Meddleton Equine Hospital

In recent years the problems stemming from the horse's foot have gained renewed attention. This is in a large part due to pioneering diagnostic imaging work done in England at the Animal Health Trust, Washington State University School of Veterinary Medicine, and the Maison Alfort school of Veterinary Medicine in Paris, France. Clinicians at these institutions started looking at the structures of the foot with MRI, CT, Nuclear scintigraphy, and high quality ultrasound.

The biggest advances in improving our understanding of injuries that occur within the foot were a result of MRI studies of equine hooves. MRI allowed the visualization of the soft tissue structures (ligaments, tendon, and synovial structures) and also gave a first glimpse at a phenomenon of bone edema or bruising; this was something that radiographs and ultrasound were unable to do. The biggest limitation of MRI was availability, but today MRI is finding its way into more and more practice across the country.

Another very important development in imaging of the equine foot came with novel techniques for ultrasonography. The discovery of soft tissue foot injuries through MRI prompted some equine ultrasonographers to find ways to examine the structures within the hoof. The problem is that the ultrasound cannot penetrate the hard hoof wall. They found a "window" by trimming down the frog to almost live tissue and thoroughly soaking the foot. This "window" through the frog allows ultrasound examination of some of the important structures within the foot; the end of the Deep Digital Flexor Tendon, Navicular Bursa, ligaments of the Navicular bone, and the flexor surface of the Navicular bone. Finally, an approach though the coronary band allowed examination of a portion of two very important collateral ligaments that hold the Coffin joint together.

Case Discussion

The following case report illustrates an example of a lameness related to one of these newly discovered foot injuries.

A 5 year old Quarter Horse gelding performance horse presented because of a mild to moderate lameness of 1 month duration. Prepurchase radiographs taken 1 year prior to presentation revealed the following: abnormal hoof-pastern angles (broken back), normal Navicular bones, and medial to lateral imbalance in both front feet evidenced by pronounced sidebone. Sidebone is a calcification of the collateral cartilages of the hoof. It is not considered a pathologic or lameness-causing change in itself, but it is associated with medial to lateral (M-L) hoof imbalance. M-L imbalance can lead to Coffin joint and surrounding ligament inflammation.

The first examination took place 3 weeks after the lameness was first noticed. Ten days of rest and 5 days on Phenylbutazone (Bute) had not helped improve the lameness. This suggested that a mild trauma to the leg or hoof was not to blame, and the owner opted for a Veterinary evaluation. The horse demonstrated a mild, bilateral forelimb lameness. There was mild sensitivity to hoof testers over the entire left sole.

Flexion stress tests of the shoulder, elbow, and Carpus (knee) were negative (did not make the horse more lame). The LF flexion of the Coffin, Pastern, and Fetlock joints (DE) did not cause an increase in lameness, but there was a 2+/5 response to flexion of the RF DE. The owner did not opt for nerve blocks at that time and radiographs of both distal extremities (DE) were taken.

M-L imbalance was still visible in both feet and the sidebone had progressed considerably. There were very mild changes (spurs) in both Pastern joints. The assessment at the time was of LF sole bruising and ligamentous strain in both DE secondary to the M-L imbalance. Very early Pastern arthritis was also considered as a possible cause of the lameness. The owner opted for a conservative treatment approach consisting of shoeing with pads using the Natural Balance method. This addressed the M-L imbalance and the LF sole pain.

He improved considerably with the new shoeing and padding, but came up lame again one month after the shoeing changes. The second lameness examination was almost identical to the first one. Nerve blocks were discussed, but again declined in favor of treatment. Based on the high likelihood of Coffin joint inflammation secondary to the chronic M-L imbalance, both Coffin joints were treated with a Corticosteroid and Hyaluronic acid. This is a joint therapy that is very effective in reducing joint inflammation for a period ranging in average of 3-12 months.

The response was excellent for 3 weeks, and then he started showing lameness again. The third examination revealed that the LF lameness had completely resolved, but he was 2/5 lame on the RF. A palmar digital nerve block did not result in improvement. This block anesthetizes most of the sole, the coffin joint, the Navicular bone and the ligaments supporting the Navicular bone. An abaxial sesamoid block of the RF resulted in soundness. In its classic definition, this block anesthetizes the pastern and the soft tissue structures below the fetlock.

Radiographs obtained earlier showed very mild Pastern arthritis, so the Pastern joints was treated (steroid and Hyaluronic acid). An ultrasound examination revealed a small tear and inflammation in the medial Superficial Digital flexor tendon (SDFT) branch at the level where it attaches to the short pastern bone. A combination of Shockwave therapy and rest were recommended to treat the tendon injury, but a referral for an MRI was recommended if there was no significant improvement after the Pastern injection.

Three weeks after Pastern joint injection, the horse had a 2/5 RF lameness. A standing MRI was performed at a referral practice. The MRI confirmed the presence of the medial SDFT branch lesion and discovered a tear in the medial collateral ligament of the distal interphalangeal joint (Coffin joint). The horse is currently being treated with rest and shockwave therapy to the injured tendon and ligament. An Equine foot specialist has also been consulted to address the shoeing in order to decrease the strains on the ligaments and tendons as much as possible. The prognosis for this injury is traditionally considered poor to guarded, but resent clinical impression using Shockwave therapy suggest that it is guarded to good.

This case illustrates a number of interesting aspects of foot lameness diagnostics. The palmar digital nerve block is generally the first one undertaken and it is classically described as anesthetizing the foot. However, it does not reliably anesthetize the collateral ligaments of the Coffin joint; approximately half of the cases will be sound after this block and the other half will only go sound after the next block is performed.

Furthermore, as illustrated by this case, ultrasound can not always visualize these injuries as a large portion of the collateral ligament is within the hoof and not accessible to ultrasounds. In the end, poor response to traditional joint therapies after localization of pain through nerve blocks prompted referral of the horse for advanced diagnostic imaging. The current case illustrated a problem with Coffin joint collateral ligaments, but similar scenarios can be seen with other sources of foot pain. Certain injuries to the portion of the Deep Digital flexor tendon that resides within the hoof do not respond to the traditional foot nerve blocks.

Alternatively, many lamenesses that block out like a classic Navicular problem end up having multiple injuries to the soft tissue structures within the foot and can only be diagnosed using an ultrasound approach through the frog as described earlier or through advanced diagnostic imaging modalities (CT, MRI). Finally, a phenomenon of bone bruising can result in confusing nerve block responses and incomplete to absent response to joint therapy options. These bone bruises are only visible with MRI. In summary, foot lameness can be very complicated and require patience and perseverance on the part of the owner, farrier, and Veterinarian.

MRI is a very useful tool in the following cases:

• Pain localized to the foot or Pastern using nerve blocks without any significant abnormalities on radiographs and ultrasound.
• Pain localized to foot or Pastern. Lesions found in joints on radiographs, but response to joint injections was incomplete or absent.
• Pain localized to foot or Pastern. Lesions found in soft tissue structures using ultrasound, but long term rest and appropriate adjunct therapies (Shockwave, A-Cell, Stem Cell, etc.) have not resulted in improvement.