Current Concepts of Contemporary Expandable Lumbar Interbody Fusion Cage Designs, Part 1: An Editorial on Their Biomechanical Characteristics.

BACKGROUND: Bidirectional expandable designs for lumbar interbody fusion cages are the latest iteration of expandable spacers employed to address some of the common problems inherent to static interbody fusion cages.
OBJECTIVE: To describe the rationales for contemporary bidirectional, multimaterial expandable lumbar interbody fusion cage designs to achieve in situ expansion for maximum anterior column support while decreasing insertion size during minimal-access surgeries.
METHODS: The authors summarize the current concepts behind expandable spinal fusion open architecture cage designs focusing on advanced minimally invasive spinal surgery techniques, such as endoscopy. A cage capable of bidirectional expansion in both height and width to address constrained surgical access problems was of particular interest to the authors while they analyzed the relationship between implant material stiffness and geometric design regarding the risk of subsidence and reduced graft loading.
CONCLUSIONS: Biomechanical advantages of new bidirectional, multimaterial expandable interbody fusion cages allow insertion through minimal surgical access and combine the advantages of proven device configurations and advanced material selection. The final construct stiffness is sufficient to provide immediate anterior column support while accommodating reduced sizes required for minimally invasive surgery applications. LEVEL OF EVIDENCE: 7. This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery.