Strong and tough mineralized PLGA nanofibers for tendon-to-bone scaffolds

Engineering complex tissues such as the tendon-to-bone insertion sites require a strong and tough biomimetic material system that incorporates both mineralized and unmineralized tissues with different strengths and stiffnesses. However, increasing strength without degrading toughness is a fundamental challenge in materials science. Here, we demonstrate a promising nanofibrous polymer-hydroxyapatite system, in which, a continuous fibrous network must function as a scaffold for both mineralized and unmineralized tissues. It is shown that the high toughness of this material system could be maintained without compromising on the strength with the addition of hydroxyapatite mineral. Individual electrospun poly (lactide-co-glycolide) (PLGA) nanofibers demonstrated outstanding strain-hardening behavior and ductility when stretched uniaxially, even in the presence of surface mineralization. This highly desirable hardening behavior which results in simultaneous nanofiber strengthening and toughening was shown to depend on the initial cross-sectional morphology of the PLGA nanofibers. For pristine PLGA nanofibers, it was shown that ellipsoidal cross-sections provide the largest increase in fiber strength by almost 200% compared to bulk PLGA. This exceptional strength accompanied by 100% elongation was shown to be retained for thin and strongly bonded conformal mineral coatings, which were preserved on the nanofiber surface even for such very large extensions.

BMP12 induces tenogenic differentiation of adipose-derived stromal cells

Adipose-derived stromal cells (ASCs) are pluripotent cells that have the capacity to differentiate into tendon fibroblasts (TFs). They are abundant in adults, easy to access, and are therefore an ideal cell source for tendon tissue engineering. Despite this potential, the molecular cues necessary for tenogenic differentiation of ASCs are unknown. Unlike other bone morphogenetic proteins (BMPs), BMP12, BMP13, and BMP14 have been reported to be less osteo-chondrogenic and to induce tendon rather than bone formation in vivo. This study investigated the effects of BMP12 and BMP14 on ASC differentiation in vitro. In canine ASCs, BMP12 effectively increased the expression of the tendon markers scleraxis and tenomodulin at both mRNA and protein levels. Consistent with these results, BMP12 induced scleraxis promoter driven-GFP and tenomodulin protein expression in mouse ASCs. Although BMP12 also enhanced the expression of the cartilage matrix gene aggrecan in ASCs, the resulting levels remained considerably lower than those detected in tendon fibroblasts. In addition, BMP12 reduced expression of the bone marker osteocalcin, but not the osteogenic transcription factor runx-2. BMP14 exhibited similar, but marginally less potent and selective effects, compared to BMP12. BMPs are known to signal through the canonical Smad pathway and the non-canonical mitogen-activated protein kinase (MAPK) pathway. BMP12 triggered robust phosphorylation of Smad1/5/8 but not Smad2/3 or p38 MAPK in ASCs. The effect was likely conveyed by type I receptors ALK2/3/6, as phosphorylation of Smad1/5/8 was blocked by the ALK2/3/6 inhibitor LDN-193189 but not by the ALK4/5/7 inhibitor SB-505124. Moreover, ALK6 was found to be the most abundant type I receptor in ASCs, with mRNA expression 100 to 10,000 times that of any other type I receptor. Collectively, results support the conclusion that BMP12 induces tenogenic differentiation of ASCs via the Smad1/5/8 pathway.

The effect of unloading on gene expression of healthy and injured rotator cuffs

Tendon unloading following rupture of one of the rotator cuff tendons can induce alterations in muscle physiology and tendon structure, which can subsequently affect reparability and healing potential. Yet little is known about the effects of muscle and tendon unloading on the molecular response of the rotator cuff. We determined the effect of mechanical unloading on gene expression and morphology of healthy supraspinatus tendons and muscles, and the same muscles after acute injury and repair. Mechanical unloading was achieved by tenotomy and/or botulinum toxin A (BTX) chemical denervation in a rat rotator cuff model of injury and repair. Gene expression profiles varied across regions of the muscle, with the greatest changes seen in the distal aspect of the muscle for most genes. Myogenic and adipogenic genes were upregulated in muscle when unloaded (tenotomy and BTX). Tendon injury, with and without repair, resulted in upregulation of fibrosis- and tendon-specific gene expression. The expression of scleraxis, a transcription factor necessary for tendon development, was upregulated in response to injury and repair. In summary, tendon detachment and repair had the greatest effect on tendon gene expression, while unloading had the greatest effect on muscle gene expression.

Muscle loading is necessary for the formation of a functional tendon enthesis

Muscle forces are essential for skeletal patterning during development. Eliminating muscle forces, e.g., through paralysis, leads to bone and joint deformities. Botulinum toxin (BtxA)-induced paralysis of mouse rotator cuffs throughout postnatal development closely mimics neonatal brachial plexus palsy, a significant clinical condition in infants. In these mice, the tendon-to-bone attachment (i.e., the tendon enthesis) presents defects in mineral accumulation and fibrocartilage formation, presumably impairing the function of the tissue. The objective of the current study was to investigate the functional consequences of muscle unloading using BtxA on the developing supraspinatus tendon enthesis. We found that the maximum endurable load and stiffness of the supraspinatus tendon attachment decreased after four and eight weeks of post-natal BtxA-muscle unloading relative to controls. Tendon cross-sectional area was not significantly reduced by BtxA-unloading, while, strength, modulus, and toughness were decreased in the BtxA-unloaded group compared to controls, indicating a decrease in tissue quality. Polarized-light microscopy and Raman microprobe analysis were used to determine collagen fiber alignment and mineral characteristics, respectively, in the tendon enthesis that might contribute to the reduced biomechanical performance in BtxA-unloaded shoulders. Collagen fiber alignment was significantly reduced in BtxA-unloaded shoulders. The mineral-to-matrix ratio in mineralized fibrocartilage was not affected by loading. However, the crystallographic atomic order of the hydroxylapatite phase (a measure of crystallinity) was reduced and the amount of carbonate (substituting for phosphate) in the hydroxylapatite crystals was increased. Taken together, these micrometer-scale structural and compositional changes partly explain the observed decreases in the mechanical functionality of the tendon enthesis in the absence of muscle loading.

Controlled delivery of mesenchymal stem cells and growth factors using a nanofiber scaffold for tendon repair

Outcomes after tendon repair are often unsatisfactory, despite improvements in surgical techniques and rehabilitation methods. Recent studies aimed at enhancing repair have targeted the paucicellular nature of tendon for enhancing repair; however, most approaches for delivering growth factors and cells have not been designed for dense connective tissues such as tendon. Therefore, we developed a scaffold capable of delivering growth factors and cells in a surgically manageable form for tendon repair. Platelet-derived growth factor BB (PDGF-BB), along with adipose-derived mesenchymal stem cells (ASCs), were incorporated into a heparin/fibrin-based delivery system (HBDS). This hydrogel was then layered with an electrospun nanofiber poly(lactic-co-glycolic acid) (PLGA) backbone. The HBDS allowed for the concurrent delivery of PDGF-BB and ASCs in a controlled manner, while the PLGA backbone provided structural integrity for surgical handling and tendon implantation. In vitro studies verified that the cells remained viable, and that sustained growth factor release was achieved. In vivo studies in a large animal tendon model verified that the approach was clinically relevant, and that the cells remained viable in the tendon repair environment. Only a mild immunoresponse was seen at dissection, histologically, and at the mRNA level; fluorescently labeled ASCs and the scaffold were found at the repair site 9days post-operatively; and increased total DNA was observed in ASC-treated tendons. The novel layered scaffold has the potential for improving tendon healing due to its ability to deliver both cells and growth factors simultaneously in a surgically convenient manner.

