Noninvasive photoelastic method to show distribution of strain in the hoof wall of a living horse.

A photoelastic method used for materials testing in industry was adapted to show the distribution of strain through the hoof wall in the living horse. Strain was a change in length per unit length in the material of the loaded hoof wall compared with the unloaded condition. Coloured fringes appeared in the photoelastic plastic where there were differences in strain between adjacent sites (strain gradients) in the hoof. Strain distribution was observed in the shod and unshod hoof wall of the front hooves of 6 sound horses with hooves that appeared 'good' to visual inspection, and one unsound horse with hoof cracks. No significant differences in strains were apparent across the hoof walls of the sound horses when the horses were standing normally. Steep strain gradients were apparent in hooves, associated with defects such as cracks, unstable nail holes, and long toes.