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The increasing appeal of cementless fixation in knee replacement surgery can be explained with rudimentary physics.
“Patients are heavier and more active, and implants will fail if cement doesn’t get stronger,” said Robert L. Barrack, M.D., a hip and knee replacement specialist at Washington University in St. Louis. Dr. Barrack discussed the growing movement toward cementless knee arthroplasty at the 2023 annual meeting of the American Association of Orthopaedic Surgeons (AAOS).
“We’re seeing exponential growth in cementless knee replacements, and I think that trend will continue,” Dr. Barrack added. “Twenty years ago, we were having the same discussion about hip replacements, and the market shifted almost entirely to cementless. We’re seeing the same appeal for consistency, efficiency and equivalent or better results in cementless knees.”
Although cemented fixation is still the predominant approach among orthopedic surgeons, the adoption of cementless knees is on the rise. The technique is used in 14% of all primary total knee replacements, according to the American Joint Replacement Registry (AJRR).
A confluence of factors is contributing to the momentum behind cementless knees. Younger, heavier and more active individuals are the fastest-growing cohort among knee replacement patients. They expect to return to their pre-surgery lifestyle after the operation. They also need implants that will last longer between primary replacements and potential revisions.
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 also 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.
Eliminating the Weak Link
Cementless fixation increases surgical efficiencies and reduces the risks of complications associated with cement impaction and debris. However, some knee replacement surgeons are concerned about the stability of implants placed without cement. They shouldn’t be, according to Dr. Barrack.
He said radiostereometric analysis, which measures early implant displacement and is used to predict long-term implant stability, shows cementless components tend to shift 1mm to 2mm after one year. One to two years after surgery, however, the trend reverses; cementless components tend to stabilize while cemented implants start to move.
Dr. Barrack said there are dozens of cement types on the market, each with various methods of application, healing properties and interactions with implant surface finishes. He also pointed to a link between high-viscosity cement, which surgeons have used more often in recent years, and implant failures.
“Cement creates too many variables that can impact outcomes,” Dr. Barrack said. “It’s the weak link of the procedure.”
Dr. Barrack said highly porous (60% to 70%) 3D-printed titanium implants with optimized tibial designs provide strong initial fixation and prevent micromotion.
The obesity epidemic in the U.S. — implant failure rates increase in line with patients’ BMIs — further supports the growing use of cementless fixation. “Studies show cementless implants consistently outperform cemented implants in obese patients,” Dr. Barrack said.
Implant Design Makes All the Difference
The popularity of cementless hip replacement doesn’t foretell widespread adoption of the technique in the knee space, according to Robert Cohen, President of Digital, Robotics and Enabling Technology at Stryker. He pointed out that the acetabular components designed for hip replacement implants are perfectly suited for the cementless approach.
“The half-cup design of the acetabular cup is press-fitted into a preparation in the acetabulum, which inherently provides peripheral forces that maintain an interference fit,” Cohen said.
In contrast, the flat-on-flat tibia components of a knee replacement system are subject to forces that can cause implant stability issues. “The designs of cementless knee systems maintain the stability of components during the three to five months after implantation as biologic ingrowth occurs,” Cohen said.
Additive manufacturing allows for the creation of new and complex porous interface surfaces that can’t be made by subtractive manufacturing methods. Stryker’s internal database houses thousands of bone morphology records, which engineers used to optimize the design of 3D-printed implant components.
The 3D printing of the tibial baseplate in Stryker’s Triathlon Tritanium cementless knee system provides a porous interconnected structure that promotes biologic ingrowth. It also creates initial fixation and stability at the front of the knee to prevent micromotion after implantation.
The unique geometry of the system’s keel-and-post design was based on information taken from thousands of knee CT scans. “We’ve designed a cementless implant for initial stability and a porous metal structure for long-term biologic in-growth,” Cohen said. “It’s an effective combination.”
Recent data from the American Joint Replacement Registry (AJRR) showed Stryker’s cementless knee had a 98.9% survivorship rate at five years.
Use of the Triathlon cementless knee is increasing quarter after quarter, according to Lisa Kloes, Vice President and General Manager of Stryker’s Knee Business Unit. “Cementless knee replacement is gaining traction because biologic fixation shows better survivorship,” she said. “Additive manufacturing gives us the ability to enhance the system’s design by putting porous titanium where it has the most impact.”
Titanium alloy is an optimal implant material for cementless implant design due to its enhanced biocompatibility and bone compatibility, according to Jennifer Guthrie, Knee Marketing Manager at Smith+Nephew.
The company’s LEGION CONCELOC cementless total knee addresses the critical elements of cementless knee replacement through its unique design, Guthrie noted. “The asymmetrical keel is designed to help achieve immediate bone fixation, while the 3D-printed structure of advanced porous titanium encourages biological ingrowth,” she said.
Robots and Cementless Fixation: Perfect Together
The growing interest in cementless fixation dovetails nicely with the continuing evolution of robotic knee replacements. Robotic-assisted surgery creates a balanced knee that leads to better initial fixation of the implant surface and porous metal for longer-term biologic in-growth, according to Cohen.
If the knee is not perfectly balanced, flexion of the joint could cause cementless components to lift off the bone. Cohen said Stryker’s Mako robot addresses this issue by identifying the optimal joint line with pinpoint accuracy.
The accuracy of the surgical cuts for cementless knees is critical to ensure a secure initial fit and maximize the implant-to-bone contact area. “Both factors are necessary for ingrowth,” said Guthrie. “Increased consideration of the interface between bone cuts and implant porous surfaces is being given to the design of implants.”
Guthrie said that Smith+Nephew’s CORI Surgical System offers the accuracy of precision milling instead of a sawblade action, which has limitations. “This feature — along with the accurate positioning and balancing of the implant — makes the platform well-suited for the cementless technique,” she added.
Robotic assistance identifies the optimal balance of the joint in individual patients, implant designs allow for initial stability after implantation and 3D-printed interface surfaces promote long-term biologic fixation.
“That’s the secret sauce that gives surgeons confidence in the stability of implants and has led to increased adoption of cementless knees,” Cohen said.
Set Up for Future Growth
Increasing numbers of knee replacements are being performed in ambulatory surgery centers (ASCs), which attract younger, more active patients. “That’s the ideal population for cementless knees,” Cohen said.
Smith+Nephew’s CORI Surgical System is a compact and portable platform and eliminates the time and cost of CT imaging. “We believe it is well-designed for an ASC environment, which is where many cementless knee replacements will be taking place,” Guthrie said.
Guthrie expected cementless implant indications to expand into the partial and revision knee space. She said Smith+Nephew is positioned for this next step with the ENGAGE Partial Knee System, the only cementless partial knee implant available in the United States.
“Industry must continue the clinical success of the modern cementless designs that have launched over the last few years,” Guthrie said. “Most large companies now have a next generation, 3D-printed tibia and patella implant. These designs must continue to show good clinical results.
“New indications and innovations in the robotic space will fuel the cementless knee evolution, allowing surgeons to achieve the best possible outcomes for their patients.”