Functional attachment of soft tissues to bone: development, healing, and tissue engineering

Connective tissues such as tendons or ligaments attach to bone across a multitissue interface with spatial gradients in composition, structure, and mechanical properties. These gradients minimize stress concentrations and mediate load transfer between the soft and hard tissues. Given the high incidence of tendon and ligament injuries and the lack of integrative solutions for their repair, interface regeneration remains a significant clinical challenge. This review begins with a description of the developmental processes and the resultant structure-function relationships that translate into the functional grading necessary for stress transfer between soft tissue and bone. It then discusses the interface healing response, with a focus on the influence of mechanical loading and the role of cell-cell interactions. The review continues with a description of current efforts in interface tissue engineering, highlighting key strategies for the regeneration of the soft tissue-to-bone interface, and concludes with a summary of challenges and future directions.

The effect of core and epitendinous suture modifications on repair of intrasynovial flexor tendons in an in vivo canine model

PURPOSE

To determine in vivo effects of modifications to core and epitendinous suture techniques in a canine intrasynovial flexor tendon repair model using clinically relevant rehabilitation. Our null hypothesis was that gap formation and rupture rates would remain consistent across repair techniques.

METHODS

We evaluated gap formation and rupture in 75 adult mongrel dogs that underwent repair of intrasynovial flexor tendon lacerations followed by standardized postoperative therapy. The current suture technique was a 4-0, 8-strand core suture with a purchase of 1.2 cm and a 5-0, epitendinous suture repair with a 2-mm purchase length and depth. We compared gap and failure by chi-square analysis to a historical group of in vivo repairs (n = 76) from the same canine model using 8-strand core suture repair with purchase of 0.75 cm and 6-0 epitendinous suture with a 1-mm purchase length and depth.

RESULTS

A total of 93% of tendons (n = 70) demonstrated gapping of less than 3 mm using the current suture technique. Five percent of tendons (n = 4) had a gap of 3 mm or greater, and there was 1 repair site failure. This was significantly improved over the comparison group of historical 8-strand core repair technique, which resulted in 82% (n = 62) of repairs with a gap of less than 3 mm and 7 failures (9%).

CONCLUSIONS

In an in vivo model, current modifications to suture techniques for intrasynovial flexor tendon repair demonstrated significant improvements in gap formation and rupture compared with a similar technique using shorter purchase lengths and shallower purchase depth.

CLINICAL RELEVANCE

Suggested repair modifications for the treatment of zone II flexor tendon transections demonstrate improvements in gap formation and tendon rupture in vivo.

Mineral distributions at the developing tendon enthesis

Tendon attaches to bone across a functionally graded interface, “the enthesis”. A gradient of mineral content is believed to play an important role for dissipation of stress concentrations at mature fibrocartilaginous interfaces. Surgical repair of injured tendon to bone often fails, suggesting that the enthesis does not regenerate in a healing setting. Understanding the development and the micro/nano-meter structure of this unique interface may provide novel insights for the improvement of repair strategies. This study monitored the development of transitional tissue at the murine supraspinatus tendon enthesis, which begins postnatally and is completed by postnatal day 28. The micrometer-scale distribution of mineral across the developing enthesis was studied by X-ray micro-computed tomography and Raman microprobe spectroscopy. Analyzed regions were identified and further studied by histomorphometry. The nanometer-scale distribution of mineral and collagen fibrils at the developing interface was studied using transmission electron microscopy (TEM). A zone (∼20 µm) exhibiting a gradient in mineral relative to collagen was detected at the leading edge of the hard-soft tissue interface as early as postnatal day 7. Nanocharacterization by TEM suggested that this mineral gradient arose from intrinsic surface roughness on the scale of tens of nanometers at the mineralized front. Microcomputed tomography measurements indicated increases in bone mineral density with time. Raman spectroscopy measurements revealed that the mineral-to-collagen ratio on the mineralized side of the interface was constant throughout postnatal development. An increase in the carbonate concentration of the apatite mineral phase over time suggested possible matrix remodeling during postnatal development. Comparison of Raman-based observations of localized mineral content with histomorphological features indicated that development of the graded mineralized interface is linked to endochondral bone formation near the tendon insertion. These conserved and time-varying aspects of interface composition may have important implications for the growth and mechanical stability of the tendon-to-bone attachment throughout development.

Effect of bone morphogenetic protein 2 on tendon-to-bone healing in a canine flexor tendon model

Tendon-to-bone healing is typically poor, with a high rate of repair-site rupture. Bone loss after tendon-to-bone repair may contribute to poor outcomes. Therefore, we hypothesized that the local application of the osteogenic growth factor bone morphogenetic protein 2 (BMP-2) would promote bone formation, leading to improved repair-site mechanical properties. Intrasynovial canine flexor tendons were injured in Zone 1 and repaired into bone tunnels in the distal phalanx. BMP-2 was delivered to the repair site using either a calcium phosphate matrix (CPM) or a collagen sponge (COL) carrier. Each animal also received carrier alone in an adjacent repair to serve as an internal control. Repairs were evaluated at 21 days using biomechanical, radiographic, and histologic assays. Although an increase in osteoid formation was noted histologically, no significant increases in bone mineral density occurred. When excluding functional failures (i.e., ruptured and gapped repairs), mechanical properties were not different when comparing BMP-2/CPM groups with carrier controls. A significantly higher percentage of BMP-2 treated specimens had a maximum force <20 N compared to carrier controls. While tendon-to-bone healing can be enhanced by addressing the bone loss that typically occurs after surgical repair, the delivery of BMP-2 using the concentrations and methods of the current study did not improve mechanical properties over carrier alone. The anticipated anabolic effect of BMP-2 was insufficient in the short time frame of this study to counter the post-repair loss of bone.

Intrasynovial flexor tendon repair: a biomechanical study of variations in suture application in human cadavera

To improve the functional outcomes of intrasynovial tendon suture, prior experiments evaluated individual technical modifications used in the repair process. Few studies, however, have assessed the combinatorial effects of those suture modifications in an integrated biomechanical manner, including a sample size sufficient to make definitive observations on repair technique. Two hundred fifty-six flexor tendon repairs were performed in human cadavera, and biomechanical properties were determined. The effects of five factors for flexor tendon repair were tested: core suture caliber (4-0 or 3-0), number of sutures crossing the repair site (four- or eight-strand), core suture purchase (0.75 or 1.2 cm), peripheral suture caliber (6-0 or 5-0), and peripheral suture purchase (superficial or 2 mm). Significant factors affecting the properties of the repair were the number of core suture strands and the peripheral suture purchase. The least significant factors were core suture purchase and peripheral suture caliber. The choice of core suture caliber affected the properties of repair marginally. Based on these results, we recommend that surgeons continue to focus on multi-strand repair methods, as the properties of eight-strand repairs were far better than those of four-strand repairs. To resist gap formation and enhance repair strength, a peripheral suture with 2 mm purchase is also recommended. Finally, since core suture caliber affected some biomechanical properties, including the failure mode, a 3-0 suture could be considered, provided that future in vivo studies can confirm that gliding properties are not adversely influenced.

Generation of controllable gradients in cell density

Making the grad(ient): a gradient in cell density was generated on a substrate that was inserted into a homogeneous suspension of cells at a specific tilt angle by taking advantage of the gradual change in the number of cells available for sedimentation. Reverse gradients were also fabricated on the same substrate using multiple sedimentation procedures.

The nanometre-scale physiology of bone: steric modelling and scanning transmission electron microscopy of collagen-mineral structure

The nanometre-scale structure of collagen and bioapatite within bone establishes bone's physical properties, including strength and toughness. However, the nanostructural organization within bone is not well known and is debated. Widely accepted models hypothesize that apatite mineral ('bioapatite') is present predominantly inside collagen fibrils: in 'gap channels' between abutting collagen molecules, and in 'intermolecular spaces' between adjacent collagen molecules. However, recent studies report evidence of substantial extrafibrillar bioapatite, challenging this hypothesis. We studied the nanostructure of bioapatite and collagen in mouse bones by scanning transmission electron microscopy (STEM) using electron energy loss spectroscopy and high-angle annular dark-field imaging. Additionally, we developed a steric model to estimate the packing density of bioapatite within gap channels. Our steric model and STEM results constrain the fraction of total bioapatite in bone that is distributed within fibrils at less than or equal to 0.42 inside gap channels and less than or equal to 0.28 inside intermolecular overlap regions. Therefore, a significant fraction of bone's bioapatite (greater than or equal to 0.3) must be external to the fibrils. Furthermore, we observe extrafibrillar bioapatite between non-mineralized collagen fibrils, suggesting that initial bioapatite nucleation and growth are not confined to the gap channels as hypothesized in some models. These results have important implications for the mechanics of partially mineralized and developing tissues.

