Biomechanical Evaluation on Hooks Pattern of the Posterior Constructs in an Unstable Burst Fracture Model

You, Jae Won; Lim, Tae Hong

Abstract

We prefer the posterior instrumentation system for the surgical treatment of the unstable burst fractures of the thoracolumbar spine. We are usually using a hooks pattern with posterior instrumentation, when we can not use a pedicle screw instrumentation in the case of an one stage operation of anterior decompression and posterior stabilization or injury of osteoporotic spine. The purpose of this study was designed to compare biomechanical flexibilities of posterior instrumentation constructs provided by hooks pattern in an unstable calf spine model. Ten fresh frozen calf spines (T10-L4) were used for this study with an anterior and middle column defect and loaded with pure unconstrained moments in flexion, extension, axial rotation, and lateral bending directions. Testing was performed on intact specimens first and then each specimens after laminar hooks insertion and ISOLA posterior implants and any kind of graft materials or transfixation devices were not used to make the worst possible case of instability of an injured spine. Three different fixation methods were instrumented. These included: (1) 2 hooks 2 levels above and 2 levels below corpectomy site in distraction modes bilaterally, (2) 2 levels above and 2 levels below hooks with one rod in distraction and the another rod in compression modes, (3) 2 levels above and 2 levels below hooks with claw hook configuration bilaterally. The results were as follows; 1. At the level of corpectomy site, the group I significantly reduced the motion in flexion and lateral bending (p<0.001), but not in extension as compared to the intact specimen. The group I provided significantly less stabilization than other fixation methods in all motions (p<0.01). The group II and III significantly reduced the motions in flexion, extension, and lateral bending as compared to the intact specimen (p<0.001). Axial rotational motions of all fixation constructs were significantly larger than the intact specimens (p<0.001), and the group I showed greater axial rotational motion than the groups II and III (p<0.001). 2. At the level below corpectomy site, all fixation methods showed the reduced motion significantly in all loading modes and the stabilization was similar with each other. 3. At the level above corpectomy site, all tested fixation methods did not improve the axial rotational stability beyond the intact case, but reduced flexion, extension, and lateral bending motions significantly(p<0.001). In conclusion, the hooks pattern construct in distraction modes is least stable at the corpectomy and above corpectomy sites. It was showed similar stabilizing effect between the claw and compression-distraction configuration. when using hooks, the hooks should be placed in either the claw or compression-distraction configuration.