Um, well ... a shank can come from a number of things, not just the 'out to in' swing everyone keeps on talking about.
The most common problem I see when people shank short
irons is an impact alignment where the hands no longer lead the club. The second most common cause I see is where the player moves towards the ball during the swing.
Practice by firstly making sure the hands lead the club into impact and keep your spine angle & height constant until after impact and that
should solve your problem, whatever it is.
If you need more help on this, post or e-mail me anytime.