Derotation of the Thoracic Spine Using Pedicle Screws: A Comparison of Concave to Convex Screws

• a - Medtronic

Correction of thoracic idiopathic scoliosis using hook, and/or wire constructs has proved adequate in the coronal and saggital planes, but disappointing in the transverse plane, leading many surgeons to add a costoplasty to address the rib hump. Segmental spinal instrumentation using pedicle screws has an ability to improve the rib hump using direct vertebral derotation in the transverse plane. However, it is not known if the derotation force can be more effectively and safely applied to the concave or the convex screws.

Method: Seven fresh frozen adult spines were instrumented with a rod connecting pedicle screws from T4 to L1, and mounted on a jig to allow testing with an MTS system. A lever grasped the head of the pedicle screw and was attached to the testing machine by a cable. The cap was loosened and the actuator moved vertically at a rate of 2mm per second. Alternating sides of each spine were tested resulting in an equal number of screws tested with a medially or laterally directed force.(a medially directed force simulates rotation through a concave screw; a laterally directed force simulates rotation through a convex screw) The rods were then removed and the screws were tested manually for clinical looseness. The screws were then removed and the vertebrae were sawed open, enabling determination of the mode of failure of each screw. Multiple regression analysis was performed using vertebral level and torque direction as independent variables and torque to failure as a dependent variable.

Results: Analysis yielded a significant effect for vertebral level (p=0.031) with a slope of 0.85Nm per level, and a significant effect for direction (P<0 .001), with a net difference of 8.1Nm between screws pulled laterally and those medially.(lateral>medial).Failure modes showed medially directed screws failed by lateral wall breach and transverse process/pedicle wall fracture. Laterally directed forces breached the spinal canal. 15 0f 59 screws felt mechanically firm despite evidence of mechanical failure. 23 of the screws had bent, 14 had breached the spinal canal and 19 had breached the lateral vertebral wall.

Conclusions: A significantly greater torque before failure can be achieved by rotating a convex screw laterally, versus a concave screw medially. Greater torque is tolerated at each level from a cephalic to a caudal direction. Screw breakout may breach the lateral vertebral wall, or the spinal canal. This information may be helpful in devising derotation strategies in thoracic idiopathic scoliosis

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