Internal pressure measurements during burst fracture formation in human lumbar vertebrae

Study Design. In a laboratory study, 21 human lumbar spine segments were used to determine whether intraosseous pressure increases occur during axial-compressive loading conditions under two displacement rates. Objective. To determine whether an intraosseous pressure rise is associated with burst fracture formation. Summary of Background Data. Burst fractures are high-speed injuries usually associated with neurologic deficit. An internal pressure rise has been implicated as a critical factor in burst fracture formation. The authors hypothesize that the internal pressure increases with increasing input velocity. Methods. The internal pressure changes were measured in spine segments using two displacement rates: 10 mm/s (slow speed) and 2500 mm/s (high speed). Failure load and energy absorption were determined for both groups. The resultant fracture types were determined from postinjury radiographs. Results. The initial peak internal pressure decreased from slow- to high-speed tests (P < 0.01). Overall peak pressure, failure load, and energy absorbed at failure were not significantly different. Slow-speed tests resulted in compression fractures, whereas high-speed tests resulted in burst and compression fractures. Conclusions. The current research did not support the current theory of burst fracture formation. There was a decrease in measured internal pressure from the slow- to high-speed groups, and burst fractures still were produced. The theory could be potentially modified to suggest that the nucleus entering the vertebral body acts as a wedge, splitting the vertebral body apart and enabling the bony fragments to be pushed into the canal space.