High-Speed Strain Mapping of Human Meniscus During Tensile Loading

The knee meniscus is a fibrous soft tissue that is frequently torn. Prevention and treatment requires an understanding of failure mechanisms. Important failure properties are currently unknown, including the magnitude and orientation of principal strains at failure. This information is needed to inform predictive failure theories. 8 human meniscal samples were failed in tension, and surface strains were tracked using a previously validated high-speed digital image correlation system. Results suggest that failures occur at 44°, along the maximum shearing plane when testing along the reinforcing fibers, and at 6° along the maximum tensile plane when testing normal to reinforcing fibers

Finite Element Modeling of Meniscal Tears Using Continuum Damage Mechanics and Digital Image Correlation

Meniscal tears are a common, painful, and debilitating knee injury with limited treatment options. Computational models that predict meniscal tears may help advance injury prevention and repair, but first these models must be validated using experimental data. Here we simulated meniscal tears with finite element analysis using continuum damage mechanics (CDM) in a transversely isotropic hyperelastic material. Finite element models were built to recreate the coupon geometry and loading conditions of forty uniaxial tensile experiments of human meniscus that were pulled to failure either parallel or perpendicular to the preferred fiber orientation. Two damage criteria were evaluated for all experiments: von Mises stress and maximum normal Lagrange strain. After we successfully fit all models to experimental force–displacement curves (grip-to-grip), we compared model predicted strains in the tear region at ultimate tensile strength to the strains measured experimentally with digital image correlation (DIC). In general, the damage models underpredicted the strains measured in the tear region, but models using von Mises stress damage criterion had better overall predictions and more accurately simulated experimental tear patterns. For the first time, this study has used DIC to expose strengths and weaknesses of using CDM to model failure behavior in soft fibrous tissue

A Validated Software Application to Measure Fiber Organization in Soft Tissue

The mechanical behavior of soft connective tissue is governed by a dense network of fibrillar proteins in the extracellular matrix. Characterization of this fibrous network requires the accurate extraction of descriptive structural parameters from imaging data, including fiber dispersion and mean fiber orientation. Common methods to quantify fiber parameters include fast Fourier transforms (FFT) and structure tensors, however, information is limited on the accuracy of these methods. In this study, we compared these two methods using test images of fiber networks with varying topology. The FFT method with a band-pass filter was the most accurate, with an error of 0.71 ± 0.43 degrees in measuring mean fiber orientation and an error of 7.4 ± 3.0% in measuring fiber dispersion in the test images. The accuracy of the structure tensor method was approximately 4 times worse than the FFT bandpass method when measuring fiber dispersion. A free software application, FiberFit, was then developed that utilizes an FFT band-pass filter to fit fiber orientations to a semicircular von Mises distribution. FiberFit was used to measure collagen fibril organization in confocal images of bovine ligament at magnifications of 63x and 20x. Grayscale conversion prior to FFT analysis gave the most accurate results, with errors of 3.3 ± 3.1 degrees for mean fiber orientation and 13.3 ± 8.2% for fiber dispersion when measuring confocal images at 63x. By developing and validating a software application that facilitates the automated analysis of fiber organization, this study can help advance a mechanistic understanding of collagen networks and help clarify the mechanobiology of soft tissue remodeling and repair

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

Development of a Pin-On-Disk Test Fixture to Quantify Meniscus Wear Parameters

Overuse and mechanical wear in meniscus tissue can increase the risk of osteoarthritis progression. In order to help prevent the advancement of this debilitating disease, meniscus tissue wear must be better understood. Current studies have investigated the friction properties of meniscus tissue samples, however wear parameters have yet to be examined. Multidirectional motion has been shown to produce clinically relevant wear rates in polyethylene joint replacements, in comparison to unidirectional motion. Therefore, to better understand physiologically relevant wear in meniscus tissue, the purpose of this project was to design a simple and repeatable test fixture that could apply multidirectional loading to sectioned human meniscus specimens. The two directions of loading (oscillating linear translations of 4 mm and continuous axial rotation at 1 RPM) were chosen to mimic the physiological loading conditions (anterior-posterior translation and tibial rotation) occurring within the knee joint. The test system was designed to mimic roughly 50% of expected physiological loads. This test fixture will enable us to characterize human meniscus wear properties that can be used to determine activities that induce wear and degeneration in soft tissue

