Collagen fiber orientation at the tendon to bone insertion and its influence on stress concentrations

The tendon to bone insertion serves the mechanical role of transferring loads from a relatively compliant tendon to a relatively rigid bone. The details of the mechanism of load transfer are of great importance, since current surgical procedures for tendon reattachment have high failure rates. We hypothesized that the microscopic structure of the insertion is optimized to minimize stress concentrations associated with this load transfer. To explore this, collagen fiber orientation distributions were measured in the supraspinatus tendons of rats. The angular deviation of fibers was fairly uniform across the insertion, and the mean angles of the local distributions deviated mildly from the tendon axis. To explore how these observed property distributions could influence load transfer, these distributions were used to derive material properties for an idealized two-dimensional mechanical model of an insertion. Comparison between stress concentrations in this idealized model and those in three comparison models suggests that the microstructure serves to (1) simultaneously reduce stress concentrations and material mass, and (2) shield the insertion's outward splay from the highest stresses.

The development of structural and mechanical anisotropy in fibroblast populated collagen gels

An in vitro model system was developed to study structure-function relationships and the development of structural and mechanical anisotropy in collagenous tissues. Fibroblast-populated collagen gels were constrained either biaxially or uniaxially. Gel remodeling, biaxial mechanical properties, and collagen orientation were determined after 72 h of culture. Collagen gels contracted spontaneously in the unconstrained direction, uniaxial mechanical constraints produced structural anisotropy, and this structural anisotropy was associated with mechanical anisotropy. Cardiac and tendon fibroblasts were compared to test the hypothesis that tendon fibroblasts should generate greater anisotropy in vitro. However, no differences were seen in either structure or mechanics of collagen gels populated with these two cell types, or between fibroblast populated gels and acellular gels. This study demonstrates our ability to control and measure the development of structural and mechanical anisotropy due to imposed mechanical constraints in a fibroblast-populated collagen gel model system. While imposed constraints were required for the development of anisotropy in this system, active remodeling of the gel by fibroblasts was not. This model system will provide a basis for investigating structure-function relationships in engineered constructs and for studying mechanisms underlying the development of anisotropy in collagenous tissues.

Delayed repair of tendon to bone injuries leads to decreased biomechanical properties and bone loss

INTRODUCTION

Repair of the torn rotator cuff tendon is a common procedure performed in the shoulder. In the clinical setting, a significant delay between rotator cuff tear and subsequent repair often exists. The purpose of this study was to investigate the biomechanical properties and bone density of the tendon to bone repair site after acute and delayed repair.

METHODS

The supraspinatus tendons in bilateral shoulders of 60 rats were transected from the bone. In the acute group, the tendons were immediately repaired with suture. In the delayed group, the tendons were allowed to retract and repaired in a second procedure after a 3-week delay. Cross sectional area and biomechanical properties were evaluated. Bone density of the humeral head was assessed using peripheral quantitative computed tomography. Histologic sections were obtained and examined.

RESULTS

At 10 days the repair tissue displayed vascular and fibroblast proliferation accompanied by predominantly mononuclear infiltrate. At 28 days the inflammatory process gradually decreased. No significant histologic differences were noted between the acute and delayed repair specimens. Cross-sectional area was higher in the delayed group at the early time points (44% at 10 days and 31% at 28 days). Viscoelastic properties were greater in the acute group at the early time points and significantly less at the latest time point, compared to the delayed group. Bone density was markedly decreased (8% and 12%, 28 and 56 days respectively) in the delay group.

DISCUSSION

Inferior rotator cuff healing was demonstrated when there was a delay between injury and repair. Viscoelastic properties of the acute repairs were increased compared to the delayed group at 10 days, indicating tendon stiffening during the 3-week delay before repair. Viscoelastic properties of the acute repairs were decreased compared to the delayed group at 56 days indicating deterioration of properties over time in the delayed group. The deterioration in properties in the delayed group coincide with bone density decreases in the greater tuberosity. These results indicate that bone loss may a significant factor in poor healing.

