Reduced thoracolumbar fascia shear strain in human chronic low back pain

Abstract

Background: The role played by the thoracolumbar fascia in chronic low back pain (LBP) is poorly understood. The thoracolumbar fascia is composed of dense connective tissue layers separated by layers of loose connective tissue that normally allow the dense layers to glide past one another during trunk motion. The goal of this study was to quantify shear plane motion within the thoracolumbar fascia using ultrasound elasticity imaging in human subjects with and without chronic low back pain (LBP).

Methods: We tested 121 human subjects, 50 without LBP and 71 with LBP of greater than 12 months duration. In each subject, an ultrasound cine-recording was acquired on the right and left sides of the back during passive trunk flexion using a motorized articulated table with the hinge point of the table at L4-5 and the ultrasound probe located longitudinally 2 cm lateral to the midline at the level of the L2-3 interspace. Tissue displacement within the thoracolumbar fascia was calculated using cross correlation techniques and shear strain was derived from this displacement data. Additional measures included standard range of motion and physical performance evaluations as well as ultrasound measurement of perimuscular connective tissue thickness and echogenicity.

Results: Thoracolumbar fascia shear strain was reduced in the LBP group compared with the No-LBP group (56.4% ± 3.1% vs. 70.2% ± 3.6% respectively, p < .01). There was no evidence that this difference was sex-specific (group by sex interaction p = .09), although overall, males had significantly lower shear strain than females (p = .02). Significant correlations were found in male subjects between thoracolumbar fascia shear strain and the following variables: perimuscular connective tissue thickness (r = -0.45, p <.001), echogenicity (r = -0.28, p < .05), trunk flexion range of motion (r = 0.36, p < .01), trunk extension range of motion (r = 0.41, p < .01), repeated forward bend task duration (r = -0.54, p < .0001) and repeated sit-to-stand task duration (r = -0.45, p < .001).

Conclusion: Thoracolumbar fascia shear strain was ~20% lower in human subjects with chronic low back pain. This reduction of shear plane motion may be due to abnormal trunk movement patterns and/or intrinsic connective tissue pathology. There appears to be some sex-related differences in thoracolumbar fascia shear strain that may also play a role in altered connective tissue function.

Figure 1

Ultrasound image acquisition method. A: Motorized articulated table capable of moving in the sagittal plane 15° at a rate of 0.5 Hz. The subject is positioned prone on the table with the hinge point at the L4-5 level. B: Location of ultrasound transducer (posterior view).

Figure 2

Ultrasound elasticity imaging method. White box indicates the region of interest (ROI) within the ultrasound image that was processed using cross correlation analyses. Arrows indicate reference axes within the ultrasound image: axial and lateral axes indicate directions parallel and perpendicular to the ultrasound beam respectively, in the plane of the ultrasound image. Elevational axis indicates direction perpendicular to the ultrasound image plane. Axial, lateral and elevational directions in the ultrasound image correspond to antero-posterior, rostro-caudal and medio-lateral anatomical directions respectively.

Figure 3

Ultrasound imaging of thoracolumbar fascia. A: Illustration of layers composing the thoracolumbar fascia corresponding to aponeuroses of back and abdominal wall muscles. Arrows indicate directions of pull for individual muscles. B-C: ultrasound image of thoracolumbar fascia in longitudinal (B) and transverse (C) planes showing echogenic (dense connective tissue) and echolucent (loose connective tissue) layers within the thoracolumbar fascia. A distinct echolucent plane (red line) is visible within the thoracolumbar fascia in the longitudinal image corresponding to the loose connective tissue layer located between the aponeurosis of the erector spinae muscles and the combined aponeuroses of the abdominal wall muscles, serratus posterior and latissimus dorsi.

Figure 4

Ultrasound data processing method. A: Location of sub-ROIs (yellow and orange boxes) used for quantification of lateral tissue motion. B: Plot of lateral tissue displacement over time. Positive displacement in B corresponds to tissue movement toward the right (rostral, red arrows in A). Negative displacement in B corresponds to tissue movement toward the left (caudal, blue arrows in A). Yellow and orange lines in B respectively correspond to deep and superficial sub-ROIs in A. C: Shear strain model and calculation method. P1 and P2 represent the mean tissue displacement in the deep (yellow) and superficial (orange) Sub-ROIs respectively at each time point as shown in B. Shear strain between the sub-ROIs was calculated as the absolute difference in lateral motion between the superficial and deep sub-ROIs divided by the distance between the centers of the two sub-ROIs (2 mm) and expressed as a percentage.

Figure 5

Cumulative lateral tissue displacement and shear strain maps. A: B-scan ultrasound image ROI. B: Sum of tissue displacement over time (cumulative displacement) during one flexion cycle of the table within the ultrasound image ROI. Red indicates tissue displacement toward the right (rostral) and blue indicates tissue displacement toward the left (caudal). C: Cumulative shear strain within the ