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Diagnosis and Management of Palmar Foot Pain
T.A. Turner
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Take Home Message—Diagnosis of palmar foot pain is based on a thorough examination that differentiates various types of pain emanating from the foot. Management is based on shoeing to improve hoof biomechanics and therapy aimed at the results of diagnostics.
I. Introduction
The palmar digital nerve block has traditionally been described as desensitizing the palmar one-third to one-half of the foot.1-3 But we know that the nerve block desensitizes much more including the coffin joint, most of the distal phalanx, and the sole of the hoof. Lamenesses in this region account for more than one-third of all chronic lamenesses in the horse. It must be understood that a palmar digital nerve block simply localizes the source of the pain the horse senses to the back of the foot. It is important to identify as specifically as one can the pathological and clinical findings. This in turn will help the clinician make their best assessment of the problem, and recommend the most specific treatment.
II. Diagnosis
The first step in developing a logical approach to the treatment of palmar hoof pain is an accurate assessment of the pain and careful evaluation of hoof structures 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.4 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 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.1,4 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 palmar foot pain.4 The hoof extension test is performed by elevating the toe with a block, holding the opposite limb, and 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 forcing the horse to stand on that foot. 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.
As part of the overall evaluation of these horses an objective assessment of hoof balance is important.3 Eleven measurements are made of each foot. The horse's weight is determined with a weight tape or scale. Seven measurements are made of the hoof length with a tape measure: medial and lateral heel lengths; medial and lateral quarter lengths, dorsomedial and dorsolateral toe lengths, and sagittal toe length. These measurements are recorded on a graph to illustrate the general shape of the foot. In addition, the frog's length and width are measured at their longest and widest points. The hoof circumference immediately below the coronary band, and the hoof angle (using a hoof gauge) are also measured. From these measurements, two additional measurements can be calculated: the frog ratio (frog width divided by length) and the body size to hoof area (horse's weight (pounds) X 12.56/square of the hoof wall circumference(C) (inches)).
A dorsopalmar(plantar) and a lateral radiograph of the hoof can also be used to determine valuable information about hoof balance.3 The horse must be standing with the metacarpus (tarsus) perpendicular to the ground, which can most easily be determined by either the use of a level placed against the cannon bone or the use of a weighted string to align the leg. The radiographic beam should be horizontal and centered on the hoof. Resting the horse's foot on a block to raise the hoof off the ground facilitates these exposures (the opposite limb should be similarly elevated).
Typically, all these before mentioned lamenesses will be improved by at least 90% after perineural anesthesia of the palmar digital nerves but it does not help differentiate these lamenesses.1 Anesthesia of the distal interphalangeal (DIP) joint or the podotrochlear bursa are additional procedures that provide information about palmar hoof pain.4 In a study reported by Turner, 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 are 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.3 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 foot pain can be divided into 5 groups,4 those horse with navicular region pain (desensitized by DIP analgesia and bursa analgesia, as well as palmar digital analgesia), those with distal interphalangeal pain (desensitized by DIP analgesia, as well as palmar digital analgesia but not bursa analgesia), those that are not desensitized by DIP analgesia but are desensitisized by bursa analgesia, as well as palmar digital analgesia, those that are improved by either DIP or bursa analgesia but are not sound but are sound after palmar digital analgesia, and those that are not desensitized by either DIP or bursal analgesia but are desensitized by palmar digital analgesia. It should be noted, podotrochlear bursa injection can be difficult compared to the DIP joint which is easier. We have found that not only is radiographic control necessary to successfully perform podotrochlear bursa injection 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.6 Injectionintothe bursa was made from the palmar surface with the limb flexed at the carpus. Injection into the bursa was made from the palmar surface with the limb flexed at the carpus. Aseptic injection techniques were used to inject a 3-ml mixture of 1:1 contrast material and local anesthetic or medication. The landmarks for needle insertion was a point just proximal to the central sulcus of the frog, with the needle directed towards the apex of the frog and in a direction parallel to the ground surface of the hoof. A 20 gauge, 3.5-in. (9 cm) needle was used. The needle was inserted until resistance was encountered; this was usually at 2/3 the length of the needle. If the needle was inserted further before encountering resistance, it usually indicated incorrect placement. A lateral radiograph of the hoof was taken to confirm the position of the needle prior to injection. Ideally the needle tip was midway between the proximal and distal borders. Once the needle position was confirmed, the bursa was injected with the contrast mixture and a second lateral hoof radiograph was taken to confirm the filling of the bursa. If the bursa had been successfully injected, then a palmaroproximal–palmarodistal (PP-PD) oblique projection of the navicular bone was obtained. The contrast material seen from a lateral view normally had the shape of an apostrophe. The contrast, seen from the PP-PD projection, was a distinct line of contrast material juxtaposed to the deep digital flexor tendon which was normally separated from the navicular cortical bone by a layer of radiolucent fibrocartilage.
The bursograms were evaluated for twelve different changes:6 (1) normal flexor fibrocartilage seen as a uniform radiolucent area 1-2mm in thickness covering the flexor surface of the navicular bone; (2) thinning or erosions of the flexor fibrocartilage, seen as a loss of the thickness of the previously mentioned radiolucent line; (3) fibrillation or splits of the deep flexor tendon within the navicular bursa, which was noted as filling defects along the bursal surface of the deep flexor tendon; (4) presence of flexor subchondral bone cystic defects, which were noted as focal filling of the flexor cortical area with contrast (5) communication of the navicular bursa with the distal interphalangeal joint, seen as leakage of the contrast from the bursa into distal interphalangeal joint; (6) complete focal loss of the dye column, which was thought to be a result of flexor tendon adhesion to the bone; (7) narrowing or enlargement of the proximal to distal borders of the bursa (bursa change) thought to represent inflammatory changes of the bursa; (8) leakage of contrast from the bursa suggesting a tear of the border of bursa; (9) marked widening of the contrast thickness thought to indicate loss of palmar support of the tendon by the distal annular ligament; (10) contrast within the body of the tendon thought to be a focal deep flexor tear; (11) contrast within the impar ligament assumed to be indicative of tearing or damage to the impar ligament; and (12) contrast within or surrounding the proximal suspensory of the navicular bone indicative of ligament injury. To date, we have had no complications to the podotrochlear bursa injection.
Recently it has become possible to examine the podotrochlear region sonographically.7 In order 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.7 The bursa is seen as a hypo echoic (fluid filled) region juxtaposed to the navicular bone. The deep flexor 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. [...]
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