The effect of tear size and nerve injury on rotator cuff muscle fatty degeneration in a rodent animal model

BACKGROUND

Irreversible muscle changes after rotator cuff tears is a well-known negative prognostic factor after shoulder surgery. Currently, little is known about the pathomechanism of fatty degeneration of the rotator cuff muscles after chronic cuff tears. The purposes of this study were to (1) develop a rodent animal model of chronic rotator cuff tears that can reproduce fatty degeneration of the cuff muscles seen clinically, (2) describe the effects of tear size and concomitant nerve injury on muscle degeneration, and (3) evaluate the changes in gene expression of relevant myogenic and adipogenic factors after rotator cuff tears using the animal model.

MATERIALS AND METHODS

Rotator cuff tears were created in rodents with and without transection of the suprascapular nerve. The supraspinatus and infraspinatus muscles were examined at 2, 8, and 16 weeks after injury for histologic evidence of fatty degeneration and expression of myogenic and adipogenic genes.

RESULTS

Histologic analysis revealed adipocytes, intramuscular fat globules, and intramyocellular fat droplets in the tenotomized and neurotomized supraspinatus and infraspinatus muscles. Changes increased with time and were most severe in the muscles with combined tenotomy and neurotomy. Adipogenic and myogenic transcription factors and markers were upregulated in muscles treated with tenotomy or tenotomy combined with neurotomy compared with normal muscles.

CONCLUSIONS

The rodent animal model described in this study produces fatty degeneration of the rotator cuff muscles similar to human muscles after chronic cuff tears. The severity of changes was associated with tear size and concomitant nerve injury.

Recent advances in shoulder research

Shoulder pathology is a growing concern for the aging population, athletes, and laborers. Shoulder osteoarthritis and rotator cuff disease represent the two most common disorders of the shoulder leading to pain, disability, and degeneration. While research in cartilage regeneration has not yet been translated clinically, the field of shoulder arthroplasty has advanced to the point that joint replacement is an excellent and viable option for a number of pathologic conditions in the shoulder. Rotator cuff disease has been a significant focus of research activity in recent years, as clinicians face the challenge of poor tendon healing and irreversible changes associated with rotator cuff arthropathy. Future treatment modalities involving biologics and tissue engineering hold further promise to improve outcomes for patients suffering from shoulder pathologies.

Electrospun nanofibers for regenerative medicine

This Progress Report reviews recent progress in applying electrospun nanofibers to the emerging field of regenerative medicine. It begins with a brief introduction to electrospinning and nanofibers, with a focus on issues related to the selection of materials, incorporation of bioactive molecules, degradation characteristics, control of mechanical properties, and facilitation of cell infiltration. Next, a number of approaches to fabricate scaffolds from electrospun nanofibers are discussed, including techniques for controlling the alignment of nanofibers and for producing scaffolds with complex architectures. The article also highlights applications of the nanofiber-based scaffolds in four areas of regenerative medicine that involve nerves, dural tissues, tendons, and the tendon-to-bone insertion site. The Progress Report concludes with perspectives on challenges and future directions for design, fabrication, and utilization of scaffolds based on electrospun nanofibers.

Bi-material attachment through a compliant interfacial system at the tendon-to-bone insertion site

The attachment of tendon to bone, one of the greatest interfacial material mismatches in nature, presents an anomaly from the perspective of interfacial engineering. Deleterious stress concentrations arising at bi-material interfaces can be reduced in engineering practice by smooth interpolation of composition, microstructure, and mechanical properties. However, following normal development, the rotator cuff tendon-to-bone "insertion site" presents an interfacial zone that is more compliant than either tendon or bone. This compliant zone is not regenerated following healing, and its absence may account for the poor outcomes observed following both natural and post-surgical healing of insertion sites such as those at the rotator cuff of the shoulder. Here, we present results of numerical simulations which provide a rationale for such a seemingly illogical yet effective interfacial system. Through numerical optimization of a mathematical model of an insertion site, we show that stress concentrations can be reduced by a biomimetic grading of material properties. Our results suggest a new approach to functional grading for minimization of stress concentrations at interfaces.

Tissue-engineering strategies for the tendon/ligament-to-bone insertion

Injuries to connective tissues are painful and disabling and result in costly medical expenses. These injuries often require reattachment of an unmineralized connective tissue to bone. The uninjured tendon/ligament-to-bone insertion (enthesis) is a functionally graded material that exhibits a gradual transition from soft tissue (i.e., tendon or ligament) to hard tissue (i.e., mineralized bone) through a fibrocartilaginous transition region. This transition is believed to facilitate force transmission between the two dissimilar tissues by ameliorating potentially damaging interfacial stress concentrations. The transition region is impaired or lost upon tendon/ligament injury and is not regenerated following surgical repair or natural healing, exposing the tissue to risk of reinjury. The need to regenerate a robust tendon-to-bone insertion has led a number of tissue engineering repair strategies. This review treats the tendon-to-bone insertion site as a tissue structure whose primary role is mechanical and discusses current and emerging strategies for engineering the tendon/ligament-to-bone insertion in this context. The focus lies on strategies for producing mechanical structures that can guide and subsequently sustain a graded tissue structure and the associated cell populations.

Enhancing the stiffness of electrospun nanofiber scaffolds with a controlled surface coating and mineralization

A new method was developed to coat hydroxyapatite (HAp) onto electrospun poly(lactic-co-glycolic acid) (PLGA) nanofibers for tendon-to-bone insertion site repair applications. Prior to mineralization, chitosan and heparin were covalently immobilized onto the surface of the fibers to accelerate the nucleation of bone-like HAp crystals. Uniform coatings of HAp were obtained by immersing the nanofiber scaffolds into a modified, 10-fold-concentrated simulated body fluid (m10SBF) for different periods of time. The new method resulted in thicker and denser coatings of mineral on the fibers compared to those produced by previously reported methods. Scanning electron microscopy measurements confirmed the formation of nanoscale HAp particles on the fibers. A mechanical property assessment demonstrated a higher stiffness with respect to previous coating methods. A combination of the nanoscale fibrous structure and bonelike mineral coating could mimic the structure, composition, and function of mineralized tissues.

Sustained delivery of transforming growth factor beta three enhances tendon-to-bone healing in a rat model

Despite advances in surgical technique, rotator cuff repairs are plagued by a high rate of failure. This failure rate is in part due to poor tendon-to-bone healing; rather than regeneration of a fibrocartilaginous attachment, the repair is filled with disorganized fibrovascular (scar) tissue. Transforming growth factor beta 3 (TGF-β3) has been implicated in fetal development and scarless fetal healing and, thus, exogenous addition of TGF-β3 may enhance tendon-to-bone healing. We hypothesized that: TGF-β3 could be released in a controlled manner using a heparin/fibrin-based delivery system (HBDS); and delivery of TGF-β3 at the healing tendon-to-bone insertion would lead to improvements in biomechanical properties compared to untreated controls. After demonstrating that the release kinetics of TGF-β3 could be controlled using a HBDS in vitro, matrices were incorporated at the repaired supraspinatus tendon-to-bone insertions of rats. Animals were sacrificed at 14-56 days. Repaired insertions were assessed using histology (for inflammation, vascularity, and cell proliferation) and biomechanics (for structural and mechanical properties). TGF-β3 treatment in vivo accelerated the healing process, with increases in inflammation, cellularity, vascularity, and cell proliferation at the early timepoints. Moreover, sustained delivery of TGF-β3 to the healing tendon-to-bone insertion led to significant improvements in structural properties at 28 days and in material properties at 56 days compared to controls. We concluded that TGF-β3 delivered at a sustained rate using a HBDS enhanced tendon-to-bone healing in a rat model.

