EQUINE VETERINARY EDUCATION / AE / NOVEMBER 2020
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length will approach the length of the toe (Fig 3). Increased stress on the hoof wall at the toe will eventually cause a concavity along the dorsal surface of the hoof wall. Stress exerted on the sole-wall junction in the toe area will cause it to widen, allowing separations to occur. Farriers have traditionally applied toe extensions to create a lever arm using a shoe or a composite to force the heel to
a)
the ground, but this will only exacerbate damage to the dorsal section of the foot, promote hoof wall separation and delay breakover (Hunt 2012; O’Grady 2017). As it is important to consider the use of a toe extension in biomechanical terms and to understand the effect of this leverage, it is necessary to briefly consider the moments about the distal
b)
interphalangeal joint. The moments about the DIPJ are the flexor moment and the extensor moment (a moment is equal to force x distance). At rest, the extensor moment is the product of the weight born by the limb (a force) and the horizontal distance from the point at which the ground reaction force GRF acts on the foot (centre of pressure CoP) and the centre of rotation of the distal interphalangeal joint (a distance). The flexor moment opposes the extensor moment and is the product of the force (tension) in the tendon and the shortest distance of the deep digital flexor tendon (DDFT) from the centre of rotation. Normally these two moments are balanced at rest, the GRF is dorsal to the centre of rotation of the joint and the heels are on the ground. In foals with a flexural deformity of the DIPJ, the tension in the tendon increases (as a result of the shortened musculotendinous unit), which increases not only the flexor moment but the opposing extensor movement which moves the CoP dorsally in the toe. At the point where the tension in the tendon increases such that the CoP is at the dorsal limit of the toe, any further increase in tendon tension cannot be balanced by movement of the CoP because it is already at the perimeter of the toe and can't move further dorsally, therefore, the heels lift off the ground (Eliashar 2012). Adding an extension to the toe of the foot allows the centre of pressure to move further dorsally in the toe in an attempt to counteract the tension in the tendon and force the heels to the ground. The ability of the toe extension to do this is dependent on the integrity of the tissues (hoof wall, lamellae), however, the tissues are not generally strong enough to withstand the additional stress and separation occurs within the dorsal hoof wall. Therefore, placing an extension or leverage on the dorsal hoof wall to counteract the forces in the shortened deep digital flexor muscle tendon unit is unrealistic and, in the author’s opinion, contraindicated (Fig 4). Furthermore, extensions may contribute to lameness due to excessive stresses on the DDFT when the foal puts full weight on its foot and at the initiation of breakover. Radiographs should be used to assess changes in the joint
Fig 3: Chronic grade 3 flexural deformity in a 3-month-old foal that has acquired a tubular shape of the hoof capsule with the coronet becoming horizontal. [Colour figure can be viewed at
wileyonlinelibrary.com]
and the integrity of the distal phalanx. The author will administer mild sedation (xylazine [0.33 mg/kg bwt, i.v.] combined with butorphanol [0.022–0.066 mg/kg bwt i.v.]) and place each of the foal’s feet on separate wooden blocks of equal height, which allows normal or equal loading of both forefeet. The forelimbs are positioned such that the third metacarpal bone is perpendicular to the ground. Lateral-to- medial weight-bearing images of both forefeet should be acquired. The degree of flexion of the DIPJ, the angle of the dorsal hoof wall and abnormalities at the margin of the distal phalanx should be assessed (Fig 5). When a marked flexural deformity is noted during radiographic examination of the feet, conservative treatment and hoof trimming alone are generally unsuccessful in resolving the foot conformation. Elevating the heels has been advocated to reduce tension in the DDFT and to promote weight-bearing on the palmar section of the hoof. However, although elevating the heels will load the palmar section of the foot, improve the hoof- pastern axis and make the foal more comfortable initially, the
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