Facet joint loading in thoracolumbar burst fractures

A combined loading of axial compression and anterior shear on a spinal motion segment causes the severe form of burst fracture, often associated with serious neurological problems. The shear force transmitted through the facet joint and the pedicles (wedging effect) to the posterior upper half of the vertebral body is an essential component of the injury mechanism. Burst fractures were created in four thoracolumbar motion segments by axial compression at 100 mm/sec. Each specimen was instrumented with two strain gages (one on the middle of the anterior wall, the other near the base of the pedicles). Two samples were tested in the neutral position while the other two were tested in full extension to maximize facet joint loading. The samples tested in the neutral position experienced crush fractures of the vertebrae, with horizontal fracture lines located at mid height of the cortical shell and minimal spread of the pedicles (failure loads: 2950 and 2004 N). The ratios of strain at the base of the pedicles to strain at the anterior wall prior to failure were 1.7 and 2.0. The samples tested in extension showed collapse of the middle column with vertical fractures in the cortical shell near the base of the pedicles and an increase in interpedicular distance (failure loads: 7377 and 6126 N). The strain ratios were 13.7 and 19.9, with tensile strains in the anterior walls. These results suggest that facet joint loading plays a pivotal role in the burst fracture injury mechanism.