Novel Rotator Cuff Repair Device Modeled After … Python Teeth?

A better way to repair torn rotator cuffs could be found by looking into a python’s mouth, researchers said.

While these animals are known for squeezing their prey to death, they also rely on peculiarly shaped teeth to keep their victims from escaping before the snakes can fully coil around them. Now, a group led by Stavros Thomopoulos, PhD, of Columbia University in New York City, has developed 3D printed devices modeled after python teeth to strengthen tendon-bone attachment beyond that achievable with conventional techniques.

Their invention passed muster in a series of lab studies, including tryouts with human cadavers, Thomopoulos and colleagues reported in Science Advances.

“Overall, our research not only introduces a device that significantly improves mechanical strength, but also, in future design iterations, aims to facilitate the delivery of biologics using bioabsorbable materials with a porous structure to improve tendon-to-bone healing,” they wrote.

Torn rotator cuffs are, of course, one of the most common orthopedic injuries. An estimated 40% of Americans older than 65 are affected, and something like 600,000 repair surgeries are performed in the U.S. each year.

Currently, the researchers explained, rotator cuff repair relies on sutures to stitch tendons onto shoulder bones. But over time, regardless of the technique used, the sutures tend to slice through the tendon in places where stress is highest.

This “cheesewiring effect” is at least partly responsible for the substantial failure rates that surgeons know all too well. “These rates range from 20% in younger patients with minor tears to a staggering 94% in elderly patients with massive tears,” Thomopoulos and colleagues wrote. They also noted that while biologic adjuncts can speed healing, they don’t add any mechanical reinforcement or protection against cheesewiring.

What’s needed, the researchers thought, is something to grasp the tendon gently. Nature, it turns out, has developed solutions to this problem, which typically appear like hooks. This is the case with python teeth, which curve inward so that when the snake bites down on a prey animal, the latter’s attempts to break free only lodge the teeth deeper, yet without tearing the flesh. (Something similar may be seen on “hitchhiker plant” burrs, rose thorns, and asparagus leaf spines, the investigators observed.)

To turn this idea into a workable medical device, Thomopoulos and colleagues used a 3D printer to create small biocompatible plastic rectangles studded with curved teeth. Experiments showed that a “curvature ratio” of 2.5 — the tooth tip’s deflection divided by its base’s width — was best for grasping pieces of bovine tendon without cutting. The group also tested different tooth arrangements and spacing to come up with an optimal design. A key advantage of 3D printing is that it makes possible devices designed specifically for individual patients, with the underside shaped to fit perfectly on the humeral head.

Thomopoulos and colleagues envisioned using sutures to hold the device in place to attach tendons to the humeral head. They tested their best design on five paired shoulder joints from human cadavers. For each joint, the researchers simulated a rotator cuff tear by cutting the supraspinatus tendon with a scalpel. Each shoulder was then randomly assigned to undergo a conventional double-row suture repair or one using the same technique to secure the python-tooth device. The devices measured 15.5-17.5 mm by 6-8 mm, each carrying 13 teeth about 3 mm in height. The repaired joints were then put under strain until they failed.

“Paired comparisons revealed that repairs incorporating the device exhibited an average increase in maximum force (i.e., strength) of 83% relative to matched controls without the device,” the researchers reported. This near-doubling in repair strength, they argued, “could significantly affect postoperative outcomes by reducing the high rerupture rates now observed.”

The investigators emphasized that they intend to explore further modifications to the design. “Future versions should consider a porous base that might better support tendon-to-bone healing and also serve as a depot for localized drug delivery,” they wrote. “We will also assess long-term outcomes through large animal model studies, investigating both mechanical integrity of the repair and healing.”

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