In SituMonitoring of Tendon Structural Changes by Elastic Scattering Spectroscopy: Correlation with Changes in Collagen Fibril Diameter and Crimp

Ultrashort echo time T2∗ values decrease in tendons with application of static tensile loads

In early stages of tendon disease, mechanical properties may become altered prior to changes in morphological anatomy. Ultrashort echo time (UTE) magnetic resonance imaging (MRI) can be used to directly detect signal from tissues with very short T2 values, including unique viscoelastic tissues such as tendons. The purpose of this study was to use UTE sequences to measure T2^∗, T1 and magnetization transfer ratio (MTR) variations of tendon samples under static tensile loads. Six human peroneal tendons were imaged before and under static loading using UTE sequences on a clinical 3T MRI scanner. Tendons were divided into two static tensile loading groups: group A that underwent one-step loading (15N) and group B that underwent two-step loading (15 and 30N). The T2^∗, T1 and MTR variations were investigated in two selected section regions of interest (ROIs), including whole and core sections. Mean T2^∗ values for the first step of loading (groups A and B) in both whole section and core section ROIs were significantly decreased by 13±7% (P=0.028) and 16±5% (P=0.017), respectively. For the second loading step (group B), there was a consistent, but non-significant reduction in T2^∗ value by 9±2% (P=0.059) and 7±5% (P=0.121) for whole and core sections, respectively. Mean T1 did not show any consistent changes for either loading steps (P>0.05). Mean MTR increased slightly, but not significantly for both loading steps (P>0.05). Significant differences were found only in T2^∗ values of tendons by static tensile load application. Therefore, T2^∗ monitoring during loading is suggested for quantitative investigation of the tendons biomechanics.

Collagen V expression is crucial in regional development of the supraspinatus tendon

Manipulations in cell culture and mouse models have demonstrated that reduction of collagen V results in altered fibril structure and matrix assembly. A tissue-dependent role for collagen V in determining mechanical function was recently established, but its role in determining regional properties has not been addressed. The objective of this study was to define the role(s) of collagen V expression in establishing the site-specific properties of the supraspinatus tendon. The insertion and midsubstance of tendons from wild type, heterozygous and tendon/ligament-specific null mice were assessed for crimp morphology, fibril morphology, cell morphology, as well as total collagen and pyridinoline cross-link (PYD) content. Fibril morphology was altered at the midsubstance of both groups with larger, but fewer, fibrils and no change in cell morphology or collagen compared to the wild type controls. In contrast, a significant disruption of fibril assembly was observed at the insertion site of the null group with the presence of structurally aberrant fibrils. Alterations were also present in cell density and PYD content. Altogether, these results demonstrate that collagen V plays a crucial role in determining region-specific differences in mouse supraspinatus tendon structure. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2154-2161, 2016.

Achilles tendons from decorin- and biglycan-null mouse models have inferior mechanical and structural properties predicted by an image-based empirical damage model

Achilles tendons are a common source of pain and injury, and their pathology may originate from aberrant structure function relationships. Small leucine rich proteoglycans (SLRPs) influence mechanical and structural properties in a tendon-specific manner. However, their roles in the Achilles tendon have not been defined. The objective of this study was to evaluate the mechanical and structural differences observed in mouse Achilles tendons lacking class I SLRPs; either decorin or biglycan. In addition, empirical modeling techniques based on mechanical and image-based measures were employed. Achilles tendons from decorin-null (Dcn(-/-)) and biglycan-null (Bgn(-/-)) C57BL/6 female mice (N=102) were used. Each tendon underwent a dynamic mechanical testing protocol including simultaneous polarized light image capture to evaluate both structural and mechanical properties of each Achilles tendon. An empirical damage model was adapted for application to genetic variation and for use with image based structural properties to predict tendon dynamic mechanical properties. We found that Achilles tendons lacking decorin and biglycan had inferior mechanical and structural properties that were age dependent; and that simple empirical models, based on previously described damage models, were predictive of Achilles tendon dynamic modulus in both decorin- and biglycan-null mice.

