Design improvements add personal touch to TKA bearings
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Historically, posterior stabilized and cruciate-retaining knee bearings have been popular bearing types among orthopedic surgeons for total knee arthroplasty.
Data from the 2022 American Joint Replacement Registry Annual Report showed posterior stabilized implants were used in more than half of all primary TKA procedures until 2019 when the rate decreased to less than 50%, while cruciate-retaining designs increased annually since 2016, reaching 49.7% in 2021.
Source: John Abbott
However, the report also showed the use of ultracongruent components increased 57% between 2012 and 2021. With the advent of more conforming, polyethylene bearing types, such as medial congruent, medial pivot and ultracongruent bearings, sources who spoke with Healio | Orthopedics Today suggested the kinematic and design changes in newer implants may alter TKA bearing usage patterns, as well as the future of total knee replacements.
“Implants themselves have greatly improved over the years,” Steven B. Haas, MD, chair of the knee service at Hospital for Special Surgery, told Healio | Orthopedics Today. “The implants are now more anatomically shaped so they look more like the patient’s anatomy that we are replacing.”
The more types of bearings available allow surgeons to treat a wider array of patients undergoing TKA, according to Kevin B. Fricka, MD, of Anderson Orthopaedic Clinic and M2 Orthopedics.
“Now, we have a lot of options as surgeons to maximize intraoperative stability, which, hopefully, will translate into excellent patient-reported outcomes,” Fricka told Healio | Orthopedics Today. “In the past, you had two choices. Now, you have, on the cruciate-retaining femur side, three different options for stability. Options are advantageous to the surgeon, as well as the patient.”
Shift in trends
Fricka said use of different congruencies in the bearings can potentially allow the implant to move in a more normal fashion. In the past, posterior stabilized bearings were designed to replace the posterior ligament with a post and cruciate-retaining bearings retained the posterior ligament with a bearing surface with minimal congruency, according to Fricka. However, he said newer implant bearings allow the posterior cruciate ligament to be retained or removed without being replaced by a post while maintaining stability with increased congruency.
“In the past, we did not necessarily have bearings that would achieve the motion of the native knee,” Fricka said. “Now, probably one of the more popular bearings that is taking shape in the industry would either be called a medial pivot, a medial congruent or a medial dish. Essentially, what that means is that the medial side is now conforming and provides some stability, anterior and posterior, to allow a medial pivot motion and allow the lateral side to roll back.”
Ultracongruent bearings
Ultracongruent and cruciate-sacrificing bearings, such as medial pivot and dual pivot designs, are increasing in use, according to James A. Browne, MD, Alfred R. Shands professor of orthopaedic surgery and executive vice-chair of orthopedics at the University of Virginia.
“The American Joint Replacement Registry as well as other national registries, such as the Australian Registry, clearly show an international trend toward increasing use of more conforming polyethylene bearings, typically at the expense of the posterior stabilized knee,” Browne told Healio | Orthopedics Today.
Giles R. Scuderi, MD, FAAOS, FACS, vice president of the orthopedic service line at Northwell Health, said the benefits of posterior stabilized bearings include easy balancing of the ligaments, implant stability and good correction of severe deformities. He added numerous clinical studies have demonstrated reproducible knee kinematics, along with long-term durability.
When compared with ultracongruent bearings, Browne said posterior stabilized bearings have some clear downsides, including retropatellar crepitus and clunk. When he reviewed more than 700 of his patients who received a posterior stabilized bearing during TKA, Browne said he found a more than 6% incidence of arthroscopy for debridement of retropatellar crepitus and scar tissue. He added posterior stabilized knees can feel mechanical, may be noisier and may not feel as natural to patients.
But these findings may not be the general experience with posterior stabilized implants and, instead, be a marked outlier for modern posterior stabilized implants, according to Haas.
“Most studies comparing posterior stabilized to cruciate-retaining knee implants find similar results for outcome scores and satisfaction,” Haas said.
Kinematics
One advantage of ultracongruent bearings is the ability to guide kinematics, which is best achieved through surface geometry, according to Browne.
“The surface geometry is the best way to influence kinematics, with some studies showing that the cam and post are not as influential in determining how the knee moves,” he said. “It is surface geometry that is a stronger determinant of kinematics. As we get into more conforming bearings, we can push this issue of kinematic function further.”
