Relationship of foot conformation and force applied to the navicular bone of sound horses at the trot

Reasons for performing study: Collapsed heels conformation has been implicated as causing radical biomechanical alterations, predisposing horses to navicular disease. However, the correlation between hoof conformation and the forces exerted on the navicular bone has not been documented. Hypothesis: The angle of the distal phalanx in relation to the ground is correlated to the degree of heel collapse and foot conformation is correlated to the compressive force exerted by the deep digital flexor tendon on the navicular bone. Methods: Thirty-one shod Irish Draught-cross type horses in routine work and farriery care were trotted over a forceplate, with 3-dimensional (3D) motion analysis system. A lateromedial radiograph of the right fore foot was obtained for each horse, and various measurements taken. Correlation coefficients were determined between hoof conformation measurements and between each of these and the force parameters at the beginning (15%) of stance phase, the middle of stance (50%) and at the beginning of breakover (86% of stance phase). Significance was defined as P<0.05. Results: The force exerted on the navicular bone was negatively correlated (P<0.05) to the angle of the distal phalanx to the ground and to the ratio between heel and toe height. This was attributed to a smaller extending moment at the distal interphalangeal joint. There was not a significant correlation between the angle of the distal phalanx and the degree of heel collapse, and heel collapse was not significantly correlated to any of the force parameters. Conclusions: Hoof conformation has a marked correlation to the forces applied to the equine foot. Heel collapse, as defined by the change in heel angle in relation to toe angle, appears to be an inaccurate parameter. The forces applied on the foot are well correlated to the changes in the ratio of heel to toe heights and the angles of the distal phalanx. Potential relevance: Assessment of hoof conformation should be judged based on these parameters, as they may have clinical significance, whereas parallelism of the heel and toe is of less importance.