Freezing does not alter multiscale tendon mechanics and damage mechanisms in tension

Elevated fluid and glycosaminoglycan content in the Achilles tendon contribute to higher intratendinous pressures: Implications for Achilles tendinopathy

BACKGROUND

Tendinopathy alters the compositional properties of the Achilles tendon by increasing fluid and glycosaminoglycan content. It has been speculated that these changes may affect intratendinous pressure, but the extent of this relationship remains unclear. Therefore, we aimed to investigate the impact of elevated fluid and glycosaminoglycan content on Achilles tendon intratendinous pressure and to determine whether hyaluronidase (HYAL) therapy can intervene in this potential relationship.

METHODS

Twenty paired fresh-frozen cadaveric Achilles tendons were mounted in a tensile-testing machine and loaded up to 5% strain. Intratendinous resting (at 0% strain) and dynamic pressure (at 5% strain) were assessed using the microcapillary infusion technique. First, intratendinous pressure was measured under native conditions before and after infusion of 2 mL physiological saline. Next, 80 mg of glycosaminoglycans were administered bilaterally to the paired tendons. The right tendons were additionally treated with 1500 units of HYAL. Finally, both groups were retested, and the glycosaminoglycan content was analyzed.

RESULTS

It was found that both elevated fluid and glycosaminoglycan content resulted in higher intratendinous resting and dynamic pressures (p < 0.001). HYAL treatment induced a 2.3-fold reduction in glycosaminoglycan content (p = 0.002) and restored intratendinous pressures.

CONCLUSION

The results of this study demonstrated that elevated fluid and glycosaminoglycan content in Achilles tendinopathy contribute to increased intratendinous resting and dynamic pressures, which can be explained by the associated increased volume and reduced permeability of the tendon matrix, respectively. HYAL degrades glycosaminoglycans sufficiently to lower intratendinous pressures and may, therefore, serve as a promising treatment.

Tendon biomechanical properties are altered by storage duration but not freeze-thaw temperatures or cycles

Tendon allograft and xenograft processing often involves one or more steps of freezing and thawing. As failure strength is an important graft consideration, this study aimed to evaluate effects on failure properties when varying freeze-thaw conditions. Kangaroo tendons, a potential xenograft source, were used to evaluate changes in ultimate tensile strength (UTS), failure strain and elastic modulus after exposure to different freezer-storage temperatures (-20°C vs. -80°C), storage durations (1, 3, 6, 9, or 12 months), number of freeze-thaw cycles (1, 2, 3, 4, 5, or 10), or freeze-thaw temperature ranges (including freezing in liquid nitrogen to thawing at 37°C). Tendons stored for 6 or more months had significantly increased UTS and elastic modulus compared with 1 or 3 months of storage. This increase occurred irrespective of the freezing temperature (-20°C vs. -80°C) or the number of freeze-thaw cycles (1 vs. 10). In contrast, UTS, failure strain and the elastic modulus were no different between storage temperatures, number of freeze-thaw cycles and multiple freeze-thaw cycles across a range of freeze and thaw temperatures. Common freeze-thaw protocols did not negatively affect failure properties, providing flexibility for graft testing, storage, transportation and decellularisation procedures. However, the change in properties with the overall storage duration has implications for assessing the consistent performance of grafts stored for short versus extended periods of time (<6 months vs. >6 months), and the interpretation of data obtained from tissues of varying or unknown storage durations.

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

Background: The treatment of patellar tendon injury has always been an unsolved problem, and mechanical characterization is very important for its repair and reconstruction. Elastin is a contributor to mechanics, but it is not clear how it affects the elasticity, viscoelastic properties, and structure of patellar tendon. Methods: The patellar tendons from six fresh adult experimental pigs were used in this study and they were made into 77 samples. The patellar tendon was specifically degraded by elastase, and the regional mechanical response and structural changes were investigated by: (1) Based on the previous study of elastase treatment conditions, the biochemical quantification of collagen, glycosaminoglycan and total protein was carried out; (2) The patellar tendon was divided into the proximal, central, and distal regions, and then the axial tensile test and stress relaxation test were performed before and after phosphate-buffered saline (PBS) or elastase treatment; (3) The dynamic constitutive model was established by the obtained mechanical data; (4) The structural relationship between elastin and collagen fibers was analyzed by two-photon microscopy and histology. Results: There was no statistical difference in mechanics between patellar tendon regions. Compared with those before elastase treatment, the low tensile modulus decreased by 75%-80%, the high tensile modulus decreased by 38%-47%, and the transition strain was prolonged after treatment. For viscoelastic behavior, the stress relaxation increased, the initial slope increased by 55%, the saturation slope increased by 44%, and the transition time increased by 25% after enzyme treatment. Elastin degradation made the collagen fibers of patellar tendon become disordered and looser, and the fiber wavelength increased significantly. Conclusion: The results of this study show that elastin plays an important role in the mechanical properties and fiber structure stability of patellar tendon, which supplements the structure-function relationship information of patellar tendon. The established constitutive model is of great significance to the prediction, repair and replacement of patellar tendon injury. In addition, human patellar tendon has a higher elastin content, so the results of this study can provide supporting information on the natural properties of tendon elastin degradation and guide the development of artificial patellar tendon biomaterials.

