Intervertebral disc distraction stiffness predicts endplate subsidence following transforaminal interbody cage expansion: an ex vivo study

Expandable cages have the potential to mitigate the currently high subsidence rates following transforaminal lumbar interbody fusion (TLIF), but are liable to over-distraction in situ. This may be due to the undefined patient-specific expansion threshold of the intervertebral disc (IVD) space. This study aimed to elucidate whether IVD properties affect the torque required to expand the cage within the IVD space, and determine the association between achieved torque, distraction stiffness, and subsidence severity. Fifteen cadaveric L3-L4 and L4-L5 samples were prepared with the TLIF approach. Under 100 N compression, the torque required to expand the cage per half-turn, alongside the changes to IVD and cage height, were recorded until maximum cage expansion. Subsidence depth was measured after subsequent cyclic loading, and the surface area of removed IVD tissue was quantified post-test. Peak torque was inversely associated with preloaded IVD height (B: –0.34, p < 0.001) and the percentage of IVD removed (B: –0.04, p < 0.01). IVD distraction stiffness was associated with preloaded IVD height only (B: -0.19, p < 0.001). There was no association with IVD or facet degeneration. When subsidence depth was normalised to bone mineral density, a positive correlation was observed with peak torque and cage expansion stiffness (both p < 0.05). The torque required to expand interbody cages in situ is relevant to subsidence risk, and depends on IVD geometry and the amount of residual tissue. Thus, short IVDs should be thoroughly prepared to alleviate excessive stiffness during cage expansion.