Effect of axial load on the flexural properties of an elastomeric total disc replacement.

STUDY DESIGN Twelve Cadisc-L devices were subjected to flexion (0°-6°) and extension (0° to -3°) motions at compressive loads between 500 N and 2000 N at a flexural rate between 0.25°/s and 3.0°/s. OBJECTIVE To quantify the change in flexural properties of the Cadisc-L (elastomeric device), when subjected to increasing magnitudes of axial load and at different flexural rates. SUMMARY OF BACKGROUND DATA The design of motion preservation devices, used to replace degenerated intervertebral discs, is commonly based on a low-friction, ball-and-socket-articulating joint. Recently, elastomeric implants have been developed that attempt to provide mechanical and motion properties that resemble those of the natural disc more closely. METHODS Twelve Cadisc-L devices (MC-10 mm-9° and MC-10 mm-12° size) were supplied by Ranier Technology Ltd (Cambridge, United Kingdom). The devices were hydrated and tested using a Bose spinal disc-testing machine (Bose Corporation, ElectroForce Systems Group, Eden Prairie, MN) in Ringer's solution at 37°C. A static load of 500 N was applied to a device and it was then subjected to motions of 0° to 6° to 0° (flexion) and 0° to -3° to 0° (extension) at a flexural rate of 0.25°/s, 0.5°/s, 1.0°/s, 1.5°/s, 2.0°/s, and 3.0°/s. Tests were repeated at 1000 N, 1500 N, and 2000 N. RESULTS Regression analyses showed a significant (R > 0.99, P < 0.05) linear increase in bending moment and flexural stiffness with flexion and extension angles (at 1000 N and higher loads)-a significant (R > 0.994, P < 0.05) linear decrease in flexural stiffness in flexion and extension as flexural rate increased. CONCLUSION The bending moment of the Cadisc-L increased linearly with flexion and extension angles at 1000 N and higher loads. Flexural stiffness increased with compressive load but decreased with flexural rate.