Characteristics of pedicle screw loading. Effect of sagittal insertion angle on intrapedicular bending moments.

STUDY DESIGN: A bending analysis of pedicle screws inserted into vertebral body analogues. Intravertebral and intrapedicular pedicle screw bending moments were studied as a function of sagittal insertion angle. OBJECTIVES: To determine how the pedicle screw bending moment is affected by changes in the insertion angle. SUMMARY OF BACKGROUND DATA: There is a significant incidence of failure when pedicle screws are used to instrument unstable spinal segments. Extrinsic factors that affect screw bending failure have been poorly characterized. Previous work has demonstrated that intrapedicular pedicle screw bending moments are significantly affected by the sagittal location and depth of pedicle screw placement. METHODS: Pedicle screw transducers were inserted in analogue vertebrae at one of three orientations: 7 degrees cephalad (toward the superior endplate), 7 degrees caudal (toward the inferior endplate), or parallel to the superior endplate (control). An axial load was applied to the superior endplate of the vertebra, and screw bending moments were recorded directly from the transducers. RESULTS: Screws angled 7 degrees cephalad developed significantly greater mean intrapedicular bending moments compared with screws inserted caudal or control screws. There was no significant difference in bending moments realized within the vertebral body for the three screw positions. CONCLUSIONS: Angulating pedicle screws toward the superior endplate increased bending moments within the pedicle. If attention to optimal screw insertion technique can reduce bending moments and potential for screw failure without increasing morbidity, surgical risk, or operative time, then proper insertion technique takes on new importance.