Biomechanical analysis of segmental lumbar lordosis and risk of cage subsidence with different cage heights and alternative placements in transforaminal lumbar interbody fusion

Cage subsidence in transforaminal lumbar interbody fusion (TLIF) is one of the concerns. The objective was to numerically assess the resulting segmental lumbar lordosis (SLL) and stresses at the bone-cage interface as functions of cage height (8- vs. 10-mm) and cage placement (oblique asymmetric, vs. anterior symmetric) for normal and osteoporotic bone quality. A L4-L5 detailed finite element model of TLIF was subjected to the functional loadings of 10 Nm in the physiological planes after the application of a 400 N follower-load. The SLL was increased by 0.9 degrees (11%) and 1.0 degrees (13%), respectively in oblique asymmetric and anterior symmetric cage placement with 8-mm height; they were 1.4 degrees (18%) and 1.7 degrees (21%) for the 10-mm cage. The maximum stresses at the cage-bone interface, in normal bone model, were increased up to 16% and 41% with the 10-mm cage and asymmetric oblique placement, respectively, and they increased up to 16% and 43% in osteoporotic bone model. The greater cage resulted to a higher simulated SLL. Oblique asymmetric placement and the use of a greater cage may increase the risk of cage subsidence. Due to the lower mechanical strength of osteoporotic bone, the risk of cage subsidence should be higher.