There are numerous causes of pain in the palmar aspect of the foot of the horse. These causes can be arbitrarily divided into conditions of the hoof wall and horn producing tissues, conditions of the third phalanx, and conditions of the podotrochlear region. Hoof problems would include hoof wall defects, such as cracks or clefts that involve the sensitive tissue; any laminar tearing, separation or inflammation; contusions of the hoof causing bruising or corn formation; abscess formation; and pododermatitis (thrush or canker). Third phalanx lameness' blocked out by palmar digital anesthesia would include wing fractures, marginal fractures, solar fractures, or deep digital flexor insertional tenopathy. Conditions of the podotrochlear region have been reported to include distal interphalangeal synovitis, deep digital flexor tendonitis, desmitis of the impar (distal navicular ligament) or collateral sesamoidean ligaments, navicular osteitis or osteopathy, and vascular disease. The common denominator of all these conditions is that they are characterized by pain that can be localized to the caudal aspect of the hoof.

The first step in developing a logical approach to the treatment of palmar hoof pain is accurate assessment of the pain and careful evaluation of hoof structure that may predispose to or cause the pain. Four diagnostic tests should be performed: hoof tester examination, distal limb flexion, hoof extension wedge test, and palmar hoof wedge test. A positive response to any of these tests is important but a negative response is equivocal and does not rule out any problem. Hoof tester examination should begin with systematic evaluation of the sole and then to the distal sesamoid (navicular) region, which includes the collateral sulci to opposite hoof wall, central sulcus to toe, and across the heels. A positive response should be repeatable, and in the distal sesamoid region the pain response should be uniform over those areas and must be evaluated in relation to examination of the remaining foot. That is, a positive response in the heels and quarters of the sole would also be expected to cause a positive response across the distal sesamoid region in the same area of the foot. Percussion utilizing a small hammer can also provide important information regarding pain in the hoof wall or sole.

Distal limb flexion test may exacerbate lameness if any of the three distal joints of the leg are affected by synovitis or osteoarthritis. A positive response could also be expected by any condition that causes induration of the tissues of the foot. This has been shown to be positive in over 95% of horses with navicular disease.

The hoof extension test is performed by elevating the toe with a block while holding the opposite limb off the ground, and then trotting the horse away after 60 seconds. The palmar hoof wedge test is performed by placing the block under the palmar two-thirds of the frog, and then forcing the horse to stand on that foot. The horse is trotted away after 60 seconds. The test can be further modified so that the wedge can be placed under either heel to determine if the pressure there causes exacerbation of the lameness.

Typically, all these before mentioned lameness' will be improved by at least 90% after perineural anesthesia of the palmar digital nerves but it does not help differentiate these lameness'. Anesthesia of the distal interphalangeal (DIP) joint or the podotrochlear bursa are additional procedures that provide information about palmar hoof pain. In a study reported by Dyson, in 95% of the horses examined using DIP and bursa anesthesia, significant new information about the pain the horse exhibited was realized. The pain relief by anesthesia of any of these three regions has been shown to overlap. The DIP joint and podotrochlear bursa do not communicate, and yet the results of injecting anesthetic into these synovial cavities is similar. Both cavities have in common the navicular bone, the impar ligament, and the collateral sesamoidean ligament (proximal suspensory ligament of the navicular bone). The neuroreceptors for the navicular bone are in those 2 ligaments and they can be anesthetized from either synovial cavity. Further, Bowker has showed that the palmar digital nerve is in very close proximity to the medial and lateral limits of the bursa and the nerve may be anesthetized at this level whenever the bursa is injected. Palmar heel pain can be divided into 2 groups, those horse with navicular region pain and those with other sources of heel pain.

Navicular region pain is diagnosed by each of the following blocks having an equal effect; palmar digital nerve block, DIP joint block, and navicular bursa block. It has also been noted recently that injection of the podotrochlear bursa can be very difficult and that it is quite easy to inject the DIP joint instead. We have found that not only is radiographic control necessary to successfully perform this block but that adding contrast media to the anesthetic to prove the limits of the block is also necessary.

This has lead to a new method of assessing navicular pathology; by evaluating the cartilage of the flexor surface of the navicular bone by contrast arthrography. In several cases we have been able to conclusively prove the presence of adhesions between the navicular bone and bursa. Radiographic examination is the imaging method most often used to assess osseous changes in the distal sesamoid bone and third phalanx. These changes with the exception of fractures are usually not pathognomonic but do provide insight into damage that has occurred to the foot. Fractures also may not be radiographically visible until 10 to 14 days after the injury occurred.

