Stability and Spine Pedicle Screws Fixation Strength—A Comparative Study of Bone Density and Insertion Angle

Study DesignAnalysis of insertion angle and bone density on the pedicle screw fixation strength with a novel testing protocol that accounts for the articular processes.ObjectiveTo analyze the relationship between pedicle screw fixation strength and bone mineral density for different transverse screw insertion angles.Summary of Background DataThe stability of the screw can become compromised by demineralization of the vertebral bone due to diseases such as osteoporosis. A weakening of the bone-screw interface, and therefore, a decrease in the fixation strength of the screw, leads to an increased probability of instrument failure, most commonly by screw loosening or screw pullout.MethodsUsing the ASTM F543 as reference, we performed pullout tests with an Instron mechanical testing machine of a posterior fixation construct mimicking two pedicle screws connected at a distance of 40 mm as suggested by the ASTM F1717 on four densities of polyurethane foam in accordance with the ASTM F1839-08 standard to simulate bone densities ranging from osteoporotic (5 pcf) to higher than normal (20 pcf) in four transverse insertion angles.ResultsA linear regression with two independent variables was found to be Y = −354.8812 + 91.8102 × X1 − 6.8747 × X2 (X1 = density [pcf], X2 = angle [degrees]), with a correlation coefficient of 0.95 for all the experimental data.ConclusionsPedicle screw insertion angle and bone density are critical to pullout strength. However, in osteoporotic bone, the insertion angle has only a marginal influence on pullout strength.Level of EvidenceV.