So, C6-7 has loose anterior screws due to a pseudoarthrosis, you are fused from S1 to T4 and have a new compression fracture of T4 due to the stress loading of the lower construct and osteoporosis.The picture is becoming clearer.
With a pseudo at C6-7, and a new compression fracture at T4 where the fusion ends, it looks like your best solution is to extend the fusion up to the cervical spine. This fixes T4 and C6-7, and gets rid of that stress riser. Make sure those anterior C6-7 screws are not prominent as not to irritate the esophagus.
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.