Mechanical properties of the human achilles tendon

Impact of cyclic mechanical stimulation on the expression of extracellular matrix proteins in human primary rotator cuff fibroblasts

PURPOSE

Mechanical stimulation plays an important role in the development and remodelling of tendons. The aim of the study was to evaluate the effects of mechanical stimulation on the expression of extracellular matrix proteins in human primary rotator cuff (RC) fibroblasts.

METHODS

RC fibroblasts were isolated from patients with degenerative RC tears and characterized using flow cytometry and immunohistochemistry. Cells were stimulated using the Flexcell FX5K™ Tension System. The stimulation regime was a uniaxial sinusoidal waveform with 10 % elongation and a frequency of 0.5 Hz, whereby each cycle consists of 10-s strain and 30-s relaxation. Data were normalized to mechanically unstimulated control groups for every experimental condition. RT-qPCR was performed to determine relative mRNA levels, and collagen production was measured by a colorimetric assay.

RESULTS

The positive expression of CD91 and CD10, and negativity for CD45 and CD4 confirmed the fibroblast phenotype of RC primary cells. RT-qPCR revealed that 10 % continuous cyclic strain for 7 and 14 days induced a significant increase in the mRNA expression both on the matrix metalloproteinases MMP1, MMP3, MMP13, and MMP14 and on the extracellular matrix proteins decorin, tenascin-C, and scleraxis. Furthermore, mechanically stimulated groups produced significantly higher amounts of total collagen.

CONCLUSION

These results may contribute to a better understanding of strain-induced tendon remodelling and will form the basis for the correct choice of applied force in rehabilitation after orthopaedic surgery. These findings underline the fact that early passive motion of the joint in order to induce remodelling of the tendon should be included within a rehabilitation protocol for rotator cuff repair.

High strains near femoral insertion site of the superficial medial collateral ligament of the Knee can explain the clinical failure pattern

The three dimensional (3D) deformation of the superficial medial collateral ligament (sMCL) of the knee might play an important role in the understanding of the biomechanics of sMCL lesions. Therefore, the strain and deformation pattern of the sMCL during the range of motion were recorded in five cadaveric knees with digital image correlation. During knee flexion, the sMCL was found to deform in the three planes. In the sagittal plane, a rotation of the proximal part of the sMCL relative to the distal part occurred with the center of this rotation being the proximal tibial insertion site of the sMCL. This deformation generated high strains near the femoral insertion site of the sMCL. These strains were significantly higher than in the other parts and were maximal at 90° with on average +3.7% of strain and can explain why most lesions in clinical practice are seen in this proximal region. The deformation also has important implications for sMCL reconstruction techniques. Only a perfect anatomic restoration of the insertion sites of the sMCL on both the proximal and distal tibial insertion sites will be able to reproduce the isometry of the sMCL and thus provide the adequate stability throughout the range of motion. The fact that knee motion between 15° and 90° caused minimal strain in the sMCL might suggest that early passive range of motion in physical therapy postoperatively should have little risk of stretching a graft out in the case of an anatomical reconstruction. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2016-2024, 2016.

Adaptive Remodeling of Achilles Tendon: A Multi-scale Computational Model

While it is known that musculotendon units adapt to their load environments, there is only a limited understanding of tendon adaptation in vivo. Here we develop a computational model of tendon remodeling based on the premise that mechanical damage and tenocyte-mediated tendon damage and repair processes modify the distribution of its collagen fiber lengths. We explain how these processes enable the tendon to geometrically adapt to its load conditions. Based on known biological processes, mechanical and strain-dependent proteolytic fiber damage are incorporated into our tendon model. Using a stochastic model of fiber repair, it is assumed that mechanically damaged fibers are repaired longer, whereas proteolytically damaged fibers are repaired shorter, relative to their pre-damage length. To study adaptation of tendon properties to applied load, our model musculotendon unit is a simplified three-component Hill-type model of the human Achilles-soleus unit. Our model results demonstrate that the geometric equilibrium state of the Achilles tendon can coincide with minimization of the total metabolic cost of muscle activation. The proposed tendon model independently predicts rates of collagen fiber turnover that are in general agreement with in vivo experimental measurements. While the computational model here only represents a first step in a new approach to understanding the complex process of tendon remodeling in vivo, given these findings, it appears likely that the proposed framework may itself provide a useful theoretical foundation for developing valuable qualitative and quantitative insights into tendon physiology and pathology.

Structural and material properties of human foot tendons

BACKGROUNDS

The aim of this study was to assess the mechanical properties of the main balance tendons of the human foot in vitro reporting mechanical structural properties and mechanical material properties separately. Tendon structural properties are relevant for clinical applications, for example in orthopedic surgery to elect suitable replacements. Tendon material properties are important for engineering applications such as the development of refined constitutive models for computational simulation or in the design of synthetic materials.

METHODS

One hundred uniaxial tensile tests were performed to obtain the mechanical response of the main intrinsic and extrinsic human foot tendons. The specimens were harvested from five frozen cadaver feet including: Extensor and Flexor tendons of all toes, Tibialis Anterior and Posterior tendons and Peroneus Brevis and Longus tendons.

FINDINGS

Cross-sectional area, load and strain failure, Young's modulus and ultimate tensile stress are reported as a reference of foot tendon mechanical properties. Two different behaviors could be differentiated. Tibialis and Peroneus tendons exhibited higher values of strain failure compared to Flexor and Extensor tendons which had higher Young's modulus and ultimate tensile stress. Stress-strain tendon curves exhibited proportionality between regions. The initial strain, the toe region and the yield point corresponded to the 15, 30 and 70% of the strain failure respectively.

INTERPRETATION

Mechanical properties of the lesser-studied human foot tendons are presented under the same test protocol for different engineering and clinical applications. The tendons that work at the inversion/eversion plane are more deformable at the same stress and strain rate than those that work at the flexion/extension plane.

