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The Truth About a “Round Knee”

We’re glad you’re interested in learning more about total knee replacement and the best implant options for you. You’ve come to the right place. Zimmer is a worldwide leader in total knee replacement with more surgeons choosing to implant Zimmer knees than any other company’s knee systems. Read on to learn more or click here to find a doctor near you.



Recently, you may have seen an advertisement entitled “GetAroundKnee” that embellishes the virtues of a round knee design. Zimmer would like to provide you a more complete perspective on this “circular argument.”

The following discussion will get more technical as you read on.  We have tried to start with the basics and provide more detail for those interested in understanding the complex motion of the natural knee.  The GetAroundKnee campaign assumes that a round knee just makes sense. Unfortunately, this simple message isn’t the whole truth.


It’s True: A Portion of Your Knee is Round

In more technical terms, that round portion is said to have a ‘single radius of curvature.’  It is that portion of a knee system design that provides for a smooth, natural motion during the majority of your normal activity, such as walking.

Zimmer has developed the most extensive single radius of curvature of any comparable knee implants, one designed to more closely match your natural knee during simple movements.  In fact, when you compare the “GetAroundKnee” (specifically, the Stryker Triathlon Knee System) to the Zimmer® NexGen® Knee System, you will see that the Stryker Triathlon Knee has a single radius of curvature from 10° degrees to 110° degrees, or an arc of 100°; while the Zimmer NexGen LPS-Flex Knee has a single radius of curvature from 17° to 130°, or an arc of 113°.

Obviously, if you are going to base your decision on which knee replacement is “more round,” then you should choose Zimmer.  With more than 15 years of use and over 4 million implanted, the Zimmer NexGen Knee System gives your surgeon the implant options he needs to better meet YOUR activity level, while providing documented long-term performance and stability. But, you shouldn’t base such an important decision on a single product feature. Several publications have shown no significant difference in performance of knee systems with different curvatures, which is the only product feature the “GetAroundKnee” advertising focuses on. In fact, a study funded by the company that manufactures the “GetAroundKnee” showed no clinical difference in knee stability or function when comparing the “GetAroundKnee” to one of their own obsolete 20-year-old “oval” knee designs. Click here to learn more.


Where Your Knee Isn’t Round Is Also Important

Factually, your knee is not a simple circle or a single radius.  You need only look at the images below of a natural knee to see the truth.


As you can see, the radius of the femur (the end of your thighbone) becomes significantly smaller in the back of the knee.  We call this the closing radius of the posterior condyles of the femur.  That’s a bit technical, but it’s this change in the shape of your natural knee that allows you to bend your knee into deep flexion.

As a leader and pioneer in knee replacement design and technology, we understand the human knee. Based on this understanding, the Zimmer NexGen Knee System incorporates the most extensive single radius that blends smoothly with the appropriate closing radius.  This provides for a more natural feeling and functioning knee.  In fact, that is why the Zimmer NexGen Knee System is documented to safely accommodate the greatest range of motion and flexion.1

During simple activities – such as walking or stair climbing – your knee movement follows a single radius of curvature; but during flexion, a single radius will actually inhibit your motion.  In fact, a recent study published at the American Academy of Orthopedic Surgeons concluded that the Stryker Triathlon Knee System (the “GetAroundKnee” system) had a total range of motion (ROM) of 104 degrees, much less than the 144 degree ROM of the Zimmer NexGen Knee System.1  While patients should understand that their ability to achieve deep flexion post-operatively depends on a variety of factors, including pre-operative ROM, surgical factors, and rehabilitation,  the study found that a NexGen Knee product  ”achieved the highest flexion” among the six modern total knee replacement implants tested.1


Implant System Single Radius of Curvature Total Range of Motion1
Stryker Triathlon Knee (“GetAroundKnee”) 100° 104°
Zimmer NexGen LPS-Flex Knee  113° 144° (greatest ROM out of the six modern knee replacements tested1)  


The table below provides a more complete summary of the findings from that study, including the range of motion for the other four implants tested.

Total Knee System Range of Motion (ROM)  Model Prediction1 Range of Motion (ROM)  Clinical Average1
Stryker Triathlon Knee (“GetAroundKnee”) 104° 108°
MatOrtho Medial Rotation Knee (MRK) 104° 105°
Stryker Duracon 105° 105°
Biomet Vanguard 117° 111°
Smith & Nephew Journey BCS 139° 118°
Zimmer NexGen LPS-Flex Fixed 144° 135°

This study looked at not only a ROM prediction model, but also compared that with clinical averages of ROM that had been reported on each implant in recent clinical studies in peer reviewed orthopaedic literature. The Zimmer NexGen Knee product showed in the study the greatest potential ROM in both the prediction model and the clinical averages.1

As you can see in the graph below, range of motion is very important to the activities you will want to do every day.  Moreover, you can understand how these activities translate into other activities, such as gardening, golfing, biking, and so on.

