A new clamping technique for biomechanical testing of tendons in small animals. aprobst@uni-muenster.de

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

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

The effect of Sildenafil, a phosphodiesterase-5 inhibitor, on tendon healing: an experimental study in rat model of achilles tendon injury

INTRODUCTION

Sildenafil Citrate has various effects on the body, including widening blood vessels, inhibiting platelet aggregation, promoting the growth of blood vessels, stimulating apoptosis and adhesion of fibroblasts, and reducing inflammation. This research aims to explore how Sildenafil Citrate affects surgically treated Achilles tendons, both in terms of tissue structure and mechanical properties.

MATERIALS AND METHODS

Forty-eight Wistar-albino rats weighing 350-400 g were randomly divided into groups, 6 in each group, as the study group was given Sildenafil Citrate and the control group given saline, respectively. The Achilles tendon rupture model was created under ketamine and xylazine anesthesia. During the entire experiment, rats were housed in eight separate cages, six of them each. The study group and control group of the first group were sacrificed at the end of 1 week, and Achilles tendon samples were taken. After that, Achilles tendon samples were taken after sacrificing the second group at 14 days, the third group at 21 days, and the fourth group at 28 days, respectively. Neovascularization, inflammation, fibrosis and fibroblastic activities of the harvested Achilles tendons were evaluated histopathologically. Biomechanically, stretching was applied to the Achilles tendons and continued until the tendon ruptured. the maximum force values at the moment of rupture were calculated.

RESULTS

The mean maximum strength value of group T21, which was given sildenafil citrate for 21 days, was 31.1 ± 4.36 N, and the mean maximum strength value of group C21, which was the control group, was 20.56 ± 6.92 N. A significant difference was observed between the groups (p: 0.008). Group T28 (45.17 ± 5.54 N) also demonstrated greater strength than group C28 (34.62 ± 3.21 N) in the comparison (p: 0.004). The study also noted significant differences between the groups in neovascularization, in the first week, 1 mild, 3 moderate and 2 prominent neovascularization was observed in group T7, in group T28, moderate neovascularization was observed in 4 specimens and prominent neovascularization was observed in 2 specimens (p: 0.001). Furthermore, the groups showed significant differences in their levels of fibrosis, inflammation and fibroblastic proliferation (p: 0.017, p: 0.036, (p: 0.035) respectively).

CONCLUSIONS

Study has demonstrated that sildenafil citrate can enhance the biomechanical and histopathological aspects of tendon healing, resulting in a stronger tendon.

Influence of the site of injury on the spontaneous healing response in a rat model of total rupture of the anterior cruciate ligament

AIM

The healing ability of the anterior cruciate ligament (ACL) injury is very poor; however, it has recently been shown to undergo self-healing with conservative treatments. In this study, we evaluated the influence of the site of injury on the healing process after complete transverse tear of ACL using a rat model.

MATERIALS AND METHODS

A total of 58 skeletally mature Wistar rats were randomly assigned to various ACL injury groups: controlled abnormal movement-mid-portion (CAM-MP), controlled abnormal movement-femoral side (CAM-FS), ACL transection-mid-portion (ACLT-MP), or ACL transection-femoral side (ACLT-FS) injury groups. The ACL was completely transected in the mid-portion in the ACLT-MP and CAM-MP groups, and on the femoral side in the ACLT-FS and CAM-FS groups. Both CAM groups underwent extra-articular braking to control for abnormal tibial translation. The animals were allowed full cage activity until sacrifice postoperatively for histological and biomechanical assessment.

RESULTS

Significant differences were found in the ratios of residual ligament lengths between the CAM-MP and CAM-FS groups, demonstrating the validity of each model. Spontaneous healing of the injured ACL was observed in the CAM-MP and CAM-FS groups but not in the ACLT-MP and ACLT-FS groups. The mechanical strength of the healing ACL did not differ between the CAM-MP and CAM-FS groups 8 weeks after injury; however, the former had better mechanical strength than the latter 12 weeks after the injury.

CONCLUSION

ACL injuries in the mid-portion and on the femoral side may be treated with conservative therapy for spontaneous healing.

Intrinsic Tendon Regeneration After Application of Purified Exosome Product: An In Vivo Study

Background:

Tendons are primarily acellular, limiting their intrinsic regenerative capabilities. This limited regenerative potential contributes to delayed healing, rupture, and adhesion formation after tendon injury.