Dynamic material characterization of the human heel pad based on in vivo experimental tests and numerical analysis

A numerical-experimental, proof-of-concept approach is described to characterize the mechanical material behavior of the human heel pad under impact conditions similar to a heel strike while running. A 3D finite-element model of the right foot of a healthy female subject was generated using magnetic resonance imaging. Based on quasi-static experimental testing of the subject's heel pad, force-displacement data was obtained. Using this experimental data as well as a numerical optimization algorithm, an inverse finite-element analysis and the 3D model, heel pad hyperelastic (long-term) material parameters were determined. Applying the same methodology, based on the dynamic experimental data from the impact test and obtained long-term parameters, linear viscoelastic parameters were established with a Prony series. Model validation was performed employing quasi-static and dynamic force-displacement data. Coefficients of determination when comparing model to experimental data during quasi-static and dynamic (initial velocity: 1480mm/s) procedure were R(2) = 0.999 and R(2) = 0.990, respectively. Knowledge of these heel pad material parameters enables realistic numerical analysis to evaluate internal stress and strain in the heel pad during different quasi-static or dynamic load conditions.

Mechanoresponsive musculoskeletal tissue differentiation of adipose-derived stem cells

Musculoskeletal tissues are constantly under mechanical strains within their microenvironment. Yet, little is understood about the effect of in vivo mechanical milieu strains on cell development and function. Thus, this review article outlines the in vivo mechanical environment of bone, muscle, cartilage, tendon, and ligaments, and tabulates the mechanical strain and stress in these tissues during physiological condition, vigorous, and moderate activities. This review article further discusses the principles of mechanical loading platforms to create physiologically relevant mechanical milieu in vitro for musculoskeletal tissue regeneration. A special emphasis is placed on adipose-derived stem cells (ADSCs) as an emerging valuable tool for regenerative musculoskeletal tissue engineering, as they are easily isolated, expanded, and able to differentiate into any musculoskeletal tissue. Finally, it highlights the current state-of-the art in ADSCs-guided musculoskeletal tissue regeneration under mechanical loading.

Low-level laser therapy modulates pro-inflammatory cytokines after partial tenotomy

Tendon injuries give rise to substantial morbidity, and current understanding of the mechanisms involved in tendon injury and repair is limited. This lesion remains a clinical issue because the injury site becomes a region with a high incidence of recurrent rupture and has drawn the attention of researchers. We already demonstrated that low-level laser therapy (LLLT) stimulates the synthesis and organization of collagen I, MMP-9, and MMP-2 and improved the gait recovery of the treated animals. The aim of this study was to evaluate the effects of LLLT in the nitric oxide and cytokines profile during the inflammatory and remodeling phases. Adult male rats were divided into the following groups: G1--intact, G2-- injured, G3--injured + LLLT (4 J/cm(2) continuous), G4--injured + LLLT (4 J/cm(2)-20 Hz--pulsed laser). According to the analysis, the animals were euthanized on different dates (1, 4, 8, or 15 days after injury). ELISA assay of TNF-α, IL-1β, IL-10, and TGF-β was performed. Western blotting of isoform of nitric oxide synthase (i-NOS) and nitric oxide dosage experiments was conducted. Our results showed that the pulsed LLLT seems to exert an anti-inflammatory effect over injured tendons, with reduction of the release of proinflammatory cytokines, such as TNF-α and the decrease in the i-NOS activity. Thanks to the pain reduction and the facilitation of movement, there was a stimulation in the TGF-β and IL-1β release. In conclusion, we believe that pulsed LLLT worked effectively as a therapy to reestablish the tendon integrity after rupture.

Ultrasound-Based Tendon Micromorphology Predicts Mechanical Characteristics of Degenerated Tendons

The purpose of this study was to explore the relationship between tendon micro-morphology quantified from a sonogram and tendon mechanical characteristics measured in vivo. Nineteen adults (nine with unilateral Achilles tendinosis) participated. A commercial ultrasound scanner was used to capture longitudinal B-mode ultrasound images from the mid-portion of bilateral Achilles tendons and a custom image analysis program was used to analyze the spatial frequency content of manually defined regions of interest; in particular, the average peak spatial frequency of the regions of interest was acquired. In addition, a dynamometer and a motion analysis system indirectly measured the tendon mechanical (stiffness) and material (elastic modulus) properties. The peak spatial frequency correlated with tendon stiffness (r = 0.74, p = 0.02) and elastic modulus (r = 0.65, p = 0.05) in degenerated tendons, but not healthy tendons. This is the first study relating the mechanical characteristics of degenerated human Achilles tendon using a non-invasive micro-morphology analysis approach.

Decellularized and Engineered Tendons as Biological Substitutes: A Critical Review

Tendon ruptures are a great burden in clinics. Finding a proper graft material as a substitute for tendon repair is one of the main challenges in orthopaedics, for which the requirement of a biological scaffold would be different for each clinical application. Among biological scaffolds, the use of decellularized tendon-derived matrix increasingly represents an interesting approach to treat tendon ruptures. We analyzed in vitro and in vivo studies focused on the development of efficient protocols for the decellularization and for the cell reseeding of the tendon matrix to obtain medical devices for tendon substitution. Our review considered also the proper tendon source and preclinical animal models with the aim of entering into clinical trials. The results highlight a wide panorama in terms of allogenic or xenogeneic tendon sources, specimen dimensions, physical or chemical decellularization techniques, and the cell type variety for reseeding from terminally differentiated to undifferentiated mesenchymal stem cells and their static or dynamic culture employed to generate implantable constructs tested in different animal models. We try to identify the most efficient approach to achieve an optimal biological scaffold for biomechanics and intrinsic properties, resembling the native tendon and being applicable in clinics in the near future, with particular attention to the Achilles tendon substitution.

Mechanical risk of rotator cuff repair failure during passive movements: A simulation-based study

BACKGROUND

Despite improvements in rotator cuff surgery techniques, re-tear rate remains above 20% and increases with tear severity. Mechanical stresses to failure of repaired tendons have been reported. While optimal immobilization postures were proposed to minimize this stress, post-operative rehabilitation protocols have never been assessed with respect to these values. Purpose was to use musculoskeletal simulation to predict when the stress in repaired tendons exceeds safety limits during passive movements. Hence, guidelines could be provided towards safer post-operative exercises.

METHODS

Sixteen healthy participants volunteered in passive three-dimensional shoulder range-of-motion and passive rehabilitation exercises assessment. Stress in all rotator cuff tendons was predicted during each movement by means of a musculoskeletal model using simulations with different type and size of tears. Safety stress thresholds were defined based on repaired tendon loads to failure reported in the literature and used to discriminate safe from unsafe ranges-of-motion.