Effect of several growth factors on canine flexor tendon fibroblast proliferation and collagen synthesis in vitro

PURPOSE

Growth factor delivery may be useful to accelerate the rate of tendon healing. Before in vivo use, however, the effects of growth factors on tendon cells need to be well characterized. The purpose of this study was to evaluate the effects of 4 growth factors on intrasynovial tendon fibroblast proliferation and collagen production in vitro. Our first hypothesis was that platelet-derived growth factor BB (PDGF-BB) and basic fibroblast growth factor (bFGF) would promote cell proliferation and collagen production. Our second hypothesis was that there would be a positive effect from the combination of PDGF-BB and bFGF.

METHODS

The growth factors PDGF-BB, bFGF, vascular endothelial growth factor (VEGF), and bone morphogenetic protein 2 (BMP-2) were evaluated in vitro with canine flexor tendon fibroblasts. The effects of single factors (PDGF-BB, bFGF, VEGF, or BMP-2) or a combination of factors (PDGF-BB and bFGF) on cell proliferation (ie, thymidine incorporation) and collagen production (ie, proline incorporation) were evaluated.

RESULTS

The results supported our hypotheses. Cell proliferation increased significantly with PDGF-BB and bFGF. Collagen production also increased significantly with PDGF-BB and bFGF. Cell proliferation and collagen production were unchanged with VEGF and BMP-2. A dose-response effect was seen for PDGF-BB combined with bFGF. The combination of PDGF-BB and bFGF led to an increase in cell proliferation but no change in collagen production compared with each factor alone.

CONCLUSIONS

The growth factors PDGF-BB and bFGF significantly increased flexor tendon fibroblast proliferation and matrix synthesis when applied singly. Administration of PDGF-BB and bFGF combined led to increased proliferation to single factors.

Variation of biomechanical, structural, and compositional properties along the tendon to bone insertion site

The tendon to bone insertion site is a complex transitional region that links two very different materials. The insertion site must transfer a complex loading environment effectively to prevent injury and provide proper joint function. In order to accomplish this load transfer effectively, the properties of the insertion site were hypothesized to vary along its length. The quasilinear viscoelastic (QLV) Model was used to determine biomechanical properties, polarized light analysis was used to quantitate collagen orientation (structure), and in situ hybridization was used to determine the expression of extracellular matrix genes (composition). All assays were performed at two insertion site locations: the tendon end of the insertion and the bony end of the insertion. Biomechanically, the apparent properties of peak strain, the coefficients (A and B) that describe the elastic component of the QLV model, and one of the coefficients (tau(1)) of the viscous component of the model were significantly higher, while another of the coefficients (C) of the viscous component was significantly lower at the tendon insertion compared to the bony insertion. The collagen was significantly more oriented at the tendon insertion compared to the bony insertion. Finally, collagen types II, IX, and X, and aggrecan were localized only to the bony insertion, while decorin and biglycan were localized only to the tendon insertion. Thus, the tendon to bony insertion site varies dramatically along its length in terms of its viscoelastic properties, collagen structure, and extracellular matrix composition.

Tendon to bone healing: differences in biomechanical, structural, and compositional properties due to a range of activity levels

Little knowledge exists about the healing process of the tendon to bone insertion, and hence little can be done to improve tissue healing. The goal of this study is to describe the healing of the supraspinatus tendon to its bony insertion under a variety of loading conditions. Tendons were surgically detached and repaired in rats. Rat shoulders were then immobilized, allowed cage activity, or exercised. Shoulders that were immobilized demonstrated superior structural (significantly higher collagen orientation), compositional (expression of extracellular matrix genes similar to the uninjured insertion), and quasilinear viscoelastic properties (A = 0.30 +/- 0.10 MPa vs. 0.16 +/- 0.08 MPa, B = 17.4 +/- 2.9 vs. 15.1 +/- 0.9, and tau 2 = 344 +/- 161 s vs. 233 +/- 40 s) compared to those that were exercised, contrary to expectations. With this knowledge of the healing response, treatment modalities for rotator cuff tears can be developed.

