CORRELATION OF MICROTRAUMA IN THE LUMBAR SPINE WITH INTRAOSSEOUS PRESSURES

This study was conducted to determine the relationship between intraos seous pressure and vertebral microtrauma in the lumbar spine. Function al spinal units were excised from human cadavers. Radio-opaque dye was injected into the nucleus. Miniature transducers were inserted into t he vertebrae to record intraosseous pressures. Compressive loading was applied quasistatically (2 mm/sec) until injury occurred. Movement of the contrast medium was monitored under fluoroscopy. The subchondral endplate was the most vulnerable component for initiation of injury to the lumbar spine segment. In the initial stages of loading, the verte bral endplates gradually bulged outward, with the contrast medium stay ing within the nucleus. However, at higher physiologic load levels, be fore reaching the limiting load, the deformations increased, resulting in buckling of one of the endplates. This was followed by the contras t medium impregnating the spongiosa. Microlevel trauma was not observe d radiographically after load removal, indicating that one cannot alwa ys equate a normal radiograph with normal spinal anatomy. Mean forces, deformations, stiffnesses, energies, and strains were 7.8 kN (+/- 1.4 ), 5.23 mm (+/- 0.78), 1940 N/mm (+/- 226), 18.7 J (+/- 4.4), and 35.5 % (+/- 3.7), respectively. Pressure in the vertebral body containing t he injured endplate before the onset of microtrauma was different (P < 0.05) from the pressure after injury; the pressures in the body conta ining the intact endplate, however, were not statistically different. Significant differences (P < 0.05) in the intraosseous pressures occur red between the two spinal levels at low-level physiologic loads befor e the onset of microtrauma. These observations indicate that microleve l injury that is not detectable by radiography can be followed by elev ated intraosseous pressure. Increased pressure in the bone has been im plicated as a source of back pain. Miniature endplate trauma may be re sponsible for acute low back pain in patients with a normal radiograph ic appearance.