During certain motions, the disc is at risk of annular injury. Axial compression coupled with various combinations of excessive flexion, lateral bending or axial rotation has been shown to lead to disc injury. However, similar injuries have also been caused by repetitive activity at lower, more physiological ranges of motion. The primary objectives of this study were to determine the regions of largest shear strain experienced by disc tissues in six degrees of freedom (DOF), since shear is considered a likely tissue failure criterion, and to identify the physiological motions that may place the disc at greatest risk of injury.

A grid of wires was inserted into the mid-transverse plane of nine human lumbar discs that were subjected to each of six principal displacements and rotations. Stereo-radiographs were taken in each position and digitised for reconstruction of the 3D position of each grid intersection. Maximum shear strains (MSS) were calculated from relative grid-intersection displacements and normalised by the input displacement or rotation. Physiological MSS were calculated using the maximum reported physiological lumbar segmental motion for each DOF.

The largest MSS were found in the posterior, posterolateral and lateral regions of the disc. For the translation motions, lateral shear and compression produced the largest MSS (approx. 9%/mm). For the rotation motions, lateral bending had significantly larger MSS than all other tests (5.8±1.6 %/°, P< 0.001).

The physiological MSS was greatest for lateral bending, being significantly larger than all other motions (57.8±16.2%, P< 0.001). In addition, physiological MSS for flexion was also significantly larger than for all remaining motions (38.3±3.3%, P< 0.001).

This study has identified lateral bending and flexion as the lumbar segmental motions that may place the disc at greatest risk of injury. The exact failure criterion for intervertebral disc tissue is not known, and MSS was used because it is related to maximum and minimum principal strains, and it was shown that disc tears may be initiated by large interlamellar shear strains that dominate over radial and circumferential annular fibre strains. These results provide improved understanding of disc behaviours under loading and may also be of value validating finite element models.

Biomechanical properties of the disc provide both flexibility and shock absorption. We hypothesised that frequency-dependent effects in shear and torsion deformations in which intrinsic viscoelasticity (solid phase) predominates would differ from compression and bending, in which fluid flow-mediated poroelasticity is also present.

Disc-vertebra-disc preparations (N=8) from human lumbar spines were subjected to each of three displacements and three rotations (6 degrees of freedom - DOF) at each of four frequencies (0.001, 0.01, 0.1, and 1 Hz) after equilibration overnight under a 0.4 MPa preload in a bath of PBS at 37C with protease inhibitors. The forces and torques were recorded along with the applied translation or rotation. The stiffness (force/displacement or torque/rotation) and the phase angle (between each force and displacement) were calculated for each degree of freedom from recorded data.

The stiffness significantly increased linearly with the log-frequency in most DOF (P< 0.001) apart for lateral bending and flexion/extension (P> 0.055). The increases over the four decades of frequency were 28%, 23% and 25% for antero-posterior (AP) shear, lateral shear and torsion respectively, and were 53%, 33% and 36% for compression, lateral bending and flexion.

The phase angle (a measure of energy absorption) significantly decreased overall with increasing frequency in all DOF (P< 0.005) apart for lateral bending. During AP and lateral shear, significant decreases in phase angle of 10% were found between 0.001 Hz compared to 0.01 Hz and 0.1 Hz (P< 0.026) with no differences found at 1 Hz. For torsion, the phase angle at 1 Hz was significantly lower by 40% compared to all slower frequencies (P< 0.001). During compression, a large significant drop in phase angle of 25%–35% occurred between 0.001 Hz and all other frequencies (P< 0.016). No significant post-hoc differences were found for flexion-extension (P> 0.057).

The dynamic effects (stiffness increase, and phase angle decrease with frequency) were consistently greater for deformation modes in which fluid flow effects are thought to be greater. Both the solid phase viscoelasticity and the fluid phase poroelasticity of the tissue appear to contribute to the disc stiffness and energy absorption, although these differences become more apparent at 1 Hz compared to the slower frequencies.