Mechanical properties of collagen fascicles from the rabbit patellar tendon

Tensile and viscoelastic properties of collagen fascicles of approximately 300 microns in diameter, which were obtained from rabbit patellar tendons, were studied using a newly designed micro-tensile tester. Their cross-sectional areas were determined with a video dimension analyzer combined with a CCD camera and a low magnification microscope. There were no statistically significant differences in tensile properties among the fascicles obtained from six medial-to-lateral locations of the patellar tendon. Tangent modulus, tensile strength, and strain at failure of the fascicles determined at about 1.5 percent/s strain rate were 216 +/- 68 MPa, 17.2 +/- 4.1 MPa, and 10.9 +/- 1.6 percent (mean +/- S.D.), respectively. These properties were much different from those of bulk patellar tendons; for example, the tensile strength and strain at failure of these fascicles were 42 percent and 179 percent of those of bulk tendons, respectively. Tangent modulus and tensile strength of collagen fascicles determined at 1 percent/s strain rate were 35 percent larger than those at 0.01 percent/s. The strain at failure was independent of strain rate. Relaxation tests showed that the reduction of stress was approximately 25 percent at 300 seconds. These stress relaxation behavior and strain rate effects of collagen fascicles differed greatly from those of bulk tendons. The differences in tensile and viscoelastic properties between fascicles and bulk tendons may be attributable to ground substances, mechanical interaction between fascicles, and the difference of crimp structure of collagen fibrils.     

Effects of static stress on the mechanical properties of cultured collagen fascicles from the rabbit patellar tendon

In-vitro tissue culture experiments were performed to study the effects of static stress on the mechanical properties of collagen fascicles obtained from the rabbit patellar tendon. After collagen fascicles having the diameter of approximately 300 microm were cultured for 1 and 2 wk under static stress between 0 and 3 MPa, their mechanical properties and crimp morphology were determined using a micro-tensile tester and a light microscope, respectively. The tensile strength and tangent modulus of the fascicles were significantly decreased by culture under no load compared to control fascicles. A statistically significant correlation, which was described by a quadratic curve, was observed between applied stress and tensile strength. The maximum tensile strength (16.7 MPa) was obtained at the applied stress of 1.2 MPa; the strength was within a range of control values. There was a similar correlation between applied stress and tangent modulus, and the modulus was maintained at control level under 1.3 MPa stress. The stress of 1.2 to 1.3 MPa is equivalent to approximately 50 percent of the peak stress developed in the intact rabbit patellar tendon by running. Strain at failure of cultured collagen fascicles was negatively correlated with applied stress, and that at 1.2 to 1.3 MPa stress was almost the same as the control value. Crimp morphology in the fascicles cultured under about 1.2 MPa stress was similar to that in control fascicles. These results indicate that cultured collagen fascicles change the mechanical properties and structure in response to static tensile stress. In addition, their mechanical properties and structure are maintained at control level if the static stress of 50 percent of in-vivo peak stress is applied.     

Effects of cyclic stress on the mechanical properties of cultured collagen fascicles from the rabbit patellar tendon

Effects of cyclic stress on the mechanical properties of collagen fascicles were studied by in vitro tissue culture experiments. Collagen fascicles (approximately 300 microns in diameter) obtained from the rabbit patellar tendon were applied cyclic load at 4 Hz for one hour per day during culture period for one or two weeks, and then their mechanical properties were determined using a micro-tensile tester. There was a statistically significant correlation between tensile strength and applied peak stress in the range of 0 to 5 MPa, and the relation was expressed by a quadratic function. The maximum strength (19.4 MPa) was obtained at the applied peak stress of 1.8 MPa. The tensile strength of fascicles were within a range of control values, if they were cultured under peak stresses between 1.1 and 2.6 MPa. Similar results were also observed in the tangent modulus, which was maintained at control level under applied peak stresses between 0.9 and 2.8 MPa. The stress of 0.9 to 1.1 MPa is equivalent to approximately 40% of the in vivo peak stress which is developed in the intact rabbit patellar tendon by running, whereas that of 2.6 to 2.8 MPa corresponds to approximately 120% of the in vivo peak stress. Therefore, the fascicles cultured under applied peak stresses of lower than 40% and higher than 120% of the in vivo peak stress do not keep the original strength and modulus. These results indicate that the mechanical properties of cultured collagen fascicles strongly depend upon the magnitude of the stress applied during culture, which are similar to our previous results observed in stress-shielded and overstressed patellar tendons in vivo.     

