The morphology of the interfacial tissue between bighorn sheep horn and bony horncore increases contact surface to enhance strength and facilitate load transfer from the horn to the horncore

AUTHORS

Luca H. Fuller, Evan C. Marcet, Laura L. Agarkov, Prisha Singh, Seth W. Donahue

ABSTRACT

The horns of bighorn sheep rams are permanent cranial appendages used for high energy head-to-head impacts during interspecific combat. The horns attach to the underlying bony horncore by a layer of interfacial tissue that facilitates load transfer between the impacted horn and underlying horncore, which has been shown to absorb substantial energy during head impact. However, the morphology and mechanical properties of the interfacial tissue were previously unknown. Histomorphometry was used to quantify the interfacial tissue composition and morphology and lap-shear testing was used to quantify its mechanical properties. Histological analyses revealed the interfacial tissue is a complex network of collagen and keratin fibers, with collagen being the most abundant protein. Sharpey's fibers provide strong attachment between the interfacial tissue and horncore bone. The inner horn surface displayed microscopic porosity and branching digitations which increased the contact surface with the interfacial tissue by approximately 3-fold. Horn-horncore samples tested by lap-shear loading failed primarily at the horn surface, and the interfacial tissue displayed non-linear strain hardening behavior similar to other soft tissues. The elastic properties of the interfacial tissue (i.e., low- and high-strain shear moduli) were comparable to previously measured values for the equine laminar junction. The interfacial tissue contact surface was positively correlated with the interfacial tissue shear strength (1.23 ± 0.21 MPa), high-strain shear modulus (4.5 ± 0.7 MPa), and strain energy density (0.38 ± 0.07 MJ/m3).

Material properties of bighorn sheep (Ovis canadensis) horncore bone with implications for energy absorption during impacts

AUTHORS

Luca H.Fuller, Seth W.Donahue

ABSTRACT

Bighorn sheep rams participate in high impact head-butting without overt signs of brain injury, thus providing a naturally occurring animal model for studying brain injury mitigation. Previously published finite element modeling showed that both the horn and bone materials play important roles in reducing brain cavity accelerations during ramming. However, in that study the elastic modulus of bone was assumed to be similar to that of human bone since the modulus of ram bone was unknown. Therefore, the goal of this study was to quantify the mechanical properties, mineral content, porosity, and microstructural organization of horncore cortical bone from juvenile and adult rams. Mineral content and elastic modulus increased with horn size, and porosity decreased. However, modulus of toughness did not change with horn size. This latter finding raises the possibility that the horncore cortical bone has not adapted exceptional toughness despite an extreme loading environment and may function primarily as an interface material between the horn and the porous bone within the horncore. Thus, geometric properties of the horn and horncore, including the porous bone architecture, may be more important for energy absorption during ramming than the horncore cortical bone. Results from this study can be used to improve accuracy of finite element models of bighorn sheep ramming to investigate these possibilities moving forward.