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Cemented implant fixation is still the predominant approach among knee replacement surgeons, but the adoption of cementless knees is gaining ground. The technique is currently used in about 20% of primary total knee replacements, according to the American Joint Replacement Registry (AJRR), and that rate is expected to increase over the next decade.
The industry has responded to the growing cementless trend with knee replacement systems designed to improve osseointegration and provide longer-term survivorship. The increased interest in cementless knees coincides with the continuing evolution of robotic surgery and the advancement of additive manufacturing. In combination, these technologies have the potential to revolutionize knee replacement outcomes.
Improved Fixation
Younger, more active individuals are among the fastest-growing cohort among knee replacement patients, and they want to return to their pre-surgery lifestyle after the operation. They also need implants that last longer so they can put off potential revisions as long as possible.
Lisa Kloes, Vice President and General Manager of the Knee Business Unit at Stryker, expects the adoption of cementless implants to accelerate as new information about their durability and performance becomes available.
“Many surgeons look for clinical data to make their decisions about implants,” she said. “Our cementless solutions now have five-year registry data with 99% survivorship. We almost have 10-year data and expect very similar results to what we’ve already seen.”
Corey Perine, Chief Operating Officer at Maxx Orthopedics, agrees that surgeon buy-in is crucial for cementless implants to achieve widespread acceptance. “Growth is driven by surgeons’ desire to preserve bone, especially in younger and more active patients who may need revisions in the future,” he said. “Removing the cement variable could extend the implant’s lifespan.”
Perine pointed out that, unlike cemented implants, cementless options have the potential to continue to improve in functionality as bone in-growth occurs. “Cemented components achieve optimal fixation the moment they’re placed in the patient,” he said. “A cemented implant probably won’t fail, but its effectiveness is not going to get any better, which is the inverse with cementless components. They have the potential to perform better over time.”
The first six postoperative weeks remain crucial for the fixation of cementless implants. “Once that timeframe is passed, solid fixation should occur between the implant and native bone,” Perine said. “But in the intervening period, implant manufacturers have integrated pegs into implant designs that attach to bone to hold femoral or tibial components in place.”
Some previous iterations of cementless implants that relied on screws for initial fixation were known for loosening prematurely, shortening the lifespan of the implant and leading to earlier revision surgeries. Given the important role that pegs play in helping cementless implants achieve stable fixation and osseointegration, it’s no surprise that orthopedic manufacturers focus a great deal of attention on their development.
For example, the pegs on Stryker’s Triathlon cementless baseplate feature a proprietary “bullet cruciform” shape. “We were very thoughtful in the dimensions of those pegs. A lot of our secret sauce and intellectual property come to bear on the design of the baseplate,” Kloes said. “Many designs were tested, but none provided greater pull-out force than the cruciform peg design.”
Advancements in additive manufacturing, including the ability to create porous surface designs, have improved the short- and long-term fixation of cementless implants. This allows for more customizable and bone-friendly surface structures and, according to Perine, a unique level of design precision, consistency, creativity and flexibility.
Perine pointed to the ability to develop implant surfaces with different roughness levels to enhance bone grip. During a posterior resection, for example, a surgeon might want the distal portion of the femoral component to be more porous, but not sharp and rough so that it grips the bone hard.
“That’s very easy to do with additive manufacturing and very difficult to do with traditional manufacturing methods,” Perine continued. “You can 3D print an implant that would be virtually impossible to machine or create with a combination of subtractive manufacturing and a secondary process like plasma spray coating.”
Kloes also emphasized that additive manufacturing allows companies to continue to refine implants quickly and easily.
“The beauty of additive is that it provides design flexibility in the placement of specific porosity at the baseplate, mimics cancellous bone and promotes necessary bony in-growth,” she said. “Our design engineers can put the material exactly where it’s needed and dial in the specificity of how they want the implant to work based on Stryker’s extensive performance data.”
Perine cites improved quality control as another benefit of additive manufacturing. “When 3D printing a device, you can test every plate and batch of parts for their chemical composition and mechanical properties, tensile and yield strength,” he said. “We can closely monitor what cementless implants are made of and how they are likely to perform rather than simply relying on verification and validation.”
Market Landscape
Cementless knee implants are a growing market trend. As such, many of the industry’s most prominent companies are developing cementless implant options. Stryker offers cementless femoral, tibial and patellar components with the Triathlon system and has established an early lead in the market with more than one million placements of the implant. Zimmer Biomet is driving the market penetration rate of cementless knees into the 50% to 60% range.
Robotics is also helping to drive the adoption of cementless implants: Smith+Nephew believes its CORI handheld robot will increase cementless knee sales; Zimmer Biomet said its Persona OsseoTi Keel Tibia for cementless knee replacement is often used in combination with the ROSA robot; and Stryker said the synergy between the company’s Triathlon cementless implant and its Mako robot is “significant.”
Smaller companies are also developing cementless implants, and Perine thinks there is room for competitors of all sizes.
“Many of the larger companies have developed proprietary porous structures and want to market them heavily,” he said. “We tend to adopt proven, widely accepted technologies and then conduct postmarket follow-up studies. If we know already that another company has used the same core structure successfully, that adds to our confidence that it will perform well for us.”
Maxx Orthopedics currently offers only a cementless femoral component, but Perine said that cementless tibial and patellar devices are in development. “Surgeons put in a hybrid system comprising the cementless femoral component and cemented versions of the other components,” he said. “That works, but it’s not a popular approach.”
Looking ahead, Kloes envisions a variety of options for cementless knee implants. “We could see components being used in revision surgeries,” she said. “That concept hasn’t been fully embraced, but cementless could play an important role in the longevity of revision implants.”
Perine believes there is much to be excited about concerning the future of cementless knees thanks to the continued evolution of implant designs, additive manufacturing and enabling technologies. “Where we are in this creative cycle is helping cementless implants as a whole show growth and become more accepted,” he said.
