collateral ligament of the navicular bone was diffusely thickened in both front digits. The left front navicular bone was severely degenerative with large cortical erosions. The right front navicular bone also has cortical erosions distally, though to a lesser degree than the left front. There were cystic structures on the palmar aspect of both front coffin bones at the insertion of the impar ligament, with the right being larger than that of the left front. There was evidence of mild synovitis and osteoarthritis of both front distal interphalangeal joints.
Impressions and Recommendations: 
Given the severity of the MRI findings, the prognosis for return to competitive athletic perfomance was guarded. Navicular bone fractures are uncommon in horses and occur more commonly in the forelimbs than the hind limbs. They may be treated conservatively or surgically with a cortical screw placed in lag fashion. Because surgery requires special instruments, is technically difficult, and expensive, conservative management is usually utilized. Conservative therapy involves a flat shoe with four 3 wedge pads stacked to reach a total of 12 of heel raise. Each following month, a pad is removed until the horse is in only a flat shoe after four months. A palmar digital neurectomy may be elected after the fracture heals, but this technique typically does not yield satisfactory results.

 Copyright 2012. Animal Imaging.