Searched: Wikipedia entry on "kneecapping";
Little Details tag "medicine: injuries: to order"
My MC is a woman, about thirty, in good general shape, and ruthless but prudent when called for.
She's been kidnapped, but has found a chance to escape and picked up something equivalent to a baseball bat.
She's sneaking up on a single guard; she needs to strike and move on quickly, so has two or at most three blows. She doesn't want to kill him - too morally and legally messy - so no full force whack to the head ... but she does want to render him immobile and unable to use his strong hand. She'd also like to hurt him bad enough he _will_ need hospitalization and even then have serious permanent damage.