FINDINGS

Increased tear size and multiple tendons tear decreased safe range-of-motion. Mostly, glenohumeral elevations below 38°, above 65°, or performed with the arm held in internal rotation cause excessive stresses in most types and sizes of injury during abduction, scaption or flexion. Larger safe amplitudes of elevation are found in scapular plane for supraspinatus alone, supraspinatus plus infraspinatus, and supraspinatus plus subscapularis tears.

INTERPRETATION

This study reinforces that passive early rehabilitation exercises could contribute to re-tear due to excessive stresses. Recommendations arising from this study, for instance to keep the arm externally rotated during elevation in case of supraspinatus or supraspinatus plus infraspinatus tear, could help prevent re-tear.

Suturing the myotendinous junction in total hip arthroplasty: A biomechanical comparison of different stitching techniques

BACKGROUND

The repair of the myotendinous junction following total hip arthroplasty is challenging as this region is the weakest part of the muscle structure. This study investigated the mechanical behaviour and the mode of failure of different suturing techniques of the myotendinous junction. A new asymmetrical stitch was compared to two widely used techniques, i.e. the simple stitch (two loops in parallel) and the figure-of-eight stitch.

METHODS

The ovine triceps brachii myotendinous junction was selected as the experimental model. Each technique was sewn in muscle belly on one side and in a polyester belt (no-tendon configuration) or in thin tendon (full configuration) on the other side. The former was chosen to determine the grasping power of the stitch on the muscle despite the tendon quality, the latter to simulate a very thin gluteus medius tendon.

FINDINGS

The new stitch showed a higher ultimate strength (+40%) compared to the two controls in the no-tendon configuration. In the full configuration, no significant increase was observed, although failure of the new stitch always occurred at the tendon side. Furthermore, the new stitch does not alter the stiffness of repair.

INTERPRETATION

The new stitch has a higher grasping power on muscle belly than the single passing-through stitches thanks to the multiple fixation points, which better distribute the load in the tissue. However, such performance can be fully exploited only in the presence of good quality tendons.

Biomimetic repeat protein derived from Xenopus tropicalis for fibrous scaffold fabrication

Collagen, silk, and elastin are the fibrous proteins consist of representative amino acid repeats. Because these proteins exhibited distinguishing mechanical properties, they have been utilized in diverse applications, such as fiber-based sensors, filtration membranes, supporting materials, and tissue engineering scaffolds. Despite their infinite prevalence and potential, most studies have only focused on a few repeat proteins. In this work, the hypothetical protein with a repeat motif derived from the frog Xenopus tropicalis was obtained and characterized for its potential as a novel protein-based material. The codon-optimized recombinant frog repeat protein, referred to as 'xetro', was produced at a high rate in a bacterial system, and an acid extraction-based purified xetro protein was successfully fabricated into microfibers and nanofibers using wet spinning and electrospinning, respectively. Specifically, the wet-spun xetro microfibers demonstrated about 2- and 1.5-fold higher tensile strength compared with synthetic polymer polylactic acid and cross-linked collagen, respectively. In addition, the wet-spun xetro microfibers showed about sevenfold greater stiffness than collagen. Therefore, the mass production potential and greater mechanical properties of the xetro fiber may result in these fibers becoming a new promising fiber-based material for biomedical engineering.

Tensile properties of craniofacial tendons in the mature and aged zebrafish

The zebrafish Danio rerio is a powerful model for the study of development, regenerative biology, and human disease. However, the analysis of load-bearing tissues such as tendons and ligaments has been limited in this system. This is largely due to technical limitations that preclude accurate measurement of their mechanical properties. Here, we present a custom tensile testing system that applies nano-Newton scale forces to zebrafish tendons as small as 1 mm in length. Tendon properties were remarkably similar to mammalian tendons, including stress-strain nonlinearity and a linear modulus (515 ± 152 MPa) that aligned closely with mammalian data. Additionally, a simple exponential constitutive law used to describe tendon mechanics was successfully fit to zebrafish tendons; the associated material constants agreed with literature values for mammalian tendons. Finally, mature and aged zebrafish comparisons revealed a significant decline in mechanical function with age. Based on the exponential constitutive model, age-related changes were primarily caused by a reduction in nonlinearity (e.g., changes in collagen crimp or fiber recruitment). These findings demonstrate the utility of zebrafish as a model to study tendon biomechanics in health and disease. Moreover, these findings suggest that tendon mechanical behavior is highly conserved across vertebrates.

A fibre-reinforced poroviscoelastic model accurately describes the biomechanical behaviour of the rat Achilles tendon

Background

Computational models of Achilles tendons can help understanding how healthy tendons are affected by repetitive loading and how the different tissue constituents contribute to the tendon’s biomechanical response. However, available models of Achilles tendon are limited in their description of the hierarchical multi-structural composition of the tissue. This study hypothesised that a poroviscoelastic fibre-reinforced model, previously successful in capturing cartilage biomechanical behaviour, can depict the biomechanical behaviour of the rat Achilles tendon found experimentally.

Materials and Methods

We developed a new material model of the Achilles tendon, which considers the tendon’s main constituents namely: water, proteoglycan matrix and collagen fibres. A hyperelastic formulation of the proteoglycan matrix enabled computations of large deformations of the tendon, and collagen fibres were modelled as viscoelastic. Specimen-specific finite element models were created of 9 rat Achilles tendons from an animal experiment and simulations were carried out following a repetitive tensile loading protocol. The material model parameters were calibrated against data from the rats by minimising the root mean squared error (RMS) between experimental force data and model output.

Results and Conclusions

All specimen models were successfully fitted to experimental data with high accuracy (RMS 0.42-1.02). Additional simulations predicted more compliant and soft tendon behaviour at reduced strain-rates compared to higher strain-rates that produce a stiff and brittle tendon response. Stress-relaxation simulations exhibited strain-dependent stress-relaxation behaviour where larger strains produced slower relaxation rates compared to smaller strain levels. Our simulations showed that the collagen fibres in the Achilles tendon are the main load-bearing component during tensile loading, where the orientation of the collagen fibres plays an important role for the tendon’s viscoelastic response. In conclusion, this model can capture the repetitive loading and unloading behaviour of intact and healthy Achilles tendons, which is a critical first step towards understanding tendon homeostasis and function as this biomechanical response changes in diseased tendons.

