The crucial rotator cuff examined in fine detail

The rotator cuff – the muscles and tendons that connect and operate your shoulder – is a crucial joint in the human body. It’s also prone to damage, with around 30% of people over the age of 60, and 60% of those over 80, having a rotator cuff tear. Surgery to repair these tears is often unsuccessful, as the connection between tendon and bone is still poorly understood.

That may be about to change, with a group of US researchers using a new imaging technique to learn more about the biomechanics of the rotator cuff. They’re hoping their discoveries will lead to better repair processes.

The researchers, who have published their findings in Science Advances, examined the shoulders of 275 mice using an X-ray imaging technique called micro computed tomography (or microCT). They combined this with electron microscopy, mechanical testing of the mouse shoulders and numerical modelling to understand the toughness of the rotator cuff.

“When [lead author] Mikhail Golman first showed us these images, we realised that much of the old picture of how tendon and bone interact had to be redrawn,” says co-author Professor Guy Genin, a mechanical engineer at Washington University in St Louis, US.

“The fibre system there seems like fibres in a rope, and we can understand much about where the toughness comes from by understanding how these fibres break sequentially when they are next to the bone. It’s a new way of thinking about how to attach different materials.”

The toughness of the rotator cuff – the amount of energy required to break it – was related to these fibres, which connect tendon and bone.

“There’s actually a trade-off between strength and toughness with these fibre systems,” says Genin.

“If you reduce the overall strength by allowing some of the fibres to break, you can actually make the structure tougher because the amount of energy absorbed increases.”

Co-author Stavros Thomopoulos, professor of orthopaedic surgery at Columbia University, US, says that the researchers’ goal “was to understand where the healthy rotator cuff gets its toughness and strength and under what conditions it ruptures.

“We found that the toughness of the rotator cuff varies as a function of shoulder position, helping to explain differences in the injury patterns seen in patients.”

Genin says that the research provided a new look at this frequently injured region, and presented new thinking about how to attach the two different materials.

“This is not just important for surgeries, but for all kinds of engineering failures that occur when you connect a material to something with a different architecture,” he says. “By merging the architectural paradigms across the materials and enabling the distributed failure of the elements that come together, you can dramatically increase toughness.”

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