Three-dimensional mechanical properties of the thoracolumbar junction.

The thoracolumbar junction region is a frequent site of spinal trauma. Accurate knowledge of the normal mechanical behavior of the intervertebral joints in this region is of importance to the clinician in treating the spinal injuries. The present study documented the complete three-dimensional motions of levels T11-T12 and T12-L1 in the thoracolumbar region. Pure moments of flexion/extension, bilateral axial torque, and bilateral lateral bending were applied to 11 three-vertebrae human cadaveric specimens (T11-L1) to a maximum of 7.5 Nm. Intervertebral motions were calculated using stereophotogrammetry and presented in the form of load-displacement curves, each containing three rotations and three translations at one intervertebral level. Average +/- SD flexion, extension, axial rotation, and lateral bending ranges of motion to one side were 2.7 +/- 1.3 degrees, 2.4 +/- 1.3 degrees, 1.8 +/- 0.7 degrees, and 3.5 +/- 1.1 degrees, respectively, at level T11-T12. The same ranges of motion at T12-L1 were 2.9 +/- 1.4 degrees, 3.9 +/- 1.4 degrees, 1.2 +/- 0.7 degrees, and 3.7 +/- 1.1 degrees, respectively. The extension and axial rotation ranges of motion at level T11-T12 were found to be significantly different than the same motions at T12-L1. The different geometry in the facet joints explains these observed differences in the mechanical behavior of T11-T12 and T12-L1.