Rotator cuff tendinosis in an animal model: role of extrinsic and overuse factors

The rat shoulder animal model has been used previously to study the role of intrinsic injury (modeled as an acute insult to the tendon), extrinsic injury (modeled as external subacromial impingement), and overuse factors on rotator cuff tendinosis. These studies demonstrated that it is possible to produce rotator cuff tendinosis with any one of these factors in isolation. The current study uses the rat shoulder model to study the roles of extrinsic compression, overuse, and overuse in combination with extrinsic compression, on the development of rotator cuff tendinosis. The results of this study demonstrate that the injury created by overuse plus extrinsic compression is greater than the injuries created by overuse or extrinsic compression alone, particularly when important biomechanical variables are considered. While ineffective in causing a change in supraspinatus tendon properties in animals with normal cage activity, extrinsic compression had a significant and dramatic effect when it was combined with overuse activity. Without an additional factor, such as overhead activity, the extrinsic compression alone may be insufficient to cause tendinosis. The results of the present study support the role of multiple factors in the etiology of some rotator cuff injuries.

The effect of fibrin clot on healing rat supraspinatus tendon defects

The possibility that fibrin clot, with its chemotactic and mitogenic factors, might improve the healing of a defect in the rat supraspinatus tendon was evaluated. Bilateral defects were surgically created in the rat supraspinatus tendon near the humeral insertion. One defect in each rat was filled with fibrin clot from a donor animal while the other side acted as a control. The tendons were evaluated at 3, 6, and 12 weeks. On histologic evaluation persistent defects were seen at all time points, whereas the healing tissue became less cellular with better collagen organization over time. Fibrin clot remained in the healing defects of treated shoulders at early time points. Biomechanically, there was improvement of properties over time, but they did not approach those of normal tendon by 12 weeks. There was no effect from addition of the fibrin clot except at 3 weeks, where it led to a decrease in material properties.

The localized expression of extracellular matrix components in healing tendon insertion sites: an in situ hybridization study

The localized expression of a number of extracellular matrix genes was evaluated over time in a novel rat rotator cuff injury model. The supraspinatus tendons of rats were severed at the bony insertion and repaired surgically. The healing response was evaluated at 1, 2, 4, and 8 weeks post-injury using histologic and in situ hybridization techniques. Expression patterns of collagens (I, II, III, IX, X, XII), proteoglycans (decorin, aggrecan, versican, biglycan, fibromodulin), and other extracellular matrix proteins (elastin, osteocalcin, alkaline phosphatase) were evaluated at the healing tendon to bone insertion site. Histologic results indicate a poor healing response to the injury, with only partial recreation of the insertion site by 8 weeks. In situ hybridization results indicate a specific pattern of genes expressed in each zone of the insertion site (i.e., tendon, fibrocartilage, mineralized cartilage, bone). Overall, expression of collagen types I and XII, aggrecan, and biglycan was increased, while expression of collagen type X and decorin was decreased. Expression of collagen type I, collagen type XII, and biglycan decreased over time, but remained above normal at 8 weeks. Results indicate that the rat supraspinatus tendon is ineffective in recreating the original insertion site, even at 8 weeks post-injury, in the absence of biological or biomechanical enhancements.

Neer Award 1999. Overuse activity injures the supraspinatus tendon in an animal model: a histologic and biomechanical study

Overuse activity has been implicated as an etiologic factor in injury to the rotator cuff and to the supraspinatus tendon in particular. Due in part to the lack of an appropriate animal model, expex85ental studies have not addressed this issue. With the use of a rat model, we measured the effects of an overuse running regimen on 36 Sprague-Dawley rats after 4 (n = 12), 8 (n = 12), or 16 (n = 12) weeks of exercise and compared them with a control group of rats (n = 10) who were allowed normal cage activity. The histologic characteristics, the gross morphologic characteristics, and the mechanical properties of the tendon tissue were evaluated. The supraspinatus tendons in the exercised animals demonstrated significant changes as a result of overuse at all time points compared with the normal group. There was an increase in cellularity and a loss of the normal collagen fiber organization consistent with what has been seen in human tendinopathy. The tendons from the exercise groups were larger than normal in cross-sectional analysis at 4 weeks (129% of control, P < .01) and continued to increase in size with time to 16 weeks (164% of control, P = .01). The mechanical properties of the tendons deteriorated in response to overuse exercise with a decreased modulus of elasticity ranging from 52% to 61% of control (P = .07 at 4 weeks, P < .05 at 8 and 16 weeks) and a decreased maximum stress of failure ranging from 51% to 63% of control (P < .007). These findings support overuse activity as an etiologic factor in the development of supraspinatus tendinopathy and begin to describe the changes in the tendons as a result of such activity. This model can now be used to study the effect of various treatment modalities on these injuries.

