Validation of a photogrammetric technique for computing equine hoof volume

Assessment of equine foot conformation is often based on linear and angular measurements performed on lateral digital photographs. However, quantification of external foot conformation requires more comprehensive assessments to capture the shape of the entire foot. Volumetric measurements of the hoof capsule represent a summary measure quantifying foot shape. The aim of this study was to develop a method for computation of virtual foot models from digital foot images allowing precise and accurate volumetric measurements. This photogrammetric technique was then assessed for the characterization of foot volume changes associated with foot trimming. Using the technique, three different photographers imaged feet from 18 cadavers at different time points and one analyst processed their images to generate virtual computer models. Volumetric measurements were obtained from these models to determine their precision in the context of 'Photographer', 'Time' and the effect of 'Trimming'. Computed tomographic (CT) imaging was used to assess the accuracy of the photogrammetric method. Pre-trim photogrammetric measurements showed excellent precision and accuracy and the results did not depend on the person acquiring the images. The accuracy of post-trim photogrammetric measurements deteriorated in comparison with the average differences measured by CT imaging (19 cm(3)). Precise volumetric measurements were obtained using the photogrammetric method, but average differences in foot volume after trimming as measured by CT imaging are likely too small to be detected with confidence.