Development of newer kinematic alignment philosophies also lends itself to the new bearings, as surgeons “do not have to worry about the post breakage or post alignment” if the tibia is in “a little bit of varus,” according to Fricka.
“That is why these new bearings are taking off, and why some are adopting newer alignment philosophies to help the patients achieve a more normal result after knee replacement,” he said.
Although research has shown ultracongruent bearings provide good results, Haas said ultracongruent bearings distort the way the knee moves.
“By nature of some ultracongruent [bearings], the cruciate ligament is loose, and the femur bone is going to slide forward on the plastic as you bend the knee, and it runs up a hill, tightens up the other ligaments and that stabilizes the knee,” Haas said. “That works and the clinical results are fine and that is why people use it.”
However, Haas added this is an unnatural way for the knee to move.
“My personal belief is that we want the knee to move more naturally and that is how we are going to get them to function more naturally,” Haas said. “More natural motion is achieved in some designs by combining a medial congruent bearing with a post for cruciate substitution.”
Personalized fit
Anatomically shaped implants and bearings as well as improvements in technology, such as robotics, accelerometers and technology to measure ligament tension, can help personalize the outcomes of each patient, according to Haas.
“With the combination of more anatomic implants with the accuracy of the technology, we can customize and match the patient’s anatomy precisely,” Haas said. “And that allows us to do the surgery better than we could before because we have to do what are called releases, which are essentially lengthening ligaments to make up for changes to alignment. Since we are restoring them more to their natural state, we have to do less of that. So, it just means less surgery for the patient and better outcomes.”
Scuderi also said the personalization of implant design will address anatomical restoration of the distal femur, as previously published literature has shown that fit follows function.
“What we are trying to do with implant design is to get a better matching of the implant to the measured anatomy,” Scuderi told Healio | Orthopedics Today. “There are now implants that have 2-mm increments, so we can closely match the femur with a more anatomic and personalized fit. That is helpful because if you restore the anatomy of the distal femur, you will get a better outcome with restoration of the joint line and good stability of the knee, clearly impacting knee kinematics.”
He added, “Prior research has shown that there is a gender difference in femoral anatomy and this led to the introduction of ‘gender’ implants that had narrow options to accommodate the female anatomy. This has carried over to more recent designs that have both narrow and standard options for the femoral component to accommodate anatomic differences in shape and size.”
Soft tissue
Surgeons are also beginning to utilize the benefits of soft tissue balancing in personalizing TKA for each patient, according to Haas.
“My personal belief is that we are just beginning to touch the surface of soft tissue,” Haas said. “Maybe the ligament tensions in a flexible female might be different from a rigid 250-pound [man]. We may be looking for different aspects of their soft tissues. We have had limited ability to study that, so we do not know the right answer.”
Scuderi said current techniques should be directed toward appropriate soft tissue balancing to achieve appropriate kinematics in the knee. He said he believes TKA is a soft tissue operation following bone resection according to the desired alignment, and added improvements in technology may help guide the next generation of surgeons during surgery.
“I am a proponent of conventional and manual instrumentation because of my decades of experience,” Scuderi said. “Yet, as we move to the future, we are looking at innovative technology with robotics and computer navigation, along with sensors that can be used in trialing of the implants, we may be able to move the qualitative feel of a balanced knee to a quantitative measure. This has the potential to give us more information for the next generation of surgeons to get a better fit, feel and a better final outcome in total knee arthroplasty.”
Implant materials
The use of better materials is also becoming increasingly important in bearings for TKA, according to Michael P. Bolognesi, MD, orthopedic surgeon at Duke University.
“As we try to create more normal knee function, having bearings or materials that have appropriate wear performance is probably even more critical because we might be creating stresses to the polyethylene or to the liner,” Bolognesi said. “You want to make sure that we have the best performing polyethylene that we can, particularly as we continue to strive to have more normal knee function.”
Haas said not only have the materials used in the bearing surface changed from metal to a metal ceramic surface to reduce friction or scratching, but the plastic components have also become more wear resistant.
“We have worked a lot to try to improve on [implant longevity] with better bearing surfaces, the use of ceramic-like materials and the use of better plastics because, historically, one of the reasons that [implants] wore out was the plastic wore,” Haas said. “Over time, as you are walking, you are probably putting 2 million cycles, at least, a year on. An elderly person, a 70-year-old, might put a million cycles a year on. So, you’re walking so many steps, the plastics can wear.”