Effects of the frequency and duration of cyclic stress on the mechanical properties of cultured collagen fascicles from the rabbit patellar tendon

The effects of frequency or duration of cyclic stress on the mechanical properties of collagen fascicles were studied by means of in vitro tissue culture experiments. Collagen fascicles of approximately 300 microm in diameter were obtained from rabbit patellar tendons. During culture, cyclic stress having the peak stress of approximately 2 MPa was applied to the fascicles at 1 Hz for 1 hour/day (1 Hz-1 h group), at 1 Hz for 4 hours/day (1 Hz-4 h group), or at 4 Hz for 1 hour/day (4 Hz-1 h group). The frequency of 4 Hz and the duration of 1 hour/day are considered to be similar to those of the in vivo stress applied to fascicles in the intact rabbit patellar tendon. After culture for 1 or 2 weeks, the mechanical properties of the fascicles were determined using a microtensile tester, and were compared to the properties of non-cultured, fresh fascicles (control group) and the fascicles cultured under no load condition (non-loaded group). The tangent modulus and tensile strength of fascicles in the 4 Hz-1 h group were similar to those in the control group; however, the fascicles of the 1 Hz-1 h and 1 Hz-4 h groups had significantly lower values than those of the control group. There was no significant difference in the tensile strength between the 1 Hz-1 h and non-loaded groups, although the strength in the 1 Hz-4 h group was significantly higher than that of the non-loaded group. It was concluded that the frequency and duration of cyclic stress significantly affect the mechanical properties of cultured collagen fascicles. If we apply cyclic stress having the frequency and duration which are experienced in vivo, the biomechanical properties are maintained at control, normal level. Lower frequencies or less cycles of applied force induce adverse effects.     

Biomechanical response of collagen fascicles to restressing after stress deprivation during culture

In vitro tissue culture experiments were performed to study the biomechanical response of collagen fascicles to restressing after exposure to non-loaded condition. Collagen fascicles of approximately 300 microm in diameter were aseptically dissected from rabbit patellar tendons. They were cultured under no-load condition for 1 week, and then under a static stress of approximately 1.2 MPa for the subsequent 1 or 2 weeks. After culture, their mechanical properties were determined with a micro-tensile tester, and were compared to those of fascicles cultured under no-load condition and non-cultured, control fascicles. Tangent modulus and tensile strength of the non-loaded fascicles were significantly lower than those of the control fascicles at 1 week and gradually decreased thereafter. However, the modulus and strength were increased by restressing. After 2-week restressing, both parameters were significantly greater than those of the time-matched, non-loaded fascicles, although these values were still significantly lower than those of the control fascicles. That is, the application of stress after exposure to non-loaded condition suppressed the deterioration of the biomechanical properties of fascicles, although it did not improve. These results indicate that a short period of stressing is not sufficient for cultured collagen fascicles to completely recover their mechanical properties, if they are once exposed to no-stress condition even for a short period of time. These are similar to previous results observed in tendons and ligaments inside the body.     

Antibody neutralization of TGF-beta enhances the deterioration of collagen fascicles in a tissue-cultured tendon matrix with ex vivo fibroblast infiltration

