Rotational stability of a spinal pedicle screw/rod system

Although the geometry of spinal instrumentation constructs may significantly affect efficacy, the variation in biomechanical data may not assist the clinician in an appropriate selection. The purpose of the present study was to quantify the effects of transverse fixators on rotational strength of a common pedicle-screw-with-rods system. Pedicle screws were mounted in blocks of polymethyl-methacrylate at angles to reproduce the configuration of placement in the human lumbar spine. Twenty cycles of +/- 12 N-m axial rotation moment was applied, and the steady-state response was used in the analysis. Configurations tested included both medial and lateral placement of longitudinal rods as well as the addition of one or two transverse rods. Up to a 20% difference in stiffness was noted between medial and lateral placement of longitudinal rods when no transverse rods were mounted. A maximum difference in flexibility of 6% was noted between the use of one and two transverse rods. For medially placed rods, a single transverse connector will add significant rotational stiffness even for shorter rod lengths; for laterally placed longitudinal rods, only the longer rod lengths need a transverse connector.