The mechanobiological aetiopathogenesis of tendinopathy: is it the over-stimulation or the under-stimulation of tendon cells?

Tendon Contraction After Cyclic Elongation Is an Age-Dependent Phenomenon: In Vitro and In Vivo Comparisons

BACKGROUND

Tendons are viscoelastic tissues that deform (elongate) in response to cyclic loading. However, the ability of a tendon to recover this elongation is unknown.

HYPOTHESIS

Tendon length significantly increases after in vivo or in vitro cyclic loading, and the ability to return to its original length through a cell-mediated contraction mechanism is an age-dependent phenomenon.

STUDY DESIGN

Controlled laboratory study.

METHODS

In vitro, rat tail tendon fascicles (RTTfs) from Sprague-Dawley rats of 3 age groups (1, 3, and 12 months) underwent 2% cyclic strain at 0.17 Hz for 2 hours, and the percentages of elongation were determined. After loading, the RTTfs were suspended for 3 days under tissue culture conditions and photographed daily to determine the amount of length contraction. In vivo, healthy male participants (n = 29; age, 19-49 years) had lateral, single-legged weightbearing radiographs taken of the knee at 60° of flexion immediately before, immediately after, and 24 hours after completing eccentric quadriceps loading exercises on the dominant leg to fatigue. Measurements of patellar tendon length were taken from the radiographs, and the percentages of tendon elongation and subsequent contraction were calculated.

RESULTS

In vitro, cyclic loading increased the length of all RTTfs, with specimens from younger (1 and 3 months) rats demonstrating significantly greater elongation than those from older (12 months) rats (P = .009). The RTTfs contracted to their original length significantly faster (P < .001) and in an age-dependent fashion, with younger animals contracting faster. In vivo, repetitive eccentric loading exercises significantly increased patellar tendon length (P < .001). Patellar tendon length decreased 24 hours after exercises (P < .001) but did not recover completely (P < .001). There was a weak but significant (R (2) = 0.203, P = .014) linear correlation between the amount of tendon contraction and age, with younger participants (<30 years) demonstrating significantly more contraction (P = .014) at 24 hours than older participants (>30 years).

CONCLUSION

Cyclic tendon loading results in a significant increase in tendon elongation under both in vitro and in vivo conditions. Tendons in both conditions demonstrated an incomplete return to their original length after 24 hours, and the extent of this return was age dependent.

CLINICAL RELEVANCE

The age- and time-dependent contraction of tendons, elongated after repetitive loading, could result in transient alterations in the mechanobiological environment of tendon cells. This, in turn, could induce the onset of catabolic changes associated with the pathogenesis of tendinopathy. These results suggest the importance of allowing time for contraction between bouts of repetitive exercise and may explain why age is a predisposing factor in tendinopathy.

Cyclic strain alters the expression and release of angiogenic factors by human tendon cells

Angiogenesis is associated with the tissue changes underlying chronic overuse tendinopathy. We hypothesized that repetitive, cyclic loading of human tendon cells would lead to increased expression and activity of angiogenic factors. We subjected isolated human tendon cells to overuse tensile loading using an in vitro model (1 Hz, 10% equibiaxial strain). We found that mechanically stimulated human tendon cells released factors that promoted in vitro proliferation and tube formation by human umbilical vein endothelial cells (HUVEC). In response to cyclic strain, there was a transient increase in the expression of several angiogenic genes including ANGPTL4, FGF-2, COX-2, SPHK1, TGF-alpha, VEGF-A and VEGF-C, with no change in anti-angiogenic genes (BAI1, SERPINF1, THBS1 and 2, TIMP1-3). Cyclic strain also resulted in the extracellular release of ANGPTL4 protein by tendon cells. Our study is the first report demonstrating the induction of ANGPTL4 mRNA and release of ANGPTL4 protein in response to cyclic strain. Tenocytes may contribute to the upregulation of angiogenesis during the development of overuse tendinopathy.

Three-dimensional deformation and transverse rotation of the human free Achilles tendon in vivo during isometric plantarflexion contraction

Freehand three-dimensional ultrasound (3DUS) was used to investigate longitudinal and biaxial transverse deformation and rotation of the free Achilles tendon in vivo during a voluntary submaximal isometric muscle contraction. Participants ( n = 8) were scanned at rest and during a 70% maximal voluntary isometric contraction (MVIC) of the plantarflexors. Ultrasound images were manually digitized to render a 3D reconstruction of the free Achilles tendon for the computation of tendon length, volume, cross-sectional area (CSA), mediolateral diameter (MLD), anteroposterior diameter (APD), and transverse rotation. Tendon longitudinal and transverse (CSA, APD, and MLD) deformation and strain at 70% MVIC were calculated relative to the resting condition. There was a significant main effect of contraction on tendon length and mean CSA, MLD, and APD ( P < 0.05), but no effect on tendon volume ( P = 0.70). Group mean transverse strains for CSA, MLD, and APD averaged over the length of the tendon were −5.5%, −8.7% and 8.7%, respectively. Peak CSA, MLD, and APD transverse strains all occurred between 40% and 60% of tendon length. Transverse rotation of the free tendon was negligible at rest but increased under load, becoming externally rotated relative to the calcaneal insertion. The relationship between longitudinal and transverse strains of the free Achilles tendon during muscle-induced elongation may be indicative of interfascicle reorganization. The finding that transverse rotation and strain peaked in midportion of the free Achilles tendon may have important implications for tendon injury mechanisms and estimation of tendon stress in vivo.

3-D ultrastructure and collagen composition of healthy and overloaded human tendon: evidence of tenocyte and matrix buckling

Achilles tendinopathies display focal tissue thickening with pain and ultrasonography changes. Whilst complete rupture might be expected to induce changes in tissue organization and protein composition, little is known about the consequences of non-rupture-associated tendinopathies, especially with regards to changes in the content of collagen type I and III (the major collagens in tendon), and changes in tendon fibroblast (tenocyte) shape and organization of the extracellular matrix (ECM). To gain new insights, we took biopsies from the tendinopathic region and flanking healthy region of Achilles tendons of six individuals with clinically diagnosed tendinopathy who had no evidence of cholesterol, uric acid and amyloid accumulation. Biochemical analyses of collagen III/I ratio were performed on all six individuals, and electron microscope analysis using transmission electron microscopy and serial block face-scanning electron microscopy were made on two individuals. In the tendinopathic regions, compared with the flanking healthy tissue, we observed: (i) an increase in the ratio of collagen III : I proteins; (ii) buckling of the collagen fascicles in the ECM; (iii) buckling of tenocytes and their nuclei; and (iv) an increase in the ratio of small-diameter : large-diameter collagen fibrils. In summary, load-induced non-rupture tendinopathy in humans is associated with localized biochemical changes, a shift from large-to small-diameter fibrils, buckling of the tendon ECM, and buckling of the cells and their nuclei.

Release of tensile strain on engineered human tendon tissue disturbs cell adhesions, changes matrix architecture, and induces an inflammatory phenotype

Mechanical loading of tendon cells results in an upregulation of mechanotransduction signaling pathways, cell-matrix adhesion and collagen synthesis, but whether unloading removes these responses is unclear. We investigated the response to tension release, with regard to matrix proteins, pro-inflammatory mediators and tendon phenotypic specific molecules, in an in vitro model where tendon-like tissue was engineered from human tendon cells. Tissue sampling was performed 1, 2, 4 and 6 days after surgical de-tensioning of the tendon construct. When tensile stimulus was removed, integrin type collagen receptors showed a contrasting response with a clear drop in integrin subunit α11 mRNA and protein expression, and an increase in α2 integrin mRNA and protein levels. Further, specific markers for tendon cell differentiation declined and normal tendon architecture was disturbed, whereas pro-inflammatory molecules were upregulated. Stimulation with the cytokine TGF-β1 had distinct effects on some tendon-related genes in both tensioned and de-tensioned tissue. These findings indicate an important role of mechanical loading for cellular and matrix responses in tendon, including that loss of tension leads to a decrease in phenotypical markers for tendon, while expression of pro-inflammatory mediators is induced.

