Navicular Disease: MRI Provides New View

Based on more than a thousand MRI exams at Washington State University (WSU), Robert Schneider, DVM, MS, an equine orthopedic surgeon at WSU, reports that the magnetic resonance imaging (MRI) will locate the pathology of navicular disease 90% percent of the time. Schneider spoke about equine foot lameness at the Western Performance Horse Forum held in Nampa, Idaho, on Feb. 15-17.

In general, radiographic signs of navicular disease do not correlate well with clinical signs until there is end-stage disease. Previously, navicular disease was based on a clinical diagnosis, but now it is also identified via MRI.

Schneider noted that a common question asked is: "How will an MRI change how I treat my horse?" After identifying the region of pain with diagnostic nerve blocks, he explained that the MRI pins down the problem; knowing the specific problem affects all further strategies such as medication, rest, arthroscopic surgery, or retirement from active athletics. Rest is expensive, especially since it often takes three to six months for resolution of a foot problem. With a subtle problem, an owner might want an immediate answer to avoid time off without risk of injuring the horse.

A large group of horses that developed clinical signs of navicular disease during the six months prior to exam were scanned with MRI. These horses demonstrated typical signs of bilateral forelimb lameness, sensitivity to hoof testers over the middle third of the frog, and diagnostic nerve blocks of the heel region that improved one leg only to worsen the other limb, which went sound with a heel block. Interestingly, all these horses had normal radiographs. MRI revealed some interesting findings:

  • Half the horses had fluid in the navicular bone region related to inflammation.
  • The impar ligament was irregular and thickened, with fluid in it;
  • The proximal suspensory ligament of the navicular bone was thickened.
  • The deep digital flexor tendon proximal to the navicular bone along the back of the pastern was thickened, irregular, and fluid was present all the way into the digital flexor sheath above the fetlock. This abnormality is difficult to identify with ultrasound.
  • Increased fluid in the navicular bursa, with occasional scar tissue, was present.
  • Increased fluid was present in the coffin joint.

In horses with chronic lameness for more than six months due to navicular disease, these MRI findings were also present. The most severe abnormalities were observed in the lamest leg in most cases, correlating well with clinical signs.

Schneider explained how maximal distention of the coffin joint occurs when the horse is pushing off the foot prior to the toe lifting off the ground. This puts backward pressure on the navicular bone, and a sore horse shortens his stride because he doesn't want to push off and extend the foot. This strains the proximal navicular suspensory ligament and impar ligament of the navicular bone. A long toe, low heel hoof configuration amplifies this abnormality to create chronic repetitive strain. Navicular bursitis might be a consequence of inflammation in the proximal suspensory ligament of the navicular bone

Deep digital flexor tendonitis might develop due to hyperextension of the coffin bone and elevation of the toe with a bad step or from repetitive use. Acute or chronic blunt trauma to the frog is also known to cause navicular bone injury and bruising. The average bone bruise might take three months to heal.

For more from the Western Performance Horse Forum click here.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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