Scintigraphy is a technique that measures gamma ray emission from a radioactive nuclide injected into the animal. The technique provides information on relative vascularity and rate of tissue metabolism. This is particularly useful in studying bone pathology and can help differentiate sites of injury in the foot.

Thermography provides information regarding skin temperature. It has been shown to be useful in assessing the relative blood flow to a region. This information is of particular interest when pre- and post exercise temperatures are determined. Exercise will normally cause a 0.5oC rise in skin temperature. Whenever, the skin temperature does not rise, poor blood flow should be considered a factor in the disease being assessed.

Ultrasonography can be used to examine the podotrochlea. The superficial horn must be pared from the frog to expose soft, spongy frog tissue. Next, sonographic gel is liberally applied to the frog. The ultrasound transducer is then applied to the frog. Images of the podotrochlea are apparent from the center of the frog to the apex. A 7.5 MHZ probe provides the best image. Generally, at the center third of the frog, the flexor surface of the navicular bone is readily noticeable as a hyper echoic line. The bursa is seen as a hypo echoic (fluid filled) region juxtaposed to the navicular bone. The deep flexor tendon fibers can be seen curving around the bone. As the probe is moved toward the apex of the frog, the distal aspect of the navicular bone can be identified as can the intersection between the deep flexor tendon and the impar ligament. As the probe reaches the apex of the frog, the deep flexor's insertion on the third phalanx becomes apparent. Ultrasound is an excellent means to visualize soft tissue structures. However, examination of the foot has been limited to the pastern because the hoof capsule served as a barrier to examination of the hoof. The proximal regions of the navicular bone could be examined if one had a special probe that would fit between the bulbs of the horse's heels. However, this gave no information as to what may be occurring further distally. The frog however, because of its high water content can serve as the hoof's standoff. By removing the hard, outer layers this exposes tissue that can transmit sound waves allowing the examiner to see this distal tissues. To date, we have performed only enough cases to have an appreciation for the anatomy. Many more cases will need to be examined to determine how useful this technique might be.

In addition to the distal podotrochlea, the collateral ligaments of the distal interphalangeal joint can also be evaluated. This is done by going through the coronary band just dorsal to the collateral cartilages. The collateral cartilages and portions of the digital cushion can also be evaluated sonographically. A thorough examination of the horse affected by pain in the palmar region of the foot can allow a more precise diagnosis to be made, whether the diagnosis reflects injury to the hoof capsule, third phalanx, or podotrochlear region. Treatment then should be based on the type of injury.

There are differences in the clinical presentation of navicular region pain (NRP) and palmar heel pain (PHP). The University of Minnesota has had an ongoing prospective study of these findings. So far approximately 54% of the cases seen are affected by NRP and 46% by other sources of PHP. Clinical signs for these two groups have shown interesting differences. Distal limb flexion has been positive in 100% of the NRP and only 88% positive in horses in the PHP group. Hoof tester examination, which is considered a cardinal sign of navicular problems, was positive in only 54% of the horses with NRP as compared to 65% for those with PHP. The frog wedge was positive in 79% of the NRP as compared to 70% of the PHP horses; whereas, the toe wedge was positive 64% in NRP and only 43% in PHP. Circulatory testing indicated that only 26% of the NRP horses had poor circulation as a component to their disease, compared to 53% of the PHP horses having compromised circulation. Scintigraphy was positive in only 62% of the NRP cases indicating that pain can be present without scintigraphic changes. Also 20% of the PHP horses have a positive bone scan indicating that the navicular bone may be involved in a complex problem of heel pain.

REFERENCES
1. Turner TA: Diagnosis and treatment of navicular syndrome in horses. Vet Clin NA Equine Pract. 5: 131-144, 1989.

2. Bowker RM, Rockershouser SJ, Linder K, et al: A silver-impregnation and Immunocytochemical study of innervation of the distal sesamoid bone and its suspensory ligaments in the horse. Equine Vet J. 26: 212-219, 1994.

3. Turner TA: The use of hoof measurements for the objective assessment of hoof balance. Proceedings of Am Assoc Eq Practnr,. 38: 389-396, 1992.

4. Stashak TS: Adams lameness in horses. 4th Ed. Philadelphia, Lea & Febiger, 1987, pp 499-514.