Biomechanical Analysis Of Pedicle Screw Density In Posterior Spine Instrumentation

Background: Posterior spinal fusion using pedicle screw instrumentation is the current standard for surgical management of adolescent idiopathic scoliosis (AIS). Decreasing the density of pedicle screw instrumentation is a method of decreasing costs associated with the procedure. Minimal biomechanical data exist evaluating pedicle screw density, and the optimal screw density has not been determined conclusively. This study evaluated the effects of decreasing pedicle screw density using in vitro robotic biomechanical testing of porcine thoracic spines. Methods: Eight porcine thoracic spines were dissected to T3-T15. Spines were instrumented with bilateral pedicle screws at levels T4-14 and were attached to a robotic system for biomechanical testing. Full instrumentation (100%) and subsequent constructs of decreased pedicle screw densities (82%, 64%, and 64% with crosslinks [64%+CL]) and noninstrumented spines were tested for total range of motion (ROM) in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) using three-dimensional segmental spinal kinematics with an optical tracking system. Results: No significant differences in ROM were noted in FE, LB, or AR when comparing full instrumentation (100%) to constructs of decreasing pedicle screw density (82% and 64%). The addition of crosslinks in the 64%+CL construct decreased ROM in AR (P=0.0634) as compared to the 64% construct. Conclusions: Decreasing pedicle screw density in biomechanical testing of nondeformed porcine thoracic spines does not result in any significant changes to ROM when comparing constructs with greater than 60% density. The addition of crosslinks may confer added stability to AR in patients with decreased density constructs.