It wasn't that long ago that X-rays were the only diagnostic tool available to veterinarians for determining the cause of lameness. But X-rays have drawbacks, including the inability to show problems with tissues other than bone and the fact that abnormalities and changes seen on X-rays aren't necessarily the cause of the horse's pain.
The advent of ultrasound was a major step forward in being able to see injury to soft tissues such as tendons and ligaments and smaller structures inside and alongside joints that function to stabilize them. However, the most important part of the horse's body when it comes to lameness is the foot, and it can't be completely evaluated by an ultrasound study because of the hoof wall.
As we stated in July 2005, this is where an MRI-magnetic resonance imaging-can be a help. An MRI produces images of incredible detail and contrast and can be set to focus precisely within the body, such as the foot. In fact, because of MRI we now know of several newly defined and important causes of foot pain including:
Collateral Desmitis Of The Distal Interphalangeal Joint. This is inflammation or tearing of stabilizing ligaments on the inside (medial) or outside (lateral) of the coffin joint. The medial ligament is the most often involved. This injury is particularly common in jumpers. Although this condition has also been described after ultrasound examinations, in one study only 32% of the horses with collateral ligament injuries could be detected by ultrasound.
Impar Ligament Problems. The impar ligament attaches the coffin bone to the bottom of the navicular bone. Tears and inflammation in this tiny ligament can result in significant pain for the horse that improves with the usual nerve blocks done for navicular disease but is invisible on X-rays to check for navicular disease.
Fluid In The Navicular Bone. The traditional theory of navicular disease is that it develops following inflammation of the navicular bursa, which sits between the bone and the deep digital flexor tendon. However, MRI studies have found that many horses with early clinical signs of navicular disease have increased fluid, indicating inflammation, inside the bone itself with no evidence of inflammation in the actual bursa.
This early inflammatory response can be seen only on MRI and occurs long before X-ray changes. In some acute cases, this fluid may in essence be actual evidence of a "bone bruise" to the navicular bone area.
Other Navicular-Related Findings. MRI is also accurate in detecting changes in the navicular bursa, the ligaments supporting the navicular bone, and along the length of and insertion of the deep digital flexor tendon within the foot. Any of these can be sources of pain in horses diagnosed with "navicular syndrome" or "heel-pain syndrome" and may in time lead to navicular disease itself.