There all replaceable. I replaced the bushing that connects the arm to the sub frame on my old bird. I think most people just replace the whole are because its just easier, you get all new bushings and ball joint, its a decent price for what you get, and the arm is not all rusty.
1995 T-bird 4.6l - Eibach lowering springs, Cobra R hood, One 12" sub woofer, Alpine type-R front speakers. 16.249 @85.11mph 1/4 mile
2001 Pontiac Trans Am WS6 - Cam, headers, HPtuned by me.