What can cause sciatica pain when mri is clear?

Piriformis Syndrome: Peripheral sciatica occurs when the sciatic nerve is compressed in the buttock or other the proximal hamstrings, and commonly, the pyriformis muscle may be a culprit. Some pts have problems with their sacro-iliac joint, which could mimic sciatic-like problems.

Several things: Piriformis syndrome could do that. Peripheral neuropathy for many reason.(diabetes) any injury to the sciatic nerve or major branches after exiting the spine.

Spine Pain Options: This chronic pain in the distribution as you suggested is the result of an irritated nerve or facet joints or other injury typically in the lumbar spine (low back) which are caused by herniated disks, spinal stenosis or degenerative disc disease, etc requiring further evaluation by a spine specialist and may be candidate for facet injections/radiofrequency ablation and epidural steroid injection.