Numbness is one of the last nerve functions to return (other than motor strength). If the time has been greater than one year since the surgical decompression, the chance of return of function diminishes greatly. Rarely, an incomplete decompression of the foramen can cause continuing numbness and if a pseudoarthrosis (lack of fusion) develops, resultant spur formation can also cause foraminal stenosis.
A new MRI might be warranted.
Dr. Corenman