Biomechanical Analysis of Derotation of the Thoracic Spine Using Pedicle Screws

In Brief Study Design. Biomechanical analysis of derotational load-to-failure of pedicle screw (PS) instrumentation in cadaveric thoracic spinal segments. Objective. To investigate the derotational torque that can be applied to the thoracic spine through different linked constructs and evaluate the modes of failure. Summary of Background Data. Thoracic derotation with PSs has been shown to provide better 3 plane correction than other methods but the effects of linked PS constructs has not been studied. Methods. Four groups of thoracic segments with different PS constructs were loaded to failure with a rotational torque applied to the construct to simulate the left to right derotational force applied to a typical idiopathic dextrorotary thoracic scoliosis curve. Single screw T4 segments instrumented on the medial (group 1M) and lateral (group 1L) sides, bilaterally-linked T5 segments (group 2), unilaterally-linked T6-T9 segments on the medial (group 3M) and lateral (group 3L) sides, and quadrangularly-linked T6-T9 segments (group 4) were loaded with MTS machine in a simulated thoracic derotation model. Results. Single T4 PSs on the medial and lateral sides failed at 4.0 ± 1.4 Nm (group 1M) and 6.1 ± 2.5 Nm (group 1L), respectively. Bilaterally-linked T5 screws failed at 11.9 ± 3.1 Nm (group 2). Unilaterally linked T6-T9 PS constructs on the medial and lateral sides failed at 21.2 ± 7.5 Nm (group 3M) and 17.9 ± 11.1 Nm (group 3L), respectively. Quadrangularly-linked PSs failed at 42.5 ± 14.5 Nm (group 4). Conclusion. A near linear increase in relative torque applied before failure was found with each additional PS linked. Linked constructs allow for significantly greater torque with less risk of PS breach of the spinal canal. A cadaveric spinal model was used to show that increasing derotational torque could be applied to vertebral segments with a linked pedicle screw construct in a thoracic scoliosis model. Single screw, bilaterally, unilaterally, and quadrangularly-linked constructs showed significantly increasing torque-to-failure with decreasing risk of spinal canal breach.