CYCLIC COMPRESSION-FLEXION LOADING OF THE HUMAN LUMBAR SPINE

Study Design. The present study was designed to investigate the biomec hanical behavior of the lumbar spine under controlled complex physiolo gic situations with chronic input. Objective. The objective was to det ermine the response of the human cadaver lumbar spinal column under re petitive compression-flexion forces. Summary of Background Data. Studi es have been conducted in the past to determine the biomechanical resp onse of the spine under uniaxial or pure forces. There is no methodolo gy that can be used to apply and continuously quantify the fatigue res ponse of the lumbar spinal column under controlled combined complex lo ading vectors (e.g., compression flexion). Methods. Intact cadaver lum bar columns (L1-L5) were mounted with the superior end in contact with a ball-transfer mount, inducing a flexion load to the spine while all owing multiple degrees of freedom. The distal portion of the specimen was attached to a six-axis load cell to quantify the force sustained b y the specimen during the entire loading cycle. The applied load and p iston deformation and the generalized six-axis force histories were ga thered as a function of time using a digital data acquisition system. Results. The stiffness versus number of cycles (K-N) response exhibite d nonlinear characteristics. The stiffness increased initially and the n stabilized after 1,0002, 000 cycles of loading, delineating the visc oelastic characteristics of the spine. The initial stiffness increase before stabilization was found to be significantly different (P < 0.02 5) compared to the stiffness beyond 2,000 cycles. Conclusions. The dat a suggest that the fatigue response can be understood by cyclically lo ading the ligamentous lumbar spine preparation to approximately 2,000 cycles.