Experimental study of pedicle screw stability on low BMD vertebrae

Objective: To conduct biomechanical study of different pedicle screws stability on spinal specimen, discuss the relationship between design parameter of screw, insertion torgue and BMD, establish the theoretical foundation for application of pedicle screw on osteoporotic patients. Methods: Six fixed lumbar cadavers were collected, the effects of design parameter, insertion torque and etc on fixation stability were determined under various BMD by using biomechanical ways. Results: According to in vitro study: (1) There was a significant difference among pullout strength of all screws (P<0.05). (2) There was a gradual accretion of pull out strength in lumbar segments from top to down. It was so in axial stiffness. (3) BMD, quality of bone will directly affect the intensity and pullout strength of screw. there was significant difference between normal group and osteoporosis group (P<0.05). (4) The fixation intensity was directly correlated with screw torgue, the bigger the torgue, the greater the fastness. And there was also positive relation between it and BMD (r=0.936). (5) Among specimen of low BMD (No.3 and No.4), the pullout strength of all screws was as follows: U2>U1>SF1>SF2>RF. Conclusions: There is a close correlated between type of screw, BMD and stability. The U-type screw displays the best fixation effect on specimen of low BMD. (authors)