Characteristics of pedicle screw loading - Effect of surgical technique onintravertebral and intrapedicular bending moments

Study Design. A static nondestructive bending analysis of pedicle screws in serted into vertebral analogues was conducted. Pedicle screw load was studi ed as a function of variables in insertion technique. Objectives. To determine how the sagittal bending moment in pedicle screws is affected by changes in pedicle screw length, insertional depth, and sagi ttal placement. Background Data. An unexpectedly high rate of clinical failure has been obs erved in pedicle screws used in short-segment instrumentation for unstable burst fractures. The majority of screws fail in sagittal bending within the pedicle. Little is known of the insertion technical factors that affect in situ loads incurred by pedicle screws. Methods. Synthetic vertebral analogues were fabricated. Pedicle screws inte rnally instrumented with strain gauges were used as load transducers to det ermine screw bending moments within the pedicle and body of the analogue. A nalogues were loaded in compression to simulate loading of an unstable burs t fracture. Results. Screw bending moments within the pedicle increased 33% and 52% whe n screws were left 3 mm and 5 mm short of full insertion. Intrapedicular mo ments increased 20% to 29% in screws inserted superiorly or inferiorly with in the pedicle. Thirty-five-millimeter screws developed intrapedicular mome nts 16% higher than 40-mm and 45-mm screws. Conclusions. In situ pedicle screw loads increased significantly as a direc t result of variations in surgical technique. Screws left short of full ins ertion, placed off center in the sagittal plane of the pedicle, or less tha n 40 mm long developed increased intrapedicular bending moments.