Introduction
Spinal vertebral body endplates have unique shapes and sizes that can change significantly with pathology. There is current debate as to optimal implant design and surgical technique. Three main surgical techniques used to prepare the interbody space for spinal fusion are to: Not remodel the endplate bone and implant an Off-The-Shelf (OTS) device; remodel the endplate bone and implant an OTS implant; and not remodel the endplate bone and implant a Patient Specific Implant (PSI). In the present study, we perform Finite Element (FE) Analysis to assess the effect of these techniques on strain distributions across cervical and lumbar vertebral endplates.
Methods
CT of pathological spines from patients who underwent ACDF and Lumbar Interbody Fusion (LIF) were used to create geometrically accurate 3D models. Post-operative CT from the same patients were processed to indicate implant positioning. Multi-material property FE models were created based on CT Hounsfield Units. The loading simulated a physiological load split between the endplate and facets according to literature reported values. Three models were created to simulate: Non-remodeled endplates with an OTS implant; remodeled endplates with an OTS implant; non-remodeled endplates with a PSI.
Results
FE modelling of the achieved construct was performed to validate the FE model. This demonstrated strain distributions that were correlated with bone remodelling evident on the post-operative CT scans.
FE results revealed that implant design and surgical technique both influence strain distributions across the vertebral body endplate.
Conclusion
The presence of an interbody cage may increase the strain on the contacting vertebral bone by either (1) loading weaker cancellous bone after burring away stronger cortical bone on the endplates, and/or (2) lacking adequate endplate contact and creating regions of strain hotspots.