Direct measurement of intervertebral disc maximum shear strain in six degrees of freedom: motions that place disc tissue at risk of injury

Abstract

Human intervertebral disc specimens were tested to determine the regions of largest maximum shear strain (MSS) experienced by disc tissues in each of three principal displacements and three rotations, and to identify the physiological rotations and displacements that may place the disc at greatest risk for large tissue strains and injury. Tearing of disc annulus may be initiated by large interlamellar shear strains. Nine human lumbar discs were tagged with radiographic markers on the endplates, disc periphery and with a grid of wires in the mid-transverse plane and subjected to each of the six principal displacements and rotations. Stereo-radiographs were taken in each position and digitized for reconstruction of the three-dimensional position of each marker. Maximum tissue shear strains were calculated from relative marker displacements and normalized by the input displacement or rotation. Lateral shear, compression, and lateral bending were the motions that produced the mean (95% confidence interval) largest mean MSS of 9.6 (0.7)%/mm, 9.0 (0.5)%/mm, and 5.8 (1.6)%/ degrees , respectively, and which occurred in the posterior, posterolateral and lateral peripheral regions of the disc. After taking into account the reported maximum physiological range of motion for each degree of freedom, motions producing the highest physiological MSS were lateral bending (57.8 (16.2)%) and flexion (38.3 (3.3)%), followed by lateral shear (14.4 (1.1)%) and compression (12.6 (0.7)%).

Figure 1

Schematic of the 3D stereo-radiography displacement measurement set up. Anterior (a) and axial views (b) show the grid of tantalum wires in the disc, endplate and calibration lead beads, and tantalum wire segments in the elastic band stretched around the periphery of the disc. Stereo-radiographs were taken before and after imposition of displacements/rotations in each of the three principal translations and rotation directions. Specimen fixation cups are not shown and diagrams are not to scale.

Figure 2

Left/right stereo-radiograph pair showing the grid of wires in the disc, endplate and calibration beads, and peripheral disc wire segments. Note: a total of ten endplate beads are visible, however, five upper endplate beads were inserted but not used for calculations in this study. Reference in the text is only made to the five lower endplate beads.

Figure 3

Axial view of the symmetrical planar intervertebral disc grid used to interpolate displacements and calculate strains. There are 148 four-noded quadrilateral elements. The grid was partitioned into anatomical regions (shaded in gray). Nine regions were defined: anterior, left/right anterolateral, left/right lateral, nucleus, left/right posterolateral and posterior.

Figure 4

Normalized internal disc average displacement vectors for anterior shear (Tx), left lateral shear (Ty), compression (Tz), right lateral bending (Rx), flexion (Ry) and left axial rotation (Rz). Note: Displacements from symmetrically opposite motions for lateral shear, lateral bending and axial rotation values were pooled, based on presumed sagittal plane symmetry. Translation displacements are magnified 6x, and rotation displacements are magnified 30x for clarity.

Figure 5

Contour plots showing the normalized maximum shear strain (%/mm) for translation tests: a) compression, b) anterior shear, c) posterior shear and d) lateral shear. Note: Lateral shear values include pooled values for left and right shear, all presented as if for left shear, based on presumed symmetry.

Figure 6

Contour plots showing the normalized maximum shear strain (%/°) for rotation tests: a) flexion, b) extension, c) axial rotation and d) lateral bending. Note: values for axial rotation and lateral bending include pooled values for both complementary rotation tests, all presented as if for left axial rotation and right lateral bending respectively, based on presumed symmetry.

Figure 7

Mean (95% confidence interval) regional normalized maximum shear strain values for translation tests: axial compression, anterior/posterior shear and lateral shear. Note: Lateral shear values include pooled values for left and right shear, all presented as if for left shear, based on presumed symmetry. Bar indicates regions with the largest regional maximum shear strain that are not significantly different.

Figure 8

Mean (95% confidence interval) regional normalized maximum shear strain values for rotation tests: flexion, extension, axial rotation and lateral bending. Note: values for axial rotation and lateral bending include pooled values for both complementary rotation tests, all presented as if for left axial rotation and right lateral bending respectively, based on presumed symmetry. Bar indicates regions with the largest regional maximum shear strain that are not significantly different.