Investigation into the differences of pull-out resistance between normal and osteoporotic cancellous bone

Abstract

Pedicle screw loosening or pullout has occurred from time to time in patients with poor bone quality. However, there have been no rigorous mechanical analysis of the pullout process, and the resistance region to pullout force remains under-investigated. The objective of the current study was to investigate the stress state of cancellous resistance to pullout in normal and osteoporotic conditions using plane finite element method. The pull-out model was simplified to a simpler two dimensional axisymmetric model by symmetry. A prescribed axial displacement of 2 mm was imposed on the screw head and a fixed boundary condition was set on edge nodes of the bone to simulate the pullout process. The results of numerical simulation showed that the high-stress area of normal bone was clearly larger than that of osteoporotic bone, and the normal bone provided the higher pullout force of 86.09 N, which was about 3 times than that for osteoporotic bone. From the study it was found that osteoporosis would diminish the active region of resistance to pullout around the screw and decrease the pullout strength about threefold. The results of the current study can be used for screw augmentation techniques.