Animal models of tendon and ligament injuries for tissue engineering applications

Improved methods are needed for prevention and treatment of injuries to the musculoskeletal soft tissues. Tissue engineering techniques have led to more effective clinical protocols for treating these injuries. Improvement of tissue healing through the addition of biologic factors, and the development of biologically active tissue engineered replacements, are two promising areas of research. An essential component of progress in this field is the use of animal models of tendon and ligament injuries, which allows for rigorous testing of hypotheses related to disease pathogenesis and treatment. Because these animal models must be appropriate for the condition being studied, no single model exists that is appropriate for all investigations. It generally is necessary to differentiate between tendon and ligament tissues. Furthermore, ligaments should be divided into intraarticular and extraarticular models, whereas tendons should be divided into intrasynovial and extrasynovial models. Other important factors in the appropriate use of an animal model include size of the animal, anatomic features, and techniques available for tissue analysis. The tissues used should be large enough to allow for accurate and reproducible manipulations (injury creation, repair, reconstruction). In addition, it is preferable to use tissues that are amenable to quantitative analysis.

Rotator cuff defect healing: a biomechanical and histologic analysis in an animal model

Rotator cuff tears are one of the most common causes of pain and disability in the upper extremity. With the use of an animal model, we studied the healing response of a controlled defect in the normal supraspinatus tendon and in a tendon with a reduced intrinsic healing capacity. In 36 Sprague-Dawley rats, defects (2 mm x 2 mm) were created in the supraspinatus tendons bilaterally. To model a tendon with an intrinsically reduced capacity to heal, the tissue adjacent to the defect area in the left shoulder was treated with in situ freezing. The contralateral tendon was not frozen. After 3 (n = 12), 6 (n = 12), and 12 (n = 12) weeks, animals were killed and underwent histologic (n = 4 from each group) and biomechanical (n = 8 from each group) evaluation. An additional group of untreated animals served as a normal control group. On histologic evaluation 78% of tendons had persistent defects (defined as incomplete closure of the defect site). Over time, the tissue from both groups demonstrated an improved histologic grade but did not reach normal levels, even at 12 weeks. No histologic differences were found between defect healing in normal tendons and in those treated with in situ freezing. On biomechanical evaluation there were also no significant differences between treatment groups. Over time, an improvement occurred in tissue properties, indicating that some healing of the defects had occurred. However, these tissue properties remained an order of magnitude lower than those of normal control tendons. These findings indicate that there is an active but inadequate repair response to the defect in the rat supraspinatus tendon, which is not significantly worsened by in situ freezing of the tissue around the defect. This model has applications toward the study of techniques to improve or accelerate cuff defect healing.

The effects of overuse combined with intrinsic or extrinsic alterations in an animal model of rotator cuff tendinosis

An in vivo animal model was used to evaluate overuse and overuse plus intrinsic tendon injury or extrinsic tendon compression in the development of rotator cuff injury. Forty-four male Sprague-Dawley rats were divided into groups of 22. Each left shoulder received an intrinsic or extrinsic injury plus overuse (treadmill running), and each right shoulder received only overuse. Eleven rats from each group were sacrificed at 4 and 8 weeks. Supraspinatus tendons were evaluated histologically or geometrically and biomechanically. Ten rats constituted a cage-activity control group. Both supraspinatus tendons of the experimental groups had increases in cellularity and collagen disorganization and changes in cell shape compared with control tendons. Tendons with injury plus overuse exhibited a worse histologic grade than those with overuse alone. The cross-sectional area of both supraspinatus tendons of the experimental rats was significantly more than in control tendons. The area of the injury plus overuse tendons was increased on average compared with overuse-alone tendons. Biomechanically, the tissue moduli of overuse/intrinsic injury tendons at 4 weeks and those of the overuse/extrinsic injury tendons at 8 weeks were significantly lower than in control tendons. Tissue moduli of the overuse/injury tendons were significantly lower than in the overuse-alone tendons at 8 weeks. This study demonstrated that damage to the supraspinatus tendon can be caused by overuse and intrinsic injury, overuse and extrinsic compression, and overuse alone.