There is no indication in the article about the surgeons who typically performed the surgeries.  Were they orthopedists or neurosurgeons?  It makes a difference:  all the difference in the world.  This variable has to be a control variable.  My husband's experience tells us that orthopedists shouldn't be doing spinal fusions.  Having more than a passing acquaintance with research we're guessing that most failed back surgeries can be attributed to orthopedists. It would be a touchy subject to investigate, but we think this hypothesis would be upheld.