Wednesday, 18 April 2012

Simulating The Repair Of Sports Injuries

We're all used to hearing about footballers and other professional athletes damaging their cruciate ligament or suffering some other tendon injury.  I remember being told once that they can repair this by cutting away more of the damaged tendon and it seemed like a crazy statement.  How can removing material make something stronger?
Time to do a little bit of medical Simulation!
I fired up SolidWorks and modelled a simple strip with two small tears on each side.  Keeping things symmetrical helped with the setup.  

Close up of the tear

A quick simulation was set up

And here's the results

Maximum stress here was just over 1GPa, almost double the yield limit of the material.
Clearly we are failing here so now for the repair!

All I did here was increase the angle of the cut from 10 to 170 degrees.  This removes 13% of the original bar.
So what did this do to the stress?  Let's take a look:

Wow!  The maximum stress has fallen by 800MPa, a 79% drop in peak stresses.  We can live to run another day!
But why does this happen?  How can removing material increase the strength of the part?
It's all to do with how the stress can spread through the part.  With the narrow/sharp tear the stresses can only act at the root because there are no stresses at the top of the tear openings, but by smoothing out the damage, stress can spread across the entire width of the part allowing more material to take up the strain.
Once again I was able to use SolidWorks Simulation to illustrate what seems to be a paradox, and get the truth.

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