Study design: Analysis of insertion angle and bone density on the pedicle screw fixation strength with a novel testing protocol that accounts for the articular processes.
Objective: To analyze the relationship between pedicle screw fixation strength and bone mineral density for different transverse screw insertion angles.
Summary of background data: The 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.
Methods: Using 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.
Results: A 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.
Conclusions: Pedicle 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 evidence: V.
Keywords: Bone density; Insertion angle; Pedicle screw; Pullout strength; Testing protocol.
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