What is the best candidate allograft for ACL reconstruction? An in vitro mechanical and histologic study in a canine model

The knee joint is generally characterized by very low friction and high wear resistance. Several previous studies have compared ACL with the commonly used allografts from tensile properties perspective. No study has reported about the graft tendons from a frictional perspective, which is an important parameter for ACL functional performance. Twenty hind legs were used to harvest FDP tendon, ACL, ACH, and patellar tendon. Samples were evaluated with surface friction testing, indentation testing for tendon compressive moduli, lubricin immunohistochemistry, and histologic analysis. Frictional force of FDP tendon and ACL was significantly less than that of patellar tendon and ACH at first and fifth cycles. At the tenth cycle, the FDP tendon, ACL, and ACH showed significantly less frictional force than patellar tendon; after 100 cycles, the FDP tendon and ACL showed significantly less frictional force than patellar tendon. The compressive moduli of the FDP tendon, ACL, and ACH were significantly greater than that of patellar tendon. Histologic results showed that FDP tendon and ACL had a smooth surface with a thin layer of epitenon cells; patellar tendon and ACH had a rough surface and a layer of paratenon. Lubricin was found on the surface and extracellular matrix of FDP tendon and ACL. There was only limited lubricin expression on the surface and extracellular matrix of the ACH and patellar tendon. The FDP tendon has friction force and lubricin expression similar to those of native ACL. However, patellar tendon and ACH show higher friction force and less lubricin expression than ACL.

The (dys)functional extracellular matrix

The extracellular matrix (ECM) is a major component of the biomechanical environment with which cells interact, and it plays important roles in both normal development and disease progression. Mechanical and biochemical factors alter the biomechanical properties of tissues by driving cellular remodeling of the ECM. This review provides an overview of the structural, compositional, and mechanical properties of the ECM that instruct cell behaviors. Case studies are reviewed that highlight mechanotransduction in the context of two distinct tissues: tendons and the heart. Although these two tissues demonstrate differences in relative cell-ECM composition and mechanical environment, they share similar mechanisms underlying ECM dysfunction and cell mechanotransduction. Together, these topics provide a framework for a fundamental understanding of the ECM and how it may vary across normal and diseased tissues in response to mechanical and biochemical cues. This article is part of a Special Issue entitled: Mechanobiology.

The past, present and future in scaffold-based tendon treatments

Tendon injuries represent a significant clinical burden on healthcare systems worldwide. As the human population ages and the life expectancy increases, tendon injuries will become more prevalent, especially among young individuals with long life ahead of them. Advancements in engineering, chemistry and biology have made available an array of three-dimensional scaffold-based intervention strategies, natural or synthetic in origin. Further, functionalisation strategies, based on biophysical, biochemical and biological cues, offer control over cellular functions; localisation and sustained release of therapeutics/biologics; and the ability to positively interact with the host to promote repair and regeneration. Herein, we critically discuss current therapies and emerging technologies that aim to transform tendon treatments in the years to come.

Evaluation of low-level laser therapy, platelet-rich plasma, and their combination on the healing of Achilles tendon in rabbits

Tendon repair is still one of the challenges for rehabilitation. Various treatments for tendon injuries have been used in recent decade. This study was established to investigate the effects of low-level laser therapy (LLLT), platelet-rich plasma (PRP) treatment alone, and using combined method on the healing of Achilles tendon in rabbits. Seventy-two healthy mature male white New Zealand rabbits were divided randomly into four groups of 18 animals each: control: partial tenotomy with no treatment, only 1 mL normal saline was injected on days 1, 8, and 15 at the site of splitting; PRP: partial tenotomy with PRP treatment on days 1, 8, and 15 at the site of splitting; LLLT: partial tenotomy with LLLT (K30 hand-held probe, AZOR, Technica, Russia, 650 nm, 30 mW, surface area = 1 cm(2), 60 S/cm(2), energy density = 1.8 J/cm(2)) for 15 consecutive days; LLLT + PRP: partial tenotomy with LLLT + PRP. At the end of trial, the rabbits were euthanatized and tendon specimens were harvested and were submitted for histopathological evaluation, hydroxyproline levels, and biomechanical measurement. The Tukey post hoc test was performed. The results for these parameters showed that PRP or LLLT alone has significant advantages over untreated animals (P < 0.05). Furthermore, it was found that the combined treatment with PRP and LLLT is even more efficient. There was no significant difference (P > 0.05) between the two groups of LLLT and PRP. However, the treatments combining PRP and LLLT showed significant results in comparison of PRP or LLLT alone (P < 0.05). Our results demonstrate that the healing time of injured tendon decreases by using the two therapies combined.

Laminar Tendon Composites with Enhanced Mechanical Properties

PURPOSE

A strong isotropic material that is both biocompatible and biodegradable is desired for many biomedical applications, including rotator cuff repair, tendon and ligament repair, vascular grafting, among others. Recently, we developed a technique, called "bioskiving" to create novel 2D and 3D constructs from decellularized tendon, using a combination of mechanical sectioning, and layered stacking and rolling. The unidirectionally aligned collagen nanofibers (derived from sections of decellularized tendon) offer good mechanical properties to the constructs compared with those fabricated from reconstituted collagen.

METHODS

In this paper, we studied the effect that several variables have on the mechanical properties of structures fabricated from tendon slices, including crosslinking density and the orientation in which the fibers are stacked.

RESULTS

We observed that following stacking and crosslinking, the strength of the constructs is significantly improved, with crosslinked sections having an ultimate tens ile strength over 20 times greater than non-crosslinked samples, and a modulus nearly 50 times higher. The mechanism of the mechanical failure mode of the tendon constructs with or without crosslinking was also investigated.

CONCLUSIONS

The strength and fiber organization, combined with the ability to introduce transversely isotropic mechanical properties makes the laminar tendon composites a biocompatiable material that may find future use in a number of biomedical and tissue engineering applications.

The effect of donor age on structural and mechanical properties of allograft tendons

BACKGROUND

Allograft tendons are commonly used in surgical ligament reconstruction. While it is commonly accepted that donor age will affect mechanical properties of graft tissue, the apparent age threshold is unknown.