The role of transforming growth factor beta isoforms in tendon-to-bone healing

The purpose of this study was to examine the role of two of the three transforming growth factor beta (TGF-β) isoforms at the healing tendon-to-bone insertion. The supraspinatus tendons of 64 rats were transected at their bony insertions and repaired to the humeral head. One shoulder of each rat received an osmotic pump for sustained delivery of the following factors at the repair site: (1) TGF-β1 and neutralizing antibodies to TGF-β2 and 3 (TGF-β1 group), (2) TGF-β3 and neutralizing antibodies to TGF-β1 and 2 (TGF-β3 group), (3) neutralizing antibodies to TGF-β1, 2, and 3 (anti-TGF-β group), and (4) saline (saline group). The contralateral shoulders received saline to serve as paired controls. The repairs were evaluated at multiple time points postmortem using histology-based assays and biomechanical testing. Treated shoulders in the TGF-β1 group showed increased type III collagen production compared to the paired control shoulders, indicative of a scar-mediated response. There was a trend toward reduced mechanical properties in the TGF-β1 group, but these changes did not reach statistical significance. The anti-TGF-β group showed no difference in tissue volume, but significantly inferior mechanical properties, compared to the paired control shoulders. The TGF-β3 group did not show any differences compared to the paired control shoulders. Although TGF-β isoforms play important roles in tendon-to-bone development and healing, application of exogenous TGF-β isoforms and neutralizing antibodies to the subacromial space using osmotic pumps did not improve supraspinatus tendon-to-bone healing.

Fibrocartilage tissue engineering: the role of the stress environment on cell morphology and matrix expression

Although much is known about the effects of uniaxial mechanical loading on fibrocartilage development, the stress fields to which fibrocartilaginous regions are subjected to during development are mutiaxial. That fibrocartilage develops at tendon-to-bone attachments and in compressive regions of tendons is well established. However, the three-dimensional (3D) nature of the stresses needed for the development of fibrocartilage is not known. Here, we developed and applied an in vitro system to determine whether fibrocartilage can develop under a state of periodic hydrostatic tension in which only a single principal component of stress is compressive. This question is vital to efforts to mechanically guide morphogenesis and matrix expression in engineered tissue replacements. Mesenchymal stromal cells in a 3D culture were exposed to compressive and tensile stresses as a result of an external tensile hydrostatic stress field. The stress field was characterized through mechanical modeling. Tensile cyclic stresses promoted spindle-shaped cells, upregulation of scleraxis and type one collagen, and cell alignment with the direction of tension. Cells experiencing a single compressive stress component exhibited rounded cell morphology and random cell orientation. No difference in mRNA expression of the genes Sox9 and aggrecan was observed when comparing tensile and compressive regions unless the medium was supplemented with the chondrogenic factor transforming growth factor beta3. In that case, Sox9 was upregulated under static loading conditions and aggrecan was upregulated under cyclic loading conditions. In conclusion, the fibrous component of fibrocartilage could be generated using only mechanical cues, but generation of the cartilaginous component of fibrocartilage required biologic factors in addition to mechanical cues. These studies support the hypothesis that the 3D stress environment influences cell activity and gene expression in fibrocartilage development.

Mechanisms of Bimaterial Attachment at the Interface of Tendon to Bone

The material mismatch at the attachment of tendon to bone is amongst the most severe for any tensile connection in nature. Attaching dissimilar materials is a major challenge in engineering, and has proven to be a challenge in surgical practice as well. Here, we examine the material attachment schemes employed at this connection through the lens of solid mechanics. We identify four strategies to that the body adopts to achieve effective load transfer between tendon and bone: 1) a shallow attachment angle at the insertion of transitional tissue and bone, 2) shaping of gross tissue morphology of the transitional tissue, 3) interdigitation of bone with the transitional tissue, and 4) functional grading of transitional tissue between tendon and bone. We provide solutions to model problems that highlight the first two mechanisms, discuss the third qualitatively in the context of engineering practice, and provide a review of our earlier work on the fourth. We study these strategies both in terms of ways that biomimetic attachment might benefit engineering practice, and of ways that engineering experience might serve to improve surgical healing outcomes.

The effects of exogenous basic fibroblast growth factor on intrasynovial flexor tendon healing in a canine model

BACKGROUND

Studies have demonstrated that flexor tendon repair strength fails to increase in the first three weeks following suturing of the tendon, a finding that correlates closely with the timing of many clinical failures. The application of growth factors holds promise for improving the tendon-repair response and obviating failure in the initial three weeks.

METHODS

The effects of basic fibroblast growth factor on flexor tendon healing were evaluated with use of a canine model. Operative repair followed by the sustained delivery of basic fibroblast growth factor, at two different doses, was compared with operative repair alone. Histological, biochemical, and biomechanical methods were used to evaluate the tendons twenty-one days after repair.

RESULTS

Vascularity, cellularity, and adhesion formation were increased in the tendons that received basic fibroblast growth factor as compared with the tendons that received operative repair alone. DNA concentration was increased in the tendons that received 1000 ng of basic fibroblast growth factor (mean and standard deviation, 5.7 ± 0.7 μg/mg) as compared with the tendons that received 500 ng of basic fibroblast growth factor (3.8 ± 0.7 μg/mg) and the matched control tendons that received operative repair alone (4.5 ± 0.9 μg/mg). Tendons that were treated with basic fibroblast growth factor had a lower ratio of type-I collagen to type-III collagen, indicating increased scar formation compared with that seen in tendons that received operative repair alone (3.0 ± 1.6 in the group that received 500-ng basic fibroblast growth factor compared with 4.3 ± 1.0 in the paired control group that received operative repair alone, and 3.4 ± 0.6 in the group that received 1000-ng basic fibroblast growth factor compared with 4.5 ± 1.9 in the paired control group that received operative repair alone). Consistent with the increases in adhesion formation that were seen in tendons treated with basic fibroblast growth factor, the range of motion was reduced in the group that received the higher dose of basic fibroblast growth factor than it was in the paired control group that received operative repair alone (16.6° ± 9.4° in the group that received 500 ng basic fibroblast growth factor, 13.4° ± 6.1° in the paired control group that received operative repair alone, and 29.2° ± 5.8° in the normal group [i.e., the group of corresponding, uninjured tendons from the contralateral forelimb]; and 15.0° ± 3.8° in the group that received 1000 ng basic fibroblast growth factor, 19.3° ± 5.5° in the paired control group that received operative repair alone, and 29.0° ± 8.8° in the normal group). There were no significant differences in tendon excursion or tensile mechanical properties between the groups that were treated with basic fibroblast growth factor and the groups that received operative repair alone.

CONCLUSIONS

Although basic fibroblast growth factor accelerated the cell-proliferation phase of tendon healing, it also promoted neovascularization and inflammation in the earliest stages following the suturing of the tendon. Despite a substantial biologic response, the administration of basic fibroblast growth factor failed to produce improvements in either the mechanical or functional properties of the repair. Rather, increased cellular activity resulted in peritendinous scar formation and diminished range of motion.

Musculoskeletal deformities secondary to neurotomy of the superior trunk of the brachial plexus in neonatal mice

The developmental course of musculoskeletal deformities in neonatal brachial plexus palsy (NBPP) has not been studied extensively. The goals of this study were to: (1) evaluate a new animal model of NBPP, (2) characterize the development of musculoskeletal abnormalities in paralyzed shoulders, and (3) investigate the expression of myogenic and adipogenic genes in paralyzed rotator cuff muscles. Neonatal mice were divided into neurotomy and sham groups. The neurotomy group underwent surgical transection of the superior trunk of the brachial plexus within 24 h of birth. The sham group underwent the same surgical exposure, but the brachial plexus was left intact. Musculoskeletal deformities were evaluated with radiological and histological assays at 2, 4, 8, 12, and 30 weeks after birth. The supraspinatus muscles of a separate group of mice were used to examine expression of myogenic and adipogenic genes at 8 weeks. The neurotomized forelimbs developed deformities similar to those seen in human NBPP. The deformities progressed with age. The denervated supraspinatus muscles showed intramuscular fat accumulation and upregulation of both myogenic and adipogenic genes compared to the normal. The current study presents a useful animal model for future research examining musculoskeletal changes secondary to neonatal nerve injury.

