[Finite element analysis of lumbar fusion and Topping-off surgery].

OBJECTIVE

To evaluate the effect of Topping-off surgery on the adjacent segment of PLIF.

METHODS

A finite element model of the human lumbar spine (L1-L5) was developed. The intact spinal model was validated by comparing it with previously reported models. Then, 2 models were analyzed and compared: (1) posterior lumbar interbody fusion (PLIF) at L4/5; (2) posterior lumbar interbody fusion at L4/5 and implantation of the interspinous spacer (ISP) at L3/4 (Topping-off). Then 500 N compressive loading plus 10 Nm moments simulating flexion, extension, lateral bending and axial rotation were imposed on both the L1 superior endplates. The ranges of motion, intradiscal pressures, facet stresses in L3/4, the stresses on spinous processes in L3 and L4 were investigated. All the measured data were analyzed by SPSS 21.0.

RESULTS

The effect of the Topping-off on the adjacent segment appeared mainly in flexion-extension: the ranges of motion, intradiscal pressures (annulus and nucleus pulposus), both facet stresses were lower than that of the PLIF model, and the stresses on spinous processes in L3 and L4 were larger. Besides, the facet stresses on the left side in the left lateral bending were also lower than those of the PLIF model.

CONCLUSION

Topping-off model is able to restrict the range of motion of the lumbar adjacent segment, decrease the intradiscal pressure and facet stresses, and has a potential effect of preventing adjacent segmental degeneration.