Biomechanics of lumbar fusion.

If lumbar fusion is indicated to restore stability and prevent irritating intrasegmental motion, then there are a wide range of candidate constructs from which to make a choice. The criteria for choice must include the immediate postoperative stability of the construct, the stability and strength of the eventual fusion, and the potential for decompression or restoration of normal anatomic relations. Interbody fusions are particularly sound biomechanically. Two model constructs of the lumbar interbody fusion, the tripod concept and flagpole concept, are presented. The posterior lumbar interbody fusion (PLIF), in particular, encompasses the most desirable biomechanic features: posterolateral siting of load-bearing grafts is the optimal location in relation to the load-bearing capacity of the vertebral bodies; posterior distraction is enhanced by maintaining the anterior anulus and ligament--they act as a fulcrum or pivot; and when adequate soft-tissue connections are maintained between posterior processes, or when they are supplemented by wire-ties, the graft is stable and strong enough for early weight-bearing. Variations on the theme can accommodate a wide range of presenting cases, whether previously operated or not. PLIF is gratifying to both patient and surgeon, even if technically demanding.