The effectiveness of each procedure was gauged using pain scores, post-surgical use of opioid painkillers, patient satisfaction and Oswestry Disability Index (ODI) scores, which measure how back pain affects function.

After two years, most studies showed significant improvements in pain and function, less opioid use and fewer problems in adjacent segments of the spine in ADR patients. The ADR group was also far more satisfied with their procedure overall. But by five years after surgery, researchers found no significant differences in outcomes between the ADR and fusion groups.

Vijay K. Goel, PhD, professor of orthopaedic bioengineering at the University of Toledo, in Ohio, notes that the study does not provide a complete analysis.

"The authors have not discussed the efficacy in preserving motion following ADR and fusion surgery,” says Goel, who was not involved with this study. “Besides pain mitigation, ADRs are expected to preserve motion. [If they don't], then the whole justification for their use may be questioned somewhat, irrespective of the clinical outcome."

Goel also notes that at least one brand of artificial disc has undergone a redesign, and that second- and third-generation artificial discs “are showing better promise” than older designs – but the study results lump older and newer artificial discs together. “The authors need to identify the version used in each study cited in the review."

Another missing piece of the puzzle, according to one expert in spinal biomechanics, is long-term follow-up. Anton E. Bowden, PhD, assistant professor of mechanical engineering at Brigham Young University in Provo, Utah, says most spine studies don't follow patients long enough. "What happens with adjacent-level degeneration will only be seen over the longer term," he explains.

Bowden, who helped design a new type of artificial disc intended to facilitate normal spine motion, strongly favors ADR over fusion. "By every metric we know, fusion is not a great surgery," he says. "Disc replacement is a tremendous step biomechanically toward a better solution.”

But he acknowledges it is still a newer procedure with its own challenges. “We're at the same evolutionary point we were when we stopped fusing hips and started doing hip replacement,” he says. “Fusion makes the spine too tight; artificial discs can be too loose. In both cases, you can get problems in adjacent segments of the spine.”

As a parting thought, Dr. Fischgrund stresses that any spine surgery is always a last resort.

"Patients with chronic low back pain should consider surgical intervention only after all other treatment modalities have failed, and the pain significantly interferes with the normal activities of daily living," he says.