Seven vertebral preparations of L1, with surrounding discs, facet joints, and ligaments were exposed to an instant axial dynamic force in order to produce a burst or crush fracture. The resulting fractures were similar to fractures observed clinically and showed a comminuted vertebral body with fractured vertebral end-plates, dislocated disc nucleus, bone fragments severely encroaching upon the spinal canal, and facet joint laxity. The flexion-extension range was increased considerably. This implies that this fracture type should be regarded as unstable with a risk of progressive flexion deformity, neurologic deterioration and pain. The fracture could be reduced by an axial distraction force of 400 N simulating the effect of Harrington distraction rods. However, the distraction resulted in an "empty" vertebral body with small areas of spongious bone mixed with fragments of the disc nucleus and fragments of the vertebral end-plate.