LOCAL MECHANICAL ANISOTROPY IN HUMAN CRANIAL DURA-MATER ALLOGRAFTS

Human cranial dura mater (CDM) allograft's success as a repair biomate rial is partly due to its high mechanical strength, which facilitates its ability to form water-tight barriers and resist high in-vivo mecha nical lends. Previous studies on CDM allograft mechanical behavior use d large test specimens and concluded that the allograft was mechanical ly isotropic. However, we have quantified CDM microstructure using sma ll angle light scattering (SALS) and found regions of well-aligned fib ers displaying structural symmetry between the right and left halves ( Jimenez et al., 1998). The high degree of fiber alignment in these reg ions suggests that they are mechanically anisotropic. However, identif ication of these regions using SALS requires irreversible tissue dehyd ration, which may affect mechanical propel-ties. Instead, we utilized CDM structural symmetry to estimate the fiber architecture of one half of the CDM using computer graphics to flip the SALS fiber architectur e map of the corresponding half about the plane of symmetry. Test spec imens (20 mm x 4 mm) were selected parallel and perpendicular to the p referred fiber directions and subjected to uniaxial mechanical failure testing. CDM allografts were found to be locally anisotropic, having an ultimate tensile strength (UTS) parallel to the fibers of 12.76 +/- 1.65 MPa, and perpendicular to the fibers of 5.21 +/- 1.01 MPa (mean +/- sem). These results indicate that uniaxial mechanical tests on lar ge samples used in previous studies tended to mask the local anisotrop ic nature of the smaller constituent sections. The testing methods est ablished in this study can be used in the evaluation of new CDM proces sing methods and post-implant allograft mechanical integrity.