Morphological and material properties of the equine hoof wall and laminar junction
The equine hoof wall deforms under load, the dorsal wall flattening and moving palmarly, abaxial flaring occurring caudally. Such differential deformation implies differential loading within the foot. This study examines regional differences in modulus of elasticity of the hoof wall and laminar junction, and in primary epidermal laminar morphology, and interprets the findings in terms of local loading. The influence on laminar morphology of foot shape and age is also examined. The outer hoof wall has a higher modulus of elasticity (1004 MPa) than has the inner wall (523 MPa), and material stiffness is inversely related to moisture content. This allows simultaneous weight-bearing and stress protection of the laminae. Circumferential and proximo-distal differences in stiffness of the material per se were not apparent, but abaxial expansion is allowed by reduced thickness of the wall caudally. The laminar junction has a non-linear modulus of elasticity, becoming stiffer at higher strains in all directions (proximo-distal and medio-lateral shear, and radial tension). At low strains (0-0.1 in shear. 0-0.05 in tension) the mean modulus ranged from 222 to 424 kPa, increasing by approximately one order of magnitude in all three directions at higher strains. Some regional differences in modulus were present. The morphology of the primary epidermal laminae (in a transverse plane) showed marked regional variations. In all feet the dorsal laminae were essentially straight and oriented perpendicular to the adjacent wall. At the quarters the laminae were mainly oriented heelwards (from exterior to interior) and were bent within their length, the interior portion usually being drawn heelwards. Heel laminae were more variable. Adult feet showed a gradual increase in laminar spacing from the dorsum caudally. These findings support caudal movement of the third phalanx with respect to the hoof wall when loaded, with concentration of stress at the dorsum. Neonatal feet had similar laminar morphology to adult feet, but showed less mediolateral and proximodistal variability, and lacked the dorsal concentration of laminae. Adult feet with a low toe angle had consistently wider-spaced laminae than did "normal" feet, and showed greater mediolateral symmetry.