A subject-specific finite element model evaluation of L4-5 lateral lumbar interbody fusion with pedicle screw fixation suggests decreased risk of cage subsidence using patient-specific interbodies

Introduction: Cage subsidence following lateral lumbar interbody fusion (LLIF) occurs in approximately 10% of cases, with just under 3% requiring revision surgery. Non-ideal interbody endplate apposition is associated with subsidence risk. To address this issue, patient specific implants have been used in an attempt to maximize endplate contact in commercially available finite element models with stand-alone cages. However, the benefit of expensive, custom interbody cages is not well understood.

Methods: A compression-validated subject-specific finite element model (FEM) of a human L4-L5 motion segment was used to simulate fixation and fusion with LLIF and pedicle screw and rod fixation (PSR) (7.5 mm diameter screws) involving a 3D printed cage (65 mm long x 22 mm wide) having varied central contact with upper and lower endplates. Stress distributions were studied across endplates and in PSR instrumentation during combined compression and flexion (C+FL: 400N+7.5Nm). Predicted maximum stress across endplates, screws and rods during loading were compared using three LLIF to endplate surface conformities (full, less and least) and four LLIF effective moduli (115000, 57500, 11500 and 3600 MPa).

Results: During C+FL, reduced LLIF-central endplate bone contact resulted in slightly increased flexibility, increased max stress across endplates (up to 6X) and decreased stress across screws and rods. For all LLIF to endplate bone contacts, decreased LLIF modulus led to slightly increased flexibility, decreased bone stresses and increased stresses across PSR (up to 2X). Relative effects of LLIF contact area on construct stresses were smaller with low modulus LLIF

Conclusion : This finite element model of LLIF with PSR suggests that incomplete LLIF surface contact with bony end plates and increased LLIF effective modulus increases the risk for cage subsidence. Use of patient-specific implants with customized effective modulus and complete end plate contact is likely protective.