Is this normal?

I was under my car and noticed something unexpected... the polyurethane bushing has disappeared from my lower engine mount!

I bought the mount from RSM Racing back in 2005. It has been on the car since then. How often do polyurethane bushings deteriorate like this? How often should they be replaced? Is this normal? There's no sign of the bushing material anywhere, the mount is completely free-floating with no polyurethane at all inside the tube. Was the polyurethane originally just two end-caps or did it go all the way through the tube? Could it breakdown due to heat from the engine? (Though I would assume the guys at RSM would have taken that into account) Any ideas?

Is it easy to replace the bushing on the one I have? Anyone know any deals on a new lower mount? Thanks guys! ...inspect your lower engine mounts!

Just buy a new lower mount. RK Sport makes one.