Load Transfer Mechanisms in Cylindrical Interbody Cage Constructs

STUDY DESIGN: A finite-element model was created to study parametrically the load transfer characteristics of a lower lumbar motion segment implanted with dual anteroposterior cylindrical interbody cages. OBJECTIVES: To describe the frontal plane bone-implant interface stresses acting on a generic cylindrical interbody cage, to evaluate the effect of implant material properties on these stresses, and to determine the associated load transfer mechanisms. SUMMARY OF BACKGROUND DATA: From a biomechanical perspective, the long-term success of an interbody cage fusion depends on effective load transfer. The cage must stress the graft sufficiently to promote fusion, while keeping bone-implant interface stresses in a range that will prevent implant subsidence or loosening. At this writing, no published study has described interface stresses or load transfer mechanisms for these devices. METHODS: A planar finite-element model was used to simulate uniform compression loading of the implanted segment. Material properties of the interbody cage were varied to simulate cortical bone, titanium, and stainless steel implants. Normal and shear interface stresses were output along the length of the interface. RESULTS: Magnitudes of both the normal and shear interface stresses were substantially higher at the medial and lateral sides of the interface than in the center. Interface stresses were largely independent of implant material. CONCLUSIONS: Cylindrical interbody implants have inherent limitations, including stress concentrations at the bone-implant interface and possible stress shielding of the graft. The results from the current study suggest that implants made of cortical bone have substantially the same load transfer characteristics as metal devices of similar geometry.