This superior range of motion that Zimmer’s NexGen Knee devices can accommodate is another reason why Zimmer knee systems are chosen more than any other knee system in the world.


Don’t Believe the Illusion of the “GetAroundKnee” Video

If you have seen Stryker’s knee movement video, you might think their “round knee design” looks compelling.  Isn’t animation wonderful?  Just like in the movies, Stryker is able to manipulate the movement of their “GetAroundKnee” compared to the movement of “Traditional knee replacement systems” to create the illusion that their design is unique and better.  Unfortunately, this animation is as fictional as the animated movies of Hollywood.

The circular movement promoted by Stryker looks natural because Stryker inaccurately shows its “GetAroundKnee” rotating in place without any forward or backward movement.  Yet, when Stryker shows an animation of “Traditional knee replacement systems,” it demonstrates an abrupt forward movement that looks unnatural, and even abusive to the knee ligaments.  It’s almost hard to watch.  It’s also not accurate.  The truth, in fact, is just the opposite.

According to a study published during the American Association of Orthopedic Surgeons annual meeting, the Stryker “GetAroundKnee” continues to slide forward as the knee flexes, rather than rolling back like a healthy un-operated knee would do.4 This is similar to the unnatural motion Stryker labels on their site as “Traditional knee replacement systems.”  That’s right.  The very sliding motion that Stryker suggests is bad in the “Traditional knee replacement systems” animation more closely represents the actual motion demonstrated in this study of their “GetAroundKnee” system!4


There is More to Knee Replacement than Motion and the Shape of the Femoral Implant

When Stryker talks about a round knee, they are referring to the implant attached to the end of your thighbone (your distal femur).  That’s only one of the components required for a total knee replacement system.  There is another component attached to the top of your shinbone (your proximal tibia), one to your kneecap (your patella) and a smooth, plastic articulating surface component designed to allow the knee to glide and move without pain (similar to what cartilage does in a healthy knee).

The design and shape of this plastic component is extremely important to the motion and the longevity of your knee replacement.  Unfortunately, as this plastic is exposed to the fluids in your body, it can age and wear just like your original cartilage – unless it is specifically engineered to resist such effects. One cause of age and wear in plastic is oxidation.5-8 Oxidation is a phenomenon where oxygen infiltrates the plastic material, causing it to lose strength and durability.5-8 One key design goal of plastics used in knee replacement components is to reduce oxidation.

How the plastic is created can have an effect on the oxidation of the plastic components of your knee implant. The durability of the plastic component, in many cases, can affect how long your knee replacement will last before requiring a “re-do” surgery (called a “revision”). The plastic in the “GetAroundKnee” is processed differently than many other knee implants on the market, and studies have shown increased oxidation of that plastic when compared to conventional plastic material10,11. While these specific studies did not make a prediction of long-term clinical performance as a result of the reported rate of oxidation of the Stryker plastic, the link between oxidation and its impact on long-term wear performance in plastic is well established in the scientific community.5-8

Zimmer’s plastic technology (more specifically Prolong® Highly Crosslinked electron beam irradiated and melt annealed Polyethylene) for total knee replacement has been shown to be among the most durable and wear resistant articular surface materials available for knee replacement.9  One of the main reasons for its success is that it is designed to resist the aging/oxidation process better than other plastic articulating surfaces.  This resistance to aging means it should last longer and wear less over time. This is yet another reason why NexGen Knees have among the best long-term performance records on the market.12, 13


Zimmer Knees: Proven Function, Proven Results.

In the end, while some companies want to make a big deal out of one aspect of their product, like its roundness or surface coating, the real test is the knee implant’s performance over time in the real world.  One of the best sources to assess performance over time, in the real world, is to look at national joint replacement registries, which are databases that track the outcome of every knee replacement surgery performed in the sponsoring country (without exception). Because the hospitals in those countries are required to maintain records for each and every patient who receives a knee implant, it’s a good way to monitor how long knee systems last in patients and to compare the results of the different knee systems available.

Zimmer NexGen knees are among the best performing knee implant systems in both the 2011 Australian National Registry12 and the 2011 Swedish National Registry13  and have consistently been among the best every year. These leading registries are the best indications of how knee systems are performing in the real world when used by a wide variety of surgeons and in a wide variety of patients.

In addition, a recent study published in the Journal of Bone and Joint Surgery showed 100% implant survivorship of the NexGen CR Knee System at 14 years.14  Few systems have shown such results.  Click here to read more about Zimmer’s Clinical Success.


