Biomechanical comparison of instrumented posterior lumbar interbody fusion with one or two cages by finite element analysis

Study Design. Using finite element models to study the biomechanics of lumbar instrumented posterior lumbar interbody fusion (PLIF) with one or two cages. Objective. Analyzing the biomechanics of instrumented PLIF with one or two cages as to evaluate whether a single cage is adequate for instrumented PLIF. Summary of Background Data. Implantation of a single cage in instrumented PLIF of lumbar spine is still controversial. Methods. Three validated finite element models of L3-L5 lumbar segment were established [intact model (INT), one cage model (LS-1), and two cages model (LS-2)]. The available finite element program ANSYS 6.0 (Swanson Analysis System Inc., Houston, TX) was applied. To analyze the biomechanics of these models, 10 Nm flexion, extension, rotation, and lateral bending moment with 150 N of preload were respectively imposed on the superior surfaces of the L3. Results. Compared with the INT model, the decrease of ROM in the LS-1 and LS-2 models were exaggerated from 0.67 degrees to 3.73 degrees and ranged from 37.2% to 86.1% in all motions. The mean subsidence was found to be slightly higher in the LS-1 model. Most of the cage dislodgement in both models was less than 0.03 mm. The mean dislodgement was slightly higher in the LS-1 model. The stress of cage was found to be high in the LS-2 model. The mean stress of screw was raised to 4.5% to 9.7% in the LS-1, which was higher than that in the LS-2 model. In general, stress of adjacent disc was more pronounced in the LS-2 model. The most stress distributed at the anterior portion of the adjacent disc, which could be used to interpret the clinical findings of the early adjacent disc degeneration. Conclusions. A single cage inserted in an instrumented PLIF gains approximate biomechanical stability, slight greater subsidence, and a slight increase in screw stress but less early degeneration in adjacent disc. Adjusting these factors, instrumented PLIF with one cage could be encouraged in clinical practice.