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.

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.”