Purpose:

To determine if a tendon’s intrinsic regenerative potential could be improved after the application of a purified exosome product (PEP) when loaded onto a collagen scaffold.

Study Design:

Controlled laboratory study.

Methods:

An in vivo rabbit Achilles tendon model was used and consisted of 3 groups: (1) Achilles tenotomy with suture repair, (2) Achilles tenotomy with suture repair and collagen scaffold, and (3) Achilles tenotomy with suture repair and collagen scaffold loaded with PEP at 1 × 10¹² exosomes/mL. Each group consisted of 15 rabbits for a total of 45 specimens. Mechanical and histologic analyses were performed at both 3 and 6 weeks.

Results:

The load to failure and ultimate tensile stress were found to be similar across all groups (P ≥ .15). The tendon cross-sectional area was significantly smaller for tendons treated with PEP compared with the control groups at 6 weeks, which was primarily related to an absence of external adhesions (P = .04). Histologic analysis confirmed these findings, demonstrating significantly lower adhesion grade both macroscopically (P = .0006) and microscopically (P = .0062) when tendons were treated with PEP. Immunohistochemical staining showed a greater intensity for type 1 collagen for PEP-treated tendons compared with collagen-only or control tendons.

Conclusion:

Mechanical and histologic results suggested that healing in the PEP-treated group favored intrinsic healing (absence of adhesions) while control animals and animals treated with collagen only healed primarily via extrinsic scar formation. Despite a smaller cross-sectional area, treated tendons had the same ultimate tensile stress. This pilot investigation shows promise for PEP as a means of effectively treating tendon injuries and enhancing intrinsic healing.

Clinical Relevance:

The production of a cell-free, off-the-shelf product that can promote tendon regeneration would provide a viable solution for physicians and patients to enhance tendon healing and decrease adhesions as well as shorten the time required to return to work or sports.

Tendon and motor phenotypes in the Crtap-/- mouse model of recessive osteogenesis imperfecta

Osteogenesis imperfecta (OI) is characterized by short stature, skeletal deformities, low bone mass, and motor deficits. A subset of OI patients also present with joint hypermobility; however, the role of tendon dysfunction in OI pathogenesis is largely unknown. Using the Crtap-/- mouse model of severe, recessive OI, we found that mutant Achilles and patellar tendons were thinner and weaker with increased collagen cross-links and reduced collagen fibril size at 1- and 4-months compared to wildtype. Patellar tendons from Crtap-/- mice also had altered numbers of CD146+CD200+ and CD146-CD200+ progenitor-like cells at skeletal maturity. RNA-seq analysis of Achilles and patellar tendons from 1-month Crtap-/- mice revealed dysregulation in matrix and tendon marker gene expression concomitant with predicted alterations in TGF-β, inflammatory, and metabolic signaling. At 4-months, Crtap-/- mice showed increased αSMA, MMP2, and phospho-NFκB staining in the patellar tendon consistent with excess matrix remodeling and tissue inflammation. Finally, a series of behavioral tests showed severe motor impairments and reduced grip strength in 4-month Crtap-/- mice - a phenotype that correlates with the tendon pathology.

Microsurgical reconstruction affects the outcome in a translational mouse model for Achilles tendon healing

Background

Animal models are one of the first steps in translation of basic science findings to clinical practice. For tendon healing research, transgenic mouse models are important to advance therapeutic strategies. However, the small size of the structures complicates surgical approaches, histological assessment, and biomechanical testing. In addition, available models are not standardized and difficult to compare. How surgery itself affects the healing outcome has not been investigated yet. The focus of the study was to develop a procedure that includes a transection and microsurgical reconstruction of the Achilles tendon but, unlike other models, preserves the sciatic nerve. We wanted to examine how distinct parts of the technique influenced healing.

Methods

For this animal model study, we used 96 wild-type male C57BL/6 mice aged 8–12 weeks. We evaluated different suture techniques and macroscopically confirmed the optimal combination of suture material and technique to minimize tendon gap formation. A key element is the detailed, step-by-step illustration of the surgery. In addition, we assessed histological (Herovici and Alcian blue staining) outcome parameters at 1–16 weeks postoperatively. Microcomputed tomography (micro-CT) was performed to measure the bone volume of heterotopic ossifications (HOs). Biomechanical analyses were carried out using a viscoelastic protocol on the biomechanical testing machine LM1.