Comparison of the Biomechanical Properties between Healthy and Whole Human and Porcine Stomachs

Gastric cancer poses a societal and economic burden, prompting an exploration into the development of materials suitable for gastric reconstruction. However, there is a dearth of studies on the mechanical properties of porcine and human stomachs. Therefore, this study was conducted to elucidate their mechanical properties, focusing on interspecies correlations. Stress relaxation and tensile tests assessed the hyperelastic and viscoelastic characteristics of porcine and human stomachs. The thickness, stress-strain curve, elastic modulus, and stress relaxation were assessed. Porcine stomachs were significantly thicker than human stomachs. The stiffness contrast between porcine and human stomachs was evident. Porcine stomachs demonstrated varying elastic modulus values, with the highest in the longitudinal mucosa layer of the corpus and the lowest in the longitudinal intact layer of the fundus. In human stomachs, the elastic modulus of the longitudinal muscular layer of the antrum was the highest, whereas that of the circumferential muscularis layer of the corpus was the lowest. The degree of stress relaxation was higher in human stomachs than in porcine stomachs. This study comprehensively elucidated the differences between porcine and human stomachs attributable to variations across different regions and tissue layers, providing essential biomechanical support for subsequent studies in this field.

Viscoelastic materials are most energy efficient when loaded and unloaded at equal rates

Biological springs can be used in nature for energy conservation and ultra-fast motion. The loading and unloading rates of elastic materials can play an important role in determining how the properties of these springs affect movements. We investigate the mechanical energy efficiency of biological springs (American bullfrog plantaris tendons and guinea fowl lateral gastrocnemius tendons) and synthetic elastomers. We measure these materials under symmetric rates (equal loading and unloading durations) and asymmetric rates (unequal loading and unloading durations) using novel dynamic mechanical analysis measurements. We find that mechanical efficiency is highest at symmetric rates and significantly decreases with a larger degree of asymmetry. A generalized one-dimensional Maxwell model with no fitting parameters captures the experimental results based on the independently characterized linear viscoelastic properties of the materials. The model further shows that a broader viscoelastic relaxation spectrum enhances the effect of rate-asymmetry on efficiency. Overall, our study provides valuable insights into the interplay between material properties and unloading dynamics in both biological and synthetic elastic systems.

Guidelines for ex vivo mechanical testing of tendon

Tendons are critical for the biomechanical function of joints. Tendons connect muscles to bones and allow for the transmission of muscle forces to facilitate joint motion. Therefore, characterizing the tensile mechanical properties of tendons is important for the assessment of functional tendon health and efficacy of treatments for acute and chronic injuries. In this guidelines paper, we review methodological considerations, testing protocols, and key outcome measures for mechanical testing of tendons. The goal of the paper is to present a simple set of guidelines to the nonexpert seeking to perform tendon mechanical tests. The suggested approaches provide rigorous and consistent methodologies for standardized biomechanical characterization of tendon and reporting requirements across laboratories.

Multimodal and multiscale characterization reveals how tendon structure and mechanical response are altered by reduced loading

Tendons are collagen-based connective tissues where the composition, structure and mechanics respond and adapt to the local mechanical environment. Adaptation to prolonged inactivity can result in stiffer tendons that are more prone to injury. However, the complex relation between reduced loading, structure, and mechanical performance is still not fully understood. This study combines mechanical testing with high-resolution synchrotron X-ray imaging, scattering techniques and histology to elucidate how reduced loading affects the structural properties and mechanical response of rat Achilles tendons on multiple length scales. The results show that reduced in vivo loading leads to more crimped and less organized fibers and this structural inhomogeneity could be the reason for the altered mechanical response. Unloading also seems to change the fibril response, possibly by altering the strain partitioning between hierarchical levels, and to reduce cell density. This study elucidates the relation between in vivo loading, the Achilles tendon nano-, meso‑structure and mechanical response. The results provide fundamental insights into the mechanoregulatory mechanisms guiding the intricate biomechanics, tissue structural organization, and performance of complex collagen-based tissues. STATEMENT OF SIGNIFICANCE: Achilles tendon properties allow a dynamic interaction between muscles and tendon and influence force transmission during locomotion. Lack of physiological loading can have dramatic effects on tendon structure and mechanical properties. We have combined the use of cutting-edge high-resolution synchrotron techniques with mechanical testing to show how reduced loading affects the tendon on multiple hierarchical levels (from nanoscale up to whole organ) clarifying the relation between structural changes and mechanical performance. Our findings set the first step to address a significant healthcare challenge, such as the design of tailored rehabilitations that take into consideration structural changes after tendon immobilization.

A Strainer-Based Platform for the Collection and Immunolabeling of Mouse Intestinal Organoids

Intestinal organoids have emerged as powerful model systems for studying the complex structure and function of the intestine. However, there is a lack of widely applicable methods for the collection, labeling, and imaging of intestinal organoids. In this study, we developed a novel method for loading and labeling intestinal organoids, a method that efficiently collects the organoids and facilitates imaging of their three-dimensional (3D) structure. Based on this strainer platform, mouse intestinal organoids were adequately collected and immobilized, facilitating the immunolabeling workflow to target proteins of the organoids. After evaluation, the strainer size of 40 μm was considered to be more conducive to the collection and labeling of mouse intestinal organoids. More extensive research on organoids of multiple types and species origins will contribute to broadening the applicability of the methodology. Overall, our study proposes an innovative workflow for loading and analyzing intestinal organoids. The combination of a strainer-based collection method, fluorescent labeling, and 3D reconstruction provides valuable insights into the organization and complexity of these tissue models, thereby offering new avenues for investigating intestinal development, disease modeling, and drug discovery.

