The GetAround Knee from Stryker

[Sam Hamner]

Over the holidays, I noticed this commercial for the "GetAround" knee replacement from Stryker: http://youtu.be/VAAJg_cYcMw

The commercial makes the claim that the GetAround knee is "designed to replace the knee's natural circular motion", as opposed to the "oval motion" of other knee replacements. This was a surprising claim to me, as in my work with musculoskeletal simulation, we have typically modeled the healthy knee with a non-circular or elliptical motion (Yamaguchi and Zajac, J Biomech, 1989; Seth et al., Nonlinear Dyn, 2010).

I am curious to hear from others in the Biomch-L community. Is there is any biomechanical basis to the claim made by Stryker in this commercial; or is this just clever marketing to sell more knee replacements?

-Samuel Hamner

[Ross Miller]

Re: The GetAround Knee from Stryker

That's interesting, I hadn't seen that before.

I don't think traditional knee replacements are as sloppy as they show there (e.g. contact point slipping forward in early flexion) but I don't really know for sure. The femoral condyles are definitely not perfect circles but the question for us would be how much of a difference does it make? For the range of knee motion in walking I don't think it would have much effect on moment arms, contact forces, etc. but for movements with deep flexion maybe it makes a bigger difference.

This made me think of a pedaling study I like:

http://www.ncbi.nlm.nih.gov/pubmed/18395213

Maybe a similar thing could be done to optimize joint replacement shapes.

Ross

[John Rasmussen]

Re: The GetAround Knee from Stryker

There was recently the biannual knee research forum in Leuven, Belgium, organized by Smith & Nephew. One of the matters discussed was whether it makes sense to get an artefact made of metal and plastic to mimic an anatomical knee made of bone and soft tissue, given the fact that the materials of the real knee are at least an order of magnitude more compliant than those of the replacement.

I think this is a very reasonable question and the engineer in me says that we would never design a mechanical joint for a machine made of metal and plastic that in any way resembles the design of an anatomical knee. It is really a good question whether an artificial knee should be anatomical or artificial.

John Rasmussen

[Ken Fischer]

Re: The GetAround Knee from Stryker

This is a GREAT marketing approach! I love the elliptical wheels! Did they really do that to a suburban? My guess is computer graphics.

But how much difference is it really? I took a screen shot of the video of both knees in the same position and overlayed them in Photoshop with the top 50% transparent. The result? The tibia is exactly the same. The shape of the femoral condyles in the region they put the circle is almost identical. The main difference appears to be in the proximal portion that does not contact the tibia (e.g. trochlear groove). My conclusion: it just depends on WHAT you draw and WHERE you put the center (of the drawing). The animation of the motion is slightly different, but I'm not sure why (except to make the "traditional knee replacement" look worse). In other words, it seems to be a great marketing ploy, but the implant is NOT substantially different!

Ken Fischer

[Michael Lawrence]

Re: The GetAround Knee from Stryker

Did anyone else notice the amount of 'wiggle' the LCL has in the traditional knee replacement? It is an entertaining commercial though.

[Greg Freisinger]

Re: The GetAround Knee from Stryker

I also noticed this commercial and wanted to look into their claims, since the current knee replacement geometry is obviously not as haphazardly designed as they make it seem. I just ran across "The Kinematics and Stability of Single-Radius Versus Multi-Radius Femoral Components Related to Mid-Range Instability after TKA" by Stoddard et al. 2012. This study was funded by Stryker and they talk through the possible issues of having a multiple radius curve on the femoral component vs. a single radius design. They used 8 cadaver limbs and looked at knee kinematics under specific loading of the natural knee, Kinemax (older multi-radius), and Triathlon (single radius) prosthesis. You can read through the paper for more details but in the author's words "Although these results are design-specific for the prostheses used in the experiments, they do not support reports in literature of mid-range instability being related to the shape of the femoral condyle." So the change to a single radius femoral component did not seem to change the kinematics of these cadaver knees under load.

Given that they didn't see any major differences between these two femoral components, I do think that prosthesis geometry is important. Especially when trying to determine how sensitive a specific prosthesis is to slight changes in alignment. I am interested in seeing how variability in alignment and soft tissue balancing, done in the OR, influence the secondary kinematic motions of the knee and how that impacts biomechanical and clinical outcomes.

-Greg Freisinger

Over the holidays, I noticed this commercial for the "GetAround" knee replacement from Stryker: http://youtu.be/VAAJg_cYcMw

The commercial makes the claim that the GetAround knee is "designed to replace the knee's natural circular motion", as opposed to the "oval motion" of other knee replacements. This was a surprising claim to me, as in my work with musculoskeletal simulation, we have typically modeled the healthy knee with a non-circular or elliptical motion (Yamaguchi and Zajac, J Biomech, 1989; Seth et al., Nonlinear Dyn, 2010).

I am curious to hear from others in the Biomch-L community. Is there is any biomechanical basis to the claim made by Stryker in this commercial; or is this just clever marketing to sell more knee replacements?

-Samuel Hamner

[Edward Morra]

Re: The GetAround Knee from Stryker

The marketing video suggests that the GetAround knee has natural "circular" motion and its competitors have an unnatural and awkward "oval" motion. Natural intact un-operated knee motion is neither circular nor oval. The lateral side of the femoral bone slides posteriorly while rotating in increasing flexion, the medial side rotates in place. At higher flexion angles, both sides slide posteriorly while continuing to rotate, which assists in achieving deep flexion activities.

Both designs in the video actually have the same femoral component shapes, just one has a circle drawn on it and the other an oval, and a graphic artist makes them move differently during high flexion activity. The GetAround Knee is depicted as rotating in place (that is where the idea of circular motion comes from because it looks like a wheel), the competing design slides anteriorly and bumps about awkwardly, much like the oval wheeled bicycle in the shot right before the product motion is displayed.

I have actually published a lot on the topic of total knee kinematics for a wide variety of designs, and compared their motion during high flexion activities to that of the intact, un-operated knee. You can download an example AAOS exhibit handout with an embedded animation of six designs here <http://www.orl-inc.com/knee_publications/2008/High%20Flexion%20TKR%20Kinematics%202008r1-dynamic.pdf> for comparison.

Ironically, the GetAround knee (rebranded name for the Triathlon knee) actually moves very similar to the depiction of the "oval" knee in the video, because the tibial insert is pretty flat and allows anterior translation of the femoral component. The MRK has a similar "single radius" femoral component as the GetAround, but the medial compartment of the tibial insert is deep dished and forms a loose ball and socket joint that promotes "circular" motion. So my results are pretty much the opposite of the marketing video.

That being said, apparently the commercial has been successful at driving interest in the GetAround product: <http://www.mmm-online.com/stryker-rebrands-knee-sees-spike-in-web-traffic/article/256060/>

Chalk one up for the clever Stryker rebranding marketing team!