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New screws can advance ACL reconstruction

Stanley Asnis, MD, holds his twist-lock interface screw, which promises to limit bone loss and damage to graft ligaments during ACL repair

Teaming with his son, Stanley Asnis, MD, developed the twist-lock interface screw, which promises to limit bone loss and damage to graft ligaments during ACL repair

Stanley Asnis, MD, has a long history as a medical device innovator, holding multiple patents on products that have reshaped orthopedic surgery.

His son, Peter, is a prominent orthopedic surgeon at Massachusetts General Hospital, and the sports physician for New England Patriots, Boston Bruins and Red Sox.

The father-son team knows hardware, and they recently teamed up to develop another invention — the twist-lock interface screw, which they believe will improve first-time anterior cruciate ligament reconstruction and reduce the risk of re-intervention.

ACL reconstruction is a $2 billion-plus a year business in the US. Revision of the initial surgery makes it even more costly while minimizing efficacy and extending recovery times.

Not your typical surgical screw

The twist-lock interface screw isn’t your typical surgical screw. It has threads on just two sides and promises to limit bone loss and damage to graft ligaments during ACL repair surgery by offering clinicians a stable, precise, low-friction, low-stress fixation device.

“With an older screw, there is a scrunch, scrunch, scrunch happening inside the bone because of the threads continuously driving into the bone,” said Dr. Asnis, Northwell’s chief of joint replacement and reconstruction. “Here, you slide the new screw into the slot and just turn it, so it compresses the ligament to the bone.”

How it works

As opposed to classic screws that are driven in by a screwdriver, the twist-lock interface screw is inserted rather seamlessly.

After surgeons pilot a hole in the desired placement, they then use an apparatus to hold the twist-lock screw, push it into the piloted hole and turn to lock. The apparatus is then detached, leaving the screw in place.

“Turn 90 degrees and you’re locked,” Dr. Asnis said. “The anterior cruciate is the ligament across the knee. If you tear it, surgeons make a new one by taking a piece of the patella tendon and a piece of the tibial tubercle, and you put it into a knee by locking them with this screw.

“When classic screws compress the graph it can move. The beauty of the new screw is it’s got sharp threads on the two sides. It doesn’t have all that friction. Friction with classic screws actually is destroying the bone. This will just cut in and stay locked.”

Getting to market

The prototype for the new Asnis screw was made in Northwell’s 3D Design and Innovation Center. The health system is utilizing its partnership with i360medical, an Irish company dedicated to curating internal ideas, testing the product, identifying manufacturers and getting the device to market.

Dr. Asnis said the screw has already been tested on human bone and graft substitutes. And i360 is shopping it to a manufacturer, who will build and sell the product.

He doesn’t think it will take that long. “The selling point is ease,” he said. “When you’re putting the screw driver into a classic screw, it’s destroying bone.”
 

quotation mark With an older screw, there is a scrunch, scrunch, scrunch happening inside the bone because of the threads continuously driving into the bone. Here, you slide the new screw into the slot and just turn it, so it compresses the ligament to the bone.
Stanley Asnis, MD

A history of surgical innovation

The twist-lock interface screw is one of many orthopedic devices Dr. Asnis has developed.

In 2001, he worked with Biomet to develop the Rx 90 Total Hip System, a sterile implantable principle component of a total hip prosthesis designed to replace the femoral head and neck. The device improves fixation and stability by promoting bone ingrowth.

“The total hip was not a game changer, but it was a commercially available and well-used hip system for about 15 years,” Dr. Asnis said.

While it wasn’t a “game-changer,” the Asnis cannulated screw was. In the 1980s, these screws helped accelerate the advancement of minimally invasive surgery in orthopedics. Now in its third generation, the Asnis cannulated screws feature a low-profile screw head, and a wide range of lengths and diameters for foot and ankle surgeons to choose from.

“The cannulated screws mean you put in a guide pin and the screw goes over it. No one did that then,” he said. “So therefore, you’re doing a hip fracture, there was a real problem.

"You would have to put in a solid screw. If you’re wrong, pull it out and make a big hole."

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