Nucleus pulposus deformation in response to rotation at L1-2 and L4-5

In vivo intervertebral disc deformation: intratissue strain patterns within adjacent discs during flexion-extension

The biomechanical function of the intervertebral disc (IVD) is a critical indicator of tissue health and pathology. The mechanical responses (displacements, strain) of the IVD to physiologic movement can be spatially complex and depend on tissue architecture, consisting of distinct compositional regions and integrity; however, IVD biomechanics are predominately uncharacterized in vivo. Here, we measured voxel-level displacement and strain patterns in adjacent IVDs in vivo by coupling magnetic resonance imaging (MRI) with cyclic motion of the cervical spine. Across adjacent disc segments, cervical flexion-extension of 10° resulted in first principal and maximum shear strains approaching 10%. Intratissue spatial analysis of the cervical IVDs, not possible with conventional techniques, revealed elevated maximum shear strains located in the posterior disc (nucleus pulposus) regions. IVD structure, based on relaxometric patterns of T₂ and T_1ρ images, did not correlate spatially with functional metrics of strain. Our approach enables a comprehensive IVD biomechanical analysis of voxel-level, intratissue strain patterns in adjacent discs in vivo, which are largely independent of MRI relaxometry. The spatial mapping of IVD biomechanics in vivo provides a functional assessment of adjacent IVDs in subjects, and provides foundational biomarkers for elastography, differentiation of disease state, and evaluation of treatment efficacy.

Lumbar spine angles and intervertebral disc characteristics with end-range positions in three planes of motion in healthy people using upright MRI

Understanding changes in lumbar spine (LS) angles and intervertebral disc (IVD) behavior in end-range positions in healthy subjects can provide a basis for developing more specific LS models and comparing people with spine pathology. The purposes of this study are to quantify 3D LS angles and changes in IVD characteristics with end-range positions in 3 planes of motion using upright MRI in healthy people, and to determine which intervertebral segments contribute most in each plane of movement. Thirteen people (average age = 24.4 years, range 18-51 years; 9 females; BMI = 22.4 ± 1.8 kg/m²) with no history of low back pain were scanned in an upright MRI in standing, sitting flexion, sitting axial rotation (left, right), prone on elbows, prone extension, and standing lateral bending (left, right). Global and local intervertebral LS angles were measured. Anterior-posterior length of the IVD and location of the nucleus pulposus was measured. For the sagittal plane, lower LS segments contribute most to change in position, and the location of the nucleus pulposus migrated from a more posterior position in sitting flexion to a more anterior position in end-range extension. For lateral bending, the upper LS contributes most to end-range positions. Small degrees of intervertebral rotation (1-2°) across all levels were observed for axial plane positions. There were no systematic changes in IVD characteristics for axial or coronal plane positions.

A quantitative assessment of the mechanical effects on the lumbar spine and the effects on straight leg raising and lumbar flexion of segmental sustained rotation

[Purpose] This study were to examine the strength and relative direction of the applied force from lumbar segmental sustained rotation (LSSR) on the lumbar spinous process, and to clarify the effects of LSSR on straight leg raising (SLR) and lumbar flexion (LF). [Subjects] 18 pain-free healthy adults volunteered for this study. [Methods] Applied force and direction were measured between the L5–S1 segments using tri-axial pressure sensors. Subjects participated in 3 trials. Subjects underwent localized right rotation, held for 10 seconds, of the L5 in relation to the S1. Sham group subjects followed LSSR group protocols; however L5–S1 rotation was absent. Control subjects rested on a plinth. SLR and LF were measured pre and post-trial. [Results] Outcome data for LSSR forces were as follows; x (0.06N (±0.29)), y (‒5.26N (±0.01)), z (6.16N (±1.33)), and resultant vector magnitude (8.19N (±1.12)). LSSR relative direction results were as follows: x-axis angle, 89. 6 ° (±1.5); y-axis, 130.9 ° (±5.6); and z-axis, 41.6 ° (±4.7). The LSSR group’s LF and SLR were significantly increased compared with those of the sham and control groups. [Conclusion] The identified resultant vector magnitude was 8.19N, less than other techniques. LSSR effectively improves LF and bilateral SLR.

Comparable effect of simulated side bending and side gliding positions on the direction and magnitude of lumbar disc hydration shift: in vivo MRI mechanistic study

OBJECTIVES

To investigate the direction and magnitude of mechanical influence to the lumbar disc in side bending and side gliding positions by considering shift of disc hydration.

METHODS

Twenty asymptomatic subjects completed this study. Direction of the hydration shift (θ), magnitude of the shift, and segmental lateral flexion and rotation angles from L1/L2 to L5/S1 during left side bend and side glide in lying were measured by magnetic resonance imaging (MRI) and compared using paired t-tests.

RESULTS

A significant difference (P<0.001) was detected in the segmental lateral flexion angle at L1/L2 between the side bending position (mean [SD], 5.1° [2.2°] left lateral flexion) and the side gliding position (mean [SD], 2.1° [2.7°] left lateral flexion). However, there was neither significant difference (P>0.05) in the lateral flexion angle at other segments nor rotation angles at each segment between the two lumbar positions. There was also no significant difference (P>0.05) in the θ value and magnitude of the hydration shift between the two lumbar positions. The disc hydration generally shifted to the right in the left side bending and side gliding positions at all disc levels.

DISCUSSION

This is the first study to investigate mechanical influence to each lumbar disc in the side gliding position using the shift of disc hydration on axial MRI. The comparability in the direction and magnitude of the hydration shift in the side bending and side gliding positions indicates that the maneuver of side gliding can produce comparable ipsilateral mechanical influence to each lumbar disc in comparison to side bending.

In vivo morphological features of human lumbar discs

Recent biomechanics studies have revealed distinct kinematic behavior of different lumbar segments. The mechanisms behind these segment-specific biomechanical features are unknown. This study investigated the in vivo geometric characteristics of human lumbar intervertebral discs. Magnetic resonance images of the lumbar spine of 41 young Chinese individuals were acquired. Disc geometry in the sagittal plane was measured for each subject, including the dimensions of the discs, nucleus pulposus (NP), and annulus fibrosus (AF). Segmental lordosis was also measured using the Cobb method.In general, the disc length increased from upper to lower lumbar levels, except that the L4/5 and L5/S1 discs had similar lengths. The L4/5 NP had a height of 8.6±1.3 mm, which was significantly higher than all other levels (P<0.05). The L5/S1 NP had a length of 21.6±3.1 mm, which was significantly longer than all other levels (P<0.05). At L4/5, the NP occupied 64.0% of the disc length, which was significantly less than the NP of the L5/S1 segment (72.4%) (P<0.05). The anterior AF occupied 20.5% of the L4/5 disc length, which was significantly greater than that of the posterior AF (15.6%) (P<0.05). At the L5/S1 segment, the anterior and posterior AFs were similar in length (14.1% and 13.6% of the disc, respectively). The height to length (H/L) ratio of the L4/5 NP was 0.45±0.06, which was significantly greater than all other segments (P<0.05). There was no correlation between the NP H/L ratio and lordosis. Although the lengths of the lower lumbar discs were similar, the geometry of the AF and NP showed segment-dependent properties. These data may provide insight into the understanding of segment-specific biomechanics in the lower lumbar spine. The data could also provide baseline knowledge for the development of segment-specific surgical treatments of lumbar diseases.