Effects of stress-shielding on the dynamic viscoelasticity and ordering of the collagen fibers in rabbit Achilles tendon

We investigated the effects of stress-shielding on both viscoelastic properties and microstructure of collagen fibers in the Achilles tendon by proton double-quantum filtered ((1) H-DQF) NMR spectroscopy. The right hind-limbs of 20 Japanese white rabbits were immobilized for 4 weeks in a cast with the ankle in plantarflexion. Dynamic viscoelasticity of the Achilles tendons was measured using a viscoelastic spectrometer. Proton DQF NMR signals were analyzed to determine the residual dipolar coupling of bound water molecules in the Achilles tendons. Both the dynamic storage modulus (E') and dynamic loss modulus (E″) decreased significantly in the Achilles tendons of the stress-shielding group. The results of the (1) H-DQF NMR examination demonstrated significantly reduced residual dipolar coupling in the Achilles tendons of this same group. The disorientation of collagen fibers by stress-shielding should contribute to degradation of the dynamic storage and loss moduli. The alterations of the collagen fiber orientation that contributed to the function of tendinous tissue can be evaluated by performing an analysis of (1) H DQF NMR spectroscopy.

Role of moderate exercising on Achilles tendon collagen crimping patterns and proteoglycans

In this study, the morphological and morphometric changes in the collagen crimping pattern of Achilles tendon and metabolism/expression of tenocytes explanted from tendons of running (RUN) and sedentary (SED) rats were investigated to assess the effects of 12 weeks moderate running exercise. The number, the top angle width and the base length of each crimp in three different regions (proximal, central and distal) of RUN and SED tendons were measured with a polarized light microscope. The most significant morphometric differences in the crimps were detectable in the central region of the RUN tendons. In this region, crimps were fewer, larger and more flattened than those of other regions as a consequence of a functional adaptation of extracellular matrix to running, in order to increase tendon stiffness and force transmission efficiency. Conversely, the top angle width of the crimps reduced in proximal and distal regions of the RUN tendons, suggesting that these crimps might act as more reactive mechanical springs, able to store and improve the release of the stored strain energy in most loaded regions. Tenocytes explanted from Achilles tendons of both RUN and SED groups were cultured. Running influenced tenocytes which showed a significant increase in collagen type-I synthesis and proteoglycans production, suggesting enhancement of the loading transmission efficiency and facilitate inter-fibril and inter-fiber sliding.

Collagen--emerging collagen based therapies hit the patient

The choice of biomaterials available for regenerative medicine continues to grow rapidly, with new materials often claiming advantages over the short-comings of those already in existence. Going back to nature, collagen is one of the most abundant proteins in mammals and its role is essential to our way of life. It can therefore be obtained from many sources including porcine, bovine, equine or human and offer a great promise as a biomimetic scaffold for regenerative medicine. Using naturally derived collagen, extracellular matrices (ECMs), as surgical materials have become established practice for a number of years. For clinical use the goal has been to preserve as much of the composition and structure of the ECM as possible without adverse effects to the recipient. This review will therefore cover in-depth both naturally and synthetically produced collagen matrices. Furthermore the production of more sophisticated three dimensional collagen scaffolds that provide cues at nano-, micro- and meso-scale for molecules, cells, proteins and bulk fluids by inducing fibrils alignments, embossing and layered configuration through the application of plastic compression technology will be discussed in details. This review will also shed light on both naturally and synthetically derived collagen products that have been available in the market for several purposes including neural repair, as cosmetic for the treatment of dermatologic defects, haemostatic agents, mucosal wound dressing and guided bone regeneration membrane. There are other several potential applications of collagen still under investigations and they are also covered in this review.

Observations of multiscale, stress-induced changes of collagen orientation in tendon by polarized Raman spectroscopy

Collagen is a versatile structural molecule in nature and is used as a building block in many highly organized tissues, such as bone, skin, and cornea. The functionality and performance of these tissues are controlled by their hierarchical organization ranging from the molecular up to macroscopic length scales. In the present study, polarized Raman microspectroscopic and imaging analyses were used to elucidate collagen fibril orientation at various levels of structure in native rat tail tendon under mechanical load. In situ humidity-controlled uniaxial tensile tests have been performed concurrently with Raman confocal microscopy to evaluate strain-induced chemical and structural changes of collagen in tendon. The methodology is based on the sensitivity of specific Raman scattering bands (associated with distinct molecular vibrations, such as the amide I) to the orientation and the polarization direction of the incident laser light. Our results, based on the changing intensity of Raman lines as a function of orientation and polarization, support a model where the crimp and gap regions of collagen hierarchical structure are straightened at the tissue and molecular level, respectively. However, the lack of measurable changes in Raman peak positions throughout the whole range of strains investigated indicates that no significant changes of the collagen backbone occurs with tensing and suggests that deformation is rather redistributed through other levels of the hierarchical structure.

