Finite element analysis of the effects of pedicle screw fixation nut loosening on lumbar interbody fusion based on the elasto-plateau plasticity of bone characteristics

Abstract

Study design: Cylindrical cages and pedicle screw fixation (PSF) have been used for the setting of a lumbar interbody fusion.

Objective: To investigate the effects of PSF-nut loosening on lumbar interbody fusion based on the elasto-plateau plasticity of bone characteristics.

Summary of background data: The stress-strain curves of bone materials beneath the cages show linear elastic behavior before yielding and then seem to be plateau plastic deformation with further compression strain until opposing cell walls contact.

Methods: Cancellous bone was assumed to be linearly elastic followed by nonlinearly plastic such as crushable foam behavior. The deformation and failure processes caused by loosening of PSF-nuts, in particular, are addressed by means of finite element analysis. Two finite element models, including anterior (or posterior) lumbar interbody fusion for the human lumbar L4-L5 segments, were constructed. In the respective models, 2 types of PSF were considered such as a solid PSF without relative movement between rod and screw, and a tightened PSF with relative axial movement of the rod between them due to the slight loosening of 2 nuts of L4.

Results: The slight loosening of nuts showed much greater levels of bone strain with relative motion at the bone-implant interface compared with the PSF with firmly tightened nuts. The loosening 2 of the 4 nuts imposed an excessive burden on the remaining screws.

Conclusion: By the consideration of the elasto-plateau plastic behavior of cancellous bone and the relative motion at the contact interface between screw and rod, much more exact behaviors of bone failure were predicted. Slight loosening of the nut would cause a possible risk of segmental instability. With difficulty in either tightening the nut firmly during actual operation or keeping the nut-locking mechanism functioning perfectly in the postoperative state, an enhanced design methodology with more effective locking mechanism is required. Slight PSF-nut loosening may increase risks of failure processes such as rod migration, rod breakage, screw breakage, screw loosening or cage loosening. Different mechanisms however, can be considered; rod breakage and screw breakages are related to the higher stress level of the rod and screw while rod migration is associated with slight nut-loosening. Especially, in the case of a nonsymmetric nut-loosening the risk would be increased. Thus, further study is necessary for improving the current design of PSFs.