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If side bending of the lumbar spine is introduced when the lumbosacral spine is in either flexion or extension, non-neutral coupling occurs. The sacrum responds to non-neutral lumbar spine coupling by moving around an oblique axis, resulting in sacral torsion. By convention, the left oblique axis runs from the upper extremity of the left sacroiliac joint to the lower end of the right sacroiliac joint, and the right oblique axis runs from the upper end of the right sacroiliac joint to the lower extremity of the left sacroiliac joint. Although the exact biomechanics of the torsional movements of the sacrum are unknown, the hypothetical left and right oblique axes are useful for descriptive purposes.

Sacral torsion occurs in an anterior direction when the lumbosacral juction is extended and side bending is introduced. When the lumbar spine moves into extension, the sacrum responds with symmetric (bilateral) flexion (anterior nutation). If the extended lumbar spine then side bends to the left with coupled rotation to the right, the sacrum will rotate to the right on the right oblique axis.

Similarly, when the lumbar spine moves into extension followed by right side bending with coupled rotation to the left, the sacrum will rotate to the left on the left oblique axis.

Sacral torsion occurs in a posterior direction when the lumbosacral junction is flexed and side bending is introduced. When the lumbar spine moves into flexion, the sacrum responds with symmetric (bilateral) extension (posterior nutation). If the flexed lumbar spine then side bends to the left with coupled rotation to the left, the sacrum will rotate to the right on the left oblique axis.

Similarly, with lumbar flexion followed by right side bending with coupled rotation to the right, the sacrum will rotate to the left on the right oblique axis.

Click here to join Dr. Philip E. Greenman, D.O., F.A.A.O., as he describes normal non-neutral mechanics.