If they meet the criteria , outpatients presenting with degenerative disc disease , facet joint arthrosis or spondylolisthesis will be included in the study and randomized into two groups : a control group undergoing conventional fusion surgery ( PLIF - posterior lumbar intervertebral fusion ) , and an intervention group undergoing fusion surgery using a new flexible pedicle screw system ( PLIF + `` topping off '' ) , which was brought on the market in 2013 .