The Hoof
Anatomy
Bone is continually remade. However, the microscopic architecture of hoof keratin is derived from the topology of the basement membrane and is fixed after the generative cells have died. Material properties do change with relative hydration. The horse hoof has two gradients of hydration in the hoof wall (Thomason et al. 1992). The hoof wall is thickest at the toe. It becomes thinner through the quarters to the heels and thicker again at the angles of the heels. This thickened portion forms the bars. It is the wall of the hoof that makes contact with the ground. The concave sole does not touch the ground except where it meets the white line. The hoof is joined to the pastern at the coronet.
The hoof wall grows 8 to 10mm per month. Growth is slower in a cold environment or a dry environment. Irregular lines around the wall may be normal. These indicate severe changes in season or changes in nutritional status, which have affected growth. Abnormal rings can be caused by laminitis, febrile episodes, blistering the coronet and low ringbone (Kainer 1989).
Figure 1. Anatomy of the Hoof. The figure shows a cut away of the side view of the hoof and the bottom of the hoof and anatomical features. Permission from www.spequine.com
Motion
Hoof keratin has a multidirectional composite. This allows it to tolerate strains in any direction. Strain orientation is unaffected by the speed or gait during the stance phase (Thomason et al. 1992). However, many of the non-directional factors associated with hoof stain depend upon the gait of the horse, the speed at which the horse moves, and the surface on which the horse is moving. The motion of the digit has been examined at the walk and the trot in detail. As the speed of the horse increases and the support time decreases, the angular velocity in the digit increases (Schryver et al 1978).
The hoof is able to handle two types of loading; high velocity impacts between the ground and transmission of forces between the skeleton and the ground. The distortion in the hoof of each animal is affected in a regular repeatable manner. Irregular substrata can result in unpredictable loading patterns, as does cornering, acceleration, deceleration, jumping and shoeing (Thomason et al 1992).
The hoof angle of the hindlimb is greater at impact than the hoof angle of the forelimb. As a result, the front hoof moves into a flat position earlier than does the rear foot. Horses often land toe-first in the forelimbs. Velocity at impact and resulting hoof acceleration in a given direction seems to be due to the velocity with which the hoof lands. The maximal friction force is proportional to the vertical force in all normal movements. The front hoof has a higher vertical velocity than the rear hoof and therefore has more bounce at impact. The rear hoof has a higher horizontal velocity and therefore slides more at impact. The maximal horizontal hoof velocity during the swing phase was reached earlier in the hindlimb than in the forelimb (Back et al. 1995c).