The mature athlete: aging tendon and ligament

CONTEXT

Aging changes the biology, healing capacity, and biomechanical function of tendons and ligaments and results in common clinical pathologies that present to orthopedic surgeons, primary care physicians, physical therapists, and athletic trainers. A better understanding of the age-related changes in these connective tissues will allow better patient care.

EVIDENCE ACQUISITION

The PubMed database was searched in December 2012 for English-language articles pertaining to age-related changes in tendons and ligaments.

LEVEL OF EVIDENCE

Level 5.

RESULTS

The mature athlete faces challenges associated with age-dependent changes in the rotator cuff, Achilles tendon, lateral humeral epicondylar tendons, quadriceps tendon, and patellar tendon. The anterior cruciate ligament and the medial collateral ligament are the most studied intra-articular and extra-articular ligaments, and both are associated with age-dependent changes.

CONCLUSION

Tendons and ligaments are highly arranged connective tissue structures that maintain joint motion and joint stability. These structures are subject to vascular and compositional changes with increasing age that alter their mechanotransduction, biology, healing capacity, and biomechanical function. Emerging research into the etiology of age-dependent changes will provide further information to help combat the age-related clinical complications associated with the injuries that occur to tendons and ligaments.

Fascicles from energy-storing tendons show an age-specific response to cyclic fatigue loading

Some tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT), act as energy stores, stretching and recoiling to increase efficiency during locomotion. Our previous observations of rotation in response to applied strain in SDFT fascicles suggest a helical structure, which may provide energy-storing tendons with a greater ability to extend and recoil efficiently. Despite this specialization, energy-storing tendons are prone to age-related tendinopathy. The aim of this study was to assess the effect of cyclic fatigue loading (FL) on the microstructural strain response of SDFT fascicles from young and old horses. The data demonstrate two independent age-related mechanisms of fatigue failure; in young horses, FL caused low levels of matrix damage and decreased rotation. This suggests that loading causes alterations to the helix substructure, which may reduce their ability to recoil and recover. By contrast, fascicles from old horses, in which the helix is already compromised, showed greater evidence of matrix damage and suffer increased fibre sliding after FL, which may partially explain the age-related increase in tendinopathy. Elucidation of helix structure and the precise alterations occurring owing to both ageing and FL will help to develop appropriate preventative and repair strategies for tendinopathy.

What is the impact of inflammation on the critical interplay between mechanical signaling and biochemical changes in tendon matrix?

Mechanical loading can influence tendon collagen homeostasis in animal models, while the dynamics of the human adult tendon core tissue are more debatable. Currently available data indicate that human tendon adaptation to loading may happen primarily in the outer tendon region. A role of inflammation in this peritendinous adaptation is supported by a rise in inflammatory mediators in the peritendinous area after physiological mechanical loading in humans. This plays a role in the exercise-induced rise in tendon blood flow and peritendinous collagen synthesis. Although inflammatory activity can activate proteolytic pathways in tendon, mechanical loading can protect against matrix degradation. Acute tendon injury displays an early inflammatory response that seems to be lowered when mechanical loading is applied during regeneration of tendon. Chronically overloaded tendons (tendinopathy) do neither at rest nor after acute exercise display any enhanced inflammatory activity, and thus the basis for using anti-inflammatory medication to treat tendon overuse seems limited.

The role of mechanical loading in tendon development, maintenance, injury, and repair

Tendon injuries often result from excessive or insufficient mechanical loading, impairing the ability of the local tendon cell population to maintain normal tendon function. The resident cell population composing tendon tissue is mechanosensitive, given that the cells are able to alter the extracellular matrix in response to modifications of the local loading environment. Natural tendon healing is insufficient, characterized by improper collagen fibril diameter formation, collagen fibril distribution, and overall fibril misalignment. Current tendon repair rehabilitation protocols focus on implementing early, well-controlled eccentric loading exercises to improve repair outcome. Tissue engineers look toward incorporating mechanical loading regimens to precondition cell populations for the creation of improved biological augmentations for tendon repair.

Cyclical strain modulates metalloprotease and matrix gene expression in human tenocytes via activation of TGFβ

Tendinopathies are a range of diseases characterised by degeneration and chronic tendon pain and represent a significant cause of morbidity. Relatively little is known about the underlying mechanisms; however onset is often associated with physical activity. A number of molecular changes have been documented in tendinopathy such as a decrease in overall collagen content, increased extracellular matrix turnover and protease activity. Metalloproteinases are involved in the homeostasis of the extracellular matrix and expression is regulated by mechanical strain. The aims of this study were to determine the effects of strain upon matrix turnover by measuring metalloproteinase and matrix gene expression and to elucidate the mechanism of action. Primary Human Achilles tenocytes were seeded in type I rat tail collagen gels in a Flexcell™ tissue train system and subjected to 5% cyclic uniaxial strain at 1 Hz for 48 h. TGFβ1 and TGFβRI inhibitor were added to selected cultures. RNA was measured using qRT-PCR and TGFβ protein levels were determined using a cell based luciferase assay. We observed that mechanical strain regulated the mRNA levels of multiple protease and matrix genes anabolically, and this regulation mirrored that seen with TGFβ stimulation alone. We have also demonstrated that the inhibition of the TGFβ signalling pathway abrogated the strain induced changes in mRNA and that TGFβ activation, rather than gene expression, was increased with mechanical strain. We concluded that TGFβ activation plays an important role in mechanotransduction. Targeting this pathway may have its place in the treatment of tendinopathy.

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Mechanical strain regulates multiple protease and matrix genes at the mRNA level.

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Changes in mRNA level are analogous to those induced by TGFβ stimulation.

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The inhibition of the TGFβ signalling pathway abrogated the strain-induced changes.

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A SMAD activatory soluble factor is increased in activity in response to mechanical load.

Tendon cell ciliary length as a biomarker of in situ cytoskeletal tensional homeostasis

To determine if tendon cell ciliary length could be used as a biomarker of cytoskeletal tensional homeostasis, 20 mm long adult rat tail tendons were placed in double artery clamps set 18 mm apart to create a 10% laxity. The tendons were allowed to contract over a 7 day period under culture conditions. At 0, 1, 5, and 7 days the tendon cell cilia were stained and ciliary length measured using confocal imaging. There was a significant (p<0.001) increase in ciliary length at 1 day. At day 5 (when the tendon became visibly taut) there was a significant (p<0.001) decrease in ciliary length compared to day 1. By day 7 the tendon remained taut and ciliary length returned to day zero values (p=0.883). These results suggest that cilia length reflects the local mechanobiological environment of tendon cells and could be used as a potential in situ biomarker of altered cytoskeletal tensional homeostasis.

Arguments for an increasing differentiation towards fibrocartilaginous components in midportion Achilles tendinopathy

PURPOSE

The objective of this study was to investigate the fibrocartilaginous differentiation occurring in midportion Achilles tendinopathy.

METHODS

Tendon samples were retrospectively collected from 23 patients, who had undergone surgery for midportion Achilles tendinopathy resistant to conservative treatment. Based on histological scores, the biopts were subdivided into three categories: a light, moderate and severe histopathological stage. Throughout these stages, immunohistochemical staining was performed against biglycan, aggrecan and collagen type II, components characteristic for fibrocartilage. Staining of these components was evaluated using a semi-quantitative scoring method.

RESULTS

The immunohistochemical scores of biglycan and aggrecan were statistically significant between the histopathological stages (P < 0.001). The immunohistochemical scores were positively correlated with the increasing histopathological stages [Spearman's correlation coefficient = 0.93 for biglycan and 0.78 for aggrecan (P < 0.001)]. Staining for collagen type II remained negative throughout these stages.

CONCLUSION

Immunohistochemical staining of the fibrocartilaginous components biglycan and aggrecan showed a progressive increase, correlated with a further evolved histopathological stage. This observation gave arguments for an increased differentiation towards fibrocartilaginous components at protein level in midportion Achilles tendinopathy.