A tissue-cultured tendon matrix infiltrated with cultured fibroblasts can be regarded as an ideal tissue-engineered tendon model. To clarify the role of TGF-beta in a tissue-cultured tendon matrix during ex vivo cellular infiltration, the present ex vivo study was conducted to test the following hypothesis that antibody neutralization of TGF-beta enhances weakening of the collagen fascicles of the patellar tendon matrix in response to ex vivo fibroblast infiltration. In skeletally mature female rabbits, fibroblasts were isolated from the right patellar tendons using an explant culture technique, and the left patellar tendons underwent multiple freeze/thaw treatment with liquid nitrogen to obtain an acellular tendon matrix. Each acellular tendon was placed in a collagen gel containing cultured fibroblasts and then incubated with or without anti-TGF-beta1 antibody for 6 weeks. We found that antibody neutralization of TGF-beta enhanced the decrease in the tensile strength and tensile modulus of the collagen fascicles of the patellar tendon matrix in response to ex vivo fibroblast infiltration. The present study indicates a possibility that TGF-beta may have a role in suppressing the material deterioration of the fascicles in the tendon during ex vivo cellular infiltration.     

Biomechanical studies of the rabbit patellar tendon after removal of its one-fourth or a half.

Effects of the overstressing induced by the harvest of grafts from the patellar tendon on the mechanical properties and morphometry of remaining tendon were studied using a rabbit model. The width of the patellar tendon was reduced by one-fourth or one-half equally removing the medial and lateral portions; by this surgery, the cross-sectional area was decreased by 25 or 50 percent from the original area. After all the rabbits were allowed unrestricted activities in cages for 3 to 12 weeks, their patellar tendons were harvested for mechanical and histological studies. The one-fourth removal induced no significant changes in the mechanical properties, but significantly increased the cross-sectional area. In the case of one-half removal, tensile strength and tangent modulus did not change in some tendons, although the cross-sectional area increased significantly. In the other central half tendons, mechanical strength decreased markedly, while the cross-sectional area increased; hypercellular areas and breakage of collagen bundles were observed in these tendons. These results indicate that the patellar tendon has an ability of functionally adapting to overstressing by changing the cross-sectional area, while keeping the mechanical properties unchanged, if the extent of overstressing is less than 30 percent.     

Effects of stress shielding on the transverse mechanical properties of rabbit patellar tendons

With the aim of studying mechanisms of the remodeling of tendons and ligaments, the effects of stress shielding on the rabbit patellar tendon were studied by performing tensile and stress relaxation tests in the transverse direction. The tangent modulus, tensile strength, and strain at failure of non-treated, control patellar tendons in the transverse direction were 1272 kPa, 370 kPa, and 40.5 percent, respectively, whereas those of the tendons stress-shielded for 1 week were 299 kPa, 108 kPa, and 40.4 percent, respectively. Stress shielding markedly decreased tangent modulus and tensile strength in the transverse direction, and the decreases were larger than those in the longitudinal direction, which were determined in our previous study. For example, tensile strength in the transverse and longitudinal direction decreased to 29 and 50 percent of each control value, respectively, after 1 week stress shielding. In addition, the stress relaxation in the transverse direction of stress-shielded patellar tendons was much larger than that of nontreated, control ones. In contrast to longitudinal tensile tests for the behavior of collagen, transverse tests reflect the contributions of ground substances such as proteoglycans and mechanical interactions between collagen fibers. Ground substances provide lubrication and spacing between fibers, and also confer viscoelastic properties. Therefore, the results obtained from the present study suggest that ground substance matrix, and interfiber and fiber-matrix interactions have important roles in the remodeling response of tendons to stress.     

Local administration of interleukin-1 receptor antagonist inhibits deterioration of mechanical properties of the stress-shielded patellar tendon

