Biomechanical evaluation of two-level oblique lumbar interbody fusion combined with posterior four-screw fixation：A finite element analysis.OBJECTIVE
 By constructing the three-dimensional finite element model of two-level OLIF lumbar spine, the aim of this study was to demonstrate the feasibility and effectiveness of posterior four-screw fixation for treatment of two-level lumbar degenerative diseases from the perspective of biomechanics.


 An intact L3-S1 segment nonlinear lumbar finite element model (M0) was constructed from the CT scanning data of a healthy adult.  After verification, two-level OLIF procedure were simulated, and three patterns of finite element analysis models were constructed: two-level stand-alone OLIF group (M1), two-level OLIF + four-screw fixation group (M2) and two-level OLIF + six-screw fixation group (M3).  Range of motion, stress of the cage, and stress of fixation were evaluated in the different models.


 Under various motion modes，the ROM of M2 and M3 were significantly lower than those of M1.  The ROM reduction of M2 relative to M1 was much greater than that of M3 relative to M2.  In terms of the maximum stresses of cages, M2 and M3 were essentially identical.  Besides, the maximum stresses of posterior instrumentation in M2 and M3 were similar, which were mainly concentrated at the root of pedicle screws.


 CONCLUSION
 There were no significant differences between M2 and M3 from the biomechanical analysis.  In two-level OLIF, posterior four-screw fixation can replace six-screw fixation, which reduces surgical trauma and decreases economic burden of patients, and will be a cost-effective alternative.
C