A Hand-Held Device to Apply Instrument-Assisted Soft Tissue Mobilization at Targeted Compression Forces and Stroke Frequencies

Instrument-assisted soft tissue mobilization (IASTM) is a manual therapy technique that is commonly used to treat dysfunctions in ligaments and other musculoskeletal tissues. The objective of this study was to develop a simple hand-held device that helps users accurately apply targeted compressive forces and stroke frequencies during IASTM treatments. This portable device uses a force sensor, tablet computer, and custom software to guide the application of user-specified loading parameters. To measure performance, the device was used to apply a combination of targeted forces and stroke frequencies to foam blocks and silicone pads. Three operators using the device applied targeted forces between 0.3 and 125 N with less than 10% error and applied targeted stroke frequencies between 0.25 and 1.0 Hz with less than 3% error. The mean error in applying targeted forces increased significantly at compressive forces less than 0.2 N and greater than 125 N. For experimental validation, the device was used to apply a series of IASTM treatments over three-weeks to rodents with a ligament injury, and the targeted compressive force and stroke frequency were repeatedly applied with an average error less than 5%. This validated device can be used to investigate the effect of IASTM loading parameters on tissue healing in animal and human studies, and therefore can support the optimization and adoption of IASTM protocols that improve patient outcomes

A Repeatable Method for Joint Alignment of Disarticulated Human Knees

Misalignment of human knee joints can significantly increase joint forces, kinematic errors, and cause specimen damage. While previous studies have developed methods to accurately align whole human knees prior to testing, no such study has developed a method to re-establish natural alignment for disarticulated knees. A surrogate knee bone model was used to address this problem. Embedded and reference coordinate systems were created by digitizing kinematic markers on each bone. The initial position was digitized prior to disarticulation. A custom MATLAB code was used to quantify clinical translations and rotations using Grood-Suntay convention. This provided adjustments to guide the disarticulated joint to within ±0.5° and 1.0 mm of the initial position. Final positions were digitized three times and compared to the initial position to determine the repeatability of the method. After three trials, errors were found to be insignificant with p \u3c 0.05. Average differences in rotation and translation were 0.33° and 0.50 mm, respectively. For the first time, this study provides a method to re-establish natural joint alignment accurately and repeatedly for disarticulated human knees. By improving joint alignment, this study can help to advance clinically impactful research in knee biomechanics and TKA technology

A Computational Tool to Automate the Analysis of Stress-Strain Data from Tensile Tests of Soft Tissue

Stress-strain curves are commonly generated to analyze the mechanical tensile behavior of biological materials and evaluate several key points. There is no standard method to find the transition point. Additionally, there is a need for a user-friendly program that automates the calculation of mechanical properties from stress-strain curves. A custom graphical user interface (GUI) was created using Python, allowing users to upload stress-strain data where it automatically generates a stress-strain curve with marked points of interest, as well as a .csv file that appends those points. The software\u27s accuracy was evaluated using tensile test data from our previous study on human meniscus and synthetic stress-strain curves with known transition points generated using FEBio studio. The GUI closely predicted the same transition point calculated using a more complex FE optimization routine. We found that a 3% slope deviation returned the lowest error for transition stress in this study. The error in calculating the transition point was insensitive to changes in the modulus and damage rate of the stress-strain curve. In conclusion, by creating an accurate computational tool to automate the analysis of stress-strain curves, this study can advance the standardization of biomechanical testing in soft fibrous tissue

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)