Guidelines for ex vivo mechanical testing of tendon

Mechanical properties of the bicipital aponeurosis

As a biarticular muscle, the biceps brachii both supinates the forearm and flexes the elbow and shoulder, thus allowing the upper limb to perform a variety of activities of daily living (ADL). The biceps brachii originates on the coracoid apex as well as the supraglenoid tubercle and inserts on the radial tuberosity. At the distal end, the bicipital aponeurosis (BA) provides a transition of the biceps tendon into the antebrachial fascia. Previous work has reported the importance of the bicipital aponeurosis in stabilizing distal tendons. Other studies have reported the supination effect that the BA has on the forearm at the radioulnar joint, where it also protects the brachial artery and median nerve (neurovascular bundle). In addition, it has been speculated to have a proprioceptive function. However, despite the important functions fulfilled by this structure, the mechanical properties of the BA are yet to be quantified. Mechanical properties for eight fresh frozen BA specimens (82 ± 12 years, 5 females, 5 right) were quantified using a Cellscale Biaxial (Waterloo, ON) testing machine. Three samples (approximately 7 × 7mm each) were harvested from the proximal, middle and distal regions along the length of the BA. Samples were tested on a biaxial testing machine while maintaining the alignment of the longitudinal collagen fiber orientation with the X-axis of the tester. The testing protocol included 10 preconditioning sinusoidal cycles at 9% strain, at a strain rate of 1%/s, followed by biaxial testing to a maximum strain of 12% at a strain rate of 1%/s. Young's modulus was quantified for all biaxial tests from the linear portion of the resulting stress-strain relation. Results showed that elastic modulus values were significantly greater in the longitudinal direction aligned with the collagen fibers. The outcomes of this study will provide input values for future models of distal biceps repair, thus aiding surgical planning by providing insight into the potential load sharing contributions of the BA.

Smaller Width Quadriceps Tendon Grafts Maintain Advantageous Biomechanical Properties for ACL Reconstruction

Background

Despite clinical evidence of risks in knee arthrofibrosis and graft impingement with larger grafts, the optimal size for quadriceps tendon (QT) autografts in anterior cruciate ligament reconstruction (ACLR) has not been established.

Purpose/Hypothesis

This study aimed to evaluate the mechanical properties of full-thickness 6-mm and 8-mm wide QT grafts compared with 10-mm patellar tendon (PT) and 10-mm QT grafts. The hypothesis was that both the 6- and 8-mm QT grafts would exhibit similar or superior ultimate tensile strength compared with the 10-mm PT graft.

Study Design

Controlled laboratory study.

Methods

A total of 18 matched pairs of cadaveric knees were used in this study. From each pair, a 10-mm wide full-thickness QT was harvested from 1 knee. Based on randomization, a 6-mm wide or 8-mm wide full-thickness QT along with a 10-mm wide PT were harvested from the contralateral knee. Each tendon was clamped, tensioned, and cycled on a servohydraulic testing machine before final loading to failure.

Results

The ultimate failure load was 1286 ± 237.3 N for the 10-mm QT, 1056 ± 226.7 N for the 8-mm QT, 935.1 ± 283.8 N for the 6-mm QT, and 816 ± 192.7 N for the 10-mm PT. Ultimate tensile strength differed significantly between the 10-mm and 8-mm QT (P = .004), 10-mm and 6-mm QT (P < .001), 10-mm QT and 10-mm PT (P < .001), and 8-mm QT and 10-mm PT grafts (P < .001), but not between the 6-mm QT and 10-mm PT grafts (P = .152).

Conclusion

The 8-mm QT had higher ultimate tensile strength than the 10-mm PT, and the 6-mm QT was comparable to the 10-mm PT. Full-thickness QT grafts <10 mm in width may maintain sufficient tensile strength for ACLR.

Clinical Relevance

Given these biomechanical properties, smaller QT graft sizes may be advantageous in minimizing arthrofibrosis risk while maintaining graft strength.

