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Barbara Pickard, Ph.D., didn’t know the burrs that stuck to her socks as she hiked through a southwestern desert would someday have the potential to revolutionize how surgeons attach tendon to bone. Still, she suspected the bothersome plants had some inherent benefit.
“The late, great Pickard, a pioneer of mechanobiology, didn’t simply discard the burrs — she latched onto the idea that nature could provide novel solutions in unexpected places,” said Guy Genin, Ph.D., Co-Director of the Center for Engineering MechanoBiology (CEMB) at Washington University in St. Louis.
Dr. Genin and his research team used the spacing and mechanics of the burrs’ attachment system to develop a novel method of connecting tendon to bone. The method, which balances the force across sutures and reduces stress on individual tendons, could double the repair strength of current suturing techniques.
A fortuitous conversation that Dr. Pickard, a then CEMB faculty fellow, had with graduate student Ethan D. Hoppe, Ph.D., decades after her hike led to the “aha” moment that helped the Washington University researchers solve a surgical problem they had been grappling with for years.
“When surgeons repair a rotator cuff, they remove the body’s natural connectors, which have evolved for the complex task of transitioning from hard bone to soft tendon, and place sutures that concentrate force in a tiny area. That’s what leads to the high failure rate for that procedure,” Dr. Genin said. “Nature has shown us how hard materials like the stiff hooks on a burr can attach very effectively to soft materials like socks. We just needed to figure out how burrs compare to sutures and how this natural solution might be applied in practice.”
Making the Most of a Sticky Situation
The research team at CEMB had joked about using Velcro to make tendon repairs simple and resilient. The concept made sense. Velcro resists pulling out from fibrous material, and tendons comprise fibrous tissue. After further debate, the researchers rightfully assumed that surgeons would not want to work with a Velcro-like material during surgery.
The conversation about Velcro could have ended there, but Dr. Hoppe realized after his conversation with Dr. Pickard that the way burrs naturally balance forces resembled the hoop-and-loop properties of Velcro.
It was the breakthrough the team needed to advance their research, but a challenge came in finding plants that grow in arid climates to test in downtown St. Louis.
After a long search, the researchers found Harpagonella palmeri plants — the same species that had stuck to Dr. Pickard’s socks — at the Santa Barbara Botanic Garden, which curates native California plant species.
The hooks found on the fruits of the plants have multiple hierarchies of grabbing mechanisms, with many small-scale features that improve stickiness to fibers.
“The mechanism that grabbed our attention at the larger scale was the way the hooks share mechanical loads,” Dr. Genin said. “Once the fruit hooks into a material, a shear force exerted to brush it off aligns the fruit so that the longest and least-stiff hook is in the direction of the pulling.”
The research team used these properties to develop a mathematical model that demonstrated the unique way that the plant balances forces across relatively few attachment points, regardless of the material to which it attaches.
Their work was published in the March 1 issue of Proceedings of the Royal Society A.
“When we analyzed their grappling hooks with the attachment models we had developed, the result amazed all of us,” Dr. Genin said. “Across all specimens, the three hooks on the plant’s fruits were shaped in a way that caused them to share force equally. The fruits used physics that we could modify and apply to surgical reattachment.”
Dr. Genin said that if the hooks were all the same length and stiffness, a force would be concentrated on the first hook. “But because the first hook is less stiff, the next takes on more force,” he added. “By distributing the force equally across all hooks, the attachment becomes stronger.”
Real-world Surgical Applications
Connecting tendon and bone, which have vastly different properties, is a challenging mechanical problem to solve, according to Stavros Thomopoulos, Ph.D., Professor of Biomechanics in Orthopedic Surgery and Biomedical Engineering at Columbia University.
“The body has solved this problem through a number of structural and compositional mechanisms, but none of these characteristics are regenerated during healing, leaving the repaired tendon-to-bone connection prone to failure,” Dr. Thomopoulos said. “Researchers have assessed mechanical and biological approaches to improve repairs.”
Mechanical approaches include python tooth-inspired fixation devices and mussel-inspired adhesives. Biologic approaches include stem cell and growth factor therapies inspired by developmental biology. Neither approach is particularly effective.
Rotator cuff repairs in young patients with isolated tears fail in 10% to 20% of cases. Repairs in elderly patients with massive tears fail in over 90% of cases — and are currently considered inoperable. “Reducing these failure rates would transform the care of rotator cuff injuries,” Dr. Thomopoulos said.
He’s currently conducting pre-clinical laboratory testing to evaluate how the suturing technique discovered by the Washington University research team affects the strength of rotator cuff repairs.
Dr. Thomopoulos said there are two practical ways to apply the attachment properties of Harpagonella palmeri for immediate and long-term results. “The immediate application is to re-consider the spacing and number of anchor points used in current rotator cuff repair methods,” he explained. “The long-term application is to develop a fixation device inspired by the plant.”
Such a fixation device, which would have hooks with geometries and spacings that mimic those of the plant, would be placed between tendon and bone. Surgeons could use the device to mechanically augment their current repair techniques.
Additional potential applications of the Harpagonella palmeri’s attachment properties include ACL and meniscus repairs, according to Dr. Thomopoulos. “We are very excited to implement this concept in a real-world surgical setting,” he said.