Results

A modified 4-strand suture combined with a cerclage for immobilization without transection of the sciatic nerve reliably eliminated gap formation. The maximal dorsal extension of the hindlimb at the upper ankle joint from the equinus position (limited by the immobilization cerclage) increased over time postoperatively (operation: 28.8 ± 2.2°; 1 week: 54 ± 36°; 6 weeks: 80 ± 11.7°; 16 weeks: 96 ± 15.8°, p > 0.05). Histological staining revealed a maturation of collagen fibres within 6 weeks, whereas masses of cartilage were visible throughout the healing period. Micro-CT scans detected the development of HOs starting at 4 weeks and further progression at 6 and 16 weeks (bone volume, 4 weeks: 0.07604 ± 0.05286 mm³; 6 weeks: 0.50682 ± 0.68841 mm³; 16 weeks: 2.36027 ± 0.85202 mm³, p > 0.001). In-depth micro-CT analysis of the different surgical elements revealed that an injury of the tendon is a key factor for the development of HOs. Immobilization alone does not trigger HOs. Biomechanical properties of repaired tendons were greatly altered and remained inferior 6 weeks after surgery.

Conclusion

With this study, we demonstrated that the microsurgical technique greatly influences the short- and longer-term healing outcome. When the sciatic nerve is preserved, the best surgical reconstruction of the tendon defect is achieved by a 4-strand core suture in combination with a tibiofibular cerclage for postoperative immobilization. The cerclage promotes a gradual increase in the range of motion of the upper ankle joint, comparable with an early mobilization rehabilitation protocol. HO, as a key mechanism for poor tendon healing, is progressive and can be monitored early in the model.

The translational potential of this article

The study enhances the understanding of model dependent factors of healing. The described reconstruction technique provides a reproducible and translational rodent model for future Achilles tendon healing research. In combination with transgenic strains, it can be facilitated to advance therapeutic strategies to improve the clinical results of tendon injuries.

Biomechanical Testing of Murine Tendons

Tendon disorders are common, affect people of all ages, and are often debilitating. Standard treatments, such as anti-inflammatory drugs, rehabilitation, and surgical repair, often fail. In order to define tendon function and demonstrate efficacy of new treatments, the mechanical properties of tendons from animal models must be accurately determined. Murine animal models are now widely used to study tendon disorders and evaluate novel treatments for tendinopathies; however, determining the mechanical properties of mouse tendons has been challenging. In this study, a new system was developed for tendon mechanical testing that includes 3D-printed fixtures that exactly match the anatomies of the humerus and calcaneus to mechanically test supraspinatus tendons and Achilles tendons, respectively. These fixtures were developed using 3D reconstructions of native bone anatomy, solid modeling, and additive manufacturing. The new approach eliminated artifactual gripping failures (e.g., failure at the growth plate failure rather than in the tendon), decreased overall testing time, and increased reproducibility. Furthermore, this new method is readily adaptable for testing other murine tendons and tendons from other animals.

Calcaneal Tendon Plasticity Following Gastrocnemius Muscle Injury in Rat

Cross-talk between skeletal muscle and tendon is important for tissue homeostasis. Whereas the skeletal muscle response to tendon injury has been well-studied, to the best of our knowledge the tendon response to skeletal muscle injury has been neglected. Thus, we investigated calcaneal tendon extracellular matrix (ECM) remodeling after gastrocnemius muscle injury using a rat model. Wistar rats were randomly divided into four groups: control group (C; animals that were not exposed to muscle injury) and harvested at different time points post gastrocnemius muscle injury (3, 14, and 28 days) for gene expression, morphological, and biomechanical analyses. At 3 days post injury, we observed mRNA-level dysregulation of signaling pathways associated with collagen I accompanied with disrupted biomechanical properties. At 14 days post injury, we found reduced collagen content histologically accompanied by invasion of blood vessels into the tendon proper and an abundance of peritendinous sheath cells. Finally, at 28 days post injury, there were signs of recovery at the gene expression level including upregulation of transcription factors related to ECM synthesis, remodeling, and repair. At this time point, tendons also presented with increased peritendinous sheath cells, decreased adipose cells, higher Young’s modulus, and lower strain to failure compared to the uninjured controls and all post injury time points. In summary, we demonstrate that the calcaneal tendon undergoes extensive ECM remodeling in response to gastrocnemius muscle injury leading to altered functional properties in a rat model. Tendon plasticity in response to skeletal muscle injury merits further investigation to understand its physiological relevance and potential clinical implications.