Tendon mechanobiology: experimental models require mathematical underpinning

Mathematical and computational modeling is in demand to help address current challenges in mechanobiology of musculoskeletal tissues. In particular for tendon, the high clinical importance of the tissue, the huge mechanical demands placed on it and its ability to adapt to these demands, require coupled, multiscale models incorporating complex geometrical and microstructural information as well as time-based descriptions of cellular activity and response.This review introduces the information sources required to develop such multiscale models. It covers tissue structure and biomechanics, cell biomechanics, the current understanding of tendon's ability in health and disease to update its properties and structure and the few already existing multiscale mechanobiological models of the tissue. Finally, a sketch is provided of what such models could achieve ideally, pointing out where experimental data and knowledge are still missing.

Age-related changes in the cellular, mechanical, and contractile properties of rat tail tendons

Tendon laxity following injury, cyclic creep, or repair has been shown to alter the normal homeostasis of tendon cells, which can lead to degenerative changes in the extracellular matrix. While tendon cells have been shown to have an inherent contractile mechanism that gives them some ability to retighten lax tendons and reestablish a homeostatic cellular environment, the effect of age on this process is unknown. To determine the effect of aging on cell number, cell shape, and tensile modulus on tendons as well as the rate of cell-mediated contraction of lax tendons, tail tendon fascicles from 1-, 3-, and 12-month-old rats were analyzed. Aging results in a decrease (p < 0.001) in cell number per mm(2): 1 m (981 ± 119), 3 m (570 ± 108), and 12 m (453 ± 23), a more flattened (p < 0.001) cell nuclei shape and a higher (p < 0.001) tensile modulus (MPa) of the tendons: 1 m (291 ± 2), 3 m (527 ± 38), and 12 m (640 ± 102). Both the extent and rate of contraction over 7 days decreased with age (p = 0.007). This decrease in contraction rate with age correlates to the observed changes seen in aging tendons [increased modulus (r(2) = 0.95), decreased cell number (r(2) = 0.89)]. The ability of tendons to regain normal tension following injury or exercise-induced laxity is a key factor in the recovery of tendon function. The decreased contraction rate as a function of age observed in the current study may limit the ability of tendon cells to retighten lax tendons in older individuals. This, in turn, may place these structures at further risk for injury or altered function.

Heterotopic mineralization (ossification or calcification) in tendinopathy or following surgical tendon trauma

Heterotopic tendon mineralization (ossification or calcification), which may be a feature of tendinopathy or which may develop following surgical trauma (repair or graft harvest), has not received much attention. The purpose of this article is to review the prevalence, mechanisms and consequences of heterotopic tendon mineralization and to identify the gaps in our current understanding. We focus on endochondral heterotopic ossification and draw on knowledge of the mechanisms of this process in other tissues and conditions. Finally, we introduce a novel murine Achilles tendon needle injury model, which will enable us to further study the mechanisms and biomechanical consequences of tendon mineralization.

The effects of immobilization on the mechanical properties of the patellar tendon in younger and older men

BACKGROUND

It remains unknown if inactivity changes the mechanical properties of the human patellar tendon in younger and older healthy persons. The purpose was to examine the effects of short-term unilateral immobilization on the structural and mechanical properties of the patellar tendon in older men and younger men, in vivo.

METHODS

Eight older men and eight younger men underwent 14 days of unilateral immobilization. All individuals were assessed on both sides before and after the intervention. MRI was used to assess whole patellar tendon dimensions. The mechanical properties of the patellar tendon were assessed using simultaneous force and ultrasonographic measurements during isometric ramp contractions.

FINDINGS

In older men, tendon stiffness [Pre: mean 2949 (SD 799) vs. Post: mean 2366 (SD 774) N mm(-1), P<0.01] and Young's Modulus [Pre: mean 1.2 (SD 0.3) vs. Post: mean 1.0 (SD 0.3) GPa, P<0.05] declined with immobilization on the immobilized side. On the control side, tendon stiffness [Pre: mean 3340 (SD 1209) vs. Post: mean 2230 (SD 503), P<0.01] and Young's Modulus [Pre: mean 1.5 (SD 0.4) vs. Post: mean 0.9 (SD 0.3) GPa, P<0.05] also decreased with immobilization. In younger men, tendon stiffness [Pre: 3622 (SD 1760) vs. Post: mean 2910 (SD 1528) N mm(-1), P<0.01] and Young's Modulus [Pre: mean 1.7 (SD 1.1) vs. Post: mean 1.4 (SD 0.8) GPa, P<0.05] decreased only on the immobilized side.

INTERPRETATION

Short-term immobilization led to impaired mechanical properties of the patellar tendon on the immobilized side in both younger men and older men, which can influence the function of the muscle-tendon complex.

Re-establishment of cytoskeletal tensional homeostasis in lax tendons occurs through an actin-mediated cellular contraction of the extracellular matrix

Cytoskeletal tensional homeostasis is known to be an important factor in controlling catabolic gene expression in tendon cells. Loss of cell tension in lax rat tail tendon fascicles (RTTfs) has been associated with an upregulation of MMP-13 gene expression and protein synthesis. To determine the role of the actin cytoskeleton in re-establishing tensional homeostasis in lax tendons, RTTfs were allowed to freely contract in vitro for 8 days. The cultured RTTfs contracted rapidly, reaching 50% of their initial length by 3 days. This contraction was associated with the presence of α-smooth muscle actin positive cells within the tendon. Disruption of the actin network by cytochalasian D caused an immediate and significant elongation of the contracted RTTfs. Subsequent removal of the cytochalasian D re-initiated the contraction process. When lax RTTfs were allowed to contract between fixed clamps in culture and become taut, they demonstrated a marked decrease in MMP-13 staining intensity when compared to freely contracting RTTfs. The ability of native tendon cells to contract lax tendons and re-establish their homeostatic "set point" with respect to collagenase production may be an important mechanism in the recovery of tendons elongated by injury, surgical positioning, or cyclic, viscoelastic creep secondary to repetitive exercise.

Optimization of leukocyte concentration in platelet-rich plasma for the treatment of tendinopathy

BACKGROUND

Numerous methods are available for platelet-rich plasma (PRP) generation, but evidence defining the optimum composition is lacking. We hypothesized that leukocyte-reduced PRP would result in lower inflammatory cytokine expression compared with concentrated-leukocyte PRP and that maintaining the platelet:white blood cell (WBC) ratio would compensate for the effect of increased WBC concentration.

METHODS

Blood and flexor digitorum superficialis tendons were collected from young adult horses. Three PRP groups were generated with the same platelet concentration but different WBC concentrations: intermediate-concentration standard PRP, leukocyte-reduced PRP, and concentrated-leukocyte PRP. An additional high-concentration PRP group was generated with the same WBC concentration as the concentrated-leukocyte PRP group and the same platelet:WBC ratio as the standard PRP group. The PRP groups were used as media for flexor digitorum superficialis tendon explants in culture for seventy-two hours with 10% plasma in Dulbecco modified Eagle medium (DMEM) serving as control. Tendon gene expression for collagen types I (COL1A1) and III (COL3A1), cartilage oligomeric matrix protein (COMP), matrix metalloproteinase (MMP-13), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α) was performed.

RESULTS

The desired PRP groups were successfully generated. The expression of COMP, the COL1A1:COL3A1 ratio, and the expression of MMP-13 in flexor digitorum superficialis tendon explants was not different between PRP groups. The expression of COMP (p = 0.0027) and the COL1A1:COL3A1 ratio (p &lt; 0.0001) were increased in the PRP groups as compared with the control group, and the expression of MMP-13 was decreased in the PRP groups as compared with the control group (p &lt; 0.0001). The expression of IL-1β was lowest in leukocyte-reduced PRP and highest in concentrated-leukocyte PRP (p = 0.0001). The leukocyte-reduced PRP group and the control group had the lowest TNF-α expression, whereas the high-concentration PRP and concentrated-leukocyte PRP groups had the highest expression (p = 0.0224).