We previously found that interleukin (IL)-1beta is over-expressed in the fibroblasts of the stress-shielded patellar tendon using a stress-shielding model [Uchida, H., Tohyama, H., Nagashima, K., Ohba, Y., Matsumoto, H., Toyama, Y., Yasuda, K., 2005. Stress deprivation simultaneously induces over-expression of interleukin-1beta, tumor necrosis factor-alpha, and transforming growth factor-beta in fibroblasts and mechanical deterioration of the tissue in the patellar tendon. Journal of Biomechanics 38(4), 791-798.]. Therefore, IL-1beta may play a role in tendon deterioration in response to stress deprivation. This study was conducted to clarify the effects of local administration of interleukin-1 receptor antagonist (IL-1ra) on the mechanical properties of the stress-shielded patellar tendon as well as the tendon fascicles harvested from it. Twenty-six mature rabbits were equally divided into Groups IL-1ra and PBS after the right patellar tendon underwent the stress-shielding treatment, which completely released the patellar tendon from tension by stretching the flexible wire installed between the patella and the tibial tubercle. In Group IL-1ra, IL-1ra was injected between the patellar tendon and the infra-patellar fat pad. In Group PBS, phosphate-buffered saline was injected in the same manner as IL-1ra. All rabbits were evaluated at 3 weeks after the stress-shielding procedure. The tangent modulus and the tensile strength of the patellar tendons were significantly greater in Group IL-1ra than in Group PBS, while there was no significant difference in the strain at failure between Groups IL-1ra and PBS. Concerning the mechanical properties of the fascicles harvested from the patellar tendon, however, we could not detect any significant differences in the tangent modulus, tensile strength, or strain at failure between Groups IL-1ra and PBS. The present study suggested that IL-1 plays an important role in the deterioration of the mechanical properties of the patellar tendon in response to stress shielding and that IL-1 does not affect the fascicles themselves.     

Multi-scale characterization of swine femoral cortical bone

Multi-scale experimental work was carried out to characterize cortical bone as a heterogeneous material with hierarchical structure, which spans from nanoscale (mineralized collagen fibril), sub-microscale (single lamella), microscale (lamellar structures), to mesoscale (cortical bone) levels. Sections from femoral cortical bone from 6, 12, and 42 months old swine were studied to quantify the age-related changes in bone structure, chemical composition, and mechanical properties. The structural changes with age from sub-microscale to mesoscale levels were investigated with scanning electron microscopy and micro-computed tomography. The chemical compositions at mesoscale were studied by ash content method and dual energy X-ray absorptiometry, and at microscale by Fourier transform infrared microspectroscopy. The mechanical properties at mesoscale were measured by tensile testing, and elastic modulus and hardness at sub-microscale were obtained using nanoindentation. The experimental results showed age-related changes in the structure and chemical composition of cortical bone. Lamellar bone was a prevalent structure in 6 months and 12 months old animals, resorption sites were most pronounced in 6 months old animals, while secondary osteons were the dominant features in 42 months old animals. Mineral content and mineral-to-organic ratio increased with age. The structural and chemical changes with age corresponded to an increase in local elastic modulus, and overall elastic modulus and ultimate tensile strength as bone matured.     

Mechanical properties of the canine patellar tendon: some correlations with age and the content of collagen.

Portions of the patellar tendon (PT) are currently used for autogenous and allogeneic reconstruction of a torn or damaged anterior cruciate ligament (ACL). Age-related changes in the mechanical properties of the PT may influence its use in this reconstruction procedure. Age-dependent changes in the PT were determined in the dog, which is often used to experimentally study this reconstruction. Tensile failure experiments were performed at 100% s-1 on patella-patellar tendon-tibia preparations from dogs aged 0.5-15 yr. The contents of collagen soluble and insoluble in pepsin were also measured at each age. Fifty-nine percent (16/27) of the preparations failed by avulsion at the patella, but neither the failure load nor the mode of failure were a function of age. Failure load and energy were higher for tendon substance failures compared to avulsions of bone from the patella. While a positive, linear correlation was measured between tensile modulus of the PT and age, the slope of regression was not significantly different from zero. The content of total collagen in the PT decreased significantly with age. The content of collagen insoluble in pepsin, however, increased with age and positively correlated with tensile modulus of the tendon. These results are different from those reported for the canine CCL, by others, which degenerates with age. Age-related changes in the mechanical properties of the canine PT are qualitatively similar to earlier, limited data on human patellar tendons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Collagen fibril diameter distribution does not reflect changes in the mechanical properties of in vitro stress-deprived tendons

