A Biomechanical Assessment of Infra-Laminar Hooks as an Alternative to Supra-Laminar Hooks in Thoracolumbar Fixation

Abstract

Biomechanical study, using human cadaveric spines, of two types of posterior fixation in the thoracolumbar spine. To compare fixation in the thoracolumbar spine using pedicle screws combined with infra-laminar hooks (at both the cranial and caudal ends of the same vertebra) with fixation using pedicle screws alone. The standard method of using laminar hooks is to place supra-laminar hooks at the level above the vertebra being fixed by the cranial pedicle screws, and infra-laminar hooks at the level of the vertebra receiving the caudal pedicle screws. However, using this method, a single normal motion segment is killed. In the proposed technique, infra-laminar hooks will be used at the same level as the cranial pedicle screws, thus sparing the superior adjacent segment. PEDICLE SCREW LOOSENING TEST: Nine vertebrae (T9-L2) had pedicle screws inserted into both pedicles. On one side, the pedicle screw was connected to a fixation rod, while on the other side the pedicle screw was combined with an infra-laminar hook and connected to a rod. The rod on one side was then pushed with increasing force until loosening of the screw. The same test was repeated on the other rod until loosening of the screw. The reverse test (i.e., pulling until loosening) was done in a similar manner on another nine vertebrae. BIOMECHANICAL STIFFNESS TEST: Eight spines (T12-L2) were biomechanically tested in: axial compression, flexion, extension, left and right lateral bending, and left and right axial torsion. This sequence was applied to: 1) the intact spine; 2) after applying pedicle screws and infra-laminar hooks with rods to the destabilized spine; and 3) after removal of the hooks (with the pedicle screw and rods still inserted). From the load-deformation curves obtained, stiffness values were calculated. PEDICLE SCREW LOOSENING TEST: The pedicle screw loosening tests showed that a pedicle screw combined with an infra-laminar hook offers significantly greater fixation strength as compared to a pedicle screw alone. Biomechanical stiffness test. Fixation by pedicle screws combined with infra-laminar hooks, placed on the vertebra fixed by pedicle screws at both the cranial and caudal ends, offered a stiffer construct as compared to fixation by pedicle screws alone. This method offers a firm construct without sacrificing a normal motion segment.