Disc Stiffness.any

Main Forum
1

Hello,

I'm currently using AnyBody 8.0 (AMMR v3) for a study focused on estimating loads exerted on the spine. In my attempt to integrate disc stiffness into the model using the BMPlugin tool, I encountered an issue where the compression forces' results remained unchanged.

Upon inspecting the BodyModelConfig.any file, I discovered that the following definitions were added after enabling the nonlinear-stiffness for whole trunk:

#define BM_TRUNK_CERVICAL_DISC_STIFFNESS _DISC_STIFFNESS_NONLINEAR_
#define BM_TRUNK_LUMBAR_DISC_STIFFNESS _DISC_STIFFNESS_NONLINEAR_

Further investigation led me to the DiscStiffness.any file, where I found the following condition:

#if BM_TRUNK_DISC_STIFNESS == _DISC_STIFFNESS_NONLINEAR

This condition implies that we need to define BM_TRUNK_DISC_STIFNESS in BodyModelConfig.any, which was not automatically created by BMPlugin. Once I defined BM_TRUNK_DISC_STIFNESS myself, the results changed accordingly.

However, I couldn't locate any stiffness moment parameters for the cervical spine in the AMMR v3.

In addition, I checked the DiscStiffness.any for both AMMR.v2.4.3-Demo and AMMR.v3.0.0-Demo. The functions defined in these files were not consistent:

For AMMR.v2.4.3-Demo:

#if BM_TRUNK_DISC_STIFNESS == _DISC_STIFFNESS_NONLINEAR_
      // K.S.Han et al. 2010 (in degrees, checked against Heuer et al.)
      //  #Flexion          -0.002x3 + 0.0141x2 - 0.4726x
      //  #Lateral bending  -0.0087x2 - 0.6989x
      //  #Axial rotation   -0.0061x3 - 1.0191x   
      
      AnyFunPolynomial Flexfun = {
        PolyCoef={{-0.002, 0.0141,-0.4726 + .ligFlex,0}};    
      };

      AnyFunPolynomial Extfun = {
        PolyCoef={{-0.002, 0.0141,-0.4726 + .ligExt,0}};    
      };
      
      AnyFunPolynomial ARfun = {
        PolyCoef={{-0.0061,0,-1.0191+ .ligAxialRot,0}};
      };
      AnyFunPolynomial LBfun = {
        PolyCoef={{0, -0.0087, -0.6989+ .ligLatbend,0}};
      };
    #endif

For AMMR.v3.0.0-Demo:

 AnyFunPolynomial Flexfun = {
        PolyCoef={{0,-0.4726 + .ligFlex, 0.0141, -0.002,0}};    
      };

      AnyFunPolynomial Extfun = {
        PolyCoef={{0,-0.4726 + .ligFlex, 0.0141, -0.002,0}};    
      };
      
      AnyFunPolynomial ARfun = {
        PolyCoef={{0, -1.0191+ .ligAxialRot,0,-0.0061}}; // symmetric for R/L ax.rotations
      };
      AnyFunPolynomial LBfun_R = {
        PolyCoef={{0, -0.6989+ .ligLatbend, -0.0087,0}};
      };
      AnyFunPolynomial LBfun_L = {
        PolyCoef={{0, -0.6989+ .ligLatbend, 0.0087,0}};
      };
    #endif

I would be grateful if you could tell me which one is accurate.

Moreover, I found the same problem reported in another topic (Influence of Disc Stiffness and Ligaments) and did not find any answers.

I would appreciate any insights on whether I made an error in my approach and where I might find information regarding cervical spine disc stiffness in the AMMR v3. Moreover, How can I find the paper related to this model for this nonlinear disc stiffness.

2

Hi Mohamad

The AMMR spine model has disc stiffness only in the lumbar region. I believe, the option for cervical stiffness was created as a placeholder for some future developments.

You are on the right track about using the statement BM_TRUNK_DISC_STIFNESS.
This is the statement that controls whether the stiffness model is implemented or not. I think we need to look into it and fix the statements or document them. This is, understandably, quite confusing.

Regarding, the functions, I will recommend using the latest AMMR. In fact, if you look at the AMMR 2.4.4 changelog, you will see that the disc stiffness functions have been corrected.

Best regards,
Dave

1 Like
3

Dear Dave,

Thank you very much for your help.

I would be grateful if you could help me with finding references related to the nonlinear functions for the lumbar. I couldn't find the papers mentioned in the code.

4

Hi Mohamad,

Here is the reference:
Han KS, Zander T, Taylor WR, Rohlmann A. An enhanced and validated generic thoraco-lumbar spine model for prediction of muscle forces. Med Eng Phys. 2012 Jul;34(6):709-16. doi: 10.1016/j.medengphy.2011.09.014.

Best regards,
Dave

1 Like