Stabilizing effect of posterior lumbar interbody fusion cages before and after cyclic loading

Object. The function of intel body fusion cages is to stabilize spinal segm ents primarily by distracting them as well as by allowing bone ingrowth and fusion. An important condition for efficient formation of bone tissue is a chieving adequate spinal stability. However, the initial stability may be r educed due to repeated movements of the spine during everyday activity. The refore, in addition to immediate stability, stability after cyclic loading is of remarkable relevance; however, this has not yet been investigated. Th e object of this study was to investigate the immediate stabilizing effect of three different posterior lumbar interbody fusion cages and to clarify t he effect of cyclic loading on the stabilization. Methods. Before and directly after implantation of a Zientek, Stryker, or R ay posterior lumbar interbody fusion cage, 24 lumbar spine segment specimen s were each evaluated in a spine tester. Pure lateral bending, flexion-exte nsion. and axial rotation moments (+/- 7.5 Nm) were applied continuously. T he motion in each specimen was measured simultaneously. The specimens were then loaded cyclically (40,000 cycles, 5 Hz) with an axial compression forc e ranging from 200 to 1000 N. Finally, they were tested once again in the s pine tester. Conclusions. In general, a decrease of movement in all loading directions w as noted after insertion of the Zientek and Ray cages and an increase of mo vement after implantation of a Stryker cage. In all three cage groups great er stability was demonstrated in lateral bending and flexion than in extens ion and axial rotation. Reduced stability during cyclic loading was observe d in all three cage groups; however, loss of stability was most pronounced when the Ray cage was used.