Having a congenital fusion of the lumbar spine is unusual and two separate levels is even more unusual. Having two levels fused does not mean future surgery by itself. There are many patients who undergo two level fusions and do well the rest of their life.
Now sciatica is another issue. You might have a compression of a nerve root from a disc herniation or bone spur (see foraminal collapse on the website). A workup would be in order.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.