Lameness of the foot is the most common cause for soundness issues in today’s horse. Traditionally, hoof testers were the most popular tool for trying to understand lameness of the foot. If a horse was negative to hoof testers, but continued to be foot sore regardless of treatment, he was turned out until soundness was achieved. Radiographs would be utilized to try to identify a fracture, arthritis, significant navicular bone changes, or significant degenerative changes at the attachments of the soft tissues (and surmise injury to those structures).
Today, the quality of these radiographs is greatly improved, but still only lends a small window into the foot. Hoof testers are used to try to determine the presence of an abscess or hot nail, but a mildly positive or negative response to the testers is not the end of the diagnostic process. With superior quality ultrasound equipment available, some diagnosticians have attempted to image through the frog, but this only offers a very limited window into the complex anatomy of the foot, and tells you nothing of the physiologic processes of the few structures that can be imaged (i.e. bone contusion, or active inflammation of the deep digital flexor tendon). With increased use of MRI in sporthorse practice, identification of bone bruising, early soft tissue sprains, and significant injuries to the previously inaccessible areas of the foot (i.e., distal aspect of the collateral ligaments of the coffin joints) is now possible. A horse with a mild injury can have a targeted treatment, and the question of “compete or not” with acute lameness can easily be answered.
Recommendation of denerving surgery to the foot for certain patients which have pain referred to the foot can be made more confidently if MR imaging can confirm healthy soft tissue structures, and additionally, monitor them post-operatively to insure the health of the foot, and therefore the safety of horse and rider as they continue to compete. Hence, many of these upper level horses can be useful and continue their successful careers. Coupled with a good lameness work-up and history, Nuclear Scintigraphy, coupled with MR imaging has been paramount in advancing the diagnosis of distal limb lameness above the foot.
With nuclear scintigraphy referring the MR coil to areas of interest, horses that don’t read the book on nerve blocks can also be figured out more efficiently. Horses that block to palmar digital or abaxial sesamoid analgesia may have a “hot spot” in the fetlock on bone scan, and MR imaging will demonstrate bony cysts, subchondral bone injury or edema not visible on traditional radiographs. There are also a substantial population of horses that block to a “high suspensory;” with negative or questionable radiographic and ultrasound findings, or horses that block lower, but distal limb imaging is negative for pathology.
These horses often have increased radiopharmaceutical uptake at the level of the origin of the suspensory ligament, and placing the MR coil at the area directed by the bone scan often identifies regions of proximal suspensory injury with hock or knee involvement. These lesions dictate a different treatment and prognosis than those with soft tissue injury only. And by using the results of the advanced imaging modalities, ultrasound can be used with more confidence to monitor these injuries following definitive diagnosis.