Trust the Experts: Talk to Your Doctor

Zimmer understands that there is a lot of information out there about knee replacement. If you have spent much time looking at options for yourself or a loved one, we understand it can be confusing. In addition to looking at a product’s real world performance in national registries and clinical studies, some of the best information you can get about knee procedures and implant products is from an orthopaedic surgeon. A qualified orthopaedic surgeon can help you understand your options and what to expect from a knee replacement.

Surgery can be daunting, but many patients who have undergone a knee replacement say they wish they hadn’t waited so long. In fact, your arthritis and knee pain is likely to worsen with time. By acting early, you and your surgeon may be able to pursue early intervention alternatives that might help you to take control of your knee pain and get back to your life.

Currently, more surgeons choose Zimmer knees for their patients than any other knee system in the world. Click here to find a surgeon near you.

Here are some questions that may be helpful to ask your orthopaedic surgeon when considering knee replacement surgery. We suggest you print this list and take it with you to your appointment. It’s also a good idea to keep a record of your knee pain to share with your doctor at the same time.

  1. Would I benefit from a knee replacement procedure?
  2. Which knee implant is best for my lifestyle and why?
  3. What has been your experience with that particular implant?
  4. How long can I expect a knee replacement to last?
  5. What other factors should I consider besides how long it lasts?
  6. What should I expect in terms of recovery from knee replacement surgery?
  7. What should I do to take care of and strengthen my knee, and how can I get the most out of my new knee?
  8. Are there any knee replacement complications that I should be aware of?
  9. Is there a knee rehab or physical therapy program that I will need to attend?

More than 7,000 surgeons use Zimmer products in the U.S. To find a doctor for your knee replacement surgery, call 1-800-HIP-KNEE (447-5633) or click here.


1. Edward A. Morra, M.S.M.E, A. Seth Greenwald, D.Phil. (Oxon). Preclinical Computational Models: Promise and Progress in Total Knee Arthroplasty Design. AAOS 2012. 2. Laubenthal KN. A quantitative analysis of knee motion during activities of daily living. Phys Ther 52(1):34-43.; 1972. 3. Hazel Clarkson and Gail Gilewich. “Musculoskeletal Assessment: Joint Range of Motion & Manual Muscle Strength;”; 1989. 4.  Edward A. Morra, M.S.M.E, A. Seth Greenwald, D.Phil. (Oxon) et al. The Influence of Knee Design on High Flexion: A Kinematic Comparison with the Normal Knee. AAOS 2009. 5. Douglas D.R. Naudie, MD, FRCSC, Deborah J. Ammeen, BSc, Gerard A. Engh, MD, Cecil H. Rorabeck, MD, FRCSC Wear and Osteolysis Around Total Knee Arthroplasty. Journal of the AAOS Volume 15, Number 1, January 2007. 6. Collier JP, Sutula LC, Currier BH, et al: Overview of polyethylene as a bearing material: Comparison of sterilization methods. Clin Orthop Relat Res 1996;333:76-86. 7. Collier JP, Sperling DK, Currier JH, Sutula LC, Saum KA, Mayor MB: Impact of gamma sterilization on clinical performance of polyethylene in the knee. J Arthroplasty 1996;11: 377-389. 8. Bohl JR, Bohl WR, Postak PD, Greenwald AS: The Coventry Award: The effects of shelf life on clinical outcome for gamma sterilized polyethylene tibial components. Clin Orthop Relat Res 1999;367:28-38. 9. Rowell SL, Yi J, Micheli BR, et al.In Vivo Performance of Second Generation Highly Cross-linked UHMWPE. Poster No. 1033. ORS 2012 Annual Meeting. 10. Reinitz, S D; Currier, B H; Franklin, K J; et al. Early Indications of Oxidative Degradation in Retrieved Annealed UHMWPE Bearings Resembles Gamma Sterilized Materials. Paper No. 0307. ORS 2012 Annual Meeting. 11. Rubash, H.E., Martell, J., et al. Clinical Performance of Highly Crosslinked Polyethylene. Symposium V: Avoiding Pitfalls in Primary THA. Feb. 10, 2012 Pgs. 26-28. 12. 2011 Australian National Registry. 13. 2011 Swedish National Registry. 14. J. W. Park, Y. H. Kim, Simultaneous cemented and cementless total knee replacement in the same patients, A Prospective Comparison Of Long-Term Outcomes Using An Identical Design Of NexGen Prosthesis, J Bone Joint Surg Br., November 2011, 93-B:1479–86 VOL. 93-B, No. 11.

Note: GetAroundKnee is a trademark of Stryker Corp, and Triathlon and Duracon are trademarks of Howmedica Osteonics Corp. Medial Rotation Knee is a trademark of MatOrtho, Ltd. Vanguard is a trademark of Biomet. Journey is a trademark of Smith & Nephew.