New data about typical loading of knee implants

[Philipp Damm]

Dear Colleagues:

We now published a paper in which we evaluated data from 9 subjects and 7 activities of daily living, measured with instrumented knee implants. From this data we derived typical or ‘standard’ loads which describe the average forces and moments acting in subjects with (1) average body weight and (2) high loads in subjects with high body weight. Because these standard loads are based on the largest data pool currently available, we hope that they will be widely used for experimental and analytical studies on the loading of knee implants and (with some limitations) natural knee joints. The applications possibly also include the definition of new test standards for wear, stability and fixation of knee implants.

The paper: G. Bergmann, A. Bender, F. Graichen, J. Dymke, A. Rohlmann, A. Trepczynski, M. O. Heller, I. Kutzner:
“Standardized Loads Acting in Knee Implants.“ PLoS ONE 2014 9(1): e86035. doi:10.1371/journal.pone.0086035

can be downloaded via the public data base www.OrthoLoad.com.

Typical trials, included in the data evaluation, can be downloaded in form of numerical data or videos if you enter the data base and select this activity: Implant: Knee Joint; Activity: Gaitanalysis. From the menu ‘Test Loads’ & ‘Knee Joint’ you can also download many additional data, especially on the loads acting in the individual subjects.

If you register as a Twitter follower on OrthoLoad, you will get a message as soon as we provide new data or publications, which may be the case every 2 months or so.

We hope that you enjoy our work and that it will help you with your own scientific, medical, rehabilitation, or commercial applications.

Sincerely Yours
Georg Bergmann

Julius Wolff Institute
Charité – Universitätsmedizin Berlin

[Ross Miller]

Re: New data about typical loading of knee implants

This is a great resource, thanks much!

[Krystyna Gielo-Perczak]

Re: New data about typical loading of knee implants

Thank you for providing a link to Orthoload website. It is very helpful to be aware of these real life data.

Krystyna Gielo-Perczak

[James Shippen]

Re: New data about typical loading of knee implants

This is an excellent resource with many immediate applications - many thanks.

Jame Shippen

[Ton van den Bogert]

Re: New data about typical loading of knee implants

This really is a great resource to have online!

I was just discussing this with a colleague who was interested in visualizing the center of pressure (COP) in the knee. It occurred to me that it would be quite easy to add this capability to the orthoload software, by using the same equations as used with force plates, to determine COP and free moment from a 3D force and moment. See, for instance, equations [3] in http://www.kwon3d.com/theory/grf/cop.html. Then you can draw the force vector from the COP instead of from the knee center.

Maybe this can be incorporated in a future version of the software.

I did a rough calculation from two of the Orthoload files. In K1L_110108_1_80P,AKF, I get COPx = -My/Fz ~ -0.01 at the peaks, or about 1 cm medial from the knee center. In K7L_280710_1_28P, the My is more negative, and Fz is about the same, which means a more medial position of the COP (if I got my signs right).

Ton van den Bogert

[Philipp Damm]

Re: New data about typical loading of knee implants

Dear Ton

Thank you for your interest in our work!
Finding the point of force application within the tibial plateau is a very interesting actual topic. Unfortunately the moments we measure with our instrumented implants can be caused by two effects at the same time: a) by friction and b) by a horizontal (a-p or m-l) offset of the axial force –Fz from the centre of our coordinate system.
In the frontal plane, or better in the plane perpendicular to the plane of knee movement (both planes are nearly identical), there is no friction. Therefore the m-l application point of -Fz in the frontal plane can be calculated from the values of the moment My and the force –Fz. Because the distance of the 2 condyles is known, this allows calculating the two forces Fmedial and Flateral, acting on the medial and lateral tibial plateau. Such calculations have been performed already and are published in these 3 papers:

1. Trepczynski A, Kutzner I, Bergmann G, Taylor W, Heller M (submitted) Modulation of the relationship between external knee adduction moments and medial joint contact forces across subjects and activities. Arthritis & Rheumatism.

2. Kutzner I, Trepczynski A, Heller MO, Bergmann G (2013) Knee Adduction Moment and Medial Contact Force - Facts about Their Correlation during Gait. PLoS One 8: e81036.

3. Halder A, Kutzner I, Graichen F, Heinlein B, Beier A, et al. (2012) Influence of limb alignment on mediolateral loading in total knee replacement: in vivo measurements in five patients. J Bone Joint Surg Am 94: 1023-1029.

Currently we evaluate the medial-lateral force distribution in the knee joint for other activities than only walking, based on all 9 subjects and many trials per subject. This paper will be finished shortly.
In the sagittal plane, however, this method doesn’t function because movement takes place in this plane. One cannot distinguish between a moment Mx caused by friction or by a force offset. Only if you assume zero friction, the MAXIMUM offset of –Fz can be calculated, but this is unrealistic.

Georg Bergmann

[Ton van den Bogert]

Re: New data about typical loading of knee implants

Thank you for this additional information. I had not seen Kutzner's paper before and this is, I believe, the first time that the relationship between external adduction moment and the true contact force has been established. This is very important.

There has obviously already been considerable work on the interpretation of the data from instrumented implants, and this is not always easy.

My suggestion was only about the visualization of the 3D force-moment data. Any 3D force-moment combination is mechanically equivalent to a 3D force at a point somewhere in a defined plane (this point is then the center of pressure for that plane), plus a moment about an axis perpendicular to that plane. This is commonly done with force plate data (example), and should be equally useful for a force plate implanted on the tibial plateau. It is just a different, and more compact, way to visualize the same information. This would not be an estimation of load distribution, it still is only a resultant load.

Ton van den Bogert