The artificial disc replacement (ADR) is a generally good implant for the right individual. The problem is that there is now more stress on C6-7 due to both your genetics (the probable reason you had the C4-6 ACDF in the first place) as well as the increased strain of having a three level fusion (or even four level if C2 fracture fused to C3) above this level. The ADR has not been tested to take this load so it is difficult to know if it will hold up. The generally good news is that if this ADR goes south, the repair is an ACDF which should hold up.
You might find however that this C6-7 level is very degenerative and has very little motion based upon flexion/extension X-rays. If this is the case, a fusion (ACDF) will not change your range of motion much and you will not have to worry if an ADR will break down.
Remember that 50% of your head motion in rotation and flexion/extension occurs at C1-2 and occiput-C1 so you will preserve at least 50% of your range of motion even with this potential fusion.
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.