Designing Into 3D for Quadruped Orthotics

Abstract

The goal of this research is to leverage local equine keratin material properties and their correlation to hoof tubule density (TD), hydration state (biological and environmental), temperature (biological and environmental), and activity (related to strain rate) in order to design custom horseshoes and/or orthotics that accommodate animal- and hoof-specific conditions. A farrier may analyze castoff hoof trimmings inherent to the shoeing process to obtain an approximate TD distribution profile of the hoof capsule. The TD distribution profile paired with the local environment temperature and humidity can be used to reasonably predict the stiffness of the hoof capsule wall. The hoof capsule wall geometry can either be obtained via a three-dimensional exterior scan, an x-ray, manual measurements, or a combination of all three, and a computer aided design (CAD) model can be constructed. From there, a horseshoe can be modeled and functionally graded to match the changing stiffness of the hoof capsule perimeter. If the hoof has conformation issues or injury, this process can be leveraged for a more detailed and powerful solution. An orthotic can then be designed to assist in correcting and healing a myriad of hoof issues, including but not limited to correcting skeletal alignment, keratin capsule growth/reshaping, promoting specific heel expansions/frog engagement, and improving the digital cushion. A CAD model of the equine digit may be constructed and used to simulate a handful of shoeing scenarios to determine the best shoeing practices for a specific animal. In this dissertation, the stages of this procedure were defined and tested to prove the concept. The local equine keratin tensile properties were systematically examined under a variety of conditions, which were comprised of combinations of hydration, strain rate, tubule orientation, and temperature. Emphasis was placed upon observing the change in the modulus of elasticity under these circumstances. Thin specimens were fabricated on the micro-scale dimensions to better isolate the different tensile properties of the three hoof wall regions, the Stratum Externum (SE), Stratum Medium (SM), and Stratum Internum (SI). The stiffness of local equine keratin was correlated to the TD of the keratin. Specimens were tested at multiple strain rates (10-2, 10-3, 10-4), but only within the elastic region to facilitate multiple measurements of the elastic modulus (E) for 0-100% relative humidity (%RH). The E of proximodistal specimens ranged from 0.25-18.3GPa for hydration levels ~0-100%RH, demonstrating an exponential decline of stiffness as hydration increased. For example, proximodistal specimens with 19≤TD≤22 saw 1.02≤E≤8.26GPa with trends: y=5.361(1-0.022)x (104s-1, s.e.=0.2803, r2=0.9721), y=6.610(1-0.019)x (103s-1, s.e.=0.2419, r2=0.9803), y=9.886(1-0.020)x (102s-1, s.e.=0.5776, r2=0.9588). Comparable specimens of similar TD all demonstrated increased stress responses for faster strain rates. The same trend applied to mediolateral specimens, but with lower elastic moduli (0.23-3.25GPa) and significantly less variation. TD, activity, E and %RH significantly impact hoof wall keratin performance. Results highlight the impact of environment on hoof-wall keratin and the importance of TD assessments for comparative hoof wall studies and non-invasive hoof assessments in the field. Bulk wall keratin specimens were tension tested at a strain rate of 10-3 s-1 and a variety of hydration conditions. The bulk wall specimens saw stiffnesses ranging from 175.0-829.2 MPa. These experimental results were compared with simulated results obtained via finite element analysis (FEA). The bulk specimens were modeled, and local keratin material properties associated with the TD profiles were assigned to the specimen model thickness layers. This method of predicting equine hoof keratin properties based on TD, %RH, and temperature is demonstrably successful for applications robust enough to withstand a +10/-16% envelope with regards to stiffness, and a +/-10% envelope for a general stress response. A CAD model of the equine digit was constructed and used to simulate a handful of scenarios: a barefoot uniform hoof, a uniform hoof with a keg shoe, and a barefoot hoof with multi-toned keratin. Resulting stresses and displacements in the control scenario (keg shoe) were comparable to the literature. This approach to modeling the hoof appears to be an accessible method with the ability to customize this model to match the specific hoof wall properties of monotoned and multitoned keratin as predicted by the TD distribution. Additively manufactured ABS was analyzed to obtain various mechanical properties of different infill patterns, density settings, and layer orientation to match the typical material behavior of a barefoot equine hoof. Additionally, Formahoof Advanced Polymer was analyzed in order to obtain the mechanical properties needed to simulate gluing a horseshoe onto the hoof as an alternative to nails. A simulated equine digit with an ABS shoe glued to the hoof perimeter demonstrated a reduction and smoothening of the stress distribution in the outer capsule wall as compared to the keg shoe simulation. This overall process was pursued to field trials, where a 3D printed ABS orthotic was designed and tested on a horse. Improved movements and skeletal alignment was observed in the animal post shoeing. This procedure has proven to be effective and a worthy research path to pursue further.