Weekly Update in lieu of Recurring Meeting of Cleveland Clinic Core Team
Date: April 25, 2017 (Update for two weeks)
Provided by: Snehal Chokhandre
Ongoing Action Items:
- Review drafts of data manuscripts for submission.
- Complete studies for tissue testing reproducibility.
- Continue tissue testing of Open Knee(s) specimens.
- Continue segmentation of Open Knee(s) specimens.
- Tissue testing.
- Snehal conducted repeatability tests for cartilage unconfined compression to assess effects of multiple freeze- thaw cycles. The cartilage strip from which the sample was taken was frozen-thawed 4 times already(including the specimen dissection). 6 tests were conducted over 6 days. Sample was frozen after every test. For the first test, 10g force position was used as the starting location for preconditioning and stress relaxation and therefore not included in the assessment. For the following 5 tests, preconditioning and stress relaxation tests were started 300 microns above the 10g 'find contact' location to capture the entire span where the force starts to rise. This was done to aid with finding the actual zero force-displacement point.
- It appears that the first 3 tests in the set of 5 tests, are repeatable with ~8% variability. The last two tests however are very different. The stiffness appear to decrease by a lot. The sample appear to have dealt well with multiple freeze-thaw cycles (8) before the properties changed dramatically.
- Also, for the last two tests, it appears that the sample starts to experience force at positions before that for the repeatable tests (sample is stiffer at lower strains).
- Note the sample thickness variation is not significant. Especially for tests 4 and 5 it was the same and yet the tissue response is very different.
- As the position where the sample starts to experience force changes, the strain applied is different for each test. (If position for 10g load is used for application of strains then it remains consistent.)
- For the first test in this set Snehal accidentally entered incorrect starting position for the first stress relaxation ramp (instead of adding 300microns to the 5% strain value (~100 microns), she added 200 microns hence the indentor was still 100 microns away from the 10g find contact position when the test started), hence the strains applied are lower. Instead of discarding the data, Snehal continued that for the rest of the tests. The only difference is that 11-12% max strain was applied from the 10g position instead of 15%. This will be rectified for any future tests.
- Since 10g is significantly lower than the force experienced by the cartilage sample at 5% strain, Snehal believes it is still a suitable value for establishing full contact with the sample surface.
- Snehal modified the tissue data analysis/added a function to the python script to find the actual zero forced-displacement. She is currently working on making the whole script modular.
- Snehal has also complied a literature review for knee cartilage and updated on the Specifications/ExperiementationTiusseMechanics wiki page.
- It appears that using PBS, longer wait times between tests, modified preconditioning protocol and starting the stress relaxation tests before the contact cover all the measures to test the cartilage unconfined compression sample in a way that they aid repeatability of behavior and provide ways for varied analyses.
- Snehal is currently conducting tests with the updated protocol with starting the preconditioning and stress relaxation 300 microns off the 10g contact position. Cartilage unconfined compression tests for the 48 yr old specimen over 4 days will be conducted with sample kept in fridge between tests. Snehal plans to repeat this for the 78 yr old test specimen. In the next 3 weeks, Snehal plans to conduct several sets of repeatability tests for all cartilage-test-specimen combinations.
- Starting with unconfined compression test set for both the specimens (48 yr old and 78 yr old). Followed by week long tests for cartilage confined compression and another week of tensile tests. All these tests will be conducted in PBS and over several days. Essentially covering all the modifications made so far.
- Once these sets are complete, Snehal and Ahmet will make a decision on whether to proceed with the actual Open Knees tests or make further modifications to protocol etc.
- Data manuscripts.
- No progress.
- No progress.
- Snehal and Ahmet met last week to discuss the simulation proposal Snehal has put forth. Snehal proposed essentially, assessing effects of variations in image segmentation performed by different people and the resultant geometries on the finite element model predictions for patella cartilage, if any.
- Snehal proposed simulating variability in surface topology to represent variations in segmentation performed by different users on the same image set (using the same tools and protocols) and the effects on the peak contact stresses. Snehal proposed obtaining 6 geometries for the same sample from segmentation attempts by different people, averaging the surface and randomizing the master surface topology to simulate segmentation variations for a larger set.
- Ahmet's critique was that the simulation (randomization) may not represent a real segmentation at all as user/ manual segmentation is guided by the visual of the image. He recommended using actual multiple segmentation attempts by different people to compare the model predictions and then varying the material properties to further assess the variability in peak contact stresses.
- Snehal is currently working on running simple patella models.
- Snehal also attended a 2 day training session for the VIC-3D strain measurement system. The system will not be used for the open knees tissue tests as previously expected, but the VIC-2D software can still be used to measure 2D strains from the images obtained by the Mach-1 system during tensile tests.