Transcriptome-Optimized Hydrogel Design of a Stem Cell Niche for Enhanced Tendon Regeneration

Bioactive hydrogels have emerged as promising artificial niches for enhancing stem cell-mediated tendon repair. However, a substantial knowledge gap remains regarding the optimal combination of niche features for targeted cellular responses, which often leads to lengthy development cycles and uncontrolled healing outcomes. To address this critical gap, an innovative, data-driven materiomics strategy is developed. This approach is based on in-house RNA-seq data that integrates bioinformatics and mathematical modeling, which is a significant departure from traditional trial-and-error methods. It aims to provide both mechanistic insights and quantitative assessments and predictions of the tenogenic effects of adipose-derived stem cells induced by systematically modulated features of a tendon-mimetic hydrogel (TenoGel). The knowledge generated has enabled a rational approach for TenoGel design, addressing key considerations, such as tendon extracellular matrix concentration, uniaxial tensile loading, and in vitro pre-conditioning duration. Remarkably, our optimized TenoGel demonstrated robust tenogenesis in vitro and facilitated tendon regeneration while preventing undesired ectopic ossification in a rat tendon injury model. These findings shed light on the importance of tailoring hydrogel features for efficient tendon repair. They also highlight the tremendous potential of the innovative materiomics strategy as a powerful predictive and assessment tool in biomaterial development for regenerative medicine.

Knotless Suture Anchors Display Favorable Elongation but an Inferior Ultimate Failure Load Versus Titanium Suture Anchors and All-Suture Anchors: A Biomechanical Comparison in a Porcine Model

Background

Several types of suture anchors, which differ in their working principles, are available for fixation of ligamentous structures in knee surgery. How the choice of a suture anchor type influences the biomechanical stability of ligament fixation is largely unknown.

Purpose

To compare the biomechanical properties of different suture anchor designs regarding primary stability for tendon fixation and repair in medial collateral ligament (MCL) surgery.

Study Design

Controlled laboratory study.

Methods

The primary stability of MCL fixation was assessed in a porcine model utilizing 1 of 3 suture anchor types: a 5.5-mm titanium suture anchor (TSA), a 2.8-mm all-suture anchor (ASA), or a 5.5-mm polyether ether ketone knotless suture anchor (KLSA). Primary stability was assessed using a uniaxial material testing machine. Cyclic loading at 50 N and 100 N was applied for 500 cycles each, followed by a load-to-failure test.

Results

After 500 cycles at 50 N, the KLSA (2.4 ± 0.3 mm) showed significantly (P < .05) reduced elongation in comparison to the TSA (4.0 ± 0.9 mm) and ASA (3.6 ± 0.7 mm), and after 500 cycles at 100 N, the KLSA (6.5 ± 1.4 mm) again showed significantly (P < .05) reduced elongation in comparison to the TSA (11.0 ± 2.2 mm) and ASA (12.0 ± 3.6 mm). However, the KLSA (213 ± 27 N) showed a significantly (P < .05) inferior ultimate failure load in comparison to the TSA (300 ± 20 N) and ASA (348 ± 23 N). In comparison to the TSA (113.0 ± 11.0 N/mm) and ASA (113.6 ± 14.4 N/mm), the KLSA (150.7 ± 11.6 N/mm) displayed the highest stiffness (P < .05). No significant differences were observed regarding yield load.

Conclusion

KLSAs displayed significantly reduced elongation, at the cost of a reduced ultimate failure load, in comparison to TSAs and ASAs.

Clinical Relevance

Surgeons should be aware that differences exist between different suture anchor types regarding their biomechanical stability. KLSAs may be favorable for fixation of peripheral ligaments in knee surgery.

Alterations in mechanical properties of rabbit collateral ligaments eight weeks after anterior cruciate ligament transection

Anterior cruciate ligament (ACL) injury is a common knee ligament injury among young, active adults; however, little is known about its impact on the viscoelastic properties of the knee joint's collateral ligaments. This study aimed to characterize and compare the viscoelastic properties of rabbit collateral ligaments in healthy control knees, injured knees, and knees contralateral to the injured knees. Unilateral anterior cruciate ligament transection was performed on six New Zealand white rabbits to create an ACL injury model. Medial and lateral collateral ligaments (MCL and LCL) were collected from the injured and contralateral knees eight weeks after ACL transection. Ligaments were also harvested from both knees of four unoperated rabbits. The ligaments underwent tensile stress-relaxation testing at strain levels of 2, 4, 6, and 8 %, and a sinusoidal loading test at 8 % strain with 0.5 % strain amplitude using frequencies of 0.01, 0.05, 0.1, 0.5, 1, and 2 Hz. The results showed that collateral ligaments of ACL-transected knees relaxed slower compared to control knees. Sinusoidal testing revealed that contralateral knee LCLs had significantly higher storage and loss modulus across all test frequencies. The results indicate that contralateral knee LCLs become stiffer compared to LCLs from control and ACL-transected knees, while LCLs from ACL-transected knees become less viscous compared to LCLs from control and contralateral knees. This study suggests that knee ligaments undergo adaptations following an ACL injury that may affect the mechanics of the ACL-transected knee, which should be considered in biomechanical and rehabilitation studies of patients with an ACL injury.

