Expanding plastics give orthopedic fasteners a better grip

DN Staff

January 3, 2000

10 Min Read
Expanding plastics give orthopedic fasteners a better grip

Marlborough, MA-People get hurt. It's a fact not just known but felt by more and more weekend warriors as they blow out aging knees and shoulders on the ski slopes, tennis courts, and other battlefields of modern recreation. Innovasive Devices Inc. helps ease their pain -- with plastics.

True to its name, ConTack tacks soft tissue to underlying bone. But this orthopedic fastener also expands into the bone and features a floating head that adjusts to the contour of the soft tissue.

As a maker of polymer fastening systems that attach soft tissue to bone, Innovasive operates on the forefront of a growing trend toward minimally invasive orthopedic surgeries. "Everything we do relates to returning people from their injuries as quickly as possible," says Alan Chervitz, Innovasive's chief operating officer. "The faster a patient gets moving, the better they do." But speedy rehabilitation comes at a price: it demands high- strength fastening devices to combat the constant stresses endured by the repaired tissue whenever the patient moves. To meet today's strength requirements, Innovasive takes advantage of plastics' ductility to create "dynamic" fasteners that expand into the bone for a boost in fastening strength.

While orthopedic fasteners have traditionally been metal, Chervitz points to an ongoing shift toward plastic devices. In fact, plastics have gained enough acceptance that some of Innovasive's devices compete not only against metal but also against other plastic devices. Chervitz attributes the change to the recognition that plastics offer more design flexibility than metal. "You can do things with plastic that you can't do with metal," he says, citing radial expansion into bone as one example. Or to take another example, Innovasive often uses polymers that the body absorbs, allowing devices to slowly disappear once the patient heals.

Two of the company's most recent devices underscore the potential for plastic tissue repair devices. Citing studies from Knowledge Enterprises Inc., a consulting firm for the orthopedic industry, Chervitz estimates that a new "tack" for shoulder repairs can serve in as many as 200,000 surgeries per year. The company's latest device for repairing torn knee ligaments sees use in a procedure that affects as many as 300,000 people yearly. Both of these expanding devices also embody Innovasive's approach to maximizing fastening strength. "Our company is built on the radial expansion of plastics," Chervitz says.

Tack shoulders heavy load. The newest expanding fastener from Innovasive targets arthroscopic shoulder surgeries. Called ConTack, the device resembles nothing so much as a thumbtack for attaching torn tissue to the underlying bone. But it represents a new twist on surgical tacks that can replace suturing for simple tears.

More than a simple screw, Intrafix, a new orthopedic device used to secure tendon grafts to the tibia in reconstructive knee surgery, actually consists of two parts. A cross-shaped sheath (left) separates the graft tendons within a tunnel drilled in the tibia. A screw driven in the center of the sheath expands it, pressing the graft strands against the tunnel wall.

Rather than relying on a friction fit, ConTack works more like a molly-bolt. Its anchor body expands into the bone as the surgeon drives a center pin into an expandable sleeve, increasing the tack's diameter from 3.5 to 4.2 mm. As a result of the expansion, ConTack achieves a greater pullout (tensile) strength than passive devices, according to Mark Johanson, ConTack's project engineer and designer. He reports that Innovasive's tests in foam models of moderate-density bone show that ConTack has a pullout strength of 156 N or about twice the strength of tacks that rely on a friction fit. He adds that ConTack offers about the same strength as anchoring sutured tissue to the bone, the prevailing method of repairing torn shoulder tissue.

ConTack's design also addresses the possibility that a poorly angled tack head would cut into soft tissue. "Imagine a thumbtack pushed into paper at an angle," says Chervitz. "At some point, the edge would cut into the paper." ConTack gets around this problem with a washer-like floating head that accommodates angles up to 30 degrees . "When the shaft goes into the bone, the head rotates to the contour of the surrounding tissue," explains Chervitz.

To make ConTack bio-absorbable while preserving its strength, Innovasive combined different grades of polylactic acid (PLA): The anchor body, which bears the bulk of the load requirements during the healing process, is injection molded from an L-PLA. The head, which is designed to disappear faster, is molded from an L/DL-PLA. Due to the vagaries of current test methods, it's hard to say just how quickly these materials will break down. "What we can say is that they maintain strength for 12 weeks and are then reabsorbed, the L/DL-PLA somewhat quicker than L-PLA," Chervitz says.

While the PLA offers the opportunity to create fasteners that disappear when the body no longer needs them, the material does have a downside. According to Johanson, it can be brittle and difficult to process because it "flows like molasses" in the tool. "It's very challenging to work with," Johanson says. Its brittleness even threatened the very radial expansion that makes ConTack unique.

