Applying ASTM Standards to Tensile Tests of Musculoskeletal Soft Tissue: Methods to Reduce Grip Failures and Promote Reproducibility.

Tensile testing is an essential experiment to assess the mechanical integrity of musculoskeletal soft tissues, yet standard test methods have not been developed to ensure the quality and reproducibility of these experiments. The ASTM International standards organization has created tensile test standards for common industry materials that specify geometric dimensions of test specimens (coupons) that promote valid failures within the gage section (midsubstance), away from the grips. This study examined whether ASTM test standards for plastics, elastomers, and fiber-reinforced composites are suitable for tensile testing of bovine meniscus along the circumferential fiber direction. We found that dumbbell (DB) shaped coupons based on ASTM standards for elastomers and plastics had an 80% and 60% rate of midsubstance failures, respectively. The rate of midsubstance failures dropped to 20% when using straight (ST) coupons based on ASTM standards for fiber-reinforced composites. The mechanical properties of dumbbell shaped coupons were also significantly greater than straight coupons. Finite element models of the test coupons revealed stress distributions that supported our experimental findings. In addition, we found that a commercial deli-slicer was able to slice meniscus to uniform layer thicknesses that were within ASTM dimensional tolerances. This study provides methods, recommendations, and insights that can advance the standardization of tensile testing in meniscus and other soft fibrous tissues.

Print-A-Punch: A 3D printed device to cut dumbbell-shaped specimens from soft tissue for tensile testing.

The failure behavior and mechanical properties of soft tissue can be characterized by conducting uniaxial tensile tests on small sectioned specimens, called test coupons. An ideal coupon geometry for tensile testing is a dumbbell shape (dog-bone), yet the cost and time required to fabricate custom steel punches to cut dumbbell-shaped coupons has hindered their universal application in biomechanics research. In this study, we developed an economical and reliable cutting device that can extract dumbbell-shaped coupons from soft biological tissue. The novel device, called Print-A-Punch, uses three-dimensional (3D) printed components in combination with standard fasteners and replaceable flexible razors. We identified design factors that influence the dimensional accuracy and symmetry of elastomer coupons extracted using this cutting device, and demonstrated its use on bovine meniscus. Advantages of this 3D printed device include a fast fabrication time, low material cost, good accuracy, replaceable blades, and an ability to scale coupon dimensions for specific tissues and experiments. By reducing the cost and time to cut accurate dumbbell-shaped coupons, this technology can facilitate the broad adoption of standard test methods that improve the quality and reproducibility of tensile tests in soft biological tissue. Researchers can freely download a set of STL files from this study to build their own Print-A-Punch device (https://boisestate.edu/coen-ntm/technology/print-a-punch).

Fatigue life of bovine meniscus under longitudinal and transverse tensile loading.

The knee meniscus is composed of a fibrous extracellular matrix that is subjected to large and repeated loads. Consequently, the meniscus is frequently torn, and a potential mechanism for failure is fatigue. The objective of this study was to measure the fatigue life of bovine meniscus when applying cyclic tensile loads either longitudinal or transverse to the principal fiber direction. Fatigue experiments consisted of cyclic loads to 60%, 70%, 80% or 90% of the predicted ultimate tensile strength until failure occurred or 20,000 cycles was reached. The fatigue data in each group was fit with a Weibull distribution to generate plots of stress level vs. cycles to failure (S-N curve). Results showed that loading transverse to the principal fiber direction gave a two-fold increase in failure strain, a three-fold increase in creep, and a nearly four-fold increase in cycles to failure (not significant), compared to loading longitudinal to the principal fiber direction. The S-N curves had strong negative correlations between the stress level and the mean cycles to failure for both loading directions, where the slope of the transverse S-N curve was 11% less than the longitudinal S-N curve (longitudinal: S=108-5.9ln(N); transverse: S=112-5.2ln(N)). Collectively, these results suggest that the non-fibrillar matrix is more resistant to fatigue failure than the collagen fibers. Results from this study are relevant to understanding the etiology of atraumatic radial and horizontal meniscal tears, and can be utilized by research groups that are working to develop meniscus implants with fatigue properties that mimic healthy tissue.