The lumbar spine in backward bending.

Cadaveric lumbar motion segments were loaded to simulate backward bending (extension) movements of the lumbar spine. The motion segments' resistance was measured initially, and after the spinous processes and apophyseal joints had been cut through in turn. Compression tests were then performed on the discs while they were wedged in full extension. The results showed that extension is resisted mainly by the disc and spinous processes, and that, in hyperextension, damage usually occurs first in the spinous processes (or the soft tissue squashed between them). However, if the spinous processes are particularly widely spaced, then the apophyseal joints can become damaged first. The protection offered the disc by the neural arch is greater in young people, and after the disc height has been reduced by creep loading. The disc can be damaged in hyperextension if the spine is subjected to high compressive forces at the same time. A sudden application of compressive force can cause an anterior disc prolapse, while a cyclic (fatigue) compressive force can increase the posterior bulging of the lamellas in the posterior annulus.