Huang Bai is good stuff - it has an antimicrobial action, so it's good for external use as well as internal. As for the combo with Ze Xie, I've only seen that pair for internal use, for draining damp heat (like a urinary tract infection).

For the knees I'd certainly include Niu Xi (achyranthes). Depending on where the swelling is (i.e. along the liver channel, or along the GB channel, etc...), you could add herbs to 'guide' the actions of the others to a specific organ/channel.

Furthermore, it sounds like the complaint is of 'Hot Bi', rather than your usual traumatic 'Bi', so an internal formula may work better...

best of luck

herb ox