Effect of fiber distribution and realignment on the nonlinear and inhomogeneous mechanical properties of human supraspinatus tendon under longitudinal tensile loading

Tendon exhibits nonlinear stress-strain behavior that may be partly due to movement of collagen fibers through the extracellular matrix. While a few techniques have been developed to evaluate the fiber architecture of other soft tissues, the organizational behavior of tendon under load has not been determined. The supraspinatus tendon (SST) of the rotator cuff is of particular interest for investigation due to its complex mechanical environment and corresponding inhomogeneity. In addition, SST injury occurs frequently with limited success in treatment strategies, illustrating the need for a better understanding of SST properties. Therefore, the objective of this study was to quantitatively evaluate the inhomogeneous tensile mechanical properties, fiber organization, and fiber realignment under load of human SST utilizing a novel polarized light technique. Fiber distributions were found to become more aligned under load, particularly during the low stiffness toe-region, suggesting that fiber realignment may be partly responsible for observed nonlinear behavior. Fiber alignment was found to correlate significantly with mechanical parameters, providing evidence for strong structure-function relationships in tendon. Human SST exhibits complex, inhomogeneous mechanical properties and fiber distributions, perhaps due to its complex loading environment. Surprisingly, histological grade of degeneration did not correlate with mechanical properties.

Tensile properties and fiber alignment of human supraspinatus tendon in the transverse direction demonstrate inhomogeneity, nonlinearity, and regional isotropy

A recent study (Lake et al., 2009); reported the properties of human supraspinatus tendon (SST) tested along the predominant fiber direction. The SST was found to have a relatively disperse distribution of collagen fibers, which may represent an adaptation to multiaxial loads imposed by the complex loading environment of the rotator cuff. However, the multiaxial mechanical properties of human SST remain unknown. The objective of this study, therefore, was to evaluate the mechanical properties, fiber alignment, change in alignment with applied load, and structure-function relationships of SST in transverse testing. Samples from six SST locations were tested in uniaxial tension with samples oriented transverse to the tendon long-axis. Polarized light imaging was used to quantify collagen fiber alignment and change in alignment under applied load. The mechanical properties of samples taken near the tendon-bone insertion were much greater on the bursal surface compared to the joint surface (e.g., bursal moduli 15-30 times greater than joint; p<0.001). In fact, the transverse moduli values of the bursal samples were very similar to values obtained from samples tested along the tendon long-axis (Lake et al., 2009). This key and unexpected finding suggests planar mechanical isotropy for bursal surface samples near the insertion, which may be due to complex in vivo loading. Organizationally, fiber distributions became less aligned along the tendon long-axis in the toe-region of the stress-strain response. Alignment changes occurred to a slightly lesser degree in the linear-region, suggesting that movement of collagen fibers may play a role in mechanical nonlinearity. Transverse mechanical properties were significantly correlated with fiber alignment (e.g., for linear-region modulus rs=0.74, p<0.0001), demonstrating strong structure-function relationships. These results greatly enhance current understanding of the properties of human SST and provide clinicians and scientists with vital information in attempting to treat or replace this complex tissue.

Experimental and theoretical evaluation of rotating orthogonal polarization imaging

Rotating orthogonal polarization imaging is a new technique that provides quantitative measurements of the polarization properties of scattering media, such as tissue, which are free from surface reflections. The technique is investigated using both experiments and Monte Carlo simulations of a polarizing target embedded within a scattering medium. The technique is sensitive to the polarization properties of the target up to a depth of 17 mean free paths. Preliminary images of bovine tendon, lamb tendon, chicken breast, and human skin are also demonstrated.

Rotating orthogonal polarization imaging

Surface reflections often present a problem in the polarization difference imaging of tissue. The technique described involves illumination in a single polarization state and detection in the orthogonal polarization state. Synchronously rotating both the illumination and orthogonal detection states provides an image free from surface reflections that is sensitive to the polarization properties of the underlying tissue. Results on tissue phantoms demonstrate that polarization sensitive measurements of a test target can be made up to a depth of 12 mean free paths within a scattering medium. Preliminary images of bovine tendon are also demonstrated.