Recurring Meeting of Cleveland Clinic Core Team

Date: April 21, 2015

Time: 10:30 AM EST

Means: In person meeting

Attendees:

1. Ahmet Erdemir
2. Craig Bennetts
3. Snehal Chokhandre
4. Robb Colbrunn
5. Tara Bonner

Agenda:

1. Discuss immediate action items from the last meeting.
2. Discuss large-scale analysis of simulation parameters for FE model convergence.
3. Decide immediate action items for the next meeting.
4. Other.

Immediate Action Items:

• Snehal
  • Start wiki page on the use case for kinematics driven mechanics of cruciate ligaments.
  • Start working on a modeling problem to evaluate thickness measurements.
• Ahmet
  • Review draft document to be submitted to Biomomentum, Inc.
• Robb

  • Provide brief documentation on tdms file contents (groups & channels).

• Ahmet & Craig

  • Meet to discuss post-processing of FEBio log file to extract convergence information.

Ongoing Action Items:

• Ahmet
  • Dissemination of anatomical images.
  • Dissemination of joint mechanics testing data.
  • Detail image segmentation specifications.
• Craig
  • Anatomical imaging manuscript.
  • Scripting for large-scale analysis of simulation parameters for convergence.
• Snehal
  • Pivot shift manuscript.

  • Cruciate ligament modeling & simulation.

  • Evaluate alternative tissue testing strategy.
• Robb
  • Patellofemoral joint experimentation manuscript.
  • Conversion of patellofemoral joint contact pressure data to ASCII format.
• Tara
  • Tibiofemoral joint experimentation manuscript.

Notes:

1. Discuss immediate action items from the last meeting.
   • TBD
2. Discuss large-scale analysis of simulation parameters for FE model convergence.
   • The group had an in-depth discussion on various simulation parameters where Ahmet provided descriptions of each parameter and their potential values.
     • Convergence tolerances are defined in relative terms (percentages). dtol is for displacement (default = 0.01). etol is for energy (default = 0.1). rtol is for residuals (force); (default = 0 meaning it is not checked); ideally, residuals should be minimized. lstol is for line search. Decreasing tolerances will likely increase solution time. Increasing them may result in solution inaccuracies.
     • Time stepper parameters include dtmax, maximum time step size; dtmin, minimum time step size; max_retries, maximum number of step cutbacks which is likely to be between 10 and 50; opt_iter; a parameter used to decide increases in step size; aggressiveness, for magnitude of step cutback (0 cuts by 10% or 1, which cuts time by half, it is set to 1). Newton or quasi-Newton parameters include max_refs and max_ups. When max_ups is equal to 0 a full Newton step is taken where the stiffness matrix is calculated rather than approximated (as in quasi-Newton).
     • Other parameters include cmax, min_residual (to set a minimum residual indicating that the system should be in equilibrium).
     • Analysis type can be set to dynamic, where inertial forces may help stabilize solution. Increasing mass will likely aid convergence but may result in inaccuracies. This is not an issue for static analysis.
   • The group also discussed model parameters that can be changed to facilitate convergence.
     • Ahmet noted that the parameters should be within physiological ranges. Their values not only affect convergence but will also influence potential interpretation of tissue and joint mechanics. In a sense, changing model parameters will also provide a sensitivity analysis.
     • It may be possible to add springs to stabilize rigid body motions. The values should be large enough to facilitate convergence but should be small to ensure these parameters do not affect joint kinematics/kinetics response. As a general rule, the loads carried by these springs should not exceed more than 5% of the dominant loading magnitude.
   • The team prioritized a set of parameters for the first convergence analysis.
     • Loading scenario will be passive flexion, with the focus on simulation parameters.
     • Convergence tolerances will be left at their default values.
     • max_retries and max_refs parameters will be changed to understand their combined role on convergence speed.
     • Craig will implement configuration files to run a set of simulations to vary max_retries and max_refs.
3. Decide immediate action items for the next meeting.
   • TBD
4. Other.
   • TBD