The purpose of this study was to determine if an association exists between the tensile properties and the collagen fibril diameter distribution in in vitro stress-deprived rat tail tendons. Rat tail tendons were paired into two groups of 21 day stress-deprived and 0 time controls and compared using transmission electron microscopy (n = 6) to measure collagen fibril diameter distribution and density, and mechanical testing (n =6) to determine ultimate stress and tensile modulus. There was a statistically significant decrease in both ultimate tensile strength (control: 17.95+/-3.99 MPa, stress-deprived: 6.79+/-3.91 MPa) and tensile modulus (control: 312.8+/-89.5 MPa, stress-deprived: 176.0+/-52.7 MPa) in the in vitro stress-deprived tendons compared to controls. However, there was no significant difference between control and stress-deprived tendons in the number of fibrils per tendon counted, mean fibril diameter, mean fibril density, or fibril size distribution. The results of this study demonstrate that the decrease in mechanical properties observed in in vitro stress-deprived rat tail tendons is not correlated with the collagen fibril diameter distribution and, therefore, the collagen fibril diameter distribution does not, by itself, dictate the decrease in mechanical properties observed in in vitro stress-deprived rat tail tendons.     

Age-dependent changes in the mechanical properties of tail tendons in TGF-beta inducible early gene-1 knockout mice.

The purpose of this study is to investigate age-dependent changes in the architecture and mechanical properties of tendon in TGF-beta inducible early gene-1 (TIEG) knockout mice. Wild-type and TIEG knockout mice, aged 1, 2, and 15 mo, were used. The mechanical properties of tail tendons isolated from these mice were determined using uniaxial tensile ramp (0.05 mm/s) and relaxation (5 mm/s) tests, with a strain of 10%. Mechanical parameters (Young's modulus from the ramp test; fast and static stresses from the relaxation test) were measured and recorded. The structure of the tail tendon fascicle was characterized by transmission electron microscopy. The results of the mechanical testing revealed no significant difference between the knockout and wild-type groups at 1 or 15 mo of age. However, the fascicles of the knockout mice at 3 mo of age exhibited decreased fast and static stresses compared with those of the wild-type mice. Electron microscopy revealed an increase in fibril size in the knockout mouse tendons relative to wild-type controls at 1 and 3 mo of age. These data indicate an important role for TIEG in tendon microarchitecture and strength in adult mice.     

Regional stiffening with aging in tibialis anterior tendons of mice occurs independent of changes in collagen fibril morphology

The incidence of tendon degeneration and rupture increases with advancing age. The mechanisms underlying this increased risk remain unknown but may arise because of age-related changes in tendon mechanical properties and structure. Our purpose was to determine the effect of aging on tendon mechanical properties and collagen fibril morphology. Regional mechanical properties and collagen fibril characteristics were determined along the length of tibialis anterior (TA) tendons from adult (8- to 12-mo-old) and old (28- to 30-mo-old) mice. Tangent modulus of all regions along the tendons increased in old age, but the increase was substantially greater in the proximal region adjacent to the muscle than in the rest of the tendon. Overall end-to-end modulus increased with old age at maximum tendon strain (799 ± 157 vs. 1,419 ± 91 MPa) and at physiologically relevant strain (377 ± 137 vs. 798 ± 104 MPa). Despite the dramatic changes in tendon mechanical properties from adulthood to old age, collagen fibril morphology and packing fraction remained relatively constant in all tendon regions examined. Since tendon properties are influenced by their external loading environment, we also examined the effect of aging on TA muscle contractile properties. Maximum isometric force did not differ between the age groups. We conclude that TA tendons stiffen in a region-dependent manner throughout the life span, but the changes in mechanical properties are not accompanied by corresponding changes in collagen fibril morphology or force-generating capacity of the TA muscle.     

Influence of decorin on the mechanical, compositional, and structural properties of the mouse patellar tendon

