Steelers ILB Sean Spence's catastrophic 2012 knee injury included a damaged peroneal nerve behind his left knee.
Spence tore his ACL and LCL, dislocated the knee, and also damaged the nerve. Players are capable of returning from major knee injuries, but Spence's recovery is wholly dependent on the nerve recovery. The Steelers have no idea when the nerve will heal. It's why GM Kevin Colbert called this an "unknown injury."