Spinal fractures are common in the thoracolumbar region. Assessment of fracture instability is often made from fracture pattern seen on plain radiographs or CT scans. The purpose of this in vitro study was to correlate three-dimensional flexibility to each fracture type, i.e., endplate, wedge, and burst. Ten fresh cadaveric human spine specimens (T11-L1) were incrementally impacted in a high-speed trauma apparatus until a fracture occurred. All fractures were produced by the same mechanism (axial compression/flexion load). The occurrence of a fracture was monitored by lateral radiographs of the specimen, whose canal was lined with 1.6-mm steel balls. After each impact, the specimen was studied for its flexibility in flexion, extension, left and right lateral bindings, and left and right axial rotations. The flexibility was determined in response to the application of maximum pure moments of 7.5 Nm. Each moment was applied individually and in three load cycles. Parameters of neutral zone (NZ) and range of motion (ROM) were computed. Average flexion-extension ROM (and NZ) for intact, endplate, wedge, and burst fracture were respectively, 12.7 degrees (1.3 degrees), 13.9 degrees (1.7 degrees), 19.2 degrees (3.2 degrees), 22.0 degrees (6.0 degrees). The average lateral bending ROM (NZ) were 12.6 degrees (1.2 degrees), 13.6 degrees (1.9 degrees), 19.1 degrees (3.7 degrees), 27.2 degrees (9.8 degrees). The average axial rotation ROM (NZ) were 4.7 degrees (0.4 degree), 6.1 degrees (0.7 degrees), 7.1 degrees (1.0 degree), 12.9 degrees (3.1 degrees).(ABSTRACT TRUNCATED AT 250 WORDS)