CONCLUSIONS

A high absolute WBC concentration in PRP contributes to the expression of inflammatory cytokines in flexor digitorum superficialis tendon explants, and maintenance of the platelet:WBC ratio is not able to counteract this effect.

CLINICAL RELEVANCE

The optimum composition of PRP for the treatment of tendinopathy has not been directly investigated. Persistent inflammation results in inferior repair with scar tissue. The present study indicates that in an animal model, WBC in PRP contributes to inflammatory cytokine production. Therefore, leukocyte-reduced PRP may be the optimum preparation to stimulate superior healing without scar tissue formation.

Tendinosis: pathophysiology and nonoperative treatment

Tendinosis is a troublesome clinical entity affecting many active people. Its treatment remains a challenge to sports medicine clinicians. The etiopathophysiology of tendinosis has not been well delineated. The known pathophysiology and the recent advances in the understanding of the etiologic process of tendinosis are discussed here, including new concepts in mechanotransduction and the biochemical alterations that occur during tendon overload. The optimal, nonoperative treatment of tendinosis is not clear. This article reviews recent evidence of the clinical efficacy of the following interventions: eccentric exercise, extracorporal shock wave treatment, corticosteroid and nonsteroidal anti-inflammatory medications, sclerosing injections, nitric oxide, platelet-rich plasma injections, and matrix metalloproteinase inhibitors. Eccentric exercise has strongest evidence of efficacy. Extracorporal shock wave treatment has mixed evidence and needs further study of energy and application protocols. Sclerosing agents show promising early results but require long-term studies. Corticosteroid and nonsteroidal anti-inflammatory medications have not been shown to be effective, and many basic science studies raise possible concerns with their use. Nitric oxide has been shown in several basic science studies to be promising, but clinical efficacy has not been well established. More clinical trials are needed to assess dosing, indications, and clinical efficacy of nitric oxide. Platelet-rich plasma injections have offered encouraging short-term results. Larger and longer-term clinical trials are needed to assess this promising modality. Matrix metalloproteinase inhibitors have had few clinical studies, and their role in the treatment of tendinosis is still in the early phase of investigation.

Non-uniform displacement within the Achilles tendon during passive ankle joint motion

PURPOSE

An initial step in the understanding of Achilles tendon dynamics is to investigate the effects of passive motion, thereby minimising muscle activation and reducing internal joint forces. Internal tendon dynamics during passive ankle joint motion have direct implications for clinical rehabilitation protocols after Achilles tendon surgery. The aim of this study was to test the hypothesis that tendon tissue displacement is different in different layers of the Achilles tendon during controlled passive ankle joint movements.

METHODS

Ultrasound imaging was conducted on the right Achilles tendon of nine healthy recreationally active males. Standardised isokinetic passive dorsi-plantar-flexion movements were performed with a total range of motion of 35°. The tendon was divided into superficial, central and deep layers in the resulting B-mode ultrasound images viewed in the sagittal plane. A block-matching speckle tracking algorithm was applied post-process, with kernels for the measurement of displacement placed in each of the layers.

RESULTS

The mean (SD) displacement of the Achilles tendon during passive dorsiflexion was 8.4 (1.9) mm in the superficial layer, 9.4 (1.9) mm in the central portion and 10.4 (2.1) mm in the deep layer, respectively. In all cases, the movement of the deep layer of the tendon was greater than that of the superficial one (P < 0.01).

CONCLUSIONS

These results, achieved in vivo with ultrasonographic speckle tracking, indicated complex dynamic differences in different layers of the Achilles tendon, which could have implications for the understanding of healing processes of tendon pathologies and also of normal tendon function.

The pathogenesis of tendon microdamage in athletes: the horse as a natural model for basic cellular research

The equine superficial digital flexor tendon (SDFT) is a frequently injured structure that is functionally and clinically equivalent to the human Achilles tendon (AT). Both act as critical energy-storage systems during high-speed locomotion and can accumulate exercise- and age-related microdamage that predisposes to rupture during normal activity. Significant advances in understanding of the biology and pathology of exercise-induced tendon injury have occurred through comparative studies of equine digital tendons with varying functions and injury susceptibilities. Due to the limitations of in-vivo work, determination of the mechanisms by which tendon cells contribute to and/or actively participate in the pathogenesis of microdamage requires detailed cell culture modelling. The phenotypes induced must ultimately be mapped back to the tendon tissue environment. The biology of tendon cells and their matrix, and the pathological changes occurring in the context of early injury in both horses and people are reviewed, with a particular focus on the use of various tendon cell and tissue culture systems to model these events.

Trigger finger and tendinosis

The pathogenesis of trigger finger has generally been ascribed to primary changes in the pulley. Histological examination of the affected tendons has rarely been done. We studied biopsies from tendons of trigger fingers from 29 patients and compared these to biopsies from six intact tendons. We used a modified Movin score, which describes the tendinosis of the Achilles tendon. Trigger finger tendons had a high score (14.2; SD, 2.2) consistent with tendinosis, while the controls were almost normal (2.5; SD, 1.9). This suggests that the tendon is also affected, and that trigger finger is a form of tendinosis.

The role of mechanobiology in tendon healing

Mechanical cues affect tendon healing, homeostasis, and development in a variety of settings. Alterations in the mechanical environment are known to result in changes in the expression of extracellular matrix proteins, growth factors, transcription factors, and cytokines that can alter tendon structure and cell viability. Loss of muscle force in utero or in the immediate postnatal period delays tendon and enthesis development. The response of healing tendons to mechanical load varies depending on anatomic location. Flexor tendons require motion to prevent adhesion formation, yet excessive force results in gap formation and subsequent weakening of the repair. Excessive motion in the setting of anterior cruciate ligament reconstruction causes accumulation of macrophages, which are detrimental to tendon graft healing. Complete removal of load is detrimental to rotator cuff healing; yet, large forces are also harmful. Controlled loading can enhance healing in most settings; however, a fine balance must be reached between loads that are too low (leading to a catabolic state) and too high (leading to microdamage). This review will summarize existing knowledge of the mechanobiology of tendon development, homeostasis, and healing.

Structure of retracted tendons after staged repair following continuous traction

PURPOSE

The effect of staged repair involving continuous re-lengthening of the retracted musculotendinous unit after rotator cuff tear is not known. We quantified changes in chronically retracted tendons undergoing no repair or a staged repair involving an initial re-lengthening of the musculotendinous unit by traction in a sheep model of massive rotator cuff tear.

MATERIALS AND METHODS

Infraspinatus tendons of 12 sheep were released and allowed to retract for 4 months. Repair was performed after the retracted musculotendinous unit had been progressively returned to its original length through continuous traction in 8 sheep (group I). In the other 4 sheep (group II) traction was not successful and the tendons remained retracted. Tendon structure was assessed macroscopically, by MRI, histology, and TEM.

RESULTS

Normalized to their contralateral controls, at sacrifice, tendon thickness was unchanged in group I (116%, n.s) and increased in group II (129%, P < 0.05), however with substantial shortening. Increased collagen fiber crimping and disorganization was found in group II, whereas in group I the differences from normal tendon were less pronounced.

CONCLUSION

Retracted musculotendinous units have deteriorated tendons, characterized by increased collagen fiber crimp, and ultrastructural collagen fibril atrophy and disorganization. Continuous traction may arrest and partially restore degenerative changes in retracted tendon. The findings of this study might contribute to new approaches for the treatment of chronic "irreparable" rotator cuff tears.

