ANALYSIS OF LOAD RELAXATION IN COMPRESSED SEGMENTS OF LUMBAR SPINE

Load-relaxation was measured in 12 segments of human cadaveric lumbar spine. Each segment consisted of an intact intervertebral disc attache d to half of its adjacent vertebrae with the posterior elements remove d. Six specimens were each compressed at six different strains (corres ponding to initial loads of 0.5-2.5 kN) and, for each strain, the load -relaxation was measured for a period of 20 min at room temperature. T hese load-relaxation curves were used to plot three isochrones for eac h specimen. All isochrones were linear (r values in the range 0.95-0.9 9). This result indicated that a linear model could be used to represe nt load-relaxation. Four specimens were tested at a single strain (cor responding to an initial load of about 2 kN) at 37 degrees C for a per iod of 4-6 h. Load was plotted against the logarithm of time. The resu lting plots did not show any peaks, indicating that relaxation effects did not predominate at any particular timer during load-relaxation. H owever, it was possible to model the load-relaxation as a simple linea r system which can be represented as two Maxwell elements in parallel. These elements were characterized by relaxation times of 16 +/- 8 min and 4.6 +/- 0.8 h. Fourier transformation of the load-relaxation curv es showed a gradual increase in the storage modulus and a gradual decr ease in the loss modulus for frequencies of about 1 Hz and above. At t hese frequencies, the spine cannot function as a shock-absorber in pur e compression.