In vitro mineralization by preosteoblasts in poly(DL-lactide-co-glycolide) inverse opal scaffolds reinforced with hydroxyapatite nanoparticles

Inverse opal scaffolds made of poly(DL-lactide-co-glycolide) (PLGA) and hydroxyapatite (HAp) were fabricated using cubic-closed-packed (ccp) lattices of uniform gelatin microspheres as templates and evaluated for bone tissue engineering. The scaffolds exhibited a uniform pore size (213 +/- 4.4 microm), a porosity of approximately 75%, and an excellent connectivity in three dimensions. Three different formulations were examined: pure PLGA, HAp-impregnated PLGA (PLGA/HAp), and apatite (Ap)-coated PLGA/HAp. After seeding with preosteoblasts (MC3T3-E1), the samples were cultured for different periods of time and then characterized by X-ray microcomputed tomography (micro-CT) and scanning electron microscopy to evaluate osteoinductivity in terms of the amount and spatial distribution of mineral secreted from the differentiated preosteoblasts. Our results indicate that preosteoblasts cultured in the Ap-coated PLGA/HAp scaffolds secreted the largest amount of mineral, which was also homogeneously distributed throughout the scaffolds. In contrast, the cells in the pure PLGA scaffolds secreted very little mineral, which was mainly deposited around the perimeter of the scaffolds. These results suggest that the uniform pore structure and favorable surface properties could facilitate the uniform secretion of extracellular matrix from cells throughout the scaffold. The Ap-coated PLGA/HAp scaffold with uniform pore structure could be a promising material for bone tissue engineering.

"Aligned-to-random" nanofiber scaffolds for mimicking the structure of the tendon-to-bone insertion site

We have demonstrated the fabrication of "aligned-to-random" electrospun nanofiber scaffolds that mimic the structural organization of collagen fibers at the tendon-to-bone insertion site. Tendon fibroblasts cultured on such a scaffold exhibited highly organized and haphazardly oriented morphologies, respectively, on the aligned and random portions.

The development and morphogenesis of the tendon-to-bone insertion - what development can teach us about healing -

The attachment of dissimilar materials is a major challenge because of the high levels of stress that develop at such interfaces. An effective solution to this problem develops at the attachment of tendon (a compliant "soft tissue") to bone (a stiff "hard tissue"). This tissue, the "enthesis", transitions from tendon to bone through gradations in structure, composition, and mechanical properties. These gradations are not regenerated during tendon-to-bone healing, leading to a high incidence of failure after surgical repair. Understanding the development of the enthesis may allow scientists to develop treatments that regenerate the natural tendon-to-bone insertion. Recent work has demonstrated that both biologic and mechanical factors drive the development and morphogenesis of the enthesis. A cascade of biologic signals similar to those seen in the growth plate promotes mineralization of cartilage on the bony end of the enthesis and the formation of fibrocartilage on the tendon end of the enthesis. Mechanical loading is also necessary for the development of the enthesis. Removal of muscle load impairs the formation of bone, fibrocartilage, and tendon at the developing enthesis. This paper reviews recent work on the development of the enthesis, with an emphasis on the roles of biologic and mechanical factors.

Enhanced flexor tendon healing through controlled delivery of PDGF-BB

A fibrin/heparin-based delivery system was used to provide controlled delivery of platelet derived growth factor BB (PDGF-BB) in an animal model of intrasynovial flexor tendon repair. We hypothesized that PDGF-BB, administered in this manner, would stimulate cell proliferation and matrix remodeling, leading to improvements in the sutured tendon's functional and structural properties. Fifty-six flexor digitorum profundus tendons were injured and repaired in 28 dogs. Three groups were compared: (1) controlled delivery of PDGF-BB using a fibrin/heparin-based delivery system; (2) delivery system carrier control; and (3) repair- only control. The operated forelimbs were treated with controlled passive motion rehabilitation. The animals were euthanized at 7, 14, and 42 days, at which time the tendons were assessed using histologic (hyaluronic acid content, cellularity, and inflammation), biochemical (total DNA and reducible collagen crosslink levels), and biomechanical (gliding and tensile properties) assays. We found that cell activity (as determined by total DNA, collagen crosslink analyses, and hyaluronic acid content) was accelerated due to PDGF-BB at 14 days. Proximal interphalangeal joint rotation and tendon excursion (i.e., tendon gliding properties) were significantly higher for the PDGF-BB-treated tendons compared to the repair-alone tendons at 42 days. Improvements in tensile properties were not achieved, possibly due to suboptimal release kinetics or other factors. In conclusion, PDGF-BB treatment consistently improved the functional but not the structural properties of sutured intrasynovial tendons through 42 days following repair.

Contribution of extracellular matrix to the mechanical properties of the heart

Extracellular matrix (ECM) components play essential roles in development, remodeling, and signaling in the cardiovascular system. They are also important in determining the mechanics of blood vessels, valves, pericardium, and myocardium. The goal of this brief review is to summarize available information regarding the mechanical contributions of ECM in the myocardium. Fibrillar collagen, elastin, and proteoglycans all play crucial mechanical roles in many tissues in the body generally and in the cardiovascular system specifically. The myocardium contains all three components, but their mechanical contributions are relatively poorly understood. Most studies of ECM contributions to myocardial mechanics have focused on collagen, but quantitative prediction of mechanical properties of the myocardium, or changes in those properties with disease, from measured tissue structure is not yet possible. Circumstantial evidence suggests that the mechanics of cardiac elastin and proteoglycans merit further study. Work in other tissues used a combination of correlation, modification or digestion, and mathematical modeling to establish mechanical roles for specific ECM components; this work can provide guidance for new experiments and modeling studies in myocardium.

Nanofiber scaffolds with gradations in mineral content for mimicking the tendon-to-bone insertion site

We have demonstrated a simple and versatile method for generating a continuously graded, bonelike calcium phosphate coating on a nonwoven mat of electrospun nanofibers. A linear gradient in calcium phosphate content could be achieved across the surface of the nanofiber mat. The gradient had functional consequences with regard to stiffness and biological activity. Specifically, the gradient in mineral content resulted in a gradient in the stiffness of the scaffold and further influenced the activity of mouse preosteoblast MC3T3 cells. This new class of nanofiber-based scaffolds can potentially be employed for repairing the tendon-to-bone insertion site via a tissue engineering approach.

Recovery potential after postnatal shoulder paralysis. An animal model of neonatal brachial plexus palsy

BACKGROUND

Injury to the brachial plexus during birth results in paralysis of the upper extremity in as many as one in 250 births and can lead to substantial functional deficits in the shoulder. The goal of this study was to characterize the development of bone and joint deformities in paralyzed neonatal shoulders and to assess the improvement of these deformities after muscle function recovery with use of an animal model.

METHODS

Intramuscular injections of botulinum toxin were used to paralyze the supraspinatus, infraspinatus, and posterior deltoid of the left shoulders of mice at birth. Seventy mice were divided into three groups: Botox, recovery, and normal. The twenty-five mice in the Botox group received botulinum toxin injections until they were killed. The twenty mice in the recovery group received botulinum toxin injections for different durations and then were allowed injection-free recovery periods until they were killed. The twenty-five mice in the normal group received saline solution injections until they were killed. Radiographs were used to measure shoulder and elbow contractures. Microcomputed tomography was used to examine anatomical parameters of the supraspinatus muscle, humerus, and scapula.

RESULTS

The Botox group showed bone and joint deformities including delayed mineralization and flattening of the humeral head, hypoplasia, and introversion (i.e., anteversion) of the humerus, contractures of the shoulder and elbow, hypoplasia of shoulder muscles, hypoplasia of the scapula, and hypoplasia and retroversion of the glenoid. In the recovery group, a significant trend toward normal properties was observed with longer recovery periods (p<0.05). However, only soft-tissue contractures of the shoulder and elbow were resolved completely with the longest recovery period.

