Segmental stability and compressive strength of posterior lumbar interbodyfusion implants

Study Design. Human cadaveric study on initial segmental stability and comp ressive strength of posterior lumbar interbody fusion implants. Objectives. To compare the initial segmental stability and compressive stre ngth of a posterior lumbar interbody fusion construct using a new cortical bone spacer machined from allograft to that of titanium threaded and nonthr eaded posterior lumbar interbody fusion cages, tested as stand-alone and wi th supplemental pedicle screw fixation. Summary of Background Data. Cages were introduced to overcome the limitatio ns of conventional allografts. Radiodense cage materials impede radiographi c assessment of the fusion, however, and may cause stress shielding of the graft. Methods. Multisegmental specimens were tested intact, with posterior lumbar interbody fusion implants inserted into the L4/L5 interbody space and with supplemental pedicle screw fixation. Three posterior lumbar interbody fusi on implant constructs (Ray Threaded Fusion Cage, Contact Fusion Cage, and P LIF Allograft Spacer) were tested nondestructively in axial rotation, flexi on-extension, and lateral bending. The implant-specimen constructs then wer e isolated and compressed to failure. Changes in the neutral zone, range of motion, yield strength, and ultimate compressive strength were analyzed. Results. None of the stand-alone implant constructs reduced the neutral zon e. Supplemental pedicle screw fixation decreased the neutral zone in flexio n-extension and lateral bending. Stand-alone implant constructs decreased t he range of motion in flexion and lateral bending. Differences in the range of motion between stand-alone cage constructs were found in flexion and ex tension (marginally significant). Supplemental posterior fixation further d ecreased the range of motion in all loading directions with no differences between implant constructs. The Contact Fusion Cage and PLIF Allograft Spac er constructs had a higher ultimate compressive strength than the Ray Threa ded Fusion Cage. Conclusions. The biomechanical data did not suggest any implant construct t o behave superiorly either as a stand-alone or with supplemental posterior fixation. The PLIF Allograph Spacer is biomechanically equivalent to titani um cages but is devoid of the deficiencies associated with other cage techn ologies. Therefore, the PLIF Allograft Spacer is a valid alternative to con ventional cages.