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

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.