Cyclic loading of tendon fascicles using a novel fatigue loading system increases interleukin-6 expression by tenocytes

Repetitive strain or 'overuse' is thought to be a major factor contributing to the development of tendinopathy. The aims of our study were to develop a novel cyclic loading system, and use it to investigate the effect of defined loading conditions on the mechanical properties and gene expression of isolated tendon fascicles. Tendon fascicles were dissected from bovine-foot extensors and subjected to cyclic tensile strain (1 Hz) at 30% or 60% of the strain at failure, for 0 h (control), 15 min, 30 min, 1 h, or 5 h. Post loading, a quasi-static test to failure assessed damage. Gene expression at a selected loading regime (1 h at 30% failure strain) was analyzed 6 h post loading by quantitative real-time polymerase chain reaction. Compared with unloaded controls, loading at 30% failure strain took 5 h to lead to a significant decrease in failure stress, whereas loading to 60% led to a significant reduction after 15 min. Loading for 1 h at 30% failure strain did not create significant structural damage, but increased Collagen-1-alpha-chain-1 and interleukin-6 (IL6) expression, suggesting a role of IL6 in tendon adaptation to exercise. Correlating failure properties with fatigue damage provides a method by which changes in gene expression can be associated with different degrees of fatigue damage.

Achilles tendinosis: a morphometrical study in a rat model

This study addresses the morphopathogenesis of Achilles tendinosis, using a rat model and presenting quantitative analysis of time-dependent histological changes. Thirty Wistar rats were used, randomly split in experimental and control groups. Animals of the experimental group were submitted to a treadmill running scheme. Five animals of each group were euthanized at four, eight and sixteen weeks. Achilles tendons were collected and processed routinely for histopath sections. Slides were stained by Hematoxylin-Eosin, Picrosirius Red, Alcian Blue, AgNOR, TUNEL and evaluated morphometrically. Cellular density decreased slightly along the time and was higher in the experimental group than in controls at fourth, eighth and sixteenth weeks. Fiber microtearing, percentual of reticular fibers and glycosaminoglycans content increased along the time and were higher in experimental group than in controls at all-time intervals. AgNOR labeling here interpreted as a marker of transcription activity was higher in the experimental groups than in controls at all-time intervals. Apoptotic cells were more frequent and diffusely distributed in tendinosis samples than in control groups. These results suggest that as mechanical overload is becoming chronic, cellular turnover and matrix deposition increases leading to tendinosis. The combination of staining techniques and morphometry used here to describe the evolution of lesions occurring in a rat model system has proved to be suited for the study of induced Achilles tendinosis.

Three-dimensional-construct bioreactor conditioning in human tendon tissue engineering

Human tendon tissue engineering attempts to address the shortage of autologous tendon material arising from mutilating injuries and diseases of the hand and forearm. It is important to maximize the tissue-engineered construct's (TEC's) biomechanical properties to ensure that the construct is in its strongest possible state before reimplantation. In this study, we sought to determine the bioreactor treatment parameters that affect these properties. Using small- and large-chamber three-dimensional-construct bioreactors (SCB and LCB, respectively), we applied cyclic axial load to TECs comprising reseeded human flexor and extensor tendons of the hand. First, small-sample pilot studies using the LCB were performed on matched-paired full-length flexor tendons to establish proof of concept. Next, large-sample studies using the SCB were performed on matched-paired extensor tendon segments to determine how reseeding, load duty cycle, load magnitude, conditioning duration, and testing delay affected ultimate tensile stress (UTS) and elastic modulus (EM). We found that compared with reseeded matched-paired controls under dynamic-loading at 1.25 N per TEC for 5 days, (1) acellular TECs had lower UTS (p=0.04) and EM (p<0.01), (2) unloaded TECs had lower UTS (p=0.01) and EM (p=0.02), (3) static-loaded TECs had lower UTS (p=0.01) and EM (p<0.01), (4) TECs conditioned for 3 days had lower UTS (p=0.03) and EM (p=0.04), and (5) TECs conditioned for 8 days had higher UTS (p=0.04) and EM (p=0.01). However, TECs conditioned at higher loads (2.5 N per TEC) and lower loads (0.625 N per TEC) possessed similar UTS (p=0.83 and p=0.89, respectively) and EM (p=0.48 and p=0.89, respectively) as controls stimulated with 1.25 N per TEC. After cycle completion, there is attrition of UTS (p=0.03) and EM (p=0.04) over a 2-day period. Our study showed that the material properties of human allograft TECs can be enhanced by reseeding and dynamic-conditioning. While conditioning duration has a significant effect on material properties, the load magnitude does not. The issue of attrition in biomechanical properties with time following cycle completion must be addressed before bioreactor preconditioning can be successfully introduced as a step in the processing of these constructs for clinical application.

Tendon tissue engineering: progress, challenges, and translation to the clinic

The tissue engineering field has made great strides in understanding how different aspects of tissue engineered constructs (TECs) and the culture process affect final tendon repair. However, there remain significant challenges in developing strategies that will lead to a clinically effective and commercially successful product. In an effort to increase repair quality, a better understanding of normal development, and how it differs from adult tendon healing, may provide strategies to improve tissue engineering. As tendon tissue engineering continues to improve, the field needs to employ more clinically relevant models of tendon injury such as degenerative tendons. We need to translate successes to larger animal models to begin exploring the clinical implications of our treatments. By advancing the models used to validate our TECs, we can help convince our toughest customer, the surgeon, that our products will be clinically efficacious. As we address these challenges in musculoskeletal tissue engineering, the field still needs to address the commercialization of products developed in the laboratory. TEC commercialization faces numerous challenges because each injury and patient is unique. This review aims to provide tissue engineers with a summary of important issues related to engineering tendon repairs and potential strategies for producing clinically successful products.

Low stress tendon fatigue is a relatively rapid process in the context of overuse injuries

To stimulate healing and prevent tendinosis through optimized physical exercise, it is important to elucidate the tendon response to repetitive mechanical loading. However, the study of this response is challenging due to complex cell-matrix interactions. In an initial approximation, the authors examined tendon mechanical response only, and did not consider cellular activity. The authors investigated the hypothesis that mechanical degradation occurs relatively rapidly (< 24 h) even at very low stress levels. The authors subjected rat tail tendons to mechanical loadings oscillating between 0 and 1.5 MPa up to one of three fatigue levels: 4% strain, 8% strain, or rupture. Using non-destructive mechanical tests, changes in tendon strain and compliance over the entire fatigue testing period were evaluated. Using microscopy techniques, the structural evidence of mechanical degradation was examined. The changes in tendon strain and compliance progressed nonlinearly and accelerated before rupture which took place around the 15-h mark. Histological analyses revealed a higher degree of alteration in the collagen network at increased fatigue levels. At rupture, local zones of damage with low fibril density were evident. These results imply that a repair process must act rapidly at critical sites; otherwise, enzymatic degradation could cause further damage in the manner of a vicious cycle.

Deciphering the pathogenesis of tendinopathy: a three-stages process

Our understanding of the pathogenesis of "tendinopathy" is based on fragmented evidences like pieces of a jigsaw puzzle. We propose a "failed healing theory" to knit these fragments together, which can explain previous observations. We also propose that albeit "overuse injury" and other insidious "micro trauma" may well be primary triggers of the process, "tendinopathy" is not an "overuse injury" per se. The typical clinical, histological and biochemical presentation relates to a localized chronic pain condition which may lead to tendon rupture, the latter attributed to mechanical weakness. Characterization of pathological "tendinotic" tissues revealed coexistence of collagenolytic injuries and an active healing process, focal hypervascularity and tissue metaplasia. These observations suggest a failed healing process as response to a triggering injury. The pathogenesis of tendinopathy can be described as a three stage process: injury, failed healing and clinical presentation. It is likely that some of these "initial injuries" heal well and we speculate that predisposing intrinsic or extrinsic factors may be involved. The injury stage involves a progressive collagenolytic tendon injury. The failed healing stage mainly refers to prolonged activation and failed resolution of the normal healing process. Finally, the matrix disturbances, increased focal vascularity and abnormal cytokine profiles contribute to the clinical presentations of chronic tendon pain or rupture. With this integrative pathogenesis theory, we can relate the known manifestations of tendinopathy and point to the "missing links". This model may guide future research on tendinopathy, until we could ultimately decipher the complete pathogenesis process and provide better treatments.

