Simple SummaryHorseshoes are placed on the hooves of horses to prevent excessive wear on the hoof when horses are ridden or used for work. Typically, horseshoes are attached to the bottom of the hoof with nails. Although nails have been used for centuries, we sought to understand whether nailing a shoe to the hoof would adversely affect hoof expansion that normally occurs when weight is borne on the limb. Horseshoes were attached to cadaveric hooves by nails in three different sets of positions. Weight bearing was simulated while hoof expansion and compression and tension on the surface of the hoof were measured. We found that placing nails closer to the back of the hoof limited hoof expansion and caused changes in compressive and tensile forces on the surface of the hoof. Limiting hoof expansion interferes with the hoof’s ability to absorb concussion. Over time, the observed changes in compression and tension may cause abnormal changes in hoof shape that can affect soundness and performance.AbstractRacehorses are susceptible to underrun heel hoof conformation. Racehorses are often shod with nails placed toward the heel. It is unknown if palmar nails restrict or alter hoof deformation in a manner that could promote the development of underrun heel conformation over time with repeated loading. To determine how the addition of palmar nails affects heel deformation during limb loading, hoof expansion and hoof wall deformations were quantified using rosette strain gauges and kinematic markers during in the vitro limb loading of cadaveric limbs that simulated midstance for walk, trot, and canter loads. Nail treatments used to attach a horseshoe to the hoof included: toe nails (T), toe and quarter nails (TQ), and toe, quarter, and heel nails (TQH). The effects of nail treatment on heel expansion and hoof wall deformations were assessed using repeated measures analysis of variance (p< 0.05). Nails placed palmar to the quarters of the hoof decreased heel expansion (p< 0.001). Heel nails resulted in the largest changes in hoof wall principal strain directions distally. The application of nails palmar to the hoof quarters alters hoof wall deformation during limb loading. The continued loading of the hoof with palmer nails could alter hoof conformation over time.