Engineers often use nature to inspire new materials and designs. A discovery by a multi-institutional team of researchers and engineers about how tendon and bone attach in the shoulder joint has uncovered previously unsuspected engineering strategies for attaching dissimilar materials. The discovery also sheds new light on how the rotator cuff functions and on why rotator cuff repairs fail so frequently.
Guy Genin, the Harold and Kathleen Faught Professor of Mechanical Engineering at the McKelvey School of Engineering at Washington University in St. Louis, and Stavros Thomopoulos, the Robert E. Carroll and Jane Chace Carroll Professor of Orthopaedic Surgery at Columbia University, led a team that discovered a previously unknown fibrous architecture between the rotator cuff tendons and their bony attachments in the shoulder.
Mercury (II) chloride-stained contrast enhanced high-resolution microCT imaging revealed that, hidden within the well-known larger apparent attachment footprint area, is a smaller, much denser primary insertion site where tendon fibers insert directly into the bone. This image is a 3D volume rendering of representative intact enthesis. Image credit: Genin lab
Results of the work were published in the journal Science Advances.
Rotator cuff tears — among the most common tendon injuries in adults — occur when tendons pull away from or break near the bone. Thirty percent of adults over age 60 have a tear, and more than 60% of adults over age 80 have a tear. Surgery to repair the tears has a high failure rate, ranging anywhere from 30% to 90% depending on age and other factors. Genin, Thomopoulos and their teams have been studying the mechanobiology of these tissues for several years.
To take a closer look at the enthesis, or the transitional material where each of the four rotator cuff tendons attaches to the bone, the team applied a novel micro computed tomography (microCT) technique. They found that the toughness of the healthy rotator cuff arises from the composition, structure and position of the enthesis as the architecture of the fibrous soft tissues interacts with that of the bone.
“When (lead author) Mikhail Golman first showed us these images, we realized that much of the old picture of how tendon and bone interact had to be redrawn,” Genin said. “The fiber system there seems like fibers in a rope, and we can understand much about where the toughness comes from by understanding how these fibers break sequentially when they are next to the bone. It’s a new way of thinking about how to attach different materials.”