Innovasive ultimately overcame material limitations through smart design and processing expertise. For example, the tack's anchor body has ridged contours that require only a slight expansion to provide a mechanical lock with the bone. Also, the sleeve's clamshell design makes clever use of the parting line to create a built-in breakaway point. Johanson notes, though, that natural variations in bone density mean that the screw may split in regions other than the breakaway point when there are less dense bone regions. Either way, he says, "it splits only in one spot." And because PLA can be something of a blank slate when it comes to the final mechanical properties, Innovasive's processing expertise helped it achieve the right balance of properties for ConTack. As Johanson explains, "Processing affects your ability to create modulus in a crystalline material like PLA." Limiting crystallinity helped maximize flexural strength and elongation where they were needed most -- in ConTack's sleeve.

Kneebone connects to the shinbone. Innovasive likewise relied on the radial expansion of plastics to improve a fastening device for arthroscopic knee surgery. Called Intrafix, it secures tendon grafts to the tibia during reconstruction of a torn anterior cruciate ligament. Chervitz likens the graft material, which comes from the patient's hamstrings, to "strands of beefy shoelace" that run behind the knee bone to link the shin and thighbone.

To attach these grafts, surgeons have traditionally used a metal screw and washer. While offering the strongest attachment, the metal devices do have an unsettling drawback: "You can feel the fixation device under the skin if you kneel or rub your shin bone," says Chervitz. The resulting discomfort -- and sometimes, outright pain -- lead to secondary hardware-removal operations- in roughly 30% of the cases, according to Chervitz. By contrast, an alternative fastening device called an "interference screw" works entirely inside a hole drilled into the tibia, holding the graft strands in place while the patient heals. Though they eliminate hardware lumps under the skin, interference screws traditionally lacked the strength needed to inspire the confidence of many surgeons, says Jo Hays, engineering project manager at Innovasive and one of Intrafix's designers. "So we set out to develop a device like an interference screw that provides both internal fixation and improved strength," she says.

Unlike conventional one-piece interference screws, Intrafix consists of a cross- shaped sheath that carries the four graft strands and a separate tapered screw that inserts in the center of the sheath. The screw expands the sheath to a nearly cylindrical shape, pressing the graft against the walls of the drill hole. Chervitz notes that the channels defined by the sheath's cross shape foster a uniform circumferential compression of the graft strands, which in turn promotes faster bone ingrowth. "Earlier interference screws tended to push all the graft strands to one side," he says. With its high-molecular-weight, high-density polyethylene sheath and acetal screw, Intrafix also differs from conventional screws made from bio-absorbable plastics, which Innovasive designers deemed too brittle to handle the expansion required by the sheath or the torsional strength needed by the screw.

Tests conducted by Innovasive show a significant strength advantage for the expanding screw. According to Hays, the pullout strength in matched tibia human sets came to 700 N versus 550 N for the absorbable screws. Putting the numbers in perspective, Hays says the additional strength increases Intrafix's "margin of safety" over the 500 N that prevailing clinical opinion considers a minimum pull-out strength for these repairs. Innovasive also conducted dynamic tests to measure creep, or how far the devices migrate under load. After subjecting the two devices to 1000 cycles of loads from 50 to 250 N, Intrafix showed a maximum displacement of 7.5 mm versus 20 mm for the conventional screw, Hayes reports.

For all its strength improvements, Intrafix still doesn't beat out metal screw-and-washer systems. Hays concedes that these metal devices provide "tremendous fixation" with typical pull-out strength in the neighborhood of 1200 Nfar closer than plastic screws to the 2000 N strength of "native" ACL. But a direct comparison with metal misses the point of enhancing strength without going to a more intrusive device. "We had to ask ourselves whether it is necessary to have the strongest device," says Hays, "or the one that best improves patient's lives."

How strong is strong?

To prove that its expanding fastener for arthroscopic knee surgery could outperform interference screws and surpass clinical strength benchmarks, Innovasive conducted tests in both human bone and models made from polyurethane foam. As Innovasive project engineer Jo Hays explains, foam models are often used for comparative purposes because bone samples can vary too much. "Human bone is most credible," she says. "But it's also the most variable and deteriorates quickly."

While pullout strength and creep data remain established measures of device performance, Hays believes that the medical device industry will increasingly focus on a more telling indication: "yield load," or the highest load at which the load-versus-displacement curve is linear. Hays argues that the displacement occurring at the yield load provides the closest match to the clinically relevant displacement at which the device begins to fail. "Historically there's been lots of focus on the ultimate load," she says of the tests that measure pullout strength. "But people should really be concerned about the yield load because that's where failure begins." In this case, failure means a laxity in the ACL repair, which can result in an unsettling looseness of the lower leg and a trip back to the doctor.

The two graphs show how Intrafix outperformed a standard interference screw in two benchmarks of strength: creep and pullout strength, according to tests conducted in polyurethane foam of a similar density to bone.

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