The acute effects of posterior fusion instrumentation on kinematics and intradiscal pressure of the human lumbar spine.

Nine cadaver lumbar spines were analyzed by applying nonconstraining nondestructive bending moments while measuring global range of motion, mechanical reaction at the sacrum, applied moment at the top of the specimen, segmental range of motion at L1-L5, and IDP at L1-L4. Each specimen was examined in an intact and instrumented state (with L3-L4 posterior instrumentation) using range of motion-based biomechanical testing, while achieving a similar global ROM in the sagittal, frontal, and transverse planes. An increase in applied moment was required during instrumented testing when compared with intact, and a significant increase in segmental range of motion during instrumented testing was found at all uninstrumented levels. Significant decreases in segmental range of motion were measured at the instrumented level when compared with intact testing. The most significant decreases and increases in IDP occurred at the instrumented level during sagittal and transverse plane testing.