Thign pain after spine fused?

Thigh pain. After fusion can be multifactorial. It can be due to nerve irritation along the front of the thigh, nerve irritation after decompression, hip flexor stretch, etc. Most often this settles down in a few weeks but you should bring it up with your surgeon to have them evaluate you.
Pelvis. After the spine is fused the lower spine loses springiness and this focuses increased mechanical forces on the sacroiliac joints (sij) of the pelvis, loosening one or both. This can result in sciatica. Radiation of pain to the groin or front of the thigh is characteristic for sij disorder, all as reported in the world medical literature.