Role of a combination dietary supplement containing mucopolysaccharides, vitamin C, and collagen on tendon healing in rats

Objective

The aim of this study was to investigate the effect of mucopolysaccharide, vitamin C, and collagen supplementation on the healing of Achilles tendon in rats.

Methods

Sixteen rats were separated into 2 groups. Both Achilles tendons of all rats were transected 5 mm above the insertion and repaired using a Kessler suture. After the surgical repair, the study group received the daily recommended amount of the supplement by gastric gavage, while the control group received a placebo. At the end of the third week, the animals were sacrificed. The biomechanical properties of the groups were compared with ultimate tensile strength and stiffness tests. The biological properties of the 2 groups were assessed with a histomorphometric comparison to determine the amount of collagen type I (COL1), proliferating cell nuclear antigen (PCNA), and transforming growth factor β1 (TGF-β1) expression in 3 different tissue subgroups (collagen matrix, tenocytes, and endotenon fibroblasts).

Results

Analysis of histomorphometric results revealed that the rats receiving dietary supplements demonstrated a significant increase in PCNA (mean value of 86 in the control group and 168.85 in the trial group; p < 0.05) and TGF-β1 (mean value of 87.57 in the control group and 161.85 in the trial group; p < 0.05) in the endotenon fibroblasts of the repair site. However, there was no difference between the groups in PCNA or TGF-β1 when the collagen matrix and the tenocytes of the repair site were examined. Furthermore, no significant difference could be found between groups in COL1 in any of the 3 tissue subgroups (collagen matrix, tenocytes, and endotenon fibroblasts). The statistical analysis also indicated that the rats receiving supplements did not demonstrate a significant increase in the ultimate tendon tensile strength or stiffness.

Conclusion

The results of this study revealed no advantage to the oral administration of the trial supplement in collagen synthesis or biomechanical properties in rats after 3 weeks using the presented study design. However, the increased expression of PCNA and TGFβ1 seen in the endotenon fibroblasts of the repair site might play a role in the continuum of tendon healing.

Biomechanical properties and histology of db/db diabetic mouse Achilles tendon

Foot ulcers are a severe complication of diabetic patients resulting from nerve and tendon pathologic alterations. In diabetic patients the tendons are thicker, shorter and have increased stiffness. We examined C57BL/KsJ (BKS.Cg-Dock7(m) +/+ Lepr (db) /J) (db/db) mice tendons to determine whether they are an animal model for human diabetic tendon changes. We hypothesized that the Achilles tendons of db/db diabetic mice would be thicker, stiffer, fail at lower loads and stresses, and have degenerative changes compared to control mice. Biomechanical and histologic analyses of the Achilles tendons of 16 week old db/db and control male mice were performed. There was a significant increase in tendon diameter and significant decreases in maximum load, tensile stress, stiffness and elastic modulus in tendons from diabetic mice compared to controls. Mild degenerative and neutrophil infiltration was observed near the tendon insertions on the calcaneous in 25% of db/db mice. In summary, hyper-glycemia and obesity lead to severe changes in db/db mice will be a useful model to examine mechanisms for tendon alterations.

The effects of high-fat diet, branched-chainamino acids and exercise on female C57BL/6 mouse Achilles tendon biomechanical properties

Objectives

The goals of this study were: 1) to determine if high-fat diet (HFD) feeding in female mice would negatively impact biomechanical and histologic consequences on the Achilles tendon and quadriceps muscle; and 2) to investigate whether exercise and branched-chain amino acid (BCAA) supplementation would affect these parameters or attenuate any negative consequences resulting from HFD consumption.

Methods

We examined the effects of 16 weeks of 60% HFD feeding, voluntary exercise (free choice wheel running) and BCAA administration in female C57BL/6 mice. The Achilles tendons and quadriceps muscles were removed at the end of the experiment and assessed histologically and biomechanically.

