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

Study Design. The mechanical response of bovine intervertebral discs to axi al compression at different loading rates and hydration levels was quantifi ed. Objectives. To quantify the effects of hydration and loading rate on the me chanical 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 sta rting 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 sig nificant differences among the lower three loading rates. The highest three loading rates were significantly different from the lower rates, but not f rom each other. Group B: At the two higher loading rates, modulus was great er than in group A. At the lowest loading rate the modulus was similar to t hat 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 wa s similar to that in Group A. Group D: The modulus increased in the first 3 0 minutes and decreased in the interval from 60 to 480 minutes. Conclusions. Intervertebral disc compressive mechanical properties are sign ificantly dependent on loading rate and hydration.