Functional analysis of tenocytes gene expression in tendon fascicles subjected to cyclic tensile strain

Tenocytes are known to be mechanoresponsive and the present study tests the hypothesis that distinct mechanical stimulation regimes, associated with the short-term and extended application of cyclic tensile strain, alters the balance between anabolic and catabolic processes. Microarray technology has been used to provide a comprehensive analysis of alterations in gene expression within isolated tendon fascicles in response to cyclic tensile strain using a well-established model system. Isolated rat tail tendon fascicles were subjected to cyclic tensile strain (3% amplitude superimposed on a 2% static strain) for 1 or 24 hr. Messenger RNA expression level was assessed using Illumina microarray. The number of genes significantly altered in strained fascicles from the level of unstrained control fascicles was greater at 24 hr than 1 hr. The expression levels of many extracellular matrix components remained unchanged at both time points; however, a number of members of the matrix metalloproteinase (MMP) and a disintegrin and metalloproteinase with a thrombospondin (ADAMTS) families were significantly downregulated at 24 hr. Functional annotation revealed that upregulated genes were significantly associated with the regulation of transcription at 1 hr and translation at 24 hr. Downregulated genes were associated with inflammatory responses at 1 hr, and genes inhibited at 24 hr were significantly associated with cell apoptosis and a variety of metabolic functions. The present results suggest that the metabolic balance was shifted in favor of catabolism by the application of a small number of tensile strain cycles, whereas an extended number stimulates strong anti-catabolic effects.

Remodeling and repair of orthopedic tissue: role of mechanical loading and biologics

Orthopedic tissues respond to mechanical loads to maintain normal homeostasis and in response to injury. As the body of work on this continues to grow, it is important to synthesize the recent studies across tissues and specialties with one another and with past studies. Hence, this review highlights the knowledge gained since 2000, with only few exceptions, concerning the effects of mechanical load and biologics on remodeling and repair of orthopedic tissue.

A mechanistic study for strain rate sensitivity of rabbit patellar tendon

The ultrastructural mechanism for strain rate sensitivity of collagenous tissue has not been well studied at the collagen fibril level. Our objective is to reveal the mechanistic contribution of tendon's key structural component to strain rate sensitivity. We have investigated the structure of the collagen fibril undergoing tension at different strain rates. Tendon fascicles were pulled and fixed within the linear region (12% local tissue strain) at multiple strain rates. Although samples were pulled to the same percent elongation, the fibrils were noticed to elongate differently, increasing with strain rate. For the 0.1, 10, and 70%/s strain rates, there were 1.84±3.6%, 5.5±1.9%, and 7.03±2.2% elongations (mean±S.D.), respectively. We concluded that the collagen fibrils underwent significantly greater recruitment (fibril strain relative to global tissue strain) at higher strain rates. A better understanding of tendon mechanisms at lower hierarchical levels would help establish a basis for future development of constitutive models and assist in tissue replacement design.

bFGF and PDGF-BB for tendon repair: controlled release and biologic activity by tendon fibroblasts in vitro

Flexor tendon injuries are often encountered clinically and typically require surgical repair. Return of function after repair is limited due to adhesion formation, which leads to reduced tendon gliding, and due to a lack of repair site strength, which leads to repair site gap formation or rupture. The application of the growth factors basic fibroblastic growth factor (bFGF) and platelet derived growth factor BB (PDGF-BB) has been shown to have the potential to enhance tendon healing. The objectives of this study were to examine: (1) the conditions over which delivery of bFGF can be controlled from a heparin-binding delivery system (HBDS) and (2) the effect of bFGF and PDGF-BB released from this system on tendon fibroblast proliferation and matrix gene expression in vitro over a 10-day interval. Delivery of bFGF was controlled using a HBDS. Fibrin matrices containing the HBDS retained bFGF better than did matrices lacking the delivery system over the 10-day period studied. Delivery of bFGF and PDGF-BB using the HBDS stimulated tendon fibroblast proliferation and promoted changes in the expression of matrix genes related to tendon gliding, strength, and remodeling. Both growth factors may be effective in enhancing tendon healing in vivo.

Fibril morphology and tendon mechanical properties in patellar tendinopathy: effects of heavy slow resistance training

BACKGROUND

Patellar tendinopathy is characterized by pathologic abnormalities. Heavy slow resistance training (HSR) is effective in the management of patellar tendinopathy, but the underlying functional mechanisms remain elusive.

PURPOSE

To investigate fibril morphology and mechanical properties in patellar tendinopathy and the effect of HSR on these properties.

STUDY DESIGN

Cohort study; Level of evidence, 2.

METHODS

Eight male patients with patellar tendinopathy completed 12 weeks of HSR. Nine healthy subjects served as controls. Assessments were conducted at baseline and at 12 weeks. Patients assessed symptoms/function and maximal tendon pain during activity. Tendon biopsy samples were analyzed for fibril density, volume fraction, and mean fibril area. Tendon mechanical properties were assessed using force and ultrasonography samplings.

RESULTS

Patients improved in symptoms/function (P = .02) and maximal tendon pain during activity (P = .008). Stiffness and modulus of control and tendinopathy tendons were similar at baseline. Stiffness remained unaffected in control tendons (3487 +/- 392 to 3157 +/- 327 N/mm, P = .57) but declined in tendinopathic tendons at 12 weeks (3185 +/- 187 to 2701 +/- 201 N/mm, P = .04). At baseline, fibril volume fraction was equal, fibril density smaller (P = .03), and mean fibril area tended to be higher in tendinopathy versus controls (P = .07). Fibril morphology remained unchanged in controls but fibril density increased (70% +/- 18%, P = .02) and fibril mean area decreased (-26% +/- 21%, P = .04) in tendinopathic tendons after HSR.

CONCLUSION

Fibril morphology is abnormal in tendinopathy, but tendon mechanical properties are not. Clinical improvements after HSR were associated with changes in fibril morphology toward normal fibril density and mean fibril area. Heavy slow resistance training improved the clinical outcome of patellar tendinopathy, and these improvements were associated with normalization of fibril morphology, most likely due to a production of new fibrils.

The pathogenesis of tendinopathy: balancing the response to loading

Tendons are designed to withstand considerable loads. Mechanical loading of tendon tissue results in upregulation of collagen expression and increased synthesis of collagen protein, the extent of which is probably regulated by the strain experienced by the resident fibroblasts (tenocytes). This increase in collagen formation peaks around 24 h after exercise and remains elevated for about 3 days. The degradation of collagen proteins also rises after exercise, but seems to peak earlier than the synthesis. Despite the ability of tendons to adapt to loading, repetitive use often results in injuries, such as tendinopathy, which is characterized by pain during activity, localized tenderness upon palpation, swelling and impaired performance. Tendon histological changes include reduced numbers and rounding of fibroblasts, increased content of proteoglycans, glycosaminoglycans and water, hypervascularization and disorganized collagen fibrils. At the molecular level, the levels of messenger RNA for type I and III collagens, proteoglycans, angiogenic factors, stress and regenerative proteins and proteolytic enzymes are increased. Tendon microrupture and material fatigue have been suggested as possible injury mechanisms, thus implying that one or more 'weak links' are present in the structure. Understanding how tendon tissue adapts to mechanical loading will help to unravel the pathogenesis of tendinopathy.

Relative contributions of mechanical degradation, enzymatic degradation, and repair of the extracellular matrix on the response of tendons when subjected to under- and over- mechanical stimulations in vitro

Tendon response to mechanical loading results in either homeostasis, improvement, or degeneration of tissue condition. In an effort to better understand the development of tendinopathies, this study investigated the mechanical and structural responses of tendons subjected to under- and over-stimulations (1.2% and 1.8% strain respectively, 1 Hz). The objective was to examine three sub-processes of tendon response: mechanical degradation, enzymatic degradation, and repair of the extracellular matrix. We subjected rat tail tendons to a 10-day stimulation protocol with four periods of 6 h each day: 30 min of stimulation and 5 h 30 min of rest. To investigate the contribution of the three sub-processes, we controlled the contribution of the cells through variations in the nutrient and protease inhibitor content in the in vitro solutions. Using nondestructive cyclic tests, we evaluated the daily changes in the peak stress. To assess structural changes, we carried out microscopic analyses at the end of the study period. We observed that the relative contributions of the sub-processes differed according to the stimulation amplitude. With over-stimulation of tendons immersed in DMEM, we succeeded in reducing enzymatic degradation and increasing peak stress. In under-stimulation, the addition of protease inhibitors was required to obtain the same result.

