Novel Tissue Engineering Strategies for Tendon Repair and Regeneration

Improving the functional outcome of lacerated tendons with a combination of surgical intervention and controlled therapy is the focus of current tendon repair paradigm. Tendons undergo gliding motion to transmit the force of muscle contraction to the bone enabling limb movement. The body’s natural ability to heal these tensile tissues is very poor leading to significant patient morbidity which severely impacts their work, recreational activities and daily needs. The healing process is complicated by the need to provide appropriate and timely tension to the repair. Too less limb movement will promote adhesions with the surrounding tissues leading to friction and pain during movement. The regenerate tendon tissue will be scarred and mechanically weak. Too much tension during the early repair phase can cause re-rupture and requires extensive corrective surgery. Current approaches use either an autograft or a freeze-dried allograft to augment and reestablish a gap defect. Biodegradable polymers are an attractive alternative to overcome the extreme shortage of autografts, the immunogenicity and disease transmission associated with allografts, and to accommodate the diverse range of stiffness’s and mechanical strengths of the tendon and ligament tissues displayed in vivo.

In the research presented here, an FDA approved biodegradable synthetic polymer, poly (lactide-co-glycolide) 65:35 (PLAGA) was developed by electrospinning technique into a biocompatible scaffold composed into fibers that dimensionally mimic the collagen fiber bundles evident in native tendon tissue. These scaffolds were seeded with primary adipose derived stromal cells (ADSCs) and treated with Growth/Differentiation Factor-5 (GDF5) supplemented medium. Cell proliferation and gene expression studies were conducted to evaluate the potential of 3D nanofiber and 2D film scaffolds to upregulated tendon phenotype markers and support cell adhesion and proliferation. The gene expression and proliferation response of ADSCs seeded on the selected scaffold in response to GDF5 protein treatment was evaluated to determine suitable concentration. The GDF5 protein was immobilized onto the surface of nanofiber scaffolds to deliver bioactive concentrations to modulate neo-tendinogenesis and the maturation of the regenerate tissue. The scaffolds were evaluated in vivo in a rat Achilles tendon injury model to evaluate the potential to regenerate tendon tissue following bridging of a gap defect.

The nanofiber scaffold demonstrated an increased neo-tendinogenic response in ADSCs as compared to the film scaffold in in vitro culture; and additionally increased the tensile properties of the regenerate tendon. These fiber bundles fabricated by electrospinning technique mimic the ECM topography evident in tendon which is mostly composed of collagen fiber bundles, and is chiefly responsible for tendon tensile strength enabling limb movement. Mechanical characterization of the nanofiber scaffold showed sufficient tensile properties to support passive and resistive active motion of the flexor tendons present in the hand. Supplementation of GDF5 protein in the culture medium found increased neo-tendinogensis and ECM expression at a concentration of 100ng/mL when the ADSCs were seeded onto the nanofiber scaffold. At higher concentrations GDF5 protein exhibited inhibition of ADSC proliferation.

PLAGA 3D scaffold was surface modified to covalently bind GDF5 protein to develop a scaffold-composite that can provide bioactive GDF5 protein locally at the gap defect. The combination of nanofiber scaffold with ADSCs or GDF-5 protein established continuity of the lacerated tendon and was replaced with a regenerate tendon that showed comparatively higher collagen fiber alignment and collagen composition similar to native tendon tissue. The results indicated that the nanofiber scaffold seeded with ADSCs improved the relative ratio of collagen type I to collagen type III suggesting faster remodeling and restoration of functionality. During the early stages of healing, the scaffold provided mechanical support thus stabilizing the repair tissue which is critical to prevent re-ruptures.