Nonlinear and anisotropic tensile properties of graft materials used in soft tissue applications

Abstract

Background: The mechanical properties of extracellular matrix grafts that are intended to augment or replace soft tissues should be comparable to the native tissue. Such grafts are often used in fiber-reinforced tissue applications that undergo multi-axial loading and therefore knowledge of the anisotropic and nonlinear properties are needed, including the moduli and Poisson's ratio in two orthogonal directions within the plane of the graft. The objective of this study was to measure the tensile mechanical properties of several marketed grafts: Alloderm, Restore, CuffPatch, and OrthADAPT.

Methods: The degree of anisotropy and non-linearity within each graft was evaluated from uniaxial tensile tests and compared to their native tissue.

Findings: The Alloderm graft was anisotropic in both the toe- and linear-region of the stress-strain response, was highly nonlinear, and generally had low properties. The Restore and CuffPatch grafts had similar stress-strain responses, were largely isotropic, had a linear-region modulus of 18MPa, and were nonlinear. OrthADAPT was anisotropic in the linear-region (131 MPA vs 47MPa in the toe-region) and was highly nonlinear. The Poisson ratio for all grafts was between 0.4 and 0.7, except for the parallel orientation of Restore which was greater than 1.0.

Interpretation: Having an informed understanding of how the available grafts perform mechanically will allow for better assessment by the physician for which graft to apply depending upon its application.

Figure 1

Size and shape of each graft material with the location, size, and orientation of test samples. Five grafts were evaluated: AlloDerm, Restore, CuffPatch, and OrthADAPT.

Figure 2

Representative stress-strain response for each graft material. Symbol = experimental data and solid line = model curve fit. Alloderm and OrthADAPT (par)allel and (per)pendicular are displayed as their toe and linear region modulus are statistically different between orientations.

Figure 3

Average toe-region modulus for each graft material, where error bars represent the standard deviation. Samples were aligned in two planar orthogonal orientations described as parallel (black bars) and perpendicular (white bars). Significant differences with orientation denoted by * P<0.05

Figure 4

Average linear-region modulus for each graft material, where error bars represent the standard deviation. All samples were aligned in two planar orthogonal orientations described as parallel (black bars) and perpendicular (white bars). Significant differences with orientation denoted by * P<0.05.

Figure 5

Average Poisson's ratio for each graft material, where error bars represent the standard deviation. Samples were aligned in two planar orthogonal orientations described as parallel (black bars) and perpendicular (white bars). Significant differences with orientation denoted by * P<0.05.