Biomechanics of posterior lumbar fixation. Analysis of testing methodologies.

A variety of biomechanical methods have been used for the experimental evaluation of spine instrumentation in vitro. Consensus has not been reached for criteria to compare the performance of dissimilar devices. The range of load-displacement conditions currently used for in vitro testing of spine instrumentation was reviewed, and compared to calculated estimates for flexion, lateral bending, and torsion loads. It is recommended that future studies simulate combinations of loads or displacements associated with common patterns of motion, and that the distraction across the region of injury be measured. For cyclic testing, input frequencies of 4-5 cycles/sec are within physiologic range, and offer advantages over tests at lower frequencies. The development of consistent protocols based on physiologic conditions should facilitate the biomechanical assessment of techniques for stabilizing the human spine.