Approach to standardized material characterization of the human lumbopelvic system-Specification, preparation and storage

The complexity of the osseo-ligamentous lumbopelvic system has made it difficult to perform both, the overall preparation as well as specimen harvesting and material testing with a reasonable amount of time and personnel. The logistics of such studies present a hurdle for reproducibility. A structured procedure was developed and proved, which allows all necessary steps to be carried out reproducibly and in a reasonable time. This enables the extraction of 26 soft tissue, 33 trabecular and 32 cortical bone specimens from this anatomical region per cadaver. The integrity of the specimens remains maintained while keeping requirements within manageable limits. The practicability of the intended five-day specimen harvesting and testing procedure could be demonstrated on five test and two pre-test sequences. The intended minimization of physical, biological, and chemical external influences on specimens could be achieved. All protocols, instructions and models of preparation and storage devices are included in the supporting information. The high grade of applicability and reproducibility will lead to better comparability between different biomechanical investigations. This procedure proven on the human pelvis is transferable to other anatomical regions.

Overload in a Rat In Vivo Model of Synergist Ablation Induces Tendon Multiscale Structural and Functional Degeneration

Tendon degeneration is typically described as an overuse injury with little distinction made between magnitude of load (overload) and number of cycles (overuse). Further, in vivo, animal models of tendon degeneration are mostly overuse models, where tendon damage is caused by a high number of load cycles. As a result, there is a lack of knowledge of how isolated overload leads to degeneration in tendons. A surgical model of synergist ablation (SynAb) overloads the target tendon, plantaris, by ablating its synergist tendon, Achilles. The objective of this study was to evaluate the structural and functional changes that occur following overload of plantaris tendon in a rat SynAb model. Tendon cross-sectional area (CSA) and shape changes were evaluated by longitudinal MR imaging up to 8 weeks postsurgery. Tissue-scale structural changes were evaluated by semiquantified histology and second harmonic generation microscopy. Fibril level changes were evaluated with serial block face scanning electron microscopy (SBF-SEM). Functional changes were evaluated using tension tests at the tissue and microscale using a custom testing system allowing both video and microscopy imaging. At 8 weeks, overloaded plantaris tendons exhibited degenerative changes including increases in CSA, cell density, collagen damage area fraction (DAF), and fibril diameter, and decreases in collagen alignment, modulus, and yield stress. To interpret the differences between overload and overuse in tendon, we introduce a new framework for tendon remodeling and degeneration that differentiates between the inputs of overload and overuse. In summary, isolated overload induces multiscale degenerative structural and functional changes in plantaris tendon.

Short-term vancomycin and buffer soaking does not change rabbit achilles tendon tensile material properties

BACKGROUND

Allograft tendons are commonly used during orthopedic surgery to reconstruct tissue that is severely damaged. Soaking the tendon in an antibiotic solution, specifically vancomycin, has been shown to lower the risk of post-operative infections. While some material properties of tendon and ligament after antibiotic soaking have previously been characterized, extensive sub-failure allograft tendon material properties after soaking in antibiotic solutions have not.

METHODS

Forty tendons were dissected from rabbits and soaked in either a phosphate buffered saline (PBS) only solution or vancomycin and PBS solution for five or 30 min. Immediately after soaking, quasi-static tensile experiments were performed in a materials testing system.

FINDINGS

Tissue nominal stress, Lagrange strain, toe-region properties and elastic modulus were characterized. For all forty tendons, the average elastic modulus was found to be 455 ± 37 MPa, the average transition strain (from toe-region to linear elastic region) was 0.0487 ± 0.0035, and the average transition stress was 9.71 ± 0.79 MPa. No statistically significant differences in any of these material properties were found across soaking medium or soaking time.

INTERPRETATION

From these results, we conclude that soaking an allograft tendon in antibiotic solution for up to 30 min prior to implantation does not change the tensile material properties of tendons, supporting current clinical practice.

Stress deprivation of tendon explants or Tpm3.1 inhibition in tendon cells reduces F-actin to promote a tendinosis-like phenotype

Actin is a central mediator between mechanical force and cellular phenotype. In tendons, it is speculated that mechanical stress deprivation regulates gene expression by reducing filamentous (F)-actin. However, the mechanisms regulating tenocyte F-actin remain unclear. Tropomyosins (Tpms) are master regulators of F-actin. There are more than 40 Tpm isoforms, each having the unique capability to stabilize F-actin subpopulations. We investigated F-actin polymerization in stress-deprived tendons and tested the hypothesis that stress fiber-associated Tpm(s) stabilize F-actin to regulate cellular phenotype. Stress deprivation of mouse tail tendon down-regulated tenogenic and up-regulated protease (matrix metalloproteinase-3) mRNA levels. Concomitant with mRNA modulation were increases in G/F-actin, confirming reduced F-actin by tendon stress deprivation. To investigate the molecular regulation of F-actin, we identified that tail, Achilles, and plantaris tendons express three isoforms in common: Tpm1.6, 3.1, and 4.2. Tpm3.1 associates with F-actin in native and primary tenocytes. Tpm3.1 inhibition reduces F-actin, leading to decreases in tenogenic expression, increases in chondrogenic expression, and enhancement of protease expression in mouse and human tenocytes. These expression changes by Tpm3.1 inhibition are consistent with tendinosis progression. A further understanding of F-actin regulation in musculoskeletal cells could lead to new therapeutic interventions to prevent alterations in cellular phenotype during disease progression.

Templated freezing: a simple method may increase gripping force of the clamp on the tendon

Purpose

To evaluate the effectiveness of combining a customized mold with frozen conventional clamps against other freezing and non-freezing methods.

Methods

Forty-five porcine and 45 chicken tendons were evenly divided into five groups (n = 9 + 9/group): control group, non-freezing with gauze placed between tendon and clamp (gauze), non-freezing with suture fixation at tendon ends (suture), freezing with dry ice pocket placed at the clamps (pocket), and freezing using a templated liquid nitrogen clamp with a customized mold (mold). Tension tests were used to measure failure modes and loads.

