The Use of Toe Extensions in Foals / Flexural Deformities
Stephen E. O’Grady, DVM
With foaling season upon us or when confronted with flexural deformities in general, it may be helpful to review the biomechanical aspects of applying toe extensions. Farriers have traditionally applied toe extensions to create a lever arm using a shoe or a composite to force the heel to the ground. To understand the effect of this lever arm, it is necessary to briefly consider the moments-about the distal interphalangeal joint (DIPJ). There are two moments, 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 borne by the limb (a force) and the horizontal distance from the point at which the ground reaction force (equivalent to weight) acts on the foot (center of pressure (CoP)) and the center of rotation (COR) of the distal interphalangeal joint. The flexor moment opposes the extensor moment and is the product of the distance of the DDF from the center of rotation and the force (tension) in the tendon. Normally at rest, the extensor and flexor moment are balanced, and GRF is dorsal to the COR of the DIPJ. In foals with a flexural deformity (so-called contracted tendon) of the distal interphalangeal joint, the tension in the DDF tendon increases (for various reasons), which increases the flexor moment. If the foot is to stay flat on the ground, the extensor moment must also increase, and this occurs because the COP moves dorsally, i.e. the moment arm distance increases. At the point the tendon increases such that the center of pressure is at the toe, any further increase in tendon tension cannot be balanced by movement of the COP because it is already at the toe, so the heels come off the ground. By adding an extension to the toe of the foot, this now permits the center of pressure to move further dorsally before the heels come off the ground (though it should be noted that the toe extension is not always sufficient to keep the heels on the ground if the tension in the DDF tendon is still too high). In other words, the toe extension is creating leverage forcing 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). Unfortunately, the tissues are immature and not always strong enough to withstand the additional stress thus, the toe of the hoof capsule distorts, and separation occurs within the dorsal hoof wall.