Candace Lightner, of MicroPort Orthopaedics, discusses surgical techniques and developments in the total knee replacement market
Total knee arthroplasty has revolutionised the treatment of end-stage knee arthritis, providing relief for individuals suffering from severe pain and limited mobility. While experts widely recognize the procedure’s success in restoring function and enhancing quality of life, a disconcerting element persists: approximately 20% of patients express dissatisfaction with the results [1].
This disparity underscores the need to explore advancements in surgical techniques and alignment approaches to address patients’ concerns and improve overall satisfaction. Recent developments in the total knee replacement market have focused more on implant selection, favouring medially stabilised designs, as well as alternative alignment choices to restore joint line and stability, aiming to create a knee that not only relieves pain and restores function but also feels more natural.
Among the emerging strategies, the combination of a medial-pivot style implant paired with a kinematic alignment approach has gained popularity for its ability to enhance patient outcomes. By examining the principles and benefits of this approach, along with recent advancements in the field, orthopaedic professionals can gain insights into the growing trend of restoring normal anatomy and kinematics in knee arthroplasty, as well as understand the factors that drive patients to seek out these specific implant and alignment techniques.
Evolving trends in the total knee replacement market
In recent years, a considerable transformation has taken place in the total knee replacement market, with a growing interest in the medial-pivot philosophy. The medial-pivot knee design is intended to mimic the natural kinematics of the knee, incorporating a medial side that is concave in shape and a lateral side that is less constrained, allowing for a more mobile lateral compartment. These features create a knee that is more stable on the medial side than lateral, resembling a ball-in-socket type joint in the medial compartment and allowing for more lateral translation. This concept, which has been in existence for over 25 years, is gaining popularity as more surgeons and patients recognise its numerous benefits.
Initially considered a niche or boutique option, medial-pivot implants differed from traditional knee designs that featured a J-curve femoral component. True medial-pivot implants are characterised by a constant radius femoral component with a more concave medial compartment on the tibia, creating a highly conforming ball-in-socket design that aims to replicate the knee’s natural kinematics. This design principle is based on the understanding that the knee is inherently stable medially and more mobile laterally.
Traditional J-curve knee designs rely on different design features to provide stability of the joint. However, we know many patients complain of a loss of stability after total knee replacement and are still unsatisfied with their results. These traditional implants have been categorized as either cruciate retaining (CR), where the patient’s PCL remains intact, or posterior stabilised (PS), which features a post-and-cam mechanism to replace the patient’s PCL. And while these designs have shown great success for the last several decades, medial-pivot style implants are growing in popularity and offer a promising solution by providing both enhanced stability and a more natural range of motion.
In fact, studies comparing different knee implant designs have demonstrated that medial-pivot knees provide greater stability than traditional designs [2,3]. Other studies also have shown that patients prefer medial-pivot knees over alternative options by a three-to-one margin [4,5].
“It is a very stable knee, and as a surgeon, you notice that,” Dr. Robert Steensen, an orthopaedic surgeon with Orthopaedic One in Columbus, Ohio, said. “My patients notice that. It feels more stable to them.”
Steensen has specialised in knee surgery and replacement for over three decades. He developed a surgical technique to prevent recurring patella dislocations that is widely used today. He also has been closely involved with research efforts published in various orthopaedic journals.
One design in particular, MicroPort Orthopedics’ Evolution® medial-pivot knee, is designed to replicate the natural medial-pivot motion of the knee. This results in a more anatomically accurate and functional joint replacement. The femoral component of these implants features a constant radius in both the coronal and sagittal planes, creating a spherical geometry and the ball-in-socket concept specifically on the medial side. This design allows the knee to pivot medially during flexion and extension while allowing the lateral compartment to remain more mobile, closely mimicking the natural movement of a healthy knee.
Alignment approaches for optimal results
As the understanding of knee mechanics and the success of medial-pivot implants grows, more surgeons are recognising the importance of restoring the knee’s natural kinematics and ultimately improving patient outcomes. As a result, another approach that has gained considerable attention is kinematic alignment.
The goal of this technique is to duplicate the patient’s pre-arthritic native joint line by compensating for cartilage wear on the femur when making femoral bone cuts, resulting in a more anatomic implant alignment and natural ligament tension. The pre-arthritic femoral joint surface and ligaments should function as a cohesive unit, and kinematic alignment aims to restore ligament isometry and balance without soft tissue releases.
In contrast, mechanical alignment, which has been the gold standard for decades, revolves around the need for a tibial cut perpendicular to the mechanical axis of the knee, which is not anatomic. This, then, results in non-anatomic cuts to the femur as well, deviating from the patient’s natural joint line. Typically, this approach results in a change of limb alignment from their native knee and often requires ligament release to achieve balance. This may compromise the knee’s inherent stability and kinematics, potentially contributing to suboptimal outcomes and patient dissatisfaction. Mechanical alignment, although still a widely accepted approach used by the majority of orthopaedic surgeons, may not always offer the desired outcomes.
“It’s not a radical change,” Steensen said. “It’s a subtle change. It’s tweaking it just enough. If you drove your car, and the wheel was out of balance because they didn’t put the weights on the right way, your car would feel a little funny. If they balance it right, it feels great. It’s just a minor little tweak that makes a big difference.”
