Mechanical behavior and quantitative morphology of the equine laminar junction

Abstract

The horse's hoof is structurally modified for its mechanical functions, but studying the functional design of internal structures is hampered by the external keratinous capsule. Finite-element analysis offers one method for evaluating mechanical function of components within the capsule, such as the laminar junction. This is the epidermodermal connection that binds the hoof wall strongly to the distal phalanx. Primary epidermal laminae (PEL), projecting inward from the wall, vary in morphology and are remodeled despite being keratinous. The aim of this study is to investigate the suggestion that remodeling of PEL is influenced by mechanical stress. Circumferential and proximodistal stress distribution and relative displacement in the laminar junction are assessed by finite-element analysis (FEA) of nine hoof models. Spacing, orientation, and curvature of PEL are assessed from sections through 47 other hooves and compared with the stress and displacement data. Significant correlations are found between laminar spacing and seven displacement and stress variables, supporting the link between stresses and remodeling. Differences in external hoof shape cause regional variation in stress magnitudes around the laminar junction. This finding is in accord with previous observations that laminar morphology is individually regionally variable. This work provides the first concrete link between mechanical behavior and laminar morphology. © 2005 Wiley-Liss, Inc. Key words: finite-element analysis; quantitative morphology;horse; hoof; laminar junction; displacement; stress; biomechanics As a result of being coopted as part of the musculoskeletal system, the equine hoof shares two attributes with that system: numerous modifications for the mechanical functions the hoof performs, and the capacity to respond to variations in loading over time. Hoof anatomy, microstructure, and growth are well documented (Stump, 1967), and ground reaction forces (GRFs) during locomotion have been experimentally recorded to determine the hoof's general mechanical function (Merkens et al., 1993). But the hoof's keratinous capsule and the nature of its attachment to the dermis and skeleton impede detailed study in vivo of its functional design and of the nature and mechanisms of its biological response to variability in loading regimes. Thus, the deceptively simple smooth exterior guards its secrets better than any crenellated castle wall.Finite-element analyses (FEAs) are clearly applicable to this situation, especially given that there are good data on the external shape and loading of the hoof, the properties of its materials and strains in the wall during locomotion. Most previous FEAs of the equine hoof have focused on the capsule (Newlyn et al., 1998;Hinterhofer et al., 2000,

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