This is the second part of Dr. Greg Haines' article on MRI for horses. Read part one of the article.
Magnetic resonance imaging (MRI) may be indicated when a lameness has been localized to a specific region, generally through diagnostic nerve or joint blocks. Through limitations, other imaging modalities (X-rays, ultrasound, bone scan) may have failed to determine a specific diagnosis.
MRI has greatly advanced our diagnostic capabilities particularly within the equine foot. Common injuries within the foot that have been diagnosed with MRI include deep digital flexor tendon (DDFT) injuries and navicular bone degeneration. Below are specific examples when MRI was helpful diagnostically. The first two deal with the foot, the third with the proximal metatarsus (Mt3) – cannon bone/hock.
A mare presented with a seven-week history of forelimb lameness that failed to improve. There were no obvious clinical findings and no response to solar hoof tester placement. The lameness was worsened when the mare trotted in a circle. She showed minimal improvement to a palmar digital nerve block (blocks the heel area and sole of the foot) and was essentially sound after an abaxial sesamoid nerve block (blocks from the fetlock to the foot). X-rays failed to identify any specific abnormalities. Given the chronicity and lack of improvement, an MRI was undertaken. A marked DDFT lesion was diagnosed by MRI.
This horse had only a two-week history of forelimb lameness that was also worsened by trotting in a circle. Again, specific abnormalities on initial examination were not noted. The lameness essentially resolved after a palmar digital nerve block and radiographs failed to provide a specific diagnosis. MRI evaluation revealed marked navicular degeneration characterized by a flexor cortex erosion.
This horse had a two-month history of right rear lameness that was localized to the area of the proximal metatarsus (Mt3) – cannon bone/hock. The horse displayed a mild improvement to a lateral plantar nerve block (blocked the area of the proximal cannon bone – suspensory ligament), but a significant improvement was noted after a tibial peroneal nerve block (blocked the area of the hock). Radiographs failed to identify any significant abnormalities. An ultrasound evaluation of proximal Mt3 demonstrated proximal suspensory ligament enlargement. A bone scan of the rear limbs was considered normal. The horse was rested and had extracorporeal shock wave therapy performed to the area of the proximal suspensory ligament without any clinical improvement. An MRI evaluation revealed enlargement of the origin of the suspensory ligament with disruption of the medial bundle. The area of the hock was ruled out as a potential source of lameness.
In each instance, a specific treatment plan was initiated and a more accurate long-term prognosis for soundness was provided. It is important to remember that MRI is not a survey technique or a substitute for a thorough clinical investigation, including conventional imaging modalities. The interpretation of MRI findings is greatly enhanced by the information obtained from these evaluations. It is clearly not indicated in all situations, but may be considered in selected cases through consultation with your veterinarian.
Article written by Greg Haines, DVM, DACVS
Pilchuck Veterinary Hospital offers comprehensive diagnostic imaging options, including MRI, video endoscopy, high-resolution ultrasound and more. Please call 360.568.3111 for more information.