Understanding intervertebral disc reaction forces

[Riza Bayoglu]

Hi,

Following the tutorial, I predicted vertebral load estimates in the male model and was able to replicate exact same results. I am, however, having difficulty when interpreting disc reaction forces. Do I need to use values from the parent or child components when reporting compression and shear forces? My feeling is that I should use values from the parent components.

For example for the L5/S1 joint, child and parent component results are:

child

L5_S1_IVDjnt_on_lumbar5_in_lumbar5_fy 576.4
L5_S1_IVDjnt_on_lumbar5_in_lumbar5_fx -141.35
L5_S1_IVDjnt_on_lumbar5_in_lumbar5_fz 0.0

parent

L5_S1_IVDjnt_on_sacrum_in_sacrum_fy -588.77
L5_S1_IVDjnt_on_sacrum_in_sacrum_fx -74.6
L5_S1_IVDjnt_on_sacrum_in_sacrum_fz -0.0

Thank you.
Riza

[Jacob J. Banks]

Yes. The JRA analysis is where you get the load estimates you (I assume) are interested in. Fy should be Compression, Fx should be AP shear, and Fz should be ML shear.
The parent body should be the superior (as opposed to inferior) body relative to the joint. The Child the inferior.

It really doesn't matter which you use, so long as you report it. Shear will be a little more sensitive to the coordinate system, but you'll find the 3D resultant force to be the same.

In Anderson's lab, we often report a vertebral load, which is the average between inferior and superior joints (so for L5 vertebrae, it would include the Parent of the L5/S1 and the child of the L4/L5).

So, back to your original question... it doesn't matter, so long as you report it. I wouldn't worry about 'signs' either, so long as you report/define them.
Jake Banks
PS: Sorry about the delayed response, just seeing this... my PC was impacted by CloudStrike last Friday.