Results

HFD feeding significantly decreased the Achilles tendon modulus without histological alterations. BCAA administration significantly decreased the stiffness of Achilles tendons in the exercised normal diet mice. Exercise partially ameliorated both the weight gain and glucose levels in the HFD-fed mice, led to a significant decrease in the maximum load of the Achilles tendon, and an increase in the average fibril diameter of the quadriceps femoris muscle. There were significant correlations between body weight and several biomechanical properties, demonstrating the importance of controlling obesity for maintaining healthy tendon properties.

Conclusions

In summary, this study showed a significant impact of obesity and body weight on tendon biomechanical properties with limited effects of exercise and BCAAs.

Cite this article: Bone Joint Res 2013;2:186–92.

A novel, inexpensive and easy to use tendon clamp for in vitro biomechanical testing

Frozen clamps can hold tendons and ligaments tightly and transmit high loads, from 4 kN to 13 kN, without slippage, yet they are complex and expensive. The existing non-frozen serrated jaw clamp is simple to fabricate and use, but the maximal tensile force it can sustain is only about 2.5 kN, which is not enough in many biomechanical tests. In this study, a new type of non-frozen clamp, which has lateral block boards and asymmetrical teeth jaws, was designed. The lateral block boards made of titanium alloy were used to prevent the soft tissues from being squeezed out during compressing, while the asymmetrical teeth jaws made of nylon were used to grip and keep holding soft tissues. The capability of this new type of clamp was tested by stretching five cattle tendons to failure on the tensile and compression testing machine, none of them displayed any slippage before rupture, the maximum tension force was 6.87 kN. This non-frozen asymmetrical teeth jaw clamp was designed for gripping tendons in foot and ankle dynamic simulation test, but it can also be applied to other in vitro tests, such as hip and knee dynamic tests.

Biomechanical responses of different rat tendons to nandrolone decanoate and load exercise

Androgenic-anabolic steroids (AAS) have been associated with an increased incidence of tendon rupture. The aim of this study was to compare the biomechanical properties of the rat calcaneal tendon (CT), superficial flexor tendon (SFT), and deep flexor tendon (DFT), and to determine the effect of jump training in association with AAS. Animals were separated into four groups: sedentary, trained, AAS-treated sedentary rats (AAS), and AAS-treated and trained animals. Mechanical testing showed that the CT differed from the DFT and SFT, which showed similar mechanical properties. Jump caused the CT to exhibit an extended toe region, an increased resistance to tensional load, and a decreased elastic modulus, characteristics of an elastic tendon capable of storing energy. AAS caused the tendons to be less compliant, and the effects were reinforced by simultaneous training. The DFT was the most affected by training, AAS, and the interaction of both, likely because of its involvement in the toe-off step of jumping, which we suggest is related to the rapid transmission of force as opposed to energy storage. In conclusion, tendons are differently adapted to exercise, but responded equally to AAS, showing reduced flexibility, which is suggested to increase the risk of tendon rupture in AAS consumers.

The tibialis tendon as a valuable anterior cruciate ligament allograft substitute: biomechanical properties

The study evaluates the biomechanical properties of single-strand and single-loop tibialis (anterior and posterior) tendon allografts. A comparison was made with bone-patellar tendon-bone (BPTB) allografts. Sixty-four tendon allografts were evaluated in this study. Sixteen of these were single-strand tibialis anterior (TA) and 16 single-strand tibialis posterior (TP) tendons. Sixteen single-loop TA and TP tendons were also tested. The fourth group was composed of 16 BPTB allografts. The biomechanical properties determined were maximal load, stiffness, cross-sectional area and elongation. The results of this study showed that the maximal load of the single-loop tibialis tendons (1,553 +/- 62 N) was greater than of the BPTB (1,139 +/- 99 N), TA (776 +/- 43 N) and TP (888 +/- 64 N) tendons. The stiffness of the single-loop tibialis tendons (236 +/- 10 N/mm) was also greater than of the BPTB (168 +/- 13 N/mm), TA (60 +/- 2 N/mm) and TP (73 +/- 5 N/mm) tendons. The cross-sectional area of the BPTB tendons was 67 +/- 5 mm(2), of the single-loop tibialis tendons 36 +/- 2 mm(2), of the TA tendons 20 +/- 1 mm(2), and of the TP tendons 23 +/- 1 mm(2). The elongation of the single-loop tibialis tendons and of the BPTB tendons was almost similar (7 +/- 0.4 mm). The same applied to the TA and TP tendons (14 +/- 0.6 mm). The results of this in vitro mechanical study suggest that fresh-frozen single-loop TA and TP tendons, and BPTB allografts are an acceptable substitute for hamstrings in anterior cruciate ligament reconstruction.

