Effect of loading rate and hydration on the mechanical properties of the disc.

STUDY
DESIGN: The mechanical response of bovine intervertebral discs to axial compression at different loading rates and hydration levels was quantified.
OBJECTIVES: To quantify the effects of hydration and loading rate on the mechanical response of the intervertebral disc to compressive axial load. SUMMARY OF BACKGROUND DATA: The disc is known to be viscoelastic, but there are few experimental data showing the effect of loading rate and hydration on its response to compression.
METHODS: Hydration level reduced by creep-loading from a fully hydrated starting point. Four groups were tested: Group A: fully hydrated (n = 5), six loading rates, from 0.3 kPa/sec to 30 MPa/sec; Group B: after 30 minutes of creep (n = 4); and Group C: after 2 hours of creep (n = 4) under a static load of 1 MPa, loading rates 3 MPa/sec, 30 kPa/sec, and 0.3 kPa/sec; Group D: at 5-minute intervals, during an 8-hour period of creep (n = 3) under a static load of 1 MPa, loading rate 3 MPa/sec. Data normalized by disc area and height: nominal stress, strain, and modulus calculated.
RESULTS: Group A: Modulus increased with load and rate of loading, with significant differences among the lower three loading rates. The highest three loading rates were significantly different from the lower rates, but not from each other. Group B: At the two higher loading rates, modulus was greater than in group A. At the lowest loading rate the modulus was similar to that in Group A. Group C: At the highest loading rate, the modulus was less than that of Groups A and B. At the lower two loading rates, the modulus was similar to that in Group A. Group D: The modulus increased in the first 30 minutes and decreased in the interval from 60 to 480 minutes.
CONCLUSIONS: Intervertebral disc compressive mechanical properties are significantly dependent on loading rate and hydration.