Your spine is probably stable with a pseudoarthrosis. The pain you currently suffer from is most likely from the lack of union of this level. It is called sclerotomal pain and refers to the inside shoulder region commonly. This pain is quite commonly misdiagnosed as thoracic region generated pain when most of the time, the pain is really generated by the cervical spine.
WIthout seeing your X-rays and MRI, I can only speculate but I would generally repair this from the front. A posterior fusion is just fine but you would be in a collar for 8 weeks when an anterior fusion would have you in a collar for generally 1-2 weeks.
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.