Questions and Answers regarding Spherical Indentation of Cartilage

Questions and Answers regarding Spherical Indentation of Cartilage

Is there a suggested Poisson's ratio to use when analyzing indentations?

If an indentation is performed in 1 second or less, it can normally be considered "instantaneous" and a Poisson's ratio of 0.5 should be used (i.e. the liquid inside the sample is considered incompressible because of the rapid velocity). This is independent of the animal model used. If your indentation is slower, you can try to find a Poisson's ratio value in literature or try to find it through your own analyses by camera. We'd recommend using the 0.5 "instantaneous" testing as this method shows good data when trying to assess the state of cartilage.

What factors determine the indentation amplitude to use? For example, why is 0.2mm recommended for sheep and 0.1mm for porcine cartilage?

We always recommend indenting between 0 and 15-20% strain because this corresponds to an elastic section of the stress-strain curve. The model used to analyze the data is an elastic model (Hayes model). The exact strain and thickness are not known when performing the indentation (only after the needle penetration). The recommended amplitudes, thus, might be a little exaggerated as they are based on an estimation of the thickness of the cartilage. It is always better to indent a little more because you can always decide to analyze only the beginning of the curve later. For example, if a 0.2mm indentation is performed, you can decide later to only analyze the 0.00- 0.15mm section.

What factors help determine the size of indenter to use (e.g. 5mm diameter for human cartilage)?

You should try to aim to have the diameter be approximately 2 to 5X the thickness of the cartilage. For human cartilage, the estimated thickness is 2-3mm, so a 5mm diameter indenter is a good fit out of the choices in our indenter kit.

How is the Relaxation Time calculated in the Batch Analysis software?

The explanation can be seen in the following figure. The 37% value comes from the derivation of a spring-dashpot relaxation model such as the Maxwell model (1/e = 37%). We have found this particular value to be easy to calculate and compare between samples.

Does the instantaneous modulus correlate with proteoglycan content, water content or collagen content?

This depends. The instantaneous modulus (IM) correlates more with the structure and integrity of the cartilage than with its contents. There are correlations with the GAG and water, but they are weak. For the collagen content, we have not noticed a correlation (see figure below), but only healthy cartilage was examined. There is a higher correlation with the Mankin Score, for example, which takes into account the integrity of the fibril network and GAG quantity at the same time.
So, it is certain that you will have a good correlation between IM and GAG/Collagen/water if there are both healthy and very degenerated samples in your analysis. However, if you only have healthy or only degenerated (ICRS 0,1,2), the correlations between IM and biochemistry can be weak or non-existant.

Sim, S et al. “Non-destructive electromechanical assessment (Arthro-BST) of human articular cartilage correlates with histological scores and biomechanical properties.” Osteoarthritis and cartilage vol. 22,11 (2014): 1926-35. doi:10.1016/j.joca.2014.08.008

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