Histological and molecular analysis of the biceps tendon long head post-tenotomy

Tendinopathy is a vexing clinical problem as its onset and development is often asymptomatic and unrecognized until tendon rupture. While extensively studied in the rotator cuff, Achilles, and patellar tendons, no study to date has examined the histological and molecular characteristics of the tendinopathic biceps long-head (LHB). The anatomy of the LHB is unique in that it comprises intra- and extra-articular portions, each exposed to differing loading patterns. Eleven LHBs post-tenotomy were sectioned, fixed in formalin, and stained (H and E; Alcian Blue), and gross structural organization of collagen measured using polarized light microscopy. Protein expression of intra- and extra-articular portions of the tenotomized biceps for IGF-I, collagen III, and MMP-1, -2, -3, and -13 was determined with Western blot analyses. The intra-articular LHB exhibited significantly greater histological evidence of tendinopathy inclusive of increased proteoglycan (p < 0.05) and decreased organization as measured by polarized light microscopy (p < 0.01). The intra-articular LHB also had significantly increased expression of collagen type III (p < 0.01) and of MMP-1 and 3 (p < 0.01, p < 0.05 respectively). No significant differences were found for IGF-I or for MMP-2 and -13. The intra-articular LHB exhibited histological characteristics of tendinopathy. Protein expression of the intra-articular LHB did not universally display signs of tendinopathy in comparison to the extra-articular portion of the tendon.

Temporal growth factor release from platelet-rich plasma, trehalose lyophilized platelets, and bone marrow aspirate and their effect on tendon and ligament gene expression

Platelet-rich plasma (PRP) has generated substantial interest for tendon and ligament regeneration because of the high concentrations of growth factors in platelet alpha-granules. This study compared the temporal release of growth factors from bone marrow aspirate (BMA), PRP, and lyophilized platelet product (PP), and measured their effects on tendon and ligament gene expression. Blood and BMA were collected and processed to yield PRP and plasma. Flexor digitorum superficialis tendon (FDS) and suspensory ligament (SL) explants were cultured in 10% plasma in DMEM (control), BMA, PRP, or PP. TGF-beta1 and PDGF-BB concentrations were determined at 0, 24, and 96 h of culture using ELISA. Quantitative RT-PCR for collagen types I and III (COL1A1, COL3A1), cartilage oligomeric matrix protein (COMP), decorin, and matrix metalloproteinases-3 and 13 (MMP-3, MMP-13) was performed. TGF-beta1 and PDGF-BB concentrations were highest in PRP and PP. Growth factor quantity was unchanged in BMA, increased in PRP, and decreased in PP over 4 days. TGF-beta1 and platelet concentrations were positively correlated. Lyophilized PP and PRP resulted in increased COL1A1:COL3A1 ratio, increased COMP, and decreased MMP-13 expression. BMA resulted in decreased COMP and increased MMP-3 and MMP-13 gene expression. Platelet concentration was positively correlated with COL1A1, ratio of COL1A1:COL3A1, and COMP, and negatively correlated with COL3A1, MMP-13, and MMP-3. White blood cell concentration was positively correlated with COL3A1, MMP3, and MMP13, and negatively correlated with a ratio of COL1A1:COL3A1, COMP, and decorin. These findings support further in vivo investigation of PRP and PP for treatment of tendonitis and desmitis.

In vitro model to study chondrogenic differentiation in tendinopathy

BACKGROUND

Treatment of midportion Achilles tendinopathy is hampered by limited knowledge of the pathophysiology.

HYPOTHESIS

Chondrogenic differentiation of tendon cells might take place in midportion Achilles tendinopathy and could be used as a target for drug treatment. An in vitro model for chondrogenic differentiation would be useful to evaluate existing and future treatment opportunities.

STUDY

A controlled laboratory study.

METHODS

Perioperatively harvested tissue from human midportion Achilles tendinotic lesions and healthy Achilles tendons was analyzed by microscopy and real-time reverse transcription polymerase chain reaction. In vitro chondrogenic differentiation of tendon explants was induced using transforming-growth-factor beta. This model was modulated by removing the chondrogenic stimulus or adding triamcinolone or platelet-rich plasma.

RESULTS

Midportion Achilles tendinotic lesions had increased glycosaminoglycan staining and more rounded cell nuclei. Chondrogenic markers (sex-determining region Y)-box9, aggrecan, collagen 2, and RUNT-related transcription factor 2 were upregulated, but collagen 10 was not. Nondegenerative tendon explants cultured on chondrogenic medium had higher expression of aggrecan, collagen 2, and collagen 10 but not (sex-determining region Y)-box9 and RUNT-related transcription factor 2. Removing the chondrogenic stimulus decreased expression of aggrecan, collagen 2, and collagen 10. Both triamcinolone and platelet-rich plasma influenced the chondrogenic gene expression pattern in the in vitro model.

CONCLUSION

Chondrogenic differentiation is present in midportion Achilles tendinopathy. An in vitro model to study this chondrogenic differentiation was developed.

CLINICAL RELEVANCE

This model can be used to investigate chondrogenic differentiation as a possible target for drug treatment, contributing to the development of more successful mechanism-based treatment opportunities.

Corticosteroid injections, eccentric decline squat training and heavy slow resistance training in patellar tendinopathy

A randomized-controlled single-blind trial was conducted to investigate the clinical, structural and functional effects of peritendinous corticosteroid injections (CORT), eccentric decline squat training (ECC) and heavy slow resistance training (HSR) in patellar tendinopathy. Thirty-nine male patients were randomized to CORT, ECC or HSR for 12 weeks. We assessed function and symptoms (VISA-p questionnaire), tendon pain during activity (VAS), treatment satisfaction, tendon swelling, tendon vascularization, tendon mechanical properties and collagen crosslink properties. Assessments were made at 0 weeks, 12 weeks and at follow-up (half-year). All groups improved in VISA-p and VAS from 0 to 12 weeks (P<0.05). VISA-p and VAS improvements were maintained at follow-up in ECC and HSR but deteriorated in CORT (P<0.05). In CORT and HSR, tendon swelling decreased (-13+/-9% and -12+/-13%, P<0.05) and so did vascularization (-52+/-49% and -45+/-23%, P<0.01) at 12 weeks. Tendon mechanical properties were similar in healthy and injured tendons and were unaffected by treatment. HSR yielded an elevated collagen network turnover. At the half-year follow-up, treatment satisfaction differed between groups, with HSR being most satisfied. Conclusively, CORT has good short-term but poor long-term clinical effects, in patellar tendinopathy. HSR has good short- and long-term clinical effects accompanied by pathology improvement and increased collagen turnover.

Role of biomechanics in the understanding of normal, injured, and healing ligaments and tendons

Ligaments and tendons are soft connective tissues which serve essential roles for biomechanical function of the musculoskeletal system by stabilizing and guiding the motion of diarthrodial joints. Nevertheless, these tissues are frequently injured due to repetition and overuse as well as quick cutting motions that involve acceleration and deceleration. These injuries often upset this balance between mobility and stability of the joint which causes damage to other soft tissues manifested as pain and other morbidity, such as osteoarthritis.

The healing of ligament and tendon injuries varies from tissue to tissue. Tendinopathies are ubiquitous and can take up to 12 months for the pain to subside before one could return to normal activity. A ruptured medial collateral ligament (MCL) can generally heal spontaneously; however, its remodeling process takes years and its biomechanical properties remain inferior when compared to the normal MCL. It is also known that a midsubstance anterior cruciate ligament (ACL) tear has limited healing capability, and reconstruction by soft tissue grafts has been regularly performed to regain knee function. However, long term follow-up studies have revealed that 20–25% of patients experience unsatisfactory results. Thus, a better understanding of the function of ligaments and tendons, together with knowledge on their healing potential, may help investigators to develop novel strategies to accelerate and improve the healing process of ligaments and tendons.