Early voluntary exercise does not promote healing in a rat model of Achilles tendon injury

Mechanical stress is an important modulator of connective tissue repair. However, the effects on tendon healing are very poorly defined, preventing optimal use of mechanical stress. We hypothesized that early voluntary exercise initially retards tendon repair but results in a faster recovery rate at longer term. Male Wistar rats were injured by a collagenase injection in the Achilles tendon, and exercise was voluntarily performed on a running wheel. We observed the persistent presence of neutrophils in injured tendons of rats that began exercise immediately after the trauma [injured + early exercise (Inj+EEx)]. Early exercise also increased the concentration of ED1(+) macrophages in injured tendons after 3 and 7 days compared with ambulatory injured rats (Inj). Similar results were obtained with the subset of ED2(+) macrophages in the tendon core 3 days after the collagenase injection. Furthermore, collagen content returned to normal values more rapidly in the Inj+EEx tendons than in the Inj group, but this was not associated with an increase in cell proliferation. Surprisingly, Inj+EEx tendons roughly displayed lower stiffness and force at rupture point relative to Inj tendons at day 28. Injured tendons of rats that began exercise only from day 7 had better mechanical properties than those of early-exercised rats 28 days postinjury. We speculate that the persistence of the inflammatory response and undue mechanical loading in the Inj+EEx tendons led to fibrosis and a loss of tendon function.

The Effects of Substance P on the Biomechanic Properties of Ruptured Rat Achilles’ Tendon

OBJECTIVE

To determine whether injection of substance P into the paratendinous region of a ruptured and subsequently sutured rat Achilles' tendon alters the biomechanic properties of the tendon.

DESIGN

Interventional animal study.

SETTING

Animal laboratory at a university hospital.

ANIMALS

Ninety-six 2-month-old, male Sprague-Dawley rats.

INTERVENTION

Injection of saline, substance P (10(-6)micromol/kg of body weight [BW] or 10(-8)micromol/kg BW) associated with neutral endopeptidase inhibitors, or neutral endopeptidase inhibitors alone into the paratendinous region of ruptured and subsequently sutured rat Achilles' tendons from the second until the sixth day postoperatively.

MAIN OUTCOME MEASURES

Stress at maximal load and work to maximal load and stiffness.

RESULTS

Stress at maximal load was higher in the groups injected with substance P than in the saline group in the first, second, and sixth weeks. Work to maximal load was higher from the second until the sixth weeks in the substance P-treated groups than in the saline group. Stiffness did not differ between the 4 groups in any of the weeks.

CONCLUSIONS

Injection of substance P into the paratendinous region of ruptured and subsequently sutured rat Achilles' tendons improved tendon healing by enhancing stress at maximal load and work to maximal load. However, stiffness was not significantly affected.

Tendon healing in interleukin-4 and interleukin-6 knockout mice

Cytokines have been shown to play an important role in tendon and ligament healing by regulating cellular differentiation and activity. The majority of studies that have investigated the role of cytokines in tendon and ligament healing have added them to injured tissue and assessed their effect. Because the efficacy of exogenously applying cytokines is dependent upon many factors such as the correct dosage, timing, and frequency, conflicting results are often reported. To avoid these factors, this study used transgenic mice with knockouts of interleukin-4 (IL4 -/-) and interleukin-6 (IL6 -/-) to investigate their role in tendon healing. Because of the reported roles of both of these cytokines in inflammation and fibroplasia, it was hypothesized that the order of organizational, geometric, and mechanical properties would be (greatest to least) injured IL6 -/-, injured control, and injured IL4 -/- mice. In addition, it was hypothesized that specific cytokines would be upregulated in each knockout group, but not compensate for the lack of IL-4 or IL-6. Mechanical and organizational properties of injured tendons from IL6 -/- mice were inferior to that of control and IL4 -/- mice despite the upregulation of the pro-inflammatory cytokine TNF-alpha. Temporal levels of IL-10 and IL-13 in the IL4 -/- mice resulted in comparable and even superior properties when compared to CTL mice. This study shows that IL-6 could not be compensated for and plays an important role in tendon healing. This study also supports the use of this animal model to further investigate tendon healing.