Kinematic alignment aims to align the knee based on where the joint surfaces naturally are, rather than imposing a predetermined, non-anatomic alignment. By preserving the natural joint line and leveraging the functional relationship between the joint surface and ligaments, kinematic alignment offers improved knee function. Plus, kinematic alignment may have the potential to improve the recovery and satisfaction of knee replacement surgeries by taking into account each patient’s unique characteristics.
Patient preference for specific alignment techniques
As research and clinical experience continue to support the benefits of kinematic alignment, more surgeons are incorporating this approach into their practice [6]. Previously, kinematic alignment was not as widely accepted, and there was a stronger emphasis on standardisation and traditional approaches that have been in use for decades.
However, healthcare today recognizes the importance of customisation and personalised medicine is on the rise. There is now a growing demand for knee replacements that are more personalised for each patient. And while some companies offer custom-made knee implants, it is essential to consider not just the fit but also the alignment, particularly in terms of the medial-pivot knee.
Patients are driven to seek specific alignment techniques because they hope to achieve a knee that feels more natural. The forgotten joint score serves as a vital measure of surgical success, measuring how well patients forget they even had a joint replacement, and illustrating the level of overall satisfaction. Studies indicate that patients who undergo kinematic alignment tend to have better forgotten joint scores, suggesting a more natural feeling of their prosthesis and minimal awareness of it during daily activities [7]. This desire for a more natural outcome is one of the factors influencing the drive toward more patient-specific, alternative alignment techniques in knee arthroplasty.
While standardisation has its merits, embracing individualisation in knee replacement surgery can lead to remarkable improvements. Rather than a one-size-fits-all approach, personalised alignment techniques take into account the patient’s specific anatomy and biomechanics. This shift in perspective allows for a more accurate replication of the patient’s natural knee kinematics, resulting in enhanced stability, range of motion, and overall functionality.
Enhancing outcomes with medial-pivot and kinematic alignment
The integration of kinematic alignment with advanced implants, such as medial-pivot designs, represents a significant advancement in knee arthroplasty, further enhancing the benefits experienced by patients. By combining the advantages of medial-pivot knees with the precision of kinematic alignment techniques, patients not only benefit from improved stability but also enjoy a more natural range of motion and achieve better functional outcomes. This synergistic approach holds substantial promise in optimising patient satisfaction and restoring optimal knee function.
Surgeons who have embraced the combination of medial-pivot knees with kinematic alignment have reported remarkable results, leading to higher patient satisfaction and improved surgical outcomes [8]. In contrast, patients who undergo mechanical alignment with ligament releases may have increased surgical invasiveness, leading to potential pain, swelling, and a slower recovery process.
“I’ve done this for a long time, and I’ve used many different implants,” Steensen said. “I was a mechanical alignment surgeon in the past, and … I think you get a higher percentage of good outcomes with a medial-pivot implant and kinematic alignment. If a patient wanted a different type of implant and they wanted a different alignment, I would refer them to another surgeon. I would tell them, ‘I believe in this enough that I would feel guilty if I didn’t do this for you.’”
The integration of medial-pivot implants with kinematic alignment techniques shows promise in contributing towards improved overall functional outcomes for patients by closely replicating natural knee joint mechanics. The enhanced stability provided by this combination seeks to empower patients to carry out daily tasks and enjoy recreational activities.
References
[1] Gunaratne, Rajitha et al. “Patient Dissatisfaction Following Total Knee Arthroplasty: A Systematic Review of the Literature.” The Journal of arthroplasty vol. 32,12 (2017): 3854-3860.
[2] Jones, Christopher W et al. “Sagittal Stability and Implant Design Affect Patient Reported Outcomes After Total Knee Arthroplasty.” The Journal of arthroplasty vol. 35,3 (2020): 747-751.
[3] Wautier, Delphine, and Emmanuel Thienpont. “Changes in anteroposterior stability and proprioception after different types of knee arthroplasty.” Knee surgery, sports traumatology, arthroscopy : official journal of the ESSKA vol. 25,6 (2017): 1792-1800.
[4] Pritchett, J W. “Patient preferences in knee prostheses.” The Journal of bone and joint surgery. British volume vol. 86,7 (2004): 979-82.
[5] Pritchett, James W. “Patients prefer a bicruciate-retaining or the medial pivot total knee prosthesis.” The Journal of arthroplasty vol. 26,2 (2011): 224-8.
[6] Dossett, H. G., et al. “A randomized controlled trial of kinematically and mechanically aligned total knee replacements.” The Bone & Joint Journal, vol. 96-B, no. 7, 2014, pp. 907-913.
[7] Tran, Ton et al. “Kinematic alignment in total knee arthroplasty: a five-year prospective, multicentre, survivorship study.” Bone & joint open vol. 3,8 (2022): 656-665.
[8] Kaneda, K., Niki, Y., Kuroyanagi, Y., Kobayashi, S., Harato, K., Iwama, Y., & Nagura, T. (2021). Kinematically Aligned Total Knee Arthroplasty Using Medial Pivot Knee Prosthesis Enhances medial-pivot Motion: A Comparative Kinematic Study With Mechanically Aligned Total Knee Arthroplasty. Arthroplasty today, 13, 24–28.