HYPOTHESIS

Donor age will significantly influence the structural and mechanical properties of tibialis posterior allograft tendons.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 550 allograft posterior tibialis tendons were examined. Linear stiffness, ultimate tensile force, ultimate displacement, tensile modulus, ultimate tensile strength, and ultimate tensile strain were calculated for specimens from donors in each of 6 age groups: 15-29, 30-39, 40-49, 50-59, 60-69, and 70-79 years. Both first- and second-order polynomial regressions were performed to determine the correlation between structural and mechanical properties and age. Welch analyses of variance with Games-Howell post hoc tests were performed to facilitate comparisons among age groups.

RESULTS

All parameters displayed a weak correlation with age, with the highest R (2) term being 0.063 for ultimate tensile strength. Linear stiffness, ultimate tensile force, and tensile modulus displayed almost no correlation with age. Ultimate tensile strength increased slightly with age up to 40-49 years and then decreased with further increases in age. Slight decreases in ultimate displacement and ultimate tensile strain were observed with increasing age. Numerous statistically significant differences were observed between age groups for each outcome parameter; however, the magnitudes of the differences between age groups are relatively small (<15%) for all outcome parameters.

CONCLUSION

Age explained at most 6% of the variation in structural and mechanical properties of tibialis posterior allograft tendons.

CLINICAL RELEVANCE

Posterior tibialis tendons from all age groups displayed structural properties superior to the native anterior cruciate ligament, with higher stiffness and ultimate force, and less displacement to failure. Although statistically significant differences in structural and mechanical properties were observed between age groups, the magnitudes of the differences are small and most likely not clinically relevant. The age of the donor will not likely affect the suitability of a graft for use in surgical reconstruction.

Ultrasound-based testing of tendon mechanical properties: a critical evaluation

In the past 20 years, the use of ultrasound-based methods has become a standard approach to measure tendon mechanical properties in vivo. Yet the multitude of methodological approaches adopted by various research groups probably contribute to the large variability of reported values. The technique of obtaining and relating tendon deformation to tensile force in vivo has been applied differently, depending on practical constraints or scientific points of view. Divergence can be seen in 1) methodological considerations, such as the choice of anatomical features to scan and to track, force measurements, or signal synchronization; and 2) in physiological considerations related to the viscoelastic behavior or length measurements of tendons. Hence, the purpose of the present review is to assess and discuss the physiological and technical aspects connected to in vivo testing of tendon mechanical properties. In doing so, our aim is to provide the reader with a qualitative analysis of ultrasound-based techniques. Finally, a list of recommendations is proposed for a number of selected issues.

Preparation of photocrosslinked fish elastin polypeptide/microfibrillated cellulose composite gels with elastic properties for biomaterial applications

Photocrosslinked hydrogels reinforced by microfibrillated cellulose (MFC) were prepared from a methacrylate-functionalized fish elastin polypeptide and MFC dispersed in dimethylsulfoxide (DMSO). First, a water-soluble elastin peptide with a molecular weight of ca. 500 g/mol from the fish bulbus arteriosus was polymerized by N,N'-dicyclohexylcarbodiimide (DCC), a condensation reagent, and then modified with 2-isocyanatoethyl methacrylate (MOI) to yield a photocrosslinkable fish elastin polypeptide. The product was dissolved in DMSO and irradiated with UV light in the presence of a radical photoinitiator. We obtained hydrogels successfully by substitution of DMSO with water. The composite gel with MFC was prepared by UV irradiation of the photocrosslinkable elastin polypeptide mixed with dispersed MFC in DMSO, followed by substitution of DMSO with water. The tensile test of the composite gels revealed that the addition of MFC improved the tensile properties, and the shape of the stress-strain curve of the composite gel became more similar to the typical shape of an elastic material with an increase of MFC content. The rheology measurement showed that the elastic modulus of the composite gel increased with an increase of MFC content. The cell proliferation test on the composite gel showed no toxicity.

Proximal tendon-prosthesis junction for active tendon implants of the hand: a biomechanical comparison of 2 techniques

PURPOSE

To study the biomechanical characteristics (percent stretch, stiffness, and ultimate load) of 2 tendon-prosthesis techniques used to connect the proximal tendon stump to silicone active tendon implants used in reconstruction of flexor tendons.

METHODS

We evaluated percent stretch following cyclic loading and at failure, stiffness during load to failure, and ultimate load of 16 tendon-prosthesis junctions using cadaveric canine flexor digitorum profundus tendons to re-create 2 junction techniques: the tendon loop (TL) and the polyester weave (PW).

RESULTS

The TL junction showed greater percent stretch at a static load of 2 N, following 500 cycles of loading between 2 N and 50 N, and at peak load. The PW junction displayed greater stiffness from 50 to 150 N during load to failure. Both junctions failed at a mean ultimate load greater than 220 N.

CONCLUSIONS

The described proximal junction techniques for active tendon implants were strong enough to resist early active motion in the immediate postoperative period without significant elongation. The PW technique displayed greater stiffness and ultimate load compared with the TL.

CLINICAL RELEVANCE

Data on tendon-prosthesis characteristics of these 2 methods may aid the surgeon in choosing which junction technique to use, during surgical tensioning decisions, and in considering activity protocols after surgery. These data may also serve as a baseline for further investigations regarding active tendon implants.

Plantar pressure relief under the metatarsal heads: therapeutic insole design using three-dimensional finite element model of the foot

Therapeutic footwear with specially-made insoles is often used in people with diabetes and rheumatoid arthritis to relieve ulcer risks and pain due to high pressures from areas beneath bony prominences of the foot, in particular to the metatarsal heads (MTHs). In a three-dimensional finite element study of the foot and footwear with sensitivity analysis, effects of geometrical variations of a therapeutic insole, in terms of insole thicknesses and metatarsal pad (MP) placements, on local peak plantar pressure under MTHs and stress/strain states within various forefoot tissues, were determined. A validated musculoskeletal finite element model of the human foot was employed. Analyses were performed in a simulated muscle-demanding instant in gait. For many design combinations, increasing insole thicknesses consistently reduce peak pressures and internal tissue strain under MTHs, but the effects reach a plateau when insole becomes very thick (e.g., a value of 12.7mm or greater). Altering MP placements, however, showed a proximally- and a distally-placed MP could result in reverse effects on MTH pressure-relief. The unsuccessful outcome due to a distally-placed MP may attribute to the way it interacts with plantar tissue (e.g., plantar fascia) adjacent to the MTH. A uniform pattern of tissue compression under metatarsal shaft is necessary for a most favorable pressure-relief under MTHs. The designated functions of an insole design can best be achieved when the insole is very thick, and when the MP can achieve a uniform tissue compression pattern adjacent to the MTH.

