Biomechanics of Grade I degenerative lumbar spondylolisthesis. Part 2: Treatment with threaded interbody cages/dowels and pedicle screws

Object. The authors sought to determine the biomechanical effectiveness of threaded interbody cages or dowels compared with that achieved using pedicl e screw instrumentation in resisting Grade I lumbar spine degenerative spon dylolisthesis. Methods. Thirty-three levels obtained from seven cadaveric lumbar spines we re instrumented with cages or dowels, pedicle screw/rod instrumentation, or both. Entire specimens were loaded with nonconstraining torques. Each leve l was loaded with anteroposterior shear forces while an optical system was used to measure the specimen's motion at individual levels. Pedicle screw/rods outperformed interbody cages and dowels in treating spon dylolisthesis. Cages or dowels alone provided only moderate biomechanical s tability, and their effectiveness depended heavily on the integrity of the ligaments and remaining annulus, whereas the success of pedicle screw fixat ion relied predominantly on the integrity of the bone for solid fixation. L ittle biomechanical difference was demonstrated between cages and dowels; b oth devices were susceptible to loosening with cyclic fatigue. Conclusions. Biomechanically, cages or dowels alone were suboptimal for tre ating lumbar spondylolisthesis, especially compared with pedicle screw/rods . Threaded cages or dowels used together with pedicle screws/rods created t he most stable construct.