Kinematics Following 3-Screw Integrated Interbody Spacers in the Lumbar Spine

"Stand-alone" fusion implants attempt to alleviate the need for supplemental posterior instrumentation.

A biomechanical study was conducted to assess the stability of an integrated 3- screw interbody cage with, and without, supplemental posterior fixation.

Nondestructive biomechanical testing was performed on 19 healthy cadaver spine segments. Specimens were tested in 6 degrees of motion and a maximum pure bending moment of 10 Nm was applied. Specimens were evaluated in the following sequence: Intact, cage, cage ± facet bolts, and cage ± pedicle screws. Nonconstrained motion was measured at both the index and adjacent levels.

The index levels were L2-L3 and L5-S1. The cage alone provided a significant decrease in motion at the L2-L3 level but not at L5-S1. At L2-L3, cage + pedicle screws decreased motion more effectively than cage + facet bolts, however, both the supplemented constructs outperformed intact (P < .05). At L5-S1, both posterior fixation systems appeared to have smaller degree of displacement compared to intact; however, no significant differences were observed at L5-S1 among the various constructs. Furthermore, the adjacent segments for each level (L1-L2 and L4-L5) had no significantly increased motion, compared to intact, for all 6 degrees of motion tested.

The stand-alone cage was more effective at L2-L3, than at L5-S1, in limiting motion. At L5-S1, supplemental fixation may need to be considered. No abnormal motion was identified at the adjacent, normal segments, for the stand-alone, or the circumferential constructs at either level tested.