With thousands of new papers published in the last ten years that involve biomechanics of ligaments and tendons, there is an increasing appreciation of this subject area. Such attention has positively impacted clinical practice. On the other hand, biomechanical data are complex in nature, and there is a danger of misinterpreting them. Thus, in these review, we will provide the readers with a brief overview of ligaments and tendons and refer them to appropriate methodologies used to obtain their biomechanical properties. Specifically, we hope the reader will pay attention to how the properties of these tissues can be altered due to various experimental and biologic factors. Following this background material, we will present how biomechanics can be applied to gain an understanding of the mechanisms as well as clinical management of various ligament and tendon ailments. To conclude, new technology, including imaging and robotics as well as functional tissue engineering, that could form novel treatment strategies to enhance healing of ligament and tendon are presented.

The effect of stress-deprivation and cyclic loading on the TIMP/MMP ratio in tendon cells: an in vitro experimental study

PURPOSE

To determine the effect of stress deprivation and cyclic loading on TIMP-1/MMP-13 mRNA expression ratio in rat tail tendon (RTT) cells.

METHOD

Adult RTTs were stress-deprived for 0, 24, 48, or 72 hours in the presence or absence of a MMP inhibitor (ilomastat), or subjected to 1%, 3%, or 6% strain for 24 h under tissue culture conditions. TIMP-1 and MMP-13 (rat interstitial collagenase) mRNA expression were measured using quantitative PCR and TIMP/MMP ratios were calculated for each group.

RESULTS

The ratio of TIMP-1 to MMP-13 in control RTTs was 3.73:1 +/- 0.73. Stress deprivation for 24 h significantly decreased the TIMP-1/MMP-13 ratio (0.25:1 +/- 0.04) and MMP-13 expression continued to increase significantly with time of stress deprivation. Inhibition of MMP-13 mRNA expression with ilomastat in stress-deprived samples did not alter TIMP-1 expression when compared to normal controls. Cyclic loading significantly increased TIMP-1/MMP-13 expression at all strain levels examined.

CONCLUSIONS

RTTs normally have a positive TIMP-1/MMP-13 expression ratio. While cyclic loading increased the TIMP-1/MMP-13 ratio, loss of cellular homeostatic tension inversed this ratio through a significant increase in MMP-13 mRNA expression rather than a decrease in TIMP expression. A negative TIMP/MMP ratio has been implicated in the pathogenesis of tendinopathy. Increasing the TIMP/MMP ratios in these patients through exercise may be beneficial in the management of tendinopathy.

High-intensity focused ultrasound ablation of ex vivo bovine achilles tendon

Small tears in tendons are a common occurrence in athletes and others involved in strenuous physical activity. Natural healing in damaged tendons can result in disordered regrowth of the underlying collagen matrix of the tendon. These disordered regions are weaker than surrounding ordered regions of normal tendon and are prone to re-injury. Multiple cycles of injury and repair can lead to chronic tendinosis. Current treatment options either are invasive or are relatively ineffective in tendinosis without calcifications. High-intensity focused ultrasound (HIFU) has the potential to treat tendinosis noninvasively. HIFU ablation of tendons is based on a currently-used surgical analog, viz., needle tenotomy. This study tested the ability of HIFU beams to ablate bovine tendons ex vivo. Two ex vivo animal models were employed: a bare bovine Achilles tendon (deep digital flexor) on an acoustically absorbent rubber pad, and a layered model (chicken breast proximal, bovine Achilles tendon central and a glass plate distal to the transducer). The bare-tendon model enables examination of lesion formation under simple, ideal conditions; the layered model enables detection of possible damage to intervening soft tissue and consideration of the possibly confounding effects of distal bone. In both models, the tissues were degassed in normal phosphate-buffered saline. The bare tendon was brought to 23 degrees C or 37 degrees C before insonification; the layered model was brought to 37 degrees C before insonification. The annular array therapy transducer had an outer diameter of 33 mm, a focal length of 35 mm and a 14-mm diameter central hole to admit a confocal diagnostic transducer. The therapy transducer was excited with a continuous sinusoidal wave at 5.25 MHz to produce nominal in situ intensities from 0.23-2.6 kW/cm(2). Insonification times varied from 2-10 s. The focus was set over the range from the proximal tendon surface to 7 mm deep. The angle of incidence ranged from 0 degrees (normal to the tissue surface) to 15 degrees . After insonification, tendons were dissected and photographed, and the dimensions of the lesions were measured. Transmission electron micrographs were obtained from treated and untreated tissue regions. Insonification produced lesions that mimicked the shape of the focal region. When lesions were produced below the proximal tendon surface, no apparent damage to overlying soft tissue was apparent. The low intensities and short durations required for consistent lesion formation, and the relative insensitivity of ablation to small variations in the angle of incidence, highlight the potential of HIFU as a noninvasive treatment option for chronic tendinosis.

Patellar tendon strain is increased at the site of the jumper's knee lesion during knee flexion and tendon loading: results and cadaveric testing of a computational model

BACKGROUND

Patellar tendinopathy (jumper's knee) is characterized by localized tenderness of the patellar tendon at its origin on the inferior pole of the patella and a characteristic increase in signal intensity on magnetic resonance imaging at this location. However, it is unclear why the lesion typically occurs in this area of the patellar tendon as surface strain gauge studies of the patellar tendon through the range of motion have produced conflicting results.

HYPOTHESIS

The predicted patellar tendon strains that occur as a result of the tendon loads and patella-patellar tendon angles (PPTAs) experienced during a jump landing will be significantly increased in the area of the patellar tendon associated with patellar tendinopathy.

STUDY DESIGN

Descriptive laboratory study.

METHODS

A 2-dimensional, computational, finite element model of the patella-patellar tendon complex was developed using anatomic measurements taken from lateral radiographs of a normal knee. The patella was modeled with plane strain rigid elements, and the patellar tendon was modeled with 8-node plane strain elements with neo-Hookean material properties. A tie constraint was used to join the patellar tendon and patella. Patella-patellar tendon angles corresponding to knee flexion angles between 0 degrees and 60 degrees and patellar tendon strains ranging from 5% to 15% were used as input variables into the computational model. To determine if the location of increased strain predicted by the computational model could produce isolated tendon fascicle damage in that same area, 5 human cadaveric patella-patellar tendon-tibia specimens were loaded under conditions predicted by the model to significantly increase localized tendon strain. Pre- and posttesting ultrasound images of the patella-patellar tendon specimens were obtained to document the location of any injured fascicles.

RESULTS

Localized tendon strain at the classic location of the jumper's knee lesion was found to increase in association with an increase in the magnitude of applied patellar tendon strain and a decrease in the PPTA. The principal stresses and strains predicted by the model for this localized area were tensile and not compressive in nature. Applying the tendon strain conditions and PPTA predicted by the model to significantly increase localized strain resulted in disruption of tendon fascicles in 3 of the 5 cadaveric specimens at the classic location of the patellar tendinopathy lesion.

CONCLUSION

The localized increase in patellar tendon strain that occurs in response to the application of tendon loads and decreased PPTA could induce microdamage at the classic location of the jumper's knee lesion.

CLINICAL RELEVANCE

The association of decreasing PPTA with increasing localized tendon strain would implicate the role of knee-joint angle as well as tendon force in the etiopathogenesis of jumper's knee.

Twenty-five years of tendon and ligament research

Twenty-five years ago, the Journal of Orthopaedic Research published its first volume, which included five articles covering topics in tendon and ligament research. Since then, the body of tendon and ligament research has continued to increase exponentially. This review summarizes major advancements in tendon and ligament research since the initial publication of this journal. The purpose of this article is not to provide an in-depth review of all of tendon and ligament research, but instead to provide a concise literature review of some of the major and recurring areas of research. The general topics covered over the last 25 years include tissue properties, tendinopathy, healing, and engineered scaffolds.