INSTABILITY OF THE LUMBAR BURST FRACTURE AND LIMITATIONS OF TRANSPEDICULAR INSTRUMENTATION

Study Design. This study analyzed the changes in the load-displacement behavior of lumbar spine segments caused by burst fractures that were experimentally produced in fresh human cadaveric spines. The effect o f three transpedicular surgical constructs on stability was investigat ed in each specimen. Objectives. To quantify the loss of mechanical st iffness caused by the injury, and to evaluate the stiffness of three t ranspedicular surgical constructs. Summary of Background Data. Althoug h various investigators have studied the biomechanical characteristics of the burst fracture and surgical stabilization techniques, few have reported quantitative data on the three-dimensional biomechanical ins tability of these fractures. Methods. Load-displacement data were acqu ired in flexion, lateral bending, and axial rotation for,intact specim ens, after the L1 burst fracture was created and after the T12-L2 segm ents were stabilized using Luque plates, VSP plates, and Isola rods wi th one transverse connector. Results. Spines with burst fractures show ed a bilinear load-displacement behavior with significant instability (loss of stiffness relative to intact) at low-loads (up to 3 N . m) in flexion, lateral bending, and axial rotation. The loss of stiffness w as greatest in axial rotation over the entire load range (up to 10 N . m). If posterior element injury also was present, a significantly lar ger loss of stiffness was observed in flexion and axial rotation. The three transpedicular constructs improved the stability of the injured spine beyond that of the intact spine in flexion and lateral bending a t low loads. At high loads, they restored the stiffness to intact leve ls. However, in axial rotation they did not restore the stiffness to p re-injury level, particularly when the posterior column was disrupted. Conclusions. Reduction of the burst fracture returns the spine to its position of greatest inherent instability, essentially requiring the transpedicular instrumentation to be load bearing. To enhance mechanic al stability, it may be necessary to augment the transpedicular constr uct, particularly when the posterior column is disrupted.