Result

Slippage and avulsion were observed in non-freezing groups with significantly lower failure loads compared to freezing methods. With freezing, rupture occurred near the central point only in the mold group. The failure loads for porcine tendons in the mold group were higher (2121.651 ± 73.101 N) than the pocket group (1746.337 ± 68.849 N). The failure loads of chicken tendons in the mold (243.552 ± 15.881 N) and pocket groups (260.647 ± 22.161 N) were not statistically different.

Conclusion

Freezing clamps represent the better choice for soft tissue clamping. The customized mold method could improve gripping effectiveness.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13018-022-03209-y.

Effect of single and double hemitenotomy on equine deep digital flexor tendon length and strength in experimental load challenges

OBJECTIVE

To evaluate a double hemitenotomy (DHT) technique as an alternative to complete deep digital flexor (DDFT) tenotomy.

STUDY DESIGN

Experimental ex vivo study.

SAMPLE POPULATION

Isolated DDFTs (n = 30) and cadaveric forelimbs (n = 16).

METHODS

In part 1, 15 isolated DDFT pairs were used. Two hemitenotomies were created in 1 DDFT while the other served as reference. Monotonic tensile load was applied. Tendon lengthening, load reduction, and load at failure were recorded. In part 2, 16 cadaveric forelimb pairs were subjected to DHT followed by complete tenotomy (CT) under monotonic compressive load. Differences between DHT and controls were assessed with Wilcoxon signed rank tests or Friedman tests.

RESULTS

In isolated tendons and cadaveric forelimbs, DHT resulted in DDFT lengthening (median, +1.9 mm and + 3.05 mm) and load reduction (median, -16.7 and -11.2 kg). Less lengthening was achieved with DHT compared to CT (P = .008). Load reduction did not occur between DHT and CT was observed during compressive testing (P = 1). Load reduction following the first hemitenotomy incision was smaller when compared to the second (P = .022). Isolated DHT tendons failed at a tensile load of 195 kg, while no intact tendons failed (P = .0001).

CONCLUSION

Double hemitenotomy was comparable to CT in load reduction. It reduced tensile strength, but load at failure was similar or exceeded the estimated DDFT load at stance.

CLINICAL SIGNIFICANCE

Hemitenotomy may be a useful alternative for surgical management of horses with laminitis, but in vivo studies are needed to confirm these findings.

Tendons exhibit greater resistance to tissue and molecular-level damage with increasing strain rate during cyclic fatigue

Musculoskeletal soft connective tissues are commonly injured due to repetitive use, but the evolution of mechanical damage to the tissue structure during repeated loading is poorly understood. We investigated the strain-rate dependence of mechanical denaturation of collagen as a form of structural microdamage accumulation during creep fatigue loading of rat tail tendon fascicles. We cycled tendons at three strain rates to the same maximum stress relative to their rate-dependent tensile strength. Collagen denaturation at distinct points during the fatigue process was measured by fluorescence quantification of collagen hybridizing peptide binding. The amount of collagen denaturation was significantly correlated with fascicle creep strain, independent of the cyclic strain rate, supporting our hypothesis that tissue level creep is caused by collagen triple-helix unfolding. Samples that were loaded faster experienced more creep strain and denaturation as a function of the number of loading cycles relative to failure. Although this increased damage capacity at faster rates may serve as a protective measure during high-rate loading events, it may also predispose these tissues to subsequent injury and indicate a mechanism of overuse injury development. These results build on evidence that molecular-level collagen denaturation is the fundamental mechanism of structural damage to tendons during tensile loading. STATEMENT OF SIGNIFICANCE: This study is the first to investigate the accumulation of denatured collagen in tendons throughout fatigue loading when the maximum stress is scaled with the applied strain rate. The amount of denatured collagen was correlated with creep strain, independent of strain rate, but samples that were cycled faster withstood greater amounts of denaturation before failure. Differential accumulation of collagen damage between fast and slow repetitive loading has relevance toward understanding the prevalence of overuse musculoskeletal injuries following sudden changes in activity level. Since collagen is a ubiquitous biological structural component, the basic patterns and mechanisms of loading-induced collagen damage in connective tissues are relevant for understanding injury and disease in other tissues, including those from the cardiovascular and pulmonary systems.

Tear propagation in vaginal tissue under inflation

Vaginal tearing at childbirth is extremely common yet understudied despite the long-term serious consequences on women's health. The mechanisms of vaginal tearing remain unknown, and their knowledge could lead to the development of transformative prevention and treatment techniques for maternal injury. In this study, whole rat vaginas with pre-imposed elliptical tears oriented along the axial direction of the organs were pressurized using a custom-built inflation setup, producing large tear propagation. Large deformations of tears through propagation were analyzed, and nonlinear strains around tears were calculated using the digital image correlation technique. Second harmonic generation microscopy was used to examine collagen fiber organization in mechanically untested and tested vaginal specimens. Tears became increasingly circular under pressure, propagating slowly up to the maximum pressure and then more rapidly. Hoop strains were significantly larger than axial strains and displayed a region- and orientation-dependent response with tear propagation. Imaging revealed initially disorganized collagen fibers that aligned along the axial direction with increasing pressure. Fibers in the near-regions of tear tips aligned toward the hoop direction, hampering tear propagation. Changes in tear geometry, regional strains, and fiber orientation revealed the inherent toughening mechanisms of the vaginal tissue. STATEMENT OF SIGNIFICANCE: Women's reproductive health has historically been understudied despite alarming maternal injury and mortality rates in the world. Maternal injury and disability can be reduced by advancing our limited understanding of the large deformations experienced by women's reproductive organs. This manuscript presents, for the first time, the mechanics of tear propagation in vaginal tissue and changes to the underlying collagen microstructure near to and far from the tear. A novel inflation setup capable of maintaining the in vivo tubular geometry of the vagina while propagating a pre-imposed tear was developed. Toughening mechanisms of the vagina to propagation were examined through measurements of tear geometry, strain distributions, and reorientation of collagen fibers. This research draws from current advances in the engineering science and mechanics fields with the goal of improving maternal health care.

