Abstract: In order to improve the outcome of lumber discectomy, a biomechanical study was performed to identify the effects of posterior approach of lumber operation on the sagittal stability of lumber spine. Comparing with the displacements of normal disc group, the horizontal displacements of disc were slightly increased in the group that underwent decompression on the posterior outer annulus fibrosus(P>0.05), and significant increased in the group that underwent entire nucleus pulposus removal and the group that underwent curettage of cartilage end plate(P<0.01). The results suggested that single level discectomy of lumber spine trends to occur sagittal segmental instability after surgery. This should be given more consideration on the clinical setting.There is no relationship between the extent of sagittal segmental instability and the content of disc removal under low physiological shearing force. When the force gets higher, the outcome goes reversal.

Key words: prolapse of lumber disc, lumber instability, biomechanics