1. The structure and function of the tendon-to-bone insertion site.
The tendon-to-bone insertion is biomechanically, compositionally, and structurally complex. The four types of tissue recognized at the insertion are tendon, fibrocartilage, mineralized fibrocartilage, and bone. We are studying tendon-to-bone insertions of the rotator cuff to determine the structure-function relationships that give the tissue its unique mechanical properties. Experimentally, we have evaluated the gene expression, collagen organization, mineral content, and biomechanical properties of the insertion. Using these results we have developed theoretical models to relate structure to mechanical function. Based on our results, it is apparent that the tendon-to-bone insertion site varies dramatically along its length in collagen structure, extracellular matrix composition, mineral content, geometry, and viscoelastic properties. Our studies suggest that this gradation in properties distributes the forces more effectively across the transition from a flexible to a rigid material.
2. Mechanobiology during development of the tendon-to-bone insertion.
An understating of the natural development of the tendon-to-bone insertion site will allow us to design biomimetic strategies for insertion site tissue engineering. We developed an animal model to examine the role of mechanical loading on the postnatal development of the supraspinatus tendon-to-bone insertion. Supraspinatus intramuscular injections of botulinum toxin A were made in the left shoulders mice within 24 hours of birth (‘Botox’ group). The supraspinatus muscles of right shoulders were injected with saline to serve as contralateral controls (‘Saline’ group). The absence of load during tendon-to-bone insertion development impaired mineral accumulation, fibrocartilage formation, and collagen fiber organization at the insertion.
Figure- Muscle volume is dramatically reduced due to chemical denervation (i.e., botulinum toxin A injection).
Figure- Rotator cuff paralysis leads to bony deformities at the humeral head.
Figure- The developing supraspinatus tendon (‘s’) to humeral head (‘h’) insertion (‘i’). The saline-injected shoulders are shown on the left and the contralateral paralyzed shoulders are shown on the right. The paralyzed shoulders contain a poorly developed insertion site.
3. Tendon-to-bone healing.
The attachment of tendon to bone occurs across a complex transitional tissue with a gradation in structure, composition, and mechanical behavior. Following tendon-to-bone surgical repair, this organ presents a high incidence of recurrent failure. We believe that these high failure rates are due in part to an inability to reconstruct the mechanical complexity of the natural organ during healing. We are currently studying mechanical and biologic treatments for tendon-to-bone healing. We are manipulating the loading environment at the healing insertion in an attempt to both protect the repair and promote better tendon-to-bone integration. We are also delivering growth factors to the repair site in an attempt to promote a fibrocartilaginous transition between the healing tendon and bone for rotator cuff and flexor tendon repair.
Figure- The normal tendon-to-bone insertion (left image) has collagen fibers inserting uniformly into bone. The healing insertion (right) does not recreate this transition. Instead, there is an abrupt interface between the healing tendon and bone, leaving the tissue prone to recurrent tear.
4. Tissue engineering the tendon-to-bone insertion.
We recently demonstrated a simple and versatile method for generating a continuously graded, bonelike calcium phosphate coating on a nonwoven mat of electrospun nanofibers (Nano Letters). A linear gradient in calcium phosphate content was 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 nanofiberbased scaffolds can potentially be employed for repairing the tendon-to-bone insertion site via a tissue engineering approach.
Figure- SEM images of calcium phosphate coatings on a nonwoven mat of PLGA nanofibers. The images were taken from different regions along the scaffold. The scale bars in the insets are 2 µ
m.
Figure- Mechanical testing of the graded scaffolds. There was a gradation in mechanical properties along the length of the scaffolds (a representative PLGA scaffold is shown). (a) The strains in the x1 direction for three values of stress are shown. Localized strains are shown on the left and average strains are shown on the right. Strain increased with increasing stress and was highest on the unmineralized side of the scaffold. (b) There was a linear decrease in calcium phosphate along the length of the scaffold. (c) Young’s modulus decreased with decreasing calcium phosphate content.
5. Enhanced flexor tendon healing with sustained delivery of growth factors.
Prior studies have reported a high incidence of failure in the early period following flexor tendon repair. Adhesion formation between the repaired tendon and the digital sheath and repair site gapping can lead to loss of finger function and risk for tendon rupture. The delivery of growth factors during tendon healing can lead to improvements in the properties of the healing tissue. The overall goals of these studies is to determine the effect of PDGF-BB and bFGF on intrasynovial flexor tendon healing. We hypothesized that sustained delivery of these growth factors would promote cell proliferation and collagen synthesis leading to improved biomechanical properties of the repair. Using a heparin based delivery system, we were able to deliver both growth factors in a sustained manner. Deliver of PDGF-BB led to improvements in the functional properties of the tendons. Current studies are evaluating bFGF as well as other delivery systems for flexor tendon repair.
Figure- Sustained release of the growth factor PDGF-BB was achieved using a heparin based delivery system (DS). The release rate was dependent on the heparin to growth factor ratio (a ratio of 1:10,000 resulted in the slowest release rate).
Figure- Range of motion for the proximal interphalangeal finger joint (PIP) was significantly improved due to PDGF (CTL- repair only control, DS- delivery system control, PDGF- growth factor treatment, NORM- uninjured control).