CONCLUSIONS

This mouse model successfully simulates human neonatal brachial plexus palsy, reproducing most of the bone and joint deformities found in the human condition. The deformities started to develop early in the postnatal period in the paralyzed shoulders and progressed with longer durations of paralysis. Early restoration of muscle function completely resolved the soft-tissue contractures of the shoulder and elbow. However, osseous deformities of the humerus and scapula were never resolved completely. These findings demonstrate the time-dependence of reversibility of musculoskeletal deformities in developing shoulders with neurological deficits.

The effect of muscle loading on flexor tendon-to-bone healing in a canine model

Previous tendon and ligament studies have demonstrated a role for mechanical loading in tissue homeostasis and healing. In uninjured musculoskeletal tissues, increased loading leads to an increase in mechanical properties, whereas decreased loading leads to a decrease in mechanical properties. The role of loading on healing tissues is less clear. We studied tendon-to-bone healing in a canine flexor tendon-to-bone injury and repair model. To examine the effect of muscle loading on tendon-to-bone healing, repaired tendons were either cut proximally (unloaded group) to remove all load from the distal phalanx repair site or left intact proximally (loaded group). All paws were casted postoperatively and subjected to daily passive motion rehabilitation. Specimens were tested to determine functional properties, biomechanical properties, repair-site gapping, and bone mineral density. Loading across the repair site led to improved functional and biomechanical properties (e.g., stiffness for the loaded group was 8.2 +/- 3.9 versus 5.1 +/- 2.5 N/mm for the unloaded group). Loading did not affect bone mineral density or gapping. The formation of a gap between the healing tendon and bone correlated with failure properties. Using a clinically relevant model of flexor tendon injury and repair, we found that muscle loading was beneficial to healing. Complete removal of load by proximal transection resulted in tendon-to-bone repairs with less range of motion and lower biomechanical properties compared to repairs in which the muscle-tendon-bone unit was left intact.

The tendon-to-bone transition of the rotator cuff: a preliminary Raman spectroscopic study documenting the gradual mineralization across the insertion in rat tissue samples

We applied Raman spectroscopy to monitor the distribution of mineral and the degree of mineralization across the tendon-bone insertion site in the shoulders of five rats. We acquired Raman spectra from 100 to 4,000 Deltacm(-1) on individual 1 microm points across the 120 microm wide transition zone of each tissue sample and identified all the peaks detected in pure tendon and in pure bone, as well as in the transition zone. The intensity of the 960 Deltacm(-1) P-O stretch for apatite (normalized to either the 2,940 Deltacm(-1) C-H stretch or the 1,003 Deltacm(-1) C-C stretch for collagen) was used as an indicator of the abundance of mineral. We relate the observed histological morphology in the tissue thin section with the observed Raman peaks for both the organic component (mostly collagen) and the inorganic component (a carbonated form of the mineral apatite) and discuss spectroscopic issues related to peak deconvolution and quantification of overlapping Raman peaks. We show that the mineral-to-collagen ratio at the insertion site increases linearly (R(2) = 0.8 for five samples) over the distance of 120 microm from tendon to bone, rather than abruptly, as previously inferred from histological observations. In addition, narrowing of the 960 Deltacm(-1) band across the traverse indicates that the crystalline ordering within the apatite increases concomitantly with the degree of mineralization. This finding of mineral gradation has important clinical implications and may explain why the uninjured tendon-to-bone connection of the rotator cuff can sustain very high loads without failure. Our finding is also consistent with recent mechanical models and calculations developed to better understand the materials properties of this unusually strong interface.

Controlled-release kinetics and biologic activity of platelet-derived growth factor-BB for use in flexor tendon repair

PURPOSE

Surgically repaired intrasynovial tendons are at greatest risk of failure in the first 3 weeks after surgery. Attempts to improve the strength of repair by modifying rehabilitation parameters have not always been successful. Manipulation of the biological environment of the sutured tendon holds great promise for accelerating the repair process. The goals of this study were to examine (1) the range of conditions (eg, dosage, delivery system formulation, presence of cells) over which delivery of platelet-derived growth factor-BB (PDGF-BB) can be sustained from fibrin matrices using a heparin-binding delivery system (HBDS) and (2) the biological activity of the PDGF-BB released from this system on canine tendon fibroblasts in vitro.

METHODS

We examined in vitro release kinetics from cellular and acellular fibrin matrices using enzyme-linked immunosorbent assays. We examined the biologic activity of the PDGF-BB in vitro by measuring cell proliferation (ie, total DNA) and collagen synthesis (ie, proline incorporation).

RESULTS

The acellular release kinetics of PDGF-BB was modulated by varying the ratio of PDGF-BB to heparin (PDGF-binding sites) or the dose of PDGF-BB in the presence of the delivery system. In the presence of canine tendon fibroblasts, the delivery system prolonged the duration of PDGF-BB release from fibrin matrices, thus demonstrating that cells are able to liberate PDGF-BB retained by the HBDS. Sustained delivery of PDGF-BB promoted increased cell proliferation at doses of 0.125 microg/mL and 1.25 microg/mL compared to fibrin without delivery system. Collagen synthesis was enhanced by PDGF-BB at doses of 0.125 microg/mL and 1.25 microg/mL; however, there was an enhancement over fibrin without the delivery system only at the lower dose.

CONCLUSIONS

These results demonstrate that the PDGF-BB released from fibrin matrices containing an HBDS is biologically active and can modulate both cell proliferation and extracellular matrix synthesis, both of which are key factors in the process of tendon repair.

Collagen fiber alignment does not explain mechanical anisotropy in fibroblast populated collagen gels

Many load-bearing soft tissues exhibit mechanical anisotropy. In order to understand the behavior of natural tissues and to create tissue engineered replacements, quantitative relationships must be developed between the tissue structures and their mechanical behavior. We used a novel collagen gel system to test the hypothesis that collagen fiber alignment is the primary mechanism for the mechanical anisotropy we have reported in structurally anisotropic gels. Loading constraints applied during culture were used to control the structural organization of the collagen fibers of fibroblast populated collagen gels. Gels constrained uniaxially during culture developed fiber alignment and a high degree of mechanical anisotropy, while gels constrained biaxially remained isotropic with randomly distributed collagen fibers. We hypothesized that the mechanical anisotropy that developed in these gels was due primarily to collagen fiber orientation. We tested this hypothesis using two mathematical models that incorporated measured collagen fiber orientations: a structural continuum model that assumes affine fiber kinematics and a network model that allows for nonaffine fiber kinematics. Collagen fiber mechanical properties were determined by fitting biaxial mechanical test data from isotropic collagen gels. The fiber properties of each isotropic gel were then used to predict the biaxial mechanical behavior of paired anisotropic gels. Both models accurately described the isotropic collagen gel behavior. However, the structural continuum model dramatically underestimated the level of mechanical anisotropy in aligned collagen gels despite incorporation of measured fiber orientations; when estimated remodeling-induced changes in collagen fiber length were included, the continuum model slightly overestimated mechanical anisotropy. The network model provided the closest match to experimental data from aligned collagen gels, but still did not fully explain the observed mechanics. Two different modeling approaches showed that the level of collagen fiber alignment in our uniaxially constrained gels cannot explain the high degree of mechanical anisotropy observed in these gels. Our modeling results suggest that remodeling-induced redistribution of collagen fiber lengths, nonaffine fiber kinematics, or some combination of these effects must also be considered in order to explain the dramatic mechanical anisotropy observed in this collagen gel model system.

