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R. Buckminster Fuller should be known for more than creating the geodesic dome.
Matthew Lessin, Co-founder of Empirical Spine, referenced Fuller’s work as a major inspiration behind medical device motion preservation technology. He said that the American architect, systems theorist, designer and futurist coined the term “tensegrity” to describe the study of structurally sound constructions that feature radical separations of compression and tension.
According to Lessin, tensegrity can explain how all motion is connected within the human anatomy and the body’s relationship with medical devices. He channeled Fuller while developing a novel motion preservation solution for degenerative spondylolisthesis patients. His device, the LimiFlex Dynamic Sagittal Tether (DST), is reportedly the only implant designed to restore natural flexion stability in the lumbar spine.
Maintaining natural anatomic motion represents a dramatic break from traditional implant technologies. Orthopedic companies are taking notice and developing devices that replace fusion as the treatment option of choice. It’s part of a wave of innovation in orthopedics aimed at building devices that preserve the body’s natural movements and allow patients to maintain a long-term quality of life.
Allowing the Spine to Shift
Burgeoning technologies aimed at maintaining vertebral mobility and health represent significant developments as companies vie for business in the $10 billion spine segment.
When designing LimiFlex DST, Lessin wanted to support existing bone structures and maintain their motion without fusion, which accounts for a majority of the spine market. According to Lessin, the solution entails sophisticated biological resurfacing aimed at ligamentous support.
Traditional spine implants involve putting rods through vertebrae, Lessin said. “If there’s no motion, there can’t be a lot of pain,” he explained. “As implant technologies have advanced, the focus has shifted to removing force from bearing surfaces while maintaining their movement.”
LimiFlex DST is designed to preserve motion in the lumbar spine in patients with grade one degenerative spondylolisthesis. The device preserves motion by stiffening the targeted segment and rebuilding its elasticity, allowing it to remain functional while preventing anterior slip, according to Lessin.
“We’ve just completed our Investigational Device Exemption study, and the data indicates that LimiFlex is superior to a single-level fusion for this indication at four years,” Lessin said. “It’s the first device that uses remaining vertebrae anatomy to maintain motion while keeping the spine stable without the need for revision surgeries.”
The biggest challenge in the development of any motion-preserving device is preventing wear debris, according to Lessin. He explained that the rubbing of metal against polyethylene results in particles arousing a natural immune response that creates bone-deteriorating enzymes.
New motion preservation technologies involve metal-on-metal surfaces, and Lessin said that FDA has concerns about debris caused by these devices. “We’re about ten years from the next step of ceramics in motion preservation devices, which create virtually no debris,” he said.
Lessin predicts that LimiFlex will receive FDA clearance by the end of the year. Empirical Spine is currently raising funds for commercialization. “The spine is made to move,” he said. “If it keeps moving, it has a better chance of compensating to correct problem areas.”
Strong and Flexible Designs
Orthopedic device makers are pioneering motion preservation technologies that focus on maintaining as much of the body’s natural range of motion as possible. These devices carry special importance in the foot and ankle, where natural movement is essential for walking and maintaining balance.
“We aim to design implants that restore the patients’ true anatomy and mimic as closely as possible the normal form and function of a joint’s pre-injured or pre-diseased state,” said Brian Hatcher, President of Trauma, CMFT and Foot and Ankle at Zimmer Biomet. “This mindset allows us to create and deliver products that provide the best possible outcomes, alleviate pain and return patients to normal everyday activity as soon as possible.”
Zimmer Biomet’s Trabecular Metal Total Ankle is in a class of innovative devices focused on delivering motion preservation solutions. The platform is semi-constrained and designed to promote joint mobility by restoring natural alignment, reducing pain and preserving flexion and extension. Its bicondylar, conical articulation design promotes tendon and muscular function, resulting in a more natural motion path of the ankle.
“Traditional fusion procedures are sometimes necessary when motion in the ankle joint is causing pain,” Hatcher acknowledged. “However, implants that preserve the joint’s natural motion typically maintain functionality in a way that mimics a healthy joint.”
Maintaining natural movement is a drastically different goal for implant engineers and requires special design considerations. Hatcher points to two significant challenges in developing and designing implants that preserve motion: Creating implant shapes and sizes that accommodate a wide range of patient anatomy, and balancing structural strength and integrity with high flexibility and versatility.
“Although we aim to provide as much motion preservation as possible, the implant construct still has to be strong enough to withstand anatomic loading typical in daily activities,” Hatcher said.
Zimmer Biomet is fully committed to leveraging additive manufacturing to develop patient-specific designs for products that preserve motion in the ankle. “The ability to rapidly produce a custom product that’s sized specifically for an individual patient provides more clinical benefit than generically shaped implants that may not fit as perfectly as desired,” Hatcher said.
He believes that additive manufacturing stands at the forefront of efforts aimed at increasing access to motion-preserving devices in the foot and ankle segment because implant designs are no longer constrained by the limitation of subtractive manufacturing. R&D engineers can now build a variety of shapes and have access to advanced materials, allowing them the freedom to create devices that meet specific patient needs.
According to Hatcher, Zimmer Biomet’s product development teams deploy an array of tests to determine how new motion preservation designs will perform in practice, including static tests that provide accurate strength measurements such as bending and torque. Implants are also run through fatigue tests that repeatedly apply force loads at various anatomic angles, with the goal of running millions of cycles without fracturing the device.
Zimmer Biomet’s design team uses these tests to analyze every risk inherent in the product’s performance and mitigates them through several design iterations.
“The challenge lies in ensuring motion preservation devices still meet the intended structural integrity,” Hatcher said.
