Stability of medially and laterally malpositioned screws: a biomechanical study on cadavers.

Background context: Pedicle screw instrumentation is widely used in spine surgery. Axial screw misplacement is a common complication. In addition to the recognized neurovascular risks associated with screw misplacement, the biomechanical stability of misplaced screws remains a subject of debate.

Purpose: The present study investigates whether screw misplacement in the lumbar spine reduces mechanical screw hold.

Study design/setting: Cadaveric biomechanical study.

Methods: Pedicle screw (mis)placement was planned for 12 fresh frozen cadaveric spines between the T12 and the L5 levels. The screws were then implanted into the vertebrae with the help of 3D-printed template guides. Pre- and postinstrumentation computed tomography (CT) scans were acquired for instrumentation planning and quantification of the misplacement. The instrumented vertebrae were potted into CT transparent boxes using Polymethyl methacrylate and mounted on a standardized biomechanical setup for pull-out (PO) testing with uniaxial tensile load.

Results: The bone density of all the specimens as per HU was comparable. The predicted pull-out force (POF) for screws medially misplaced by 2 , 4, and 6 mm was respectively 985 N (SD 474), 968 N (SD 476) and 822 N (SD 478). For screws laterally misplaced by 2 , 4, and 6 mm the POF was respectively 605 N (SD 473), 411 N (SD 475), and 334 N (SD 477). Screws that did not perforate the pedicle (control) resisted pull-out forces of 837 N (SD 471).

Conclusions: Medial misplacement is associated with increased axial screw hold against static loads compared to correctly placed screws and laterally placed screws.

Clinical significance: In clinical settings, the reinsertion of medially misplaced screws should primarily aim to prevent neurological complications while the reinsertion of lateral misplaced screws should aim to prevent screw loosening.