Biomimetic Patches Improve Shoulder Surgery Outcomes

Two new studies describe a novel approach intended to solve one of the most difficult problems in orthopedic surgery: reattaching tendon to bone in rotator cuff repairs.

Researchers at Washington University (St. Louis, MO, USA) developed a scaffold consists of a mat of nanoscopic fibers that mimic the structure of collagen fibers in a tendon. The mat is then coated with a continuous gradient of hydroxyapatite--a mineral containing calcium and phosphorus that gives strength to bone--so that it is taut and bone-like toward one end, and compliant and tendon-like toward the other. Finally, the scaffold is seeded with adult mesenchymal stem cells, which can mature into osteoblasts or fibroblasts. The hypothesis is that as the fibers disintegrate over the course of a few months, the mineral gradient will promote the graduated differentiation of the stem cells; those toward the rigid end will be coerced by the presence of mineral to differentiate into osteoclasts, while the stem cells at the tendon end, surrounded by aligned, unmineralized fibers, will form fibroblasts.

To assist the process, transforming growth factor beta 3 (TGF-β3) is released in a controlled manner using a heparin/fibrin-based delivery system (HBDS). TheTGF-β3 treatment accelerated the healing process in an in vivo-rat model, with increases in inflammation, cellularity, vascularity, and cell proliferation at the early time points. 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 when compared to controls. The studies were published in the January 11, 2011, of the Journal of Orthopedic Research and the February 7, 2011, issue of Small.

"Attaching a compliant material like tendon to a relatively stiff material like bone is a fundamental engineering challenge. Soon after birth, the bone starts to mineralize and a linear gradient forms between the bone and the tendon that slowly stiffens the material and transitions it from hard bone to compliant tendon,” said lead author Stavros Thomopoulos, PhD, an associate professor of orthopaedic surgery. "This unique transitional tissue is not recreated after injury. If you fall off a ladder and your rotator cuff tears, even if the surgeons goes in and puts the tendon back on the bone, he's putting the tendon right against bone. You don't have the graded interface. Biologically, it doesn't reform.”

"A few scaffolds have been attempted in rotator cuff repair in humans, with only limited success,” added Dr. Thomopoulos. "These patches are fairly compliant materials, even compared to tendon. The hope is that they'll stimulate a better healing response, but it hasn't happened yet. Our scaffold approach is to mimic the natural tissue by stepping up gradually in stiffness from tendon to bone.”

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