UC Davis

Musculoskeletal injuries are a major cause of injury and death for racehorses. Underrun heel conformation in racehorses has been associated with catastrophic fetlock breakdowns during racing and training. While the cause of the development of underrun heels is not fully understood, two factors that could contribute to their development are the method that horseshoes are attached to the hoof and length of the shoe branches. As racehorses are often shod with shoes with shorter branches and with nails placed palmar to the widest portion of the hoof, the research presented here aimed to investigate the effect of nail placement location and shoe branch length, to determine whether either factor could play a role in the development of an underrun heel. Two in vitro experiments examined the effects of nail placement (increasing number dorsal to palmar) and 3 different shoe branch lengths on hoof wall surface strains and deformations during limb loading that simulated the middle of stance through gallop loads. Rosette strain gauges placed on the lateral side of the equine hoof in the heel and quarter regions of the lateral side of the hoof wall measured strains. Kinematic markers placed on the hoof tracked hoof expansion and distortion. A third in vivo 30 week long longitudinal crossover study examined the effect of shoes with either Full or Short branch lengths on forelimb hoof shape and growth. The most notable findings are related to nail placement configuration. In vitro, differences were observed in hoof expansion and hoof wall surface strains and deformations among hooves with nails placed in three configurations to attach the shoe to the hoof wall: toe nails (T), toe and quarter nails (TQ), and toe, quarter, and heel nails (TQH). Heel expansion decreased (P<0.001) when nails were placed under the TQH configuration. The largest changes in hoof wall principal strain directions occurred distally with the TQH configuration. With the hoof fixed to the ground, the direction of principal tensile strain in the distal heel and quarter regions became oriented to exert dorsal traction on the distal aspect of the heel, the orientation of principal shear strains changed the proximal portion of the heel to move palmarly relative to the distal portion of the heel, and the direction of principal compressive strains changed to direct compression on the heel in a distal orientation. Hoof wall deformation was altered during limb loading with nails placed palmar to the hoof quarters. The placement of nails closer to the heels (TQH) reduced hoof expansion and hoof wall deformation at the heels and altered hoof wall principal strain directions in a manner consistent with mechanisms leading to the development of underrun heels. Shoe length had smaller in vitro effects on hoof wall expansion, surface strains, and deformations when three shoe branch lengths (short shoe, full shoe, and long shoe) and no shoe were compared. Overall, principal compressive strain magnitudes were greater than principal tensile strain magnitudes for the lateral side of the hoof wall for all shoe length treatments. Increasing shoe length decreased principal compressive strains, except for the middle quarter location while shear strains increased for distal locations and decreased at the proximal heel location. Additionally, principal strain directions when viewed from the lateral side of the right hoof rotated in a counterclockwise direction at middle and distal quarter locations. Shoe length tested under these in vitro conditions did not provide evidence that would lead to the development of an underrun heel. Similarly, hoof wall expansion and lateral hoof wall distortion did not differ among in-vivo shoe length treatments. Twenty-Three Horses (10 Quarter Horses and 13 Thoroughbreds) were shod with two horseshoes of different branch length (Full and Short), over a 30-week period to determine the effect of branch length, breed, and exercise on lateral hoof wall angles, coronet lengths and shape, and hoof wall growth. Only the Palmar Tubule Distal Angle was significant for treatment (P=0.04) with Short decreasing and Full increasing (-0.1°, 2.8° respectively) the angle when adjusted over all other factors in the analysis. Over the duration of the study, Toe Distal Angle decreased in Thoroughbreds compared to Quarter Horses and decreased in horses not in training compared to those in training. Dorsal Tubule Proximal Angle decreased in Thoroughbreds and increased in Quarter Horses. Horses that were in training had decreased Toe Distal Angle in contrast with previous studies. Under the experimental conditions of this in vivo study, shoe length did not alter hoof growth or angles that would lead to the development of an underrun heel. Nail placement configurations demonstrated changes that could lead to an underrun heel during in vitro limb loading, but limb loading simulated only the middle of stance. Future studies should include a longitudinal in vivo study that examines different nail placements to validate the in vitro findings from this dissertation research. Additionally, further investigation into trimming methods used at racetracks for racehorses may provide valuable information on other aspects related to hoof maintenance and potential factors that may influence the development of underrun heels.