Badshot,
I have knee and back issues also and was totally inconsistant in my approach and my release, so my ball reaction suffered and was hard to read. I worked with a PBA pro( Joey Cruz in Virginia) and after videotaping me, he recommended that I lose the pushaway start and go to a hinge swing ... a straight drop from my setup point. It has done wonders for me ... my balance is better, approach is smoother, ball reaction is more consistent, and my average is up. It took some getting used to, but with practice, it's really helped !!! Also, it has taken pressure off of my knee and back !!!
Robb in VA