Factors for pedicle screw extraction force / 中国组织工程研究

ABSTRACT

Pedicle screw loosening and axial dislocation are main reasons for spinal internal fixation failure. Screw loosening and dislocation is caused by screw extraction force,and the screw withdrawl force is related to screw diameter,screw thread design,bone density,and operation skills. Withdrawl intensity is increased with screw diameter and length. Pedicle screw stability can be decreased by osteoporosis. For osteoporotic patients,it is difficult to increase screw diameter alone,so polymethyl methacrylate bone cement is used to enhance screw stability and anti-flexion intensity. However,polymerization heat may induce tissue injury,and long-term stay may produce toxicity and exhibit cancerogenesis. Novel additive materials with good biocompatibility such as hydroxyapatite and calcium phosphate bone cement are gradually replacing polymethyl methacrylate bone cement. In addition,screw entrance point,direction and exact location during screw implantation can decrease pullout strength reducation caused by screw re-twisting and improve screw extraction intensity and stability.