Influence of graded facetectomy and laminectomy on spinal biomechanics.

Facetectomy and laminectomy are techniques for decompressing lumbosacral spinal stenosis. Resections of posterior bony or ligamentous parts normally lead to a decrease in stability. The degree of instability depends on the extent of resection, the loading situation and the condition of the intervertebral discs. The correlation between these parameters is not well understood. In order to investigate how these parameters relate to one another, a three-dimensional, non-linear finite element model of the lumbosacral spine was created. Intersegmental rotations, intradiscal pressures, stresses, strains and forces in the facet joints were calculated while simulating an intact spine as well as different extents of resection (left and bilateral hemifacetectomy, hemilaminectomy and bilateral laminectomy, two-level laminectomy), disc conditions (intact and degenerated) and loading situations (pure moment loads, standing and forward bending). The results of the modelling showed that a unilateral hemifacetectomy increases intersegmental rotation for the loading situation of axial rotation. Expanding the resection to bilateral hemifacetectomy increases intersegmental rotation even more, while further resection up to a bilateral laminectomy has only a minor additional effect. Hemilaminectomy and laminectomy only differ in their effect for ventriflexion and muscle-supported forward bending. Two-level laminectomy increases the intersegmental rotation only for standing. Degenerated discs result in smaller intersegmental rotations and higher disc stresses at the respective levels. Decompression procedures affect the examined biomechanical parameters less markedly in degenerated than in intact discs. Resection of posterior bony or ligamentous elements has a stronger influence on the amount than on the distribution of stresses and deformations in a disc. It has only a minor effect on the biomechanical behaviour of the adjacent region. Spinal stability is decreased after a laminectomy for forward bending, and after a two-level laminectomy for standing. For axial rotation, spinal stability is decreased even after a hemifacetectomy. Patients should therefore avoid excessive axial rotation after such a treatment.