[Intervertebral cages from a biomechanical point of view]

If lumbar interbody fusion is indicated, there are several options for instrumentation of the affected motion segment. Intervertebral cages are implanted in the disc to restore disc height and to stabilize the motion segment by tensioning the ligamentous structures.

Based on a selective literature search with the focus on biomechanical aspects of intervertebral cages, experimental and clinical studies are shown, interpreted, and discussed.

In the literature, biomechanical flexibility tests of "stand alone" cages without supplemental instrumentation showed a limited stabilizing effect, particularly in extension and axial rotation, as well as an increased load transfer through the ventral column. Applying supplemental dorsal instrumentation can return the ventral/dorsal load sharing to the range of an intact motion segment and causes a marked increase of stability in all motion planes. Compared to bilateral dorsal instrumentation, unilateral dorsal instrumentation showed a reduced primary stability and leads to an asymmetrical loading of the cage which can cause unilateral loss of reduction. Nonmetallic cages with a stiffness adapted to bone allow better radiological evaluation of the bony fusion of the motion segment and theoretically have a reduced tendency to migrate.

In combination with bilateral dorsal instrumentation, cage geometry and material have only a minor influence on primary stability and the main stability is provided by the internal fixator.