Influence of decorin and biglycan on mechanical properties of multiple tendons in knockout mice

Evaluations of tendon mechanical behavior based on biochemical and structural arrangement have implications for designing tendon specific treatment modalities or replacement strategies. In addition to the well studied type I collagen, other important constituents of tendon are the small proteoglycans (PGs). PGs have been shown to vary in concentration within differently loaded areas of tendon, implicating them in specific tendon function. This study measured the mechanical properties of multiple tendon tissues from normal mice and from mice with knock-outs of the PGs decorin or biglycan. Tail tendon fascicles, patellar tendons (PT), and flexor digitorum longus tendons (FDL), three tissues representing different in vivo loading environments, were characterized from the three groups of mice. It was hypothesized that the absence of decorin or biglycan would have individual effects on each type of tendon tissue. Surprisingly, no change in mechanical properties was observed for the tail tendon fascicles due to the PG knockouts. The loss of decorin affected the PT causing an increase in modulus and stress relaxation, but had little effect on the FDL. Conversely, the loss of biglycan did not significantly affect the PT, but caused a reduction in both the maximum stress and modulus of the FDL. These results give mechanical support to previous biochemical data that tendons likely are uniquely tailored to their specific location and function. Variances such as those presented here need to be further characterized and taken into account when designing therapies or replacements for any one particular tendon.

Tendon properties in interleukin-4 and interleukin-6 knockout mice

Cytokines are known to play an important role in normal tendon development, function, and maintenance through interactions with fibroblasts and extracellular matrix proteins. However, the role of interleukins on normal tendon activity remains poorly understood. Previous studies that have researched the role of specific cytokines by exogenously applying them have often reported conflicting results. Therefore, a knockout mouse model was used to investigate the role of interleukins 4 and 6 on normal tendon organizational and biomechanical properties. It was hypothesized that interleukin-6 knockout (IL6 -/-) mice will display more organized collagen orientation and greater cross-sectional area and mechanical properties when compared to that of control mice. In addition, interleukin-4 knockout (IL4 -/-) mice will display the most disorganized collagen orientation and lowest cross-sectional area and mechanical properties. As hypothesized, IL6 -/- mice show a trend towards lower angular deviation (more organized) (p<0.1) when compared to IL4 -/- mice. In addition, the IL6 -/- mice show a trend towards a higher percent relaxation (p<0.1) and a significantly higher modulus (p<0.01) when compared to CTL and IL4 -/- mice. Unexpectedly, the IL6 -/- mice exhibited no significant differences in collagen fiber distribution and maximum stress from the other groups and actually had a smaller cross-sectional area than CTL mice (p<0.1). This study supports transgenic mice as an animal model for investigating how cytokines affect normal tendon properties. In addition, this study demonstrates that interleukins may play an important role in tendon development, function, and maintenance.

Achilles tendon healing: long-term biomechanical effects of postoperative mobilization and immobilization in a new mouse model

The aim of the study was to investigate the long-term effects of postoperative immobilization as opposed to mobilization on the biomechanical attributes of healing Achilles tendons in a new experimental mouse model. In 114 Balb-C-mice the left Achilles tendon was transected and sutured by the Kirchmayr-Kessler technique. The tendons healed either under postoperative immobilization effected by fixing the upper ankle joint in equinus position or under mobilization through a limited range of movement. The contralateral Achilles tendons served as internal control. All tendons were tested biomechanically at short intervals up to the 112th postoperative day in terms of load to failure [N], tendon deflection [mm] and tendon stiffness [N/mm], and were evaluated histologically after 8 and 112 days. Postoperative mobilization resulted in a continuous and significantly more rapid restoration of load to failure in comparison to the immobilization group. Tendon deflection was decreased by postoperative mobilization, whereas under immobilization it paradoxically increased still further in the later course. After 112 days the tendons of the mobilization group had regained their original tendon stiffness, whereas the tendons after immobilization reached only about half the values seen in the control tendons. Histologically, postoperative mobilization led to increased immigration of inflammatory cells in the early phase. In the late phase, as compared to immobilization, tendon structure was more mature, with fibre bundles arranged in parallel and interposed tendocytes. Tensile loading of the healing tendon by postoperative mobilization leads to fundamental changes in the biological process of tendon healing resulting in accelerated restoration of load to failure and reduced tendon deflection.