A hierarchical, stretchable and stiff fibrous biotemplate engineered using stagger-electrospinning for augmentation of rotator cuff tendon-healing

The regeneration of fibrocartilage at the tendon-bone insertion site in rotator cuff tears (RCTs) is challenging due to the complexity of its composition and mechanical properties. In this study, hierarchical, stretchable and stiff fibrous scaffolds composed of microfibers of poly(ε-caprolactone) (PCL) and nanofibers of chitosan (CS) were fabricated using stagger-electrospinning for the augmentation of RCT-healing. It was found that the composite PCL-CS scaffolds had significantly improved strength and failure strain compared to the control CS scaffolds and increased stiffness compared to the control PCL scaffolds. These scaffolds also showed enhanced hydrophilicity, water absorption and a faster degradation rate compared to the PCL scaffolds. Moreover, they demonstrated better fibroblast attachment and proliferation compared to the PCL scaffolds. Radiological and histological analysis revealed that the PCL-CS scaffolds enhanced new bone formation (mineralization) and collagen and glycosaminoglycan expression (major components of extracellular matrix) compared to the PCL scaffolds. Furthermore, the torn tissues at the tendon-bone insertion site regenerated with the PCL-CS scaffolds showed higher strength and failure strain as well as stiffness compared to those repaired using only the PCL scaffolds. The above mentioned results suggest that the hierarchical, stretchable and stiff fibrous scaffolds engineered using stagger-electrospinning have great potential for the augmentation of RCT-healing.

Engineering complex orthopaedic tissues via strategic biomimicry

The primary current challenge in regenerative engineering resides in the simultaneous formation of more than one type of tissue, as well as their functional assembly into complex tissues or organ systems. Tissue-tissue synchrony is especially important in the musculoskeletal system, wherein overall organ function is enabled by the seamless integration of bone with soft tissues such as ligament, tendon, or cartilage, as well as the integration of muscle with tendon. Therefore, in lieu of a traditional single-tissue system (e.g., bone, ligament), composite tissue scaffold designs for the regeneration of functional connective tissue units (e.g., bone-ligament-bone) are being actively investigated. Closely related is the effort to re-establish tissue-tissue interfaces, which is essential for joining these tissue building blocks and facilitating host integration. Much of the research at the forefront of the field has centered on bioinspired stratified or gradient scaffold designs which aim to recapitulate the structural and compositional inhomogeneity inherent across distinct tissue regions. As such, given the complexity of these musculoskeletal tissue units, the key question is how to identify the most relevant parameters for recapitulating the native structure-function relationships in the scaffold design. Therefore, the focus of this review, in addition to presenting the state-of-the-art in complex scaffold design, is to explore how strategic biomimicry can be applied in engineering tissue connectivity. The objective of strategic biomimicry is to avoid over-engineering by establishing what needs to be learned from nature and defining the essential matrix characteristics that must be reproduced in scaffold design. Application of this engineering strategy for the regeneration of the most common musculoskeletal tissue units (e.g., bone-ligament-bone, muscle-tendon-bone, cartilage-bone) will be discussed in this review. It is anticipated that these exciting efforts will enable integrative and functional repair of soft tissue injuries, and moreover, lay the foundation for the development of composite tissue systems and ultimately, total limb or joint regeneration.

Achilles tendons hypertrophy in response to high loading training

BACKGROUND

Whether the human Achilles tendon undergoes hypertrophic changes as measured by an increase in cross-sectional area, in response to endurance training exercise remains in question. We investigated the hypothesis that transition from civilian life through 6 months of elite infantry training would induce adaptive Achilles tendon hypertrophy.

METHODS

Seventy-two new elite infantry recruits had the cross-sectional area of their Achilles tendons measured at a point 2.5 cm proximal to the Achilles insertion by ultrasound before beginning elite infantry training. Measurements were repeated by the same ultrasonographer for those recruits who were still in the training program at 6 months. Prior to beginning the study the intraobserver reliability of the ultrasonographer's Achilles tendon measurements was calculated (intraclass correlation coefficient = .96). Fifty-five recruits completed 6 months of training.

RESULTS

The mean cross-sectional area of their right Achilles tendon increased from 47.0 ± 11.2 to 50.2 ± 9.6 mm(2) (P = .037) and the left Achilles tendon from 47.2 ± 8.9 to 51.1 ± 8.3 mm(2) (P = .013). The change in cross-sectional area did not correlate with subject height, weight, prior sport history, or jumping and running abilities.

CONCLUSIONS

An abrupt stimulus of 6 months of elite infantry training was adequate to induce hypertrophic changes in the Achilles tendon. This is the first human prospective study showing an increase in the Achilles tendon cross-sectional area in response to rigorous endurance type training. The finding supports the hypothesis that the Achilles tendon in response to sufficiently high and sustained loading can remodel its morphological properties and thereby strengthen itself.

LEVEL OF EVIDENCE

Level II, etiology study.

The effect of the gastrocnemius on the plantar fascia

Although anatomic and functional relationship has been established between the gastrocnemius muscle, via the Achilles tendon, and the plantar fascia, the exact role of gastrocnemius tightness in foot and plantar fascia problems is not completely understood. This article summarizes past and current literature linking these 2 structures and gives a mechanical explanation based on functional models of the relationship between gastrocnemius tightness and plantar fascia. The effect of gastrocnemius tightness on the sagittal behavior of the foot is also discussed.

Posterior cruciate ligament graft fixation angles, part 2: biomechanical evaluation for anatomic double-bundle reconstruction

BACKGROUND

Prior studies have suggested that anatomic double-bundle (DB) posterior cruciate ligament reconstruction (PCLR) reduces residual laxity compared with the intact state better than single-bundle PCLR. Although the anterolateral bundle (ALB) and posteromedial bundle (PMB) reportedly act codominantly, few studies have compared commonly used graft fixation angles and the influence that graft fixation angles have on overall graft forces and knee laxity.

