Anisotropic and viscoelastic tensile mechanical properties of aponeurosis: Experimentation, modeling, and tissue microstructure

Abstract

Aponeuroses are stiff sheath-like components of the muscle-tendon unit that play a vital role in force transmission and thus locomotion. There is clear importance of the aponeurosis in musculoskeletal function, but there have been relatively few studies of aponeurosis material properties to date. The goals of this work were to: 1) perform tensile stress-relaxation tests, 2) perform planar biaxial tests, 3) employ computational modeling to the data from 1 and 2, and 4) perform scanning electron microscopy to determine collagen fibril organization for aponeurosis tissue. Viscoelastic modeling and statistical analysis of stress-relaxation data showed that while relaxation rate differed statistically between strain levels (p=0.044), functionally the relaxation behavior was nearly the same. Biaxial testing and associated modeling highlighted the nonlinear (toe region of ~2-3% strain) and anisotropic (longitudinal direction linear modulus ~50 MPa, transverse ~2.5 MPa) tensile mechanical behavior of aponeurosis tissue.Comparisons of various constitutive formulations showed that a transversely isotropic Ogden approach balanced strong fitting (goodness of fit 0.984) with a limited number of parameters (five), while damage modeling parameters were also provided. Scanning electron microscopy showed a composite structure of highly aligned, partially wavy collagen fibrils with more random collagen cables for aponeurosis microstructure. Future work to expand microstructural analysis and use these data to inform computational modeling would benefit this work and the field.

The alteration of the structure and macroscopic mechanical response of porcine patellar tendon by elastase digestion

“…Since the theory of this model is linear viscosity hypothesis, it cannot fully describe the nonlinear viscoelastic behavior of various biological soft tissues ( Shetye et al, 2014 ). However, Prony series is widely used and proved to be a constitutive equation that can effectively express the viscoelasticity of materials ( Grega et al, 2020 ; Shearer et al, 2020 ; Li et al, 2021 ; Morrison et al, 2023 ). In this study, R 2 and NRMSE obtained by fitting the experimental data of stress relaxation of patellar tendon before and after treatment showed that Prony series had a good fitting effect and was suitable for this study.…”

Aponeurosis structure-function properties: Evidence of heterogeneity and implications for muscle function

“…Thus, materials testing of excised tissue samples under carefully controlled experimental environments are the current gold standard for material property estimation, despite the fact that these approaches remove the tissue from the native mechanical environment. To the best of the authors' knowledge, there have been two previously published studies that have combined materials testing and structural imaging of aponeurosis, both of which qualitatively showed a collagen-rich structure with aligned, wavy fibers [22,23]. Previous materials characterization works of aponeurosis have shown properties common to biological soft tissues such as anisotropy, nonlinearity, viscoelasticity, and heterogeneity [10,[22][23][24].…”

“…To the best of the authors' knowledge, there have been two previously published studies that have combined materials testing and structural imaging of aponeurosis, both of which qualitatively showed a collagen-rich structure with aligned, wavy fibers [22,23]. Previous materials characterization works of aponeurosis have shown properties common to biological soft tissues such as anisotropy, nonlinearity, viscoelasticity, and heterogeneity [10,[22][23][24]. Specifically, aponeurosis is highly anisotropic with a longitudinal modulus between 50-750 MPa and a transverse modulus of 0.3-100 MPa, which is greater than the approximate modulus of muscle (~0.1 MPa) and comparable to that of tendon (~1000 MPa) [22][23][24].…”

“…Previous materials characterization works of aponeurosis have shown properties common to biological soft tissues such as anisotropy, nonlinearity, viscoelasticity, and heterogeneity [10,[22][23][24]. Specifically, aponeurosis is highly anisotropic with a longitudinal modulus between 50-750 MPa and a transverse modulus of 0.3-100 MPa, which is greater than the approximate modulus of muscle (~0.1 MPa) and comparable to that of tendon (~1000 MPa) [22][23][24]. Aponeurosis nonlinearity can be described by a nonlinear toe-region followed by a linear region, which suggests collagen waviness plays a key role in tissue stiffness [22].…”

“…One study of gastrocnemius and soleus samples from human cadavers found that a region of the anterior gastrocnemius lateralis aponeurosis exhibited an elastic modulus of nearly twice that of a region of the anterior lateral soleus aponeurosis (324 MPa vs 164 MPa, respectively) [10]. There is also variability in species and specific muscletendon units used for aponeurosis materials testing studies, ranging from human gastrocnemius and soleus [10] to turkey gastrocnemius [22] and porcine triceps brachii [23]. Despite these numerous in vitro studies on aponeurosis mechanics, aponeurosis material properties have not been directly attributed to quantified microstructural measures.…”