The interactions of small leucine-rich proteoglycans (SLRPs) with collagen fibrils, their association with water, and their role in fibrillogenesis suggests that SLRPs may play an important role in tendon mechanics. Some studies have assessed the role of SLRPs in the mechanical response of the tendon, but the relationships between sophisticated mechanics, assembly of collagen, and SLRPs have not been well characterized. Decorin content was varied in a dose dependent manner using decorin null, decorin heterozygote, and wild type mice. Quantitative measures of mechanical (tension and compression), compositional, and structural changes of the mouse patellar tendon were evaluated. Viscoelastic, tensile dynamic modulus was increased in the decorin heterozygous tendons compared to wild type. These tendons also had a significant decrease in total collagen and no structural changes compared to wild type. Decorin null tendons did not have any mechanical changes; however, a significant decrease in the average fibril diameter was found. No differences were seen between genotypes in elastic or compressive properties, and all tendons demonstrated viscoelastic mechanical dependence on strain rate and frequency. These results suggest that decorin, a member of the SLRP family, plays a role in tendon viscoelasticity that cannot be completely explained by its role in collagen fibrillogenesis. In addition, reductions in decorin do not cause large changes in indentation compressive properties, suggesting that other factors contribute to these properties. Understanding these relationships may ultimately help guide development of tissue engineered constructs or treatment modalities.     

Lateral force transmission between human tendon fascicles.

Whether adjacent collagen fascicles transmit force in parallel is unknown. The purpose of the present study was to examine the magnitude of lateral force transmission between adjacent collagen fascicles from the human patellar and Achilles tendon. From each sample two adjacent strands of fascicles (phi 300-530 mum) enclosed in a fascicular membrane were dissected. The specimen was deformed to approximately 3% strain in three independent load-displacement cycles in a small-scale tensile testing device. Cycle 1: the fascicles and the fascicular membrane were intact. Cycle 2: one fascicle was transversally cut while the other fascicle and the fascicular membrane were kept intact. Cycle 3: both fascicles were cut in opposite ends while the fascicular membrane was left intact. A decline in peak force of 45% and 55% from cycle 1 to cycle 2, and 93% and 92% from cycle 2 to cycle 3 was observed in the patellar and Achilles tendon fascicles, respectively. A decline in stiffness of 39% and 60% from cycle 1 to cycle 2, and of 93% and 100% from cycle 2 to cycle 3 was observed in the patellar and Achilles tendon fascicles, respectively. The present data demonstrate that lateral force transmission between adjacent collagen fascicles in human tendons is small or negligible, suggesting that tendon fascicles largely act as independent structures and that force transmission principally takes place within the individual fascicles.     

Tensile properties of human collagen fibrils and fascicles are insensitive to environmental salts

To carry out realistic in vitro mechanical testing on anatomical tissue, a choice has to be made regarding the buffering environment. Therefore, it is important to understand how the environment may influence the measurement to ensure the highest level of accuracy. The most physiologically relevant loading direction of tendon is along its longitudinal axis. Thus, in this study, we focus on the tensile mechanical properties of two hierarchical levels from human patellar tendon, namely: individual collagen fibrils and fascicles. Investigations on collagen fibrils and fascicles were made at pH 7.4 in solutions of phosphate-buffered saline at three different concentrations as well as two HEPES buffered solutions containing NaCl or NaCl + CaCl₂. An atomic force microscope technique was used for tensile testing of individual collagen fibrils. Only a slight increase in relative energy dissipation was observed at the highest phosphate-buffered saline concentration for both the fibrils and fascicles, indicating a stabilizing effect of ionic screening, but changes were much less than reported for radial compression. Due to the small magnitude of the effects, the tensile mechanical properties of collagen fibrils and fascicles from the patellar tendon of mature humans are essentially insensitive to environmental salt concentration and composition at physiological pH.     

Ageing changes in the tensile properties of tendons: influence of collagen fibril volume fraction

