Mechanism of the burst fracture in the thoracolumbar spine. The effect of loading rate.

Abstract

Calf lumbar spine motion segments were randomly assigned to two groups. After insertion of a transducer capable of measuring transient occlusion of the spinal canal during impact, a low rate axial impact was applied in one group and a high rate load in the other. Post-injury computed tomography scans and peak canal occlusions were measured to determine the effect of rate of load application on occlusion of the spinal canal. This study was designed to determine if for the same direction of impact and total energy delivered, occlusion of the spinal canal postvertebral fracture was related to the rate at which the impact was delivered (time from zero to peak load). Several reports based on clinical observations have hypothesized that axial burst fractures, which displace bone fragments into the canal, occur because of internal pressurization and explosion of the vertebral body. The extent of bursting of the vertebra may depend on the rate of pressurization of the body, which could be related to the rate at which the load is applied. Using calf lumbar spines, a transducer was placed within the spinal canal, after removal of the cord, to measure canal occlusion during impact. One group received axial compressive impacts at a mean loading rate of 400 msec (zero to peak load) using a materials-testing machine. The energy of failure was determined and used to select a drop weight and distance for the high loading rate tests, which would yield equivalent impact energy. The second group received impacts at a loading rate of of 20 msec. The post-injury radiographs and canal occlusion measurements were compared. The same mean energy of impact was used in the fractures for both groups. Post-injury radiographs of the low loading rate group showed compressive fractures with a mean canal occlusion of 6.84%, whereas the high loading rate group had burst fractures with mean canal encroachment of 47.6% (P = 0.0007). For the same energy and direction of impact, a high impact loading rate produces fractures with significant canal encroachment, whereas minimal encroachment is seen for fractures produced at a low loading rate.