Mechanical characterization of fibrotic and mineralized tissue in Peyronie's disease

Peyronie's disease affects penile mechanics, but published research lacks biomechanical characterization of affected tunica albuginea. This work aims to establish mechanical testing methodology and characterize pathological tissue mechanics of Peyronie's disease. Tunica albuginea was obtained from patients (n = 5) undergoing reconstructive surgery for Peyronie's disease, sectioned into test specimens (n = 12), stored frozen at -20 °C, and imaged with micro-computed tomography (µCT). A tensile testing protocol was developed based on similar soft tissues. Correlation of mechanical summary variables (force, displacement, stiffness, work, Young's modulus, ultimate tensile stress, strain at ultimate tensile stress, and toughness) and µCT features were assessed with linear regression. Specimens empirically grouped into hard or soft stress-strain behavior were compared using a Student's t-test. Surface strain and failure patterns were described qualitatively. Specimens displayed high inter- and intra-subject variability. Mineralization volume was not correlated with mechanical parameters. Empirically hard tissue had higher ultimate tensile stress. Failure mechanisms and strain patterns differed between mineralized and non-mineralized specimens. Size, shape, and quantity of mineralization may be more important in determining Peyronie's disease plaque behavior than presence of mineralization alone, and single summary variables like modulus may not fully describe mechanical behavior.

The effect of multiple freeze-thaw cycles on the viscoelastic properties and microstructure of bovine superficial digital flexor tendon

The most common injuries of the human musculoskeletal system are related to soft tissue structures such as tendons or ligaments. To repair torn structures, surgical intervention and application of a biological or synthetic graft may be required. A typical procedure for the processing, storage, and distribution of soft tissue grafts involves at least two freezing/thawing (F/T) cycles. Even though repeated F/T cycles decrease the mechanical performance and change the structure of tendons, it is unclear whether there exists a maximum number of F/T cycles above which tendons should not be approved for use as a tissue allograft. To fill this research gap, we present an ex vivo study on the effects of repetitive F/T cycles on the biomechanical stability of bovine superficial digital flexor tendon tissue. Using mechanical testing supported with scanning electron microscopy imaging, we show that multiple F/T cycles affect the viscoelastic and structural properties of tissue by significantly reducing its tensile modulus after the 3^rd or 4^th F/T cycle (depending on the strain range), stress drop during relaxation after the 8^th F/T cycle (regardless the strain values), mechanical hysteresis after the 10^th F/T cycle, and by causing a significant decrease in collagen fibril diameter. Our results provide a deeper insight into understanding the mechanisms responsible for tissue damage during multiple F/T cycles, and thus, may be useful for the future optimization of tissue storage protocols.

Mechanical characterization of the human pia-arachnoid complex

Traumatic brain injury (TBI) is a significant problem in global health that affects a wide variety of patients. Mild forms of TBI, commonly referred to as concussion, are a result of rapid accelerations of the head from either direct or indirect impacts. Kinetic energy from the impact is transferred into deformation of the brain, leading to cellular disruption. This transfer of energy is in part mediated by the pia-arachnoid complex (PAC), a layer of anatomical structures that forms the physical connection between the brain and the skull. The importance of properly quantifying the mechanics of the PAC for use in computational models of TBI has been understood for some time, but data from human subjects has been unavailable. In this study, we quantify the normal traction modulus of the PAC in five post-mortem human subjects using hydrostatic fluid pressurization in combination with optical coherence tomography. Testing at multiple locations across each brain reveals that brain-skull stiffness is heterogeneously distributed. The material response to traction loading was linear, with a mean normal traction modulus of 12.6 ± 4.8 kPa. Modulus was 21% greater in superior regions of the brain compared to inferior regions. Comparisons with regional microstructural data suggests a potential relationship between the volume fraction of arachnoid trabeculae and modulus. Comparisons to coincident measurements of microstructural properties showed a positive correlation between arachnoid membrane thickness and normal traction modulus. This study is the first to characterize the mechanics of the human pia-arachnoid complex and quantify material properties in situ. These findings suggest implementing a heterogeneous model of the brain-skull interface in computational models of TBI may lead to more realistic injury prediction.

Tendon Multiscale Structure, Mechanics, and Damage Are Affected by Osmolarity of Bath Solution

One of the most common bath solutions used in musculoskeletal mechanical testing is phosphate buffered saline (PBS). In tendon, swelling induced by physiological PBS results in decreased tendon modulus and induces microstructural changes. It is critical to evaluate the multiscale mechanical behavior of tendon under swelling to interpret prior work and provide information to design future studies. We compared the effects of physiological PBS and 8% polyethylene glycol and saline bathing solutions on tendon multiscale tendon mechanics and damage as well as microstructure with TEM in order to understand the effect of swelling on tendon. At the tissue level, tendons in PBS had a lower modulus than SPEG samples. PBS samples also showed an increased amount of non-recoverable sliding, which is an analog for microscale damage. SPEG had a higher microscale to tissue-scale strain ratio, showing the fibrils experienced less strain attenuation. From the TEM data, we showed the fibril spacing of SPEG samples was more similar to fresh control than PBS. We concluded that swelling alters multiscale mechanics and damage in addition to tendon microstructure. Future mechanical testing should consider using SPEG as a bath solution with an osmotic pressure which preserves fresh tissue water content.