The use of uncemented implants in younger patients and more active patients has also been a growing trend to improve implant longevity, according to Haas.
“We do not have the long-term on it, but at least the midterm results are looking good at 5 years,” Haas said. “But we will need to wait to see if it does better, because we have a bar that is high.”
Data
Scuderi said before the historically popular posterior stabilized knee and cruciate-retaining knee bearings are pushed aside for newer bearing types, there are certain data-specific benchmarks that need to be reached.
“They have to show good evidence that it makes a difference from my current experience of over 30 years of doing posterior stabilized total knee arthroplasty,” Scuderi said. “There has to be a demonstrable improvement in outcomes because the clinical results that we have are predictable and durable and, at times, maybe superior to what we have seen with other implants.”
He added, “Right now, we have predictable knee kinematics along with satisfied patients and that is what matters to me. I will need to see more long-term data in order to make a significant change.”
High powered studies featuring “outcome measures sensitive enough to detect the highest level of function to avoid the ceiling effect seen with a lot of the current outcome measures” are needed on the newer bearing types to determine efficacy, according to Browne.
“Obviously, what we would love to see would be some high-quality randomized control trial data. That would be what we would want to see to drive this forward,” Browne said. “There have been a few smaller trials done looking at ultracongruent knees vs. [posterior stabilized] PS knees and vs. [cruciate-retaining] CR knees, and they have largely failed to show any clear difference in outcome.”
Future bearings
Browne added the future of TKA bearings will rely on the ability to continue honing the process of reproducing the natural kinematic function of the knee.
“As we become more comfortable with more congruent bearings, I think we will see more implants designed to improve the kinematics of a total knee arthroplasty,” Browne said. “The ability to start dialing in and reproducing individual patient kinematics is what we will see in the future as we get more sophisticated with our understanding of how to drive kinematics through surface geometry and bearings.”
Haas said future bearing designs will be more anatomically shaped, including the plastic and femur components.
“The future will have the technology where we will do more personalization to personalize alignment,” Haas said. “Robotic technology or other navigational technology will allow us to do that.”
But Fricka said there needs to be a reliable way to measure the functional differences between bearing designs.
“One of the problems right now is the tools we have may not be able to measure a specific difference between a medial pivot and a cruciate-retaining design,” Fricka said. “The Forgotten Joint Score is a good outcome score to use, but as we get more into assessing patients’ function, we will need better clinical and functional measurement tools.”
Regardless of where surgeons fall in their assessment of the modern bearing landscape, Bolognesi said surgeons should have general optimism about the expanded list of bearing options.
“We are now armed with more bearing options certainly than were available when we first started knee replacement procedures,” Bolognesi said.
He added, “The good thing is that all of these bearings have excellent results. The great thing about the procedure is that there are strong studies and registry results that support all of these bearings. It allows the surgeon to have some selection and development of their technique and implant choice to best suit their patient population.”
- References:
- American Joint Replacement Registry (AJRR): 2022 annual report. https://connect.registryapps.net/2022-ajrr-annual-report. Accessed Sept. 29, 2023.
- Heckmann ND, et al. J Am Acad Orthop Surg. 2021;doi:10.5435/JAAOS-D-20-01232.
- Hodge PE, et al. J Arthroplasty. 2023;doi:10.1016/j.arth.2023.08.010.
- Kim MS, et al. J Bone Joint Surg Am. 2021;doi:10.2106/JBJS.20.00279.
- Liu X, et al. Knee. 2022;doi:10.1016/j.knee.2022.03.010.
- Ning L, et al. Knee Surg Sports Traumatol Arthrosc. 2014;doi:10.1007/s00167-012-2275-0.
- For more information:
- Michael P. Bolognesi, MD, of Duke University, can be reached at michael.bolognesi@duke.edu.
- James A. Browne, MD, of the University of Virginia, can be reached at jab8hd@uvahealth.org.
- Kevin B. Fricka, MD, of Anderson Orthopaedic Clinic and M2 Orthopedics, can be reached at kfricka@andersonclinic.com.
- Steven B. Haas, MD, of Hospital for Special Surgery, can be reached at rennichr@hss.edu.
- Giles R. Scuderi, MD, FAAOS, FACS, of Northwell Health, can be reached at gscuderi@northwell.edu.
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