Connective tissues are biological composites comprising of collagen fibrils embedded in (and reinforcing) the hydrated proteoglycan-rich (PG) gel within the extracellular matrices (ECMs). Age-related changes to the mechanical properties of tissues are often associated with changes to the structure of the ECM, namely, fibril diameter. However, quantitative attempts to correlate fibril diameter to mechanical properties have yielded inconclusive evidence. Here, we described a novel approach that was based on the rule of mixtures for fiber composites to evaluate the dependence of age-related changes in tendon tensile strength (sigma) and stiffness (E) on the collagen fibril cross-sectional area fraction (rho), which is related to the fibril volume fraction. Tail tendons from C57BL6 mice from age groups 1.6-35.3 months old were stretched to failure to determine sigma and E. Parallel measurements of rho as a function of age were made using transmission electron microscopy. Mathematical models (rule of mixtures) of fibrils reinforcing a PG gel in tendons were used to investigate the influence of rho on ageing changes in sigma and E. The magnitudes of sigma, E, and rho increased rapidly from 1.6 months to 4.0 months (P-values <0.05) before reaching a constant (age independent) from 4.0 months to 29.0 months (P-values >0.05); this trend continued for E and rho (P-values >0.05) from 29.0 months to 35.3 months, but not for sigma, which decreased gradually (P-values <0.05). Linear regression analysis revealed that age-related changes in sigma and E correlated positively to rho (P-values <0.05). Collagen fibril cross-sectional area fraction rho is a significant predictor of ageing changes in sigma and E in the tail tendons of C57BL6 mice.     

Effects of administration of transforming growth factor (TGF)-beta1 and anti-TGF-beta1 antibody on the mechanical properties of the stress-shielded patellar tendon

Previous studies have shown that, in the stress-shielded patellar tendon, the mechanical properties of the tendon were dramatically reduced and TGF-beta was over-expressed in tendon fibroblasts. In the present study, therefore, we tested two supportive hypotheses using 40 rabbits: One was that an application of TGF-beta1 might significantly increase the tensile strength and the tangent modulus of the stress-shielded patellar tendon. The other one was that an administration of anti-TGF-beta1 antibody might significantly reduce the mechanical properties of the stress-shielded patellar tendon. In the results, an application of 4-ng TGF-beta1 significantly increased the tangent modulus of the stress-shielded patellar tendon at 3 weeks (p = 0.019), compared with the sham treatment. Concerning the tensile strength, the 4-ng TGF-beta1 application increased the average value, but a statistical significance was not reached. An application of 50-microg anti-TGF-beta1 antibody significantly reduced the tangent modulus and the tensile strength of the stress-shielded patellar tendon at 3 weeks (p = 0.0068 and p = 0.0355), compared with the sham treatment. Because the stress-shielding treatment used in this study dramatically reduces the tangent modulus and the tensile strength of the patellar tendon, the present study suggested that an administration of TGF-beta1 weakly but significantly inhibited the reduction of the mechanical properties of the stress-shielded patellar tendon, and that inactivation of TGF-beta1 with its antibody significantly enhanced the reduction of the mechanical properties that occurs in the stress-shielded patellar tendon. These results suggested that TGF-beta1 plays an important role in remodeling of the stress-shielded patellar tendon.     

Mechanical properties of human patellar tendon at the hierarchical levels of tendon and fibril

Tendons are strong hierarchical structures, but how tensile forces are transmitted between different levels remains incompletely understood. Collagen fibrils are thought to be primary determinants of whole tendon properties, and therefore we hypothesized that the whole human patellar tendon and its distinct collagen fibrils would display similar mechanical properties. Human patellar tendons (n = 5) were mechanically tested in vivo by ultrasonography. Biopsies were obtained from each tendon, and individual collagen fibrils were dissected and tested mechanically by atomic force microscopy. The Young's modulus was 2.0 ± 0.5 GPa, and the toe region reached 3.3 ± 1.9% strain in whole patellar tendons. Based on dry cross-sectional area, the Young's modulus of isolated collagen fibrils was 2.8 ± 0.3 GPa, and the toe region reached 0.86 ± 0.08% strain. The measured fibril modulus was insufficient to account for the modulus of the tendon in vivo when fibril content in the tendon was accounted for. Thus, our original hypothesis was not supported, although the in vitro fibril modulus corresponded well with reported in vitro tendon values. This correspondence together with the fibril modulus not being greater than that of tendon supports that fibrillar rather than interfibrillar properties govern the subfailure tendon response, making the fibrillar level a meaningful target of intervention. The lower modulus found in vitro suggests a possible adverse effect of removing the tissue from its natural environment. In addition to the primary work comparing the two hierarchical levels, we also verified the existence of viscoelastic behavior in isolated human collagen fibrils.