Biomechanical Properties of the Rabbit Disc Body Unit Under Cyclic Torsional Loading

Abstract

To understand the biomechanical effect of repetitive torsional loading to the spine, fresh cadaveric lumbar disc body units from Japanese white rabbits were tested. The specified angular displacement was loaded to the specimen in a cyclic manner. The amplitude of axial rotation ranged from 10° to 19°, and the average rate of loading was 24°/s. The peak values of torque were monitored as a time series. The torque angle curves were recorded intermittently during cyclic loading, which was continued until failure of the specimen occurred. As the number of torsional cycles increased, the peak torque decreased. It was represented as two portions of a linear segment plotted against the logarithm of the number of torsional cycle. The decrease of peak torque showed the two phases, which were, first, the gradual decrease phase and second, the sudden decrease phase. The torque angle curves showed that the stiffness decreased and the initial toe part of the curve increased after repetition of the load. Histological examination after testing showed that the failure site was a growth plate at the upper vertebral body. However, distortion of stratification and rupture of the laminated bands were also observed at the intervertebral disc. These were considered as the accumulation of damage resulting in the failure caused by the cyclic load. These findings suggest that the response to the repetitive loading, in other words, the fatigue phenomenon, is strongly associated with the etiology of disc degeneration and spondylosis.