Clinical assessment of pseudarthrosis or solid fusion is based on the residual motion across the “fused” segment (Kowalski et al, 2001). Dynamic flexion/extension (F/E) radiographs are commonly used to determine residual motion. Despite widespread use, it is unclear what the appropriate “cut-off” criteria to declare a fusion solid should be, with recommendations ranging from 0 to 5°. These values have not been derived by scientific methods. The present study was initiated to predict the angular sagittal motion across simulated lumbar interbody fusions (IF) using a Finite Element Model (FEM) of the ligamentous lumbar spinal segment. Anterior and posterior lumbar interbody fusions were simulated at the L3–L4 level as per the clinical procedure. Varying degrees of fusion were taken into account and the fusion mass was the simulated as a cancellous core with a cortical shell. The results indicated that 0.5° to 5.14° of angular motion can occur depending on fusion location and degree of completeness. While continuous bone might be noted at surgical exploration, this amount of motion may enable persistent loading of remaining structures, such as the annulus or spinal ligaments. In our view, this may prompt a redefinition of clinically “solid fusion”.

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