Development of the supraspinatus tendon-to-bone insertion: localized expression of extracellular matrix and growth factor genes

The adult healing response of the rotator cuff tendon-to-bone insertion site differs from the ordered process of insertion site development. Healing is characterized by disorganized scar and a lack of fibrocartilage formation, in contrast to the well organized fibrocartilaginous transition which forms during the normal development of the tendon-to-bone insertion. The purpose of this study was to localize the expression of a number of extracellular matrix and growth factor genes during insertion site development in order to guide future strategies for augmenting adult rotator cuff healing. The rotator cuff was morphologically distinct at 13.5 dpc (days postconception). Neo-tendon was evident as a condensation of cells adjacent to bone. The interface between tendon and bone did not form into a mature fibrocartilaginous insertion until 21-days postnatally, based upon the appearance of four distinct zones with a mineralized humeral head. Fibroblasts of the supraspinatus tendon expressed type I collagen at all timepoints. Type II collagen was first expressed by chondrocytes in the fibrocartilage and mineralized fibrocartilage at 7 days and persisted in the mineralized fibrocartilage at 56 days. Type X collagen was first expressed by the chondrocytes in the mineralized fibrocartilage at 14 days and persisted in the mineralized fibrocartilage at 56 days. A shift from TGF-beta3 to TGF-beta1 expression occurred at 15.5 dpc.

PDGF-BB released in tendon repair using a novel delivery system promotes cell proliferation and collagen remodeling

The purpose of this study was to promote fibroblast proliferation and collagen remodeling in flexor tendon repair through sustained delivery of platelet derived growth factor (PDGF-BB). The release kinetics of PDGF-BB from a novel fibrin matrix delivery system was initially evaluated in vitro. After the in vivo degradation rate of the fibrin matrix was determined using fluorescently tagged fibrin, PDGF-BB was delivered to the site of flexor tendon repair in vivo in a canine model. The effect of PDGF-BB on intrasynovial tendon healing was studied using histology-based assays (cell density, proliferation, and type I collagen expression) and by measuring total DNA levels and reducible collagen crosslink levels. The fibrin matrix delivery system provided sustained release of PDGF-BB in vitro at a rate modulated by the ratio of heparin to growth factor. In vivo, the fibrin matrix remained at the repair site for more than 10 days. Delivery of PDGF-BB led to a qualitative increase in cell density, cell proliferation, and type I collagen mRNA expression. PDGF-BB also led to statistically significant increases in total DNA (20% increase at 7 days, 18% increase at 14 days) and reducible collagen crosslinks (30% increase at 7 days). Sustained delivery of growth factors may be achieved using a novel fibrin-based delivery system. PDGF-BB delivery increased cell proliferation and matrix remodeling and thus may accelerate flexor tendon healing.

Decreased muscle loading delays maturation of the tendon enthesis during postnatal development

Physical environment influences the development and maintenance of musculoskeletal tissues. The current study uses an animal model to explore the role of the physical environment on the postnatal development of the supraspinatus tendon enthesis. A supraspinatus intramuscular injection of botulinum toxin A was used to paralyze the left shoulders of mice at birth. The supraspinatus muscles of right shoulders were injected with saline to serve as contralateral controls. The supraspinatus enthesis was examined after 14, 21, 28, and 56 days of postnatal development. Histologic assays were used to examine fibrocartilage morphology and percentage osteoclast surface. Micro-computed tomography was used to examine muscle geometry and bone architecture. At 14 days there were no differences between groups in fibrocartilage formation, muscle geometry, bone architecture, or osteoclast surface. When comparing groups at 21, 28, and 56 days, muscle volume was decreased, fibrocartilage development was delayed, mineralized bone was decreased, and osteoclast surface was higher at each timepoint in the botulinum group compared to the contralateral saline control group. Our results indicate that the development of the tendon enthesis is sensitive to its mechanical environment. A reduction in muscle loading delayed the development of the tendon-to-bone insertion site by impeding the accumulation of mineralized bone. Physical factors did not play a significant role in enthesis maturation in the first 14 days postnatally, implying that biologic factors may drive early postnatal development.

Long durations of immobilization in the rat result in enhanced mechanical properties of the healing supraspinatus tendon insertion site

Rotator cuff tears frequently occur and can lead to pain and decreased shoulder function. Repair of the torn tendon back to bone is often successful in relieving pain, but failure of the repair commonly occurs. Post-operative activity level is an important treatment component that has received minimal attention for the shoulder, but may have the potential to enhance tendon to bone healing. The objective of this study was to investigate the effect of short and long durations of various activity levels on the healing supraspinatus tendon to bone insertion site. Rotator cuff tears were surgically created in Sprague-Dawley rats by detaching the supraspinatus tendon from its insertion on the humerus and these tears were immediately repaired back to the insertion site. The post-operative activity level was controlled through shoulder immobilization (IM), cage activity (CA), or moderate exercise (EX) for durations of 4 or 16 weeks. The healing tissue was evaluated utilizing biomechanical testing and a quantitative polarized light microscopy method. We found that activity level had no effect on the elastic properties (stiffness, modulus) of the insertion site at four weeks post injury and repair, and a decreased activity level had a positive effect on these properties at 16 weeks (IM>CA=EX). Furthermore, a decreased activity level had the greatest positive effect on these properties over time (IM>CA=EX). The angular deviation of the collagen, a measure of disorganization, was decreased with a decrease in activity level at 4 weeks (IM<CA=EX), but was similar between groups at 16 weeks (IM=CA=EX). It appears from this study that decreasing the activity level by immobilizing the shoulder improves tendon to bone healing, which progresses by first increasing the organization of the collagen and then increasing the mechanical properties. Future studies in this area will investigate the effect of passive motion and remobilization on both tendon to bone healing and shoulder function.

Alendronate prevents bone loss and improves tendon-to-bone repair strength in a canine model

Previously we showed a loss of bone and a concomitant decrease in mechanical properties in the first 21 days after flexor tendon insertion site injury and repair in a canine model. The goal of this short-term study was to suppress bone loss after insertion site repair using alendronate in an attempt to prevent the reduction in biomechanical properties. Flexor tendons of the second and fifth digits of the right forelimbs of canines were injured and repaired. Dogs received a daily oral dose of alendronate (2 mg/kg). One digit in each dog also received a local dose of alendronate in the bone tunnel at the time of surgery. The repair was evaluated for bone mineral density (BMD) and biomechanical properties and compared to data from a previous study in which no alendronate was used. Alendronate was effective in protecting the distal phalanx from resorption during tendon-to-bone healing (BMD was 94 and 104% of control for systemic alendronate and for systemic plus local alendronate, respectively). Alendronate treatment prevented much of the decrease in ultimate load that occurs in the first 21 days. Without treatment, ultimate load was 42% of control. With systemic alendronate treatment and systemic plus local alendronate treatment, ultimate load was 78 and 69% of control, respectively. Failure mode was significantly different when comparing alendronate treatment to repair alone. A lower incidence of suture pull through was found in alendronate treated dogs, suggesting less tendon degeneration. Ultimate load can be improved in association with preventing the bone loss that normally occurs during the early period following tendon-to-bone repair. These initial short-term data demonstrate the potential for a clinical treatment that could enhance tendon-to-bone healing.

The early effects of sustained platelet-derived growth factor administration on the functional and structural properties of repaired intrasynovial flexor tendons: an in vivo biomechanic study at 3 weeks in canines

PURPOSE

A bioactive fibrin-based delivery system was used to provide sustained administration of platelet-derived growth factor (PDGF-BB) in a clinically relevant model of intrasynovial flexor tendon repair. We hypothesized that PDGF-BB administered in this manner would improve the sutured tendon's functional and structural properties 3 weeks after repair.

METHODS

A delivery system consisting of 30 microL of fibrin matrix, peptide, heparin, and 100 ng of PDGF-BB was incorporated into the repair sites of randomly selected medial or lateral forepaw flexor digitorum profundus tendons of 8 adult mongrel dogs. The remaining forepaw flexor digitorum profundus tendons were repaired without the growth-factor and fibrin-based delivery system and served as controls. The surgically treated forelimbs were treated with controlled passive motion rehabilitation. The animals were killed at 3 weeks, at which time the tendons were tested for range of motion with a motion analysis system and for tensile properties with a materials testing machine.

