Introduction
A common goal of spinal fusion is to achieve long term bone union between adjacent endplates. Why is bone growth through the graft window more important than bone on-growth to the cage from a basic science point of view?
FDA demands that fusion cages have graft windows. Conceptually, cages can either be minimal footprint, graft holding devices or they can be designed to bear majority of the load on a more permanent basis. Interbody cages with large graft windows may allow use of larger amounts of graft, however reducing cage footprint to fit a larger graft window can increase strain on the vertebral endplate and device, increasing subsidence risk.
Methods
A simplified construct modelling lumbar interbody geometry was created with and without screws. Loading was applied to simulate a physiological load. Models were created to simulate typical progression of a spinal fusion: initial fixation via screw fixation; bone on-growth to the surface of the device; and full bone growth through the graft window.
Results
Different cage footprints resulted in different strain pathways in the vertebral bodies. Regions of high strain were reduced with thicker cage walls. Even with fusion through the graft window, there is a region of low strain above the endplate which may limit endplate remodelling. Screws influence the strain distribution and magnitudes, with an increase within the graft window and between the screw and posterior cage wall.
Discussion
There is a balance between maintaining adequate graft window and footprint size, so cages should be designed with perpetual load cycles in mind.
Contiguous bone between two vertebral bodies through the graft window increases load sharing between bone and cage, reducing stress under and above the cage, which will reduce sclerotic bone and cyst formation.