Effect of storage and preconditioning of healing rat Achilles tendon on structural and mechanical properties

Tendon tissue storage and preconditioning are often used in biomechanical experiments and whether this generates alterations in tissue properties is essential to know. The effect of storage and preconditioning on dense connective tissues, like tendons, is fairly understood. However, healing tendons are unlike and contain a loose connective tissue. Therefore, we investigated if storage of healing tendons in the fridge or freezer changed the mechanical properties compared to fresh tendons, using a pull-to-failure or a creep test. Tissue morphology and cell viability were also evaluated. Additionally, two preconditioning levels were tested. Rats underwent Achilles tendon transection and were euthanized 12 days postoperatively. Statistical analyzes were done with one-way ANOVA or Student's t-test. Tissue force and stress were unaltered by storage and preconditioning compared to fresh samples, while high preconditioning increased the stiffness and modulus (p ≤ 0.007). Furthermore, both storage conditions did not modify the viscoelastic properties of the healing tendon, but altered transverse area, gap length, and water content. Cell viability was reduced after freezing. In conclusion, preconditioning on healing tissues can introduce mechanical data bias when having extensive tissue strength diversity. Storage can be used before biomechanical testing if structural properties are measured on the day of testing.

Microstructural and Mechanical Properties of Grafts Commonly Used for Cruciate Ligament Reconstruction

BACKGROUND

Injuries to the anterior cruciate ligament and posterior cruciate ligament are common, and often are treated with reconstruction. Limited quantitative data are available describing material properties of grafts used for reconstructions such as the bone-patellar tendon-bone (BPTB), hamstring tendon (HS), and quadriceps tendon (QT). The purpose of this study was to quantify and compare microstructural and mechanical properties of BPTB, HS, and QT grafts.

METHODS

Forty specimens (13 BPTB, 13 HS, and 14 QT grafts) from 24 donors were used. Specimens were subjected to preconditioning, stress relaxation, and ramp to failure. Mechanical parameters were calculated for each sample, and polarization imaging was used to evaluate the direction and strength of collagen fiber alignment during testing.

RESULTS

QT had the largest modulus values, and HS had the smallest. BPTB exhibited the least disperse collagen organization, while HS were the least strongly aligned. Microstructural properties showed more strongly aligned collagen with increasing load for all grafts. All tissues showed stress relaxation and subtle microstructural changes during the hold period.

CONCLUSIONS

The mechanical and microstructural properties differed significantly among BPTB, HS, and QT grafts. QT exhibited the largest moduli and greatest strength of collagen alignment, while HS had the smallest moduli and least strongly aligned collagen.

CLINICAL RELEVANCE

This study identified mechanical and microstructural differences among common grafts and between these grafts and the cruciate ligaments they replace. Further research is needed to properly interpret the clinical relevance of these differences.

The influence of different sample preparation on mechanical properties of human iliotibial tract

In the run-up to biomechanical testing, fresh human tissue samples are often frozen in order to inhibit initial decomposition processes and to achieve a temporal independence of tissue acquisition from biomechanical testing. The aim of this study was to compare the mechanical properties of fresh tissue samples of the human iliotibial tract (IT) to fresh-frozen samples taken from the same IT and those modified with different concentrations of Dimethylsulfoxide (DMSO) prior to freezing. All samples were partial plastinated and destructive tensile tests were conducted with a uniaxial tensile test setup. A plastination technique already established in the laboratory was modified to improve the clamping behaviour of the samples. Material failure was caused by a gradual rupture of the load-bearing collagen fibre bundles. Contrary to our expectations, no significant difference was found between the tensile strength of fresh and fresh frozen specimens. The addition of 1 wt% DMSO did not increase the tensile strength compared to fresh-frozen samples; an addition of 10 wt% DMSO even resulted in a decrease. Based on our findings, the use of simple fresh-frozen specimens to determine the tensile strength is viable; however fresh specimens should be used to generate a complete property profile.

The effect of autologous Achilles bursal tissue implants in tendon-to-bone healing of rotator cuff tears in rats

BACKGROUND

The aim of this study was to investigate the influence of autologous bursal tissue derived from the Achilles bursa on tendon-to-bone healing after rotator cuff tear repair in a rat model.

METHODS

A total of 136 Sprague-Dawley rats were randomly assigned to either an untreated or a bursal tissue application group or biomechanical testing and histologic testing after rotator cuff repair. After separating the supraspinatus tendon close to the greater tuberosity, the tendon was reattached either unaltered or with a bursal tissue interposition sewn onto the interface. Immunohistologic analysis was performed 1 and 7 weeks after supraspinatus tendon reinsertion. Biomechanical testing of the tendon occurred 6 and 7 weeks after reinsertion.

RESULTS

Immunohistologic results demonstrated a significantly higher percentage of Type II collagen (P = .04) after 1 and 7 weeks in the tendon-to-bone interface using autologous bursal tissue in comparison to control specimens. The bursa group showed a significantly higher collagen I to III quotient (P = .03) at 1 week after surgery in comparison to the 7-week postsurgery bursa groups and controls. Biomechanical assessment showed that overall tendon stiffness (P = .002) and the tendon viscoelasticity in the bursa group (P = .003) was significantly improved after 6 and 7 weeks. There was no significant difference (P = .55) in force to failure between the bursa group and the control group after 6 and 7 weeks.

CONCLUSION

Autologous bursal tissue derived from the Achilles bursa and implanted to the tendon-to-bone interface after rotator cuff repair facilitates a faster healing response to re-establish the biologic and biomechanical integrity of the rotator cuff in rats.

Multi-Scale Loading and Damage Mechanisms of Plantaris and Rat Tail Tendons

Tendinopathy, degeneration of the tendon that leads to pain and dysfunction, is common in both sports and occupational settings, but multi-scale mechanisms for tendinopathy are still unknown. We recently showed that micro-scale sliding (shear) is responsible for both load transfer and damage mechanisms in the rat tail tendon; however, the rat tail tendon is a specialized non-load-bearing tendon, and thus the load transfer and damage mechanisms are still unknown for load-bearing tendons. The objective of this study was to investigate the load transfer and damage mechanisms of load-bearing tendons using the rat plantaris tendon. We demonstrated that micro-scale sliding is a key component for both mechanisms in the plantaris tendon, similar to the tail tendon. Namely, the micro-scale sliding was correlated with applied strain, demonstrating that load was transferred via micro-scale sliding in the plantaris and tail tendons. In addition, while the micro-scale strain fully recovered, the micro-scale sliding was non-recoverable and strain-dependent, and correlated with tissue-scale mechanical parameters. When the applied strain was normalized, the % magnitudes of non-recoverable sliding was similar between the plantaris and tail tendons. Statement of clinical significance: Understanding the mechanisms responsible for the pathogenesis and progression of tendinopathy can improve prevention and rehabilitation strategies and guide therapies and the design of engineered constructs. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1827-1837, 2019.

Tendons from kangaroo rats are exceptionally strong and tough

Tendons must be able to withstand the forces generated by muscles and not fail. Accordingly, a previous comparative analysis across species has shown that tendon strength (i.e., failure stress) increases for larger species. In addition, the elastic modulus increases proportionally to the strength, demonstrating that the two properties co-vary. However, some species may need specially adapted tendons to support high performance motor activities, such as sprinting and jumping. Our objective was to determine if the tendons of kangaroo rats (k-rat), small bipedal animals that can jump as high as ten times their hip height, are an exception to the linear relationship between elastic modulus and strength. We measured and compared the material properties of tendons from k-rat ankle extensor muscles to those of similarly sized white rats. The elastic moduli of k-rat and rat tendons were not different, but k-rat tendon failure stresses were much larger than the rat values (nearly 2 times larger), as were toughness (over 2.5 times larger) and ultimate strain (over 1.5 times longer). These results support the hypothesis that the tendons from k-rats are specially adapted for high motor performance, and k-rat tendon could be a novel model for improving tissue engineered tendon replacements.

Comparative multi-scale hierarchical structure of the tail, plantaris, and Achilles tendons in the rat

Rodent tendons are widely used to study human pathologies such as tendinopathy and repair, and to address fundamental physiological questions about development, growth, and remodeling. However, how the gross morphology and multi-scale hierarchical structure of rat tendons, such as the tail, plantaris, and Achilles tendons, compare with that of human tendons are unknown. In addition, there remains disagreement about terminology and definitions. Specifically, the definitions of fascicle and fiber are often dependent on diameter sizes, not their characteristic features, and these definitions impair the ability to compare hierarchical structure across species, where the sizes of the fiber and fascicle may change with animal size and tendon function. Thus, the objective of the study was to select a single species that is commonly used for tendon research (rat) and tendons with varying mechanical functions (tail, plantaris, Achilles) to evaluate the hierarchical structure at multiple length scales using histology, SEM, and confocal imaging. With the exception of the specialized rat tail tendon, we confirmed that in rat tendons there are no fascicles and the fiber is the largest subunit. In addition, we provided a structurally based definition of a fiber as a bundle of collagen fibrils that is surrounded by elongated cells, and this definition was supported by both histologically processed and unprocessed samples. In all rat tendons studied, the fiber diameters were consistently between 10 and 50 μm, and this diameter range appears to be conserved across larger species. Specific recommendations were made highlighting the strengths and limitations of each rat tendon as a research model. Understanding the hierarchical structure of tendon can advance the design and interpretation of experiments and development of tissue-engineered constructs.

Characterization of the Continuous Elastic Parameters of Porcine Vocal Folds

This paper presents an evaluation of the elastic properties of porcine vocal folds through uniaxial tensile tests. Inferior vocal fold tissue samples were subjected to tension in the longitudinal direction while digital image correlation techniques were employed to determine strain values throughout the tests. The stress-strain results showed a low-strain linear region, followed by both a nonlinear exponential and then a higher strain linear region. Data from 16 porcine vocal fold samples were analyzed following a similar optimization method as proposed in prior studies [1] to yield continuous model parameters which describe the elastic properties of the tissue. The average low and high strain linear modulus values were found to be 17.86 kPa and 609.27 kPa, respectively. The model also identified the location of two transition points: p₁, describing the transition from the low-strain linear region to an exponential region at 0.122 ± 0.058 mm/mm and p₂, describing the transition from the exponential to the high strain linear region at 0.308 ± 0.069 mm/mm. The exponential region of the averaged data set was found to be described by the relationship [Formula: see text] kPa. In addition to locating transition points, the optimization method maintained modulus continuity across all strain values. Averaged elastic modulus values across strain from 0 to 0.40 mm/mm were compared to representative low and high strain linear modulus which were measured at 0.05 and 0.35 mm/mm, respectively. Statistically significant differences were found among all strain intervals between the two transition points and the linear modulus values. These results indicate the need to consider the location of transition points and further highlight the nonlinearity and changes in elastic modulus which are especially important when using excised porcine vocal folds as a model for phonation. The results quantify continuous linear and nonlinear parameters describing the elastic properties which can be used as a framework for future excised larynx tests and while evaluating the dynamics of sound production, which rely heavily on the elastic properties of the tissue.

