THE STRENGTH IN ANTERIOR BENDING OF LUMBAR INTERVERTEBRAL DISKS

Study Design. This was a motion segment experiment. Objectives. To det ermine the strength in bending of lumbar intervertebral discs. Methods . Eighteen ''motion segments'' consisting of two vertebrae and the int ervening disc and ligaments were loaded to simulate forward bending mo vements in life. The range of flexion and the resistance to bending of each specimen was compared before and after removal of the posterior elements. Nine of the discs were tested after the intradiscal pressure had been reduced by damaging the vertebral end plate in compression. Results. The posterior elements restricted the disc to 80% of its full range of flexion. The strength in anterior bending of isolated discs decreased with age and was 33 Nm on average. Reducing intradiscal pres sure did not affect the discs' strength in bending. Conclusions. Disc failure in bending occurs through overstretching of the outer anulus i n the vertical direction. In life, the posterior elements may not adeq uately protect the posterior anulus from fatigue damage.