Leveraging in vivo animal models of tendon loading to inform tissue engineering approaches

Tendon injuries disrupt successful transmission of force between muscle and bone, resulting in reduced mobility, increased pain, and significantly reduced quality of life for affected patients. There are currently no targeted treatments to improve tendon healing beyond conservative methods such as rest and physical therapy. Tissue engineering approaches hold great promise for designing instructive biomaterials that could improve tendon healing or for generating replacement graft tissue. More recently, engineered microphysiological systems to model tendon injuries have been used to identify therapeutic targets. Despite these advances, current tissue engineering efforts that aim to regenerate, replace, or model injured tendons have largely failed due in large part to a lack of understanding of how the mechanical environment of the tendon influences tissue homeostasis and how altered mechanical loading can promote or prevent disease progression. This review article draws inspiration from what is known about tendon loading from in vivo animal models and identifies key metrics that can be used to benchmark success in tissue engineering applications. Finally, we highlight important challenges and opportunities for the field of tendon tissue engineering that should be taken into consideration in designing engineered platforms to understand or improve tendon healing.

Effect of Tear Size and Location on Supraspinatus Tendon Strain During Activities of Daily Living and Physiotherapy

The supraspinatus tendon plays a crucial role in shoulder abduction, making it one of the common structures affected by injury. Clinically, crescent-shaped tears are the most commonly seen tear shape. By developing six specimen-specific, three-dimensional, supraspinatus-infraspinatus finite element model with heterogeneous material properties, this study aimed to examine the changes in tissue deformation (maximum principal strain) of the supraspinatus tendon due to specimen-specific material properties and rotator cuff tear size. FE models with small- and medium-sized full-thickness crescent-shaped tears were subjected to loads seen during activities of daily living and physiotherapy. Six fresh-frozen cadaveric shoulders were dissected to mechanically test the supraspinatus tendon and develop and validate FE models that can be used to assess changes in strain due to small (< 1 cm, equivalent to 20-30% of the tendon width) and medium-sized (1-3 cm, equivalent to 40-50% of the tendon width) tears that are located in the middle and posterior regions of the supraspinatus tendon. FE predictions of maximum principal strain at the tear tips were examined to determine whether failure strain was reached during activities of daily living (drinking and brushing teeth) and physiotherapy exercises (prone abduction and external rotation at 90° abduction). No significant differences were observed between the middle and posterior tear failure loads for small- and medium-sized tears. For prone abduction, there was a potential risk for tear progression (exceeded failure strain) for medium-sized tears in the supraspinatus tendon's middle and posterior regions. For external rotation at 90° abduction, one model with a middle tear and two with posterior tears experienced failure. For all daily activity loads, the strain never exceeded the failure strain. Our three-dimensional supraspinatus-infraspinatus FE model shows that small tears appear unlikely to progress based on the regional strain response; however, medium-sized tears are at higher risk during more strenuous physiotherapy exercises. Furthermore, differences in patient-specific tendon material properties are important in determining whether the tear will progress. Therefore, patient-specific management plans based on tear size may be beneficial to improve clinical outcomes.

The anisotropic and region-dependent mechanical response of wrap-around tendons under tensile, compressive and combined multiaxial loads

The present work reports on the multiaxial region and orientation-dependent mechanical properties of two porcine wrap-around tendons under tensile, compressive and combined loads based on an extensive study with n=175 samples. The results provide a detailed dataset of the anisotropic tensile and compressive longitudinal properties and document a pronounced tension-compression asymmetry. Motivated by the physiological loading conditions of these tendons, which include transversal compression at bony abutments in addition to longitudinal tension, we systematically investigated the change in axial tension when the tendon is compressed transversally along one or both perpendicular directions. The results reveal that the transversal compression can increase axial tension (proximal-distal direction) in both cases to orders of 30%, yet by a larger amount in the first case (transversal compression in anterior-posterior direction), which seems to be more relevant for wrap-around tendons in-vivo. These quantitative measurements are in line with earlier findings on auxetic properties of tendon tissue, but show for the first time the influence of this property on the stress response of the tendon, and may thus reveal an important functional principle within these essential elements of force transmission in the body. STATEMENT OF SIGNIFICANCE: The work reports for the first time on multiaxial region and orientation-dependent mechanical properties of wrap-around tendons under various loads. The results indicate that differences in the mechanical properties exist between zones that are predominantly in a uniaxial tensile state and those that experience complex load states. The observed counterintuitive increase of the axial tension upon lateral compression points at auxetic properties of the tendon tissue which may be pivotal for the function of the tendon as an element of the musculoskeletal system. It suggests that the tendon's performance in transmitting forces is not diminished but enhanced when the action line is deflected by a bony pulley around which the tendon wraps, representing an important functional principle of tendon tissue.

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.

Shape or size matters? Towards standard reporting of tensile testing parameters for human soft tissues: systematic review and finite element analysis

Material properties of soft-tissue samples are often derived through uniaxial tensile testing. For engineering materials, testing parameters (e.g., sample geometries and clamping conditions) are described by international standards; for biological tissues, such standards do not exist. To investigate what testing parameters have been reported for tensile testing of human soft-tissue samples, a systematic review of the literature was performed using PRISMA guidelines. Soft tissues are described as anisotropic and/or hyperelastic. Thus, we explored how the retrieved parameters compared against standards for engineering materials of similar characteristics. All research articles published in English, with an Abstract, and before 1 January 2023 were retrieved from databases of PubMed, Web of Science, and BASE. After screening of articles based on search terms and exclusion criteria, a total 1,096 articles were assessed for eligibility, from which 361 studies were retrieved and included in this review. We found that a non-tapered shape is most common (209 of 361), followed by a tapered sample shape (92 of 361). However, clamping conditions varied and were underreported (156 of 361). As a preliminary attempt to explore how the retrieved parameters might influence the stress distribution under tensile loading, a pilot study was performed using finite element analysis (FEA) and constitutive modeling for a clamped sample of little or no fiber dispersion. The preliminary FE simulation results might suggest the hypothesis that different sample geometries could have a profound influence on the stress-distribution under tensile loading. However, no conclusions can be drawn from these simulations, and future studies should involve exploring different sample geometries under different computational models and sample parameters (such as fiber dispersion and clamping effects). Taken together, reporting and choice of testing parameters remain as challenges, and as such, recommendations towards standard reporting of uniaxial tensile testing parameters for human soft tissues are proposed.

Contributions of collagen and elastin to elastic behaviours of tendon fascicle

Tendon exhibits the capacity to be stretched and to return to its original length without suffering structural damage in vivo, a capacity known as elastic recoil. Collagen fibres are aligned longitudinally and elastin fibres mostly run parallel to collagen fibres in tendon. However, their interactions and contributions to tendon elastic behaviours are not well understood. The present study examined functional roles of collagen and elastin in tendon elastic behaviours using a variety of mechanical tests. We prepared three types of fascicle specimens from mouse tail tendon: fascicles freshly isolated, those digested with elastase in PBS to selectively remove elastin, and those incubated in PBS without elastase. A quasi-static tensile test demonstrated that elastase-treated fascicles had higher tangent moduli and strength compared to fresh and PBS fascicles. Cyclic stretching tests showed that fresh and PBS fascicles could withstand cyclic strain at both small and large amplitudes, but elastase-treated fascicles could only behave elastically to a limited degree. Fibre-sliding analysis revealed that fresh fascicles could be elongated both through stretching of collagen fibers and through movement of the fibres. However, elastase-treated fascicles could be stretched only via fibre stretching. This evidence suggests that normal tendons can be extended through both fibre stretching and fibre sliding, whereas tendons without elastin can only extend as much as collagen fibers can withstand. Accordingly, collagen fibres mainly contribute to tendon elastic behaviours by furnishing rigidity and elasticity, whereas elastin provides tendon viscoelasticity and also enables sliding of collagen fibres during elastic behaviours. STATEMENT OF SIGNIFICANCE: The present study revealed distinct mechanical functions of collagen and elastin fibres in elastic behaviours of mouse tail tendon fascicle using a variety of mechanical tests at both microscopic and macroscopic levels. It was demonstrated that collagen mainly governs tendon fascicle rigidity and elasticity, but only possesses limited extensibility, whereas elastin contributes to viscoelasticity and collagen fibre sliding, enabling elastic recoil behaviour against relatively large deformation. By their interactions, tendon can be elongated without suffering major structural damage and withstand a large magnitude of tensile force in response to mechanical loading. Such information should be particularly useful in designing collagen-based biomaterials such as artificial tendons, in that previous studies have merely considered collagen without incorporation of elastin.