[Effects on single segmental lumbar stability after graded facetectomy and laminectomy: a finite element study]. 
To explore the effects on degenerative single segmental lumbar spinal stability after posterior graded laminectomy and facetectomy by a finite element method.  A finite element model of L3-S1 segments with a single segmental degeneration at L4-5 level was established.  Different models of L4-L5 segmental instability after posterior graded laminectomy and facetectomy were established.  And interior 1/3, 1/2, 2/3 of laminar and bilateral facet joints were resected.  Physical loads were applied to the models and the changes of range of motion (ROM) at L4-5 level in different models were recorded during flexion, extension, lateral bending and rotation.  As compared to preoperative model, after resecting 1/3 bilateral facet joints at L4-5 level, L4-5 segmental ROM increased 18% during flexion, 27% during extension and 45% during torsion.  And the increased degree of ROM became more obvious along with a greater resection range of bilateral facet joints.  No significant difference existed in L4-5 segmental ROM during lateral bending in all models.  Facet joints play vital roles in lumbar segmental stability.  And segmental instability may occur when 1/3 bilateral facet joints are resected.