RESULTS

Proximal interphalangeal joint and distal interphalangeal joint rotation values were significantly higher for the PDGF-BB-treated tendons compared with the repair-alone tendons. Excursion values were also significantly higher in the PDGF-BB-treated tendons. There were no significant differences in tensile properties when comparing PDGF-BB-treated with repair-alone tendons.

CONCLUSIONS

The functional properties of repaired intrasynovial flexor tendons were significantly improved with the sustained administration of PDGF-BB. The failure to achieve improvements in ultimate load, stiffness, and strain in the experimental group may have been due to suboptimal PDGF-BB dosage or suboptimal release kinetics.

Nicotine delays tendon-to-bone healing in a rat shoulder model

BACKGROUND

Many studies have shown that nicotine negatively impacts fracture healing and bone fusion processes. However, very little is known about its effect on tendon and ligament healing. The goal of the present study was to evaluate the effect of nicotine on tendon-to-bone healing.

METHODS

Supraspinatus tendons in both shoulders of seventy-two rats were transected and repaired to the humeral head. Osmotic pumps were implanted subcutaneously, and nicotine or saline solution was delivered for ten, twenty-eight, or fifty-six days. Cell morphology was evaluated with use of histologic sections. Cells were counted, and proliferating cell nuclear antigen (PCNA) immunohistochemistry was performed to assess cellular proliferation. In situ hybridization was performed to measure type-I collagen mRNA expression. Biomechanical and geometric properties were assessed.

RESULTS

Inflammation persisted longer in the nicotine group than in the saline solution group. Cellular proliferation was higher in the saline solution group than in the nicotine group at the early time-points. Type-I collagen expression was higher in the saline solution group at twenty-eight days. Mechanical properties increased over time in both groups. Maximum stress was significantly lower in the nicotine group than in the saline solution group at ten days. Maximum force was significantly lower in the nicotine group than in the saline solution group at twenty-eight days. Maximum force was significantly higher in the nicotine group than in the saline solution group at fifty-six days. Stiffness was not different between the groups at any time-point.

CONCLUSIONS

Nicotine caused a delay in tendon-to-bone healing in a rat rotator cuff animal model. Mechanical properties increased over time in both groups, but the properties in the nicotine group lagged behind those in the saline solution group. Chronic inflammation and decreased cell proliferation may partly explain the inferior biomechanical properties in the nicotine group as compared with the saline solution group.

CLINICAL RELEVANCE

Failure of rotator cuff repair is a major clinical problem. The adverse effect of nicotine on rotator cuff healing noted in this clinically appropriate animal model may be an important clinical consideration.

Early healing of flexor tendon insertion site injuries: Tunnel repair is mechanically and histologically inferior to surface repair in a canine model

Orthopedic injuries often require surgical reattachment of tendon to bone. Tendon ends can be sutured to bone by direct apposition to the bone surface or by placement within a bone tunnel. Our objective was to compare early healing of a traditional surface versus a novel tunnel method for repair of the flexor digitorum profundus (FDP) tendon insertion site in a canine model. A total of 70 tendon-bone specimens were analyzed 0, 5, 10 or 21 days after injury and repair, using tensile and range of motion mechanical testing, histology and densitometry. Ultimate force (a measure of repair strength) did not differ between surface and tunnel repairs at day 0. Both repair types had reduced strength at 10 and 21 days compared to 0 days, indicative of deterioration of suture grasping strength (tendon softening). At 21 days, tendons repaired in a bone tunnel had 38% lower ultimate force compared to surface repairs (p = 0.017). Histological findings were comparable between repair groups at 5 and 10 days but differed at 21 days, when we saw evidence of maturation of the tendon-bone interface in the surface repairs compared to an immature fibrous interface with no evidence of tendon-bone integration in the tunnel repairs. After accounting for bone removed by the tunnel, no difference in bone mineral density or trabecular bone volume existed between surface and tunnel repairs. If the results of our animal study extend to healing of the human FDP insertion, they indicate that FDP tendons should be reattached to the distal phalanx by suture to the cortical surface rather than suture in a bone tunnel.

Characteristics of the rat supraspinatus tendon during tendon-to-bone healing after acute injury

Rotator cuff repair is known to have a high failure rate. Little is known about the natural healing process of the rotator cuff repair site, hence little can be done to improve the tendon's ability to heal. The purpose of this study was to investigate the collagen formation at the early repair site and to localize TGFbeta-1 and 3 during early healing and compare their levels to cell proliferation and histological changes. Bilateral supraspinatus tendons were transected and repaired in 60 rats. Specimens were harvested and evaluated at 0, 1, 3, 7, 10, 28, and 56 days. Histological sections were evaluated for cell morphology. Immunohistochemistry and in situ hybridization was performed to localize protein and mRNA for collagen types I and III and TGFbeta-1 and 3. Proliferating cell nuclear antigen (PCNA) assay was performed to measure cell proliferation, and cells were counted to determine cell density. Biomechanical properties were evaluated. Repair tissue demonstrated an initial inflammatory response with multinucleated cells present at 1 and 3 days, and lymphocytes and plasma cells presents at 7 and 10 days. Capillary proliferation began at 3 days and peaked at 10 days. Ultimate force increased significantly over the time period studied. Collagen I protein and mRNA significantly increased at 10 days, and reached a plateau by 28 and 56 days. Collagen III showed a similar trend, with an early increase, and remained high until 56 days. TGFbeta-1 was localized to the forming scar tissue and showed a distinct peak at 10 days. TGFbeta-3 was not seen at the healing insertion site. Cell proliferation and density followed the same trend as TGFbeta-1. A wound healing response does occur at the healing rotator cuff insertion site, however, the characteristics of the tendon after healing differ significantly from the uninjured tendon insertion site at the longest time-point studied. A distinctive collagen remodeling process occurred with an initial increase in the formation of collagen types I and III followed by a decrease toward baseline levels seen at time 0. Growth factor TGFbeta-1 was localized to repair tissue and coincided with a peak in cell proliferation and cellularity. Repair sites remained unorganized histologically and biomechanically inferior in comparison to previously described uninjured insertion sites.

Decreased collagen organization and content are associated with reduced strength of demineralized and intact bone in the SAMP6 mouse

To examine the link between bone material properties and skeletal fragility, we analyzed the mechanical, histological, biochemical, and spectroscopic properties of bones from a murine model of skeletal fragility (SAMP6). Intact bones from SAMP6 mice are weak and brittle compared with SAMR1 controls, a defect attributed to reduced strength of the bone matrix. The matrix weakness is attributed primarily to poorer organization of collagen fibers and reduced collagen content.

INTRODUCTION

The contribution of age-related changes in tissue material properties to skeletal fragility is poorly understood. We previously reported that bones from SAMP6 mice are weak and brittle versus age-matched controls. Our present objectives were to use the SAMP6 mouse to assess bone material properties in a model of skeletal fragility and to relate defects in the mechanical properties of bone to the properties of demineralized bone and to the structure and organization of collagen and mineral.

MATERIALS AND METHODS

Femora from 4- and 12-month-old SAMR1 (control) and SAMP6 mice were analyzed using bending and torsional mechanical testing of intact bones, tensile testing of demineralized bone, quantitative histology (including collagen fiber orientation), collagen cross-links biochemistry, and Raman spectroscopic analysis of mineral and collagen.

RESULTS

Intact bones from SAMP6 mice have normal elastic properties but inferior failure properties, with 60% lower fracture energy versus SAMR1 controls. The strength defect in SAMP6 bones was associated with a 23% reduction in demineralized bone strength, which in turn was associated with poorer collagen fiber organization, lower collagen content, and higher hydroxylysine levels. However, SAMP6 have normal levels of collagen cross-links and normal apatite mineral structure.

CONCLUSIONS

Bones from SAMP6 osteoporotic mice are weak and brittle because of a defect in the strength of the bone matrix. This defect is attributed primarily to poorer organization of collagen fibers and reduced collagen content. These findings highlight the role of the collagen component of the bone matrix in influencing skeletal fragility.