Short cracks in knee meniscus tissue cause strain concentrations, but do not reduce ultimate stress, in single-cycle uniaxial tension

Tears are central to knee meniscus pathology and, from a mechanical perspective, are crack-like defects (cracks). In many materials, cracks create stress concentrations that cause progressive local rupture and reduce effective strength. It is currently unknown if cracks in meniscus have these consequences; if they do, this would have repercussions for management of meniscus pathology. The objective of this study was to determine if a short crack in meniscus tissue, which mimics a preclinical meniscus tear, (a) causes crack growth and reduces effective strength, (b) creates a near-tip strain concentration and (c) creates unloaded regions on either side of the crack. Specimens with and without cracks were tested in uniaxial tension and compared in terms of macroscopic stress-strain curves and digital image correlation strain fields. The strain fields were used as an indicator of stress concentrations and unloaded regions. Effective strength was found to be insensitive to the presence of a crack (potential effect < 0.86 s.d.; β = 0.2), but significant strain concentrations, which have the potential to lead to long-term accumulation of tissue or cell damage, were observed near the crack tip.

Age-related dataset on the mechanical properties and collagen fibril structure of tendons from a murine model

Connective tissues such as tendon, ligament and skin are biological fibre composites comprising collagen fibrils reinforcing the weak proteoglycan-rich ground substance in extracellular matrix (ECM). One of the hallmarks of ageing of connective tissues is the progressive and irreversible change in the tissue mechanical properties; this is often attributed to the underlying changes to the collagen fibril structure. This dataset represents a comprehensive screen of the mechanical properties and collagen fibril structure of tendon from the tails of young to old (i.e. 1.6-35.3 month-old) C57BL6/B mice. The mechanical portion consists of the load-displacement data, as well as the derived tensile properties; the structure data consists of transmission electron micrographs of collagen fibril cross section, as well as the derived cross-sectional parameters. This dataset will allow other researchers to develop and demonstrate the utility of innovative multiscale models and approaches of the extra-cellular and physiological events of ageing of current interest to ageing research, by reducing the current reliance on conducting new mammalian experiments.

Effects of maturation and advanced glycation on tensile mechanics of collagen fibrils from rat tail and Achilles tendons

Connective tissues are ubiquitous throughout the body and consequently affect the function of many organs. In load bearing connective tissues like tendon, the mechanical functionality is provided almost exclusively by collagen fibrils that in turn are stabilized by covalent cross-links. Functionally distinct tendons display different cross-link patterns, which also change with maturation, but these differences have not been studied in detail at the fibril level. In the present study, a custom built nanomechanical test platform was designed and fabricated to measure tensile mechanics of individual fibrils from rat tendons. The influence of animal maturity (4 vs. 16 week old rats) and functionally different tendons (tail vs. Achilles tendons) were examined. Additionally the effect of methylglyoxal (MG) treatment in vitro to form advanced glycation end products (AGEs) was investigated. Age and tissue type had no significant effect on fibril mechanics, but MG treatment increased strength and stiffness without inducing brittleness and gave rise to a distinct three-phase mechanical response corroborating that previously reported in human patellar tendon fibrils. That age and tissue had little mechanical effect, tentatively suggest that variations in enzymatic cross-links may play a minor role after initial tissue formation.

STATEMENT OF SIGNIFICANCE

Tendons are connective tissues that connect muscle to bone and carry some of the greatest mechanical loads in the body, which makes them common sites of injury. A tendon is essentially a biological rope formed by thin strands called fibrils made of the protein collagen. Tendon function relies on the strength of these fibrils, which in turn depends on naturally occurring cross-links between collagen molecules, but the mechanical influence of these cross-links have not been measured before. It is believed that beneficial cross-linking occurs with maturation while additional cross-linking with aging may lead to brittleness, but this study provides evidence that maturation has little effect on mechanical function and that age-related cross-linking does not result in brittle collagen fibrils.

Investigating mechanisms of tendon damage by measuring multi-scale recovery following tensile loading

Tendon pathology is associated with damage. While tendon damage is likely initiated by mechanical loading, little is known about the specific etiology. Damage is defined as an irreversible change in the microstructure that alters the macroscopic mechanical parameters. In tendon, the link between mechanical loading and microstructural damage, resulting in macroscopic changes, is not fully elucidated. In addition, tendon damage at the macroscale has been proposed to initiate when tendon is loaded beyond a strain threshold, yet the metrics to define the damage threshold are not determined. We conducted multi-scale mechanical testing to investigate the mechanism of tendon damage by simultaneously quantifying macroscale mechanical and microstructural changes. At the microscale, we observe full recovery of the fibril strain and only partial recovery of the interfibrillar sliding, indicating that the damage initiates at the interfibrillar structures. We show that non-recoverable sliding is a mechanism for tendon damage and is responsible for the macroscale decreased linear modulus and elongated toe-region observed at the fascicle-level, and these macroscale properties are appropriate metrics that reflect tendon damage. We concluded that the inflection point of the stress-strain curve represents the damage threshold and, therefore, may be a useful parameter for future studies. Establishing the mechanism of damage at multiple length scales can improve prevention and rehabilitation strategies for tendon pathology.

STATEMENT OF SIGNIFICANCE

Tendon pathology is associated with mechanically induced damage. Damage, as defined in engineering, is an irreversible change in microstructure that alters the macroscopic mechanical properties. Although microstructural damage and changes to macroscale mechanics are likely, this link to microstructural change was not yet established. We conducted multiscale mechanical testing to investigate the mechanism of tendon damage by simultaneously quantifying macroscale mechanical and microstructural changes. We showed that non-recoverable sliding between collagen fibrils is a mechanism for tendon damage. Establishing the mechanism of damage at multiple length scales can improve prevention and rehabilitation strategies for tendon pathology.