If you have a fusion from T11-S1, there are two areas that need to be watched- the sacroiliac joints and the T10-11 level. You can mitigate the wear by doing just what you stated- low impact activities. The spine fusion should not affect your hips, knees and ankles.
Swimming, cycling including mountain biking, hiking (without a heavy pack), rowing, reasonable gym workouts, cross-country skiing and snowshoeing are all activities that in general, would not harm your back.
Intermediate activities that could cause increased wear are skiing groomed slopes, hiking with a heavy pack, heavy weight lifting and water skiing.
Acivities to avoid are all impact activities such as running, tennis, volleyball, basketball, soccer as well as others that increase the load on the back.
Normally, both a CT scan on a 64 slice scanner and an MRI on at least a 1.5 tesla machine are necessary for the workup. In-office standing x-rays with flexion and extension view are necessary.
Dr. Corenman
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.