THE EFFECT OF SPINAL DESTABILIZATION AND INSTRUMENTATION ON LUMBAR INTRADISCAL PRESSURE AN IN-VITRO BIOMECHANICAL ANALYSIS

Study Design. In vitro biomechanical testing was performed in human ca daveric lumbar spines, using pressure needle transducers toanalyie the effects of spinal destabilization and instrumentation on lumbar intra discal pressures. Objectives, To quantify changes in lumbar intradisca l pressures at three adjacent disc levels under conditions of spinal r econstruction, and to evaluate the possibility Of pressure;induced dis c pathology secondary to spinal instrumentation. Summary of Background Data. Lumbar intradiscal pressures under in vivo and in vitro conditi ons and the use and development of Spinal instrumentation have been in vestigated comprehensively. However, the effects of spinal destabiliza tion and instrumentation on lumbar intradiscal pressure have not been delineated clearly. Methods. In 11 human cadaveric lumbosacral specime ns, specially designed pressure needle transducers quantified intradis cal pressure changes at three adjacent disc levels (L2-L3, proximal; L 3-L4, operative; and L4-L5, distal) under four Conditions of spinal st ability: intact, :destabilized, laminar hook and pedicle screw reconst ructions. Biomechanical testing was performed under axial compression (0-600 N), anterior flexion (+12.5 degrees) and extension (-12.5 degre es), after which the level of degeneration and disc area (cm(2)) were quantified, Results. In response to destabilization and instrumentatio n, proximal disc pressures increased as much as 45%, and operative pre ssure levels decreased 41-55% (P < 0.05), depending on the instrumenta tion technique. Linear regression and correlation analyses comparing i ntradiscal pressure to the grade of disc degeneration were not signifi cant:(r = 0.24): Conclusions. Changes in: segmental intradiscal pressu re levels occur in response to spinal destabilization and instrumentat ion (P < 0.05). Intradiscal cyclic pressure differentials drive the me tabolic production and exchange of disc substances. Conditions of high or low disc pressure secondary to spinal instrumentation may serve as the impetus for altered metabolic exchange and predispose operative a nd adjacent levels to disc pathology.