HYPOTHESIS

Graft fixation angle combinations of 0°/75° (PMB/ALB), 0°/90°, 0°/105°, 15°/75°, 15°/90°, and 15°/105° would significantly reduce knee laxity from the sectioned PCL state while preventing in vitro graft forces from being overloaded between any of the graft fixation angles.

STUDY DESIGN

Controlled laboratory study.

METHODS

Nine cadaveric knees were evaluated for the kinematics of the intact, PCL-sectioned, and DB PCLR techniques. The DB technique was varied by fixing the PMB and ALB grafts at the following 6 randomly ordered fixation angle combinations: 0°/75° (PMB/ALB), 0°/90°, 0°/105°, 15°/75°, 15°/90°, and 15°/105°. A 6 degrees of freedom robotic testing system subjected each specimen to an applied 134-N posterior tibial load at 0° to 120° of flexion and 5-N·m external, 5-N·m internal, and 10-N·m valgus rotation torques applied at 60°, 75°, 90°, 105°, and 120° of flexion. The ALB and PMB grafts were fixed to load cells that concurrently measured graft forces throughout kinematic testing. t tests compared the kinematics between groups, and 2-factor models assessed the contribution of ALB and PMB grafts after DB PCLR (P < .05).

RESULTS

Consistently, DB PCLR significantly reduced posterior translation compared with the sectioned PCL and was comparable with the intact state during applied posterior tibial loads at flexion angles of greater than 90°; a mean residual laxity of 1.5 mm remained compared with the intact state during applied posterior tibial loads. Additionally, fixing the PMB graft at 15° resulted in significantly larger PMB graft forces compared with fixation at 0° during applied posterior loading, internal rotation, external rotation, and valgus rotation. Similarly, fixing the ALB graft at 75° resulted in significantly larger ALB graft forces compared with fixation of the ALB graft at 90° or 105° during all loading conditions.

CONCLUSION

Fixation of the PMB graft at 0° to 15° and the ALB graft at 75° to 105° during DB PCLR were successful in significantly reducing knee laxity from the sectioned state. However, fixation of the PMB graft at 15° versus 0° resulted in significantly increased loads through the PMB graft, and fixation of the ALB graft at 75° versus 90° or 105° resulted in significantly increased loads through the ALB graft.

CLINICAL RELEVANCE

This study found that all 6 fixation angle combinations significantly improved knee kinematics compared with the sectioned state at time zero; however, it is recommended that fixation of the PMB graft be performed at 0° because of the significant increases in PMB graft loading that occur with fixation at 15° and that fixation of the ALB graft be performed at 90° or 105° rather than 75° to minimize ALB graft forces, which could lead to graft attenuation or failure over time.

Tenogenic Induction of Human MSCs by Anisotropically Aligned Collagen Biotextiles

A novel biofabrication modality, electrophoretic compaction with macromolecular alignment, was utilized to make collagen threads that mimic the native tendon's structure and mechanical properties. A device with kinematic electrodes was designed to fabricate collagen threads in continuous length. For the first time, a 3D-biotextile was woven purely from collagen. Mechanical properties and load-displacement behavior of the biotextile mimicked those of the native tendon while presenting a porosity of 80%. The open pore network facilitated cell seeding across the continuum of the bioscaffold. Mesenchymal stem cells (MSCs) seeded in the woven scaffold underwent tenogenic differentiation in the absence of growth factors and synthesized a matrix that was positive for tenomodulin, COMP and type I collagen. Up-regulation of tenomodulin, a tendon specific marker, was 11.6 ± 3.5 fold, COMP was up-regulated 16.7 ± 5.5 fold, and Col I was up-regulated 6.9 ± 2.7 fold greater on ELAC threads when compared to randomly oriented collagen gels. These results demonstrate that a bioscaffold woven by using collagen threads with densely compacted and anisotropically aligned substrate texture stimulates tenogenesis topographically, rendering the electrochemically aligned collagen as a promising candidate for functional repair of tendons and ligaments.

Mechanical properties vary for different regions of the finger extensor apparatus

The extensor apparatus, an aponeurosis that covers the dorsal side of each finger, transmits force from a number of musculotendons to the phalanges. Multiple tendons integrate directly into the structure at different sites and the extensor apparatus attaches to the phalanges at multiple points. Thus, prediction of the force distribution within the extensor apparatus, or hood, and the transmission to the phalanges is challenging, especially as knowledge of the underlying mechanical properties of the tissue is limited. We undertook quantification of some of these properties through material testing of cadaver specimens. We punched samples at specified locations from 19 extensor hood specimens. Material testing was performed to failure for each sample with a custom material testing device. Testing revealed significant differences in ultimate load, ultimate strain, thickness, and tangent modulus along the length of the extensor hood. Specifically, thickness, ultimate load, and ultimate strain were greater in the more proximal sections of the extensor hood, while the tangent modulus was greater in the more distal sections. The variations in mechanical properties within the hood may impact prediction of force transmission and, thus, should be considered when modeling the action of the extensor apparatus. Across the extensor hood, tangent modulus values were substantially smaller than values reported for other soft tissues, such as the Achilles tendon and knee ligaments, while ultimate strains were much greater. Thus, the tissue in the extensor apparatus seems to have greater elasticity, which should be modeled accordingly.

Regionalizing muscle activity causes changes to the magnitude and direction of the force from whole muscles-a modeling study

Skeletal muscle can contain neuromuscular compartments that are spatially distinct regions that can receive relatively independent levels of activation. This study tested how the magnitude and direction of the force developed by a whole muscle would change when the muscle activity was regionalized within the muscle. A 3D finite element model of a muscle with its bounding aponeurosis was developed for the lateral gastrocnemius, and isometric contractions were simulated for a series of conditions with either a uniform activation pattern, or regionally distinct activation patterns: in all cases the mean activation from all fibers within the muscle reached 10%. The models showed emergent features of the fiber geometry that matched physiological characteristics: with fibers shortening, rotating to greater pennation, adopting curved trajectories in 3D and changes in the thickness and width of the muscle belly. Simulations were repeated for muscle with compliant, normal and stiff aponeurosis and the aponeurosis stiffness affected the changes to the fiber geometry and the resultant muscle force. Changing the regionalization of the activity resulted to changes in the magnitude, direction and center of the force vector from the whole muscle. Regionalizing the muscle activity resulted in greater muscle force than the simulation with uniform activity across the muscle belly. The study shows how the force from a muscle depends on the complex interactions between the muscle fibers and connective tissues and the region of muscle that is active.

Dynamic loading of electrospun yarns guides mesenchymal stem cells towards a tendon lineage

Alternative strategies are required when autograft tissue is not sufficient or available to reconstruct damaged tendons. Electrospun fibre yarns could provide such an alternative. This study investigates the seeding of human mesenchymal stem cells (hMSC) on electrospun yarns and their response when subjected to dynamic tensile loading. Cell seeded yarns sustained 3600 cycles per day for 21 days. Loaded yarns demonstrated a thickened cell layer around the scaffold׳s exterior compared to statically cultured yarns, which would suggest an increased rate of cell proliferation and/or matrix deposition, whilst maintaining a predominant uniaxial cell orientation. Tensile properties of cell-seeded yarns increased with time compared to acellular yarns. Loaded scaffolds demonstrated an up-regulation in several key tendon genes, including collagen Type I. This study demonstrates the support of hMSCs on electrospun yarns and their differentiation towards a tendon lineage when mechanically stimulated.

A novel electrospun-aligned nanoyarn-reinforced nanofibrous scaffold for tendon tissue engineering

An electrospun-aligned nanoyarn-reinforced nanofibrous scaffold (NRS) was developed for tendon tissue engineering to improve mechanical strength and cell infiltration. The novel scaffold composed of aligned nanoyarns and random nanofibers was fabricated via electrospinning using a two-collector system. The aim of the present study was to investigate three different types of electrospun scaffolds (random nanofibrous scaffold, aligned nanofibrous scaffold and NRS) based on silk fibroin (SF) and poly(l-lactide-co-caprolactone) blends. Morphological analysis demonstrated that the NRS composed of aligned nanoyarns and randomly distributed nanofibers formed a 3D microstructure with relatively large pore sizes and high porosity. Biocompatibility analysis revealed that bone marrow-derived mesenchymal stem cells exhibited a higher proliferation rate when cultured on the NRS compared with the other scaffolds. The mechanical testing results indicated that the tensile properties of the NRS were reinforced in the direction parallel to the nanoyarns and satisfied the mechanical requirements for tendon repair. In addition, cell infiltration was significantly enhanced on the NRS. In conclusion, with its improved porosity and appropriate mechanical properties, the developed NRS shows promise for tendon tissue engineering applications.

Digital image correlation as a tool for three-dimensional strain analysis in human tendon tissue

BACKGROUND

Determining the mechanical behaviour of tendon and ligamentous tissue remains challenging, as it is anisotropic, non-linear and inhomogeneous in nature.

METHODS

In this study, three-dimensional (3D) digital image correlation (DIC) was adopted to examine the strain distribution in the human Achilles tendon. Therefore, 6 fresh frozen human Achilles tendon specimens were mounted in a custom made rig for uni-axial loading. 3D DIC measurements of each loading position were obtained and compared to 2 linear variable differential transformers (LVDT's).

RESULTS

3D DIC was able to calculate tendon strain in every region of all obtained images. The scatter was found to be low in all specimens and comparable to that obtained in steel applications. The accuracy of the 3D DIC measurement was higher in the centre of the specimen where scatter values around 0.03% strain were obtained. The overall scatter remained below 0.3% in all specimens. The spatial resolution of 3D DIC on human tendon tissue was found to be 0.1 mm(2). The correlation coefficient between the 3D DIC measurements and the LVDT measurements showed an excellent linear agreement in all specimens (R(2) = 0.99). Apart from the longitudinal strain component, an important transverse strain component was revealed in all specimens. The strain distribution of both components was of a strongly inhomogeneous nature, both within the same specimen and amongst different specimens.

CONCLUSION

DIC proved to be a very accurate and reproducible tool for 3D strain analysis in human tendon tissue.

Sutured tendon repair; a multi-scale finite element model

Following rupture, tendons are sutured to reapproximate the severed ends and permit healing. Several repair techniques are employed clinically, with recent focus towards high-strength sutures, permitting early active mobilisation thus improving resultant joint mobility. However, the arrangement of suture repairs locally alters the loading environment experienced by the tendon. The extent of the augmented stress distribution and its effect on the tissue is unknown. Stress distribution cannot be established using traditional tensile testing, in vivo, or ex vivo study of suture repairs. We have developed a 3D finite element model of a Kessler suture repair employing multiscale modelling to represent tendon microstructure and incorporate its highly orthotropic behaviour into the tissue description. This was informed by ex vivo tensile testing of porcine flexor digitorum profundus tendon. The transverse modulus of the tendon was 0.2551 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm $$\end{document}± 0.0818 MPa and 0.1035 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm $$\end{document}± 0.0454 MPa in proximal and distal tendon samples, respectively, and the interfibrillar tissue modulus ranged from 0.1021 to 0.0416 MPa. We observed an elliptically shaped region of high stress around the suture anchor, consistent with a known region of acellularity which develop 72 h post-operatively and remain for at least a year. We also observed a stress shielded region close to the severed tendon ends, which may impair collagen fibre realignment during the remodelling stage of repair due to the lack of tensile stress.

Mechanical compromise of partially lacerated flexor tendons

Tendons function to transmit loads from muscle to move and stabilize joints and absorb impacts. Functionality of lacerated tendons is diminished, however clinical practice often considers surgical repair only after 50% or more of the tendon is lacerated, the "50% rule." Few studies provide mechanical insight into the 50% rule. In this study cyclic and static stress relaxation tests were performed on porcine flexor tendons before and after a 0.5, 1.0, 2.0, or 2.75 mm deep transverse, midsubstance laceration. Elastic and viscoelastic properties, such as maximum stress, change in stress throughout each test, and stiffness, were measured and compared pre- and post-laceration. Nominal stress and stiffness parameters decreased, albeit disproportionately in magnitude, with increasing percent loss of cross-sectional area. Conversely, mean stress at the residual area (determined using remaining intact area at the laceration cross section) exhibited a marked increase in stress concentration beginning at 47.2% laceration using both specified load and constant strain analyses. The marked increase in stress concentration beginning near 50% laceration provides mechanical insight into the 50% rule. Additionally, a drastic decrease in viscoelastic stress parameters after only an 8.2% laceration suggests that time-dependent mechanisms protecting tissues during impact loadings are highly compromised regardless of laceration size.