Damping properties of the nucleus pulposus.

BACKGROUND The nucleus pulposus is extremely deformable and it is not uncommon to observe strain amplitudes as large as 12.5% in physiological loading conditions. It has been shown that the nucleus pulposus contributes to the damping properties of the intervertebral disc. The quantification of the damping properties of the nucleus pulposus under physiological large deformations is then a key aspect for its mechanical characterization and for the design of nucleus replacement devices. METHODS A specific mechanical device has been developed to encapsulate nucleus pulposus tissues into a deformable and permeable device, while quantifying its water content. The specific damping capacity was defined by dividing the energy loss by the work input. With this device and definition, the specific damping capacity of the bovine coccygeal nucleus pulposus was quantified in large compressive deformations (12.5%) and for frequencies ranging between 10(-2) and 10(1)Hz. FINDINGS It is found that the specific damping capacity of the nucleus pulposus of the bovine coccygeal ranged between 18 and 36%. The lowest values of specific damping capacity are found for frequencies corresponding to the dynamics of loads in all day activities such as walking (0.1 to 1Hz). INTERPRETATION The nucleus pulposus contributes to dissipate energy under physiological large deformations. However, it seems that the nucleus pulposus is designed so that damping is minimal for frequencies corresponding to moderate daily activities.