get these. i've had tibial bruising 3 times now (aka a bone bruise, not just a normal skin wah wah bruise) and the first time it took me off the mats for 7 weeks, b/c the doctor told me, 'bone bruising is one step before an actual break. stay off it.' the next time it happened, i had these. i don't really like training with them on, but i was able to train, and stay on the mats. it healed up i'd say in a couple weeks where i wouldn't feel it at all.
it's not the same knee issues you're describing, but could be helpful.
Are you wearing construction style knee pads with the straps? I prefer the ones that are sleeves made of neoprene since the ones with a strap on the top and bottom give me hotspots in the back of my leg.
The ones I use are wrestling knee pads. Only bad thing is I have to take my shoes off if I want to remove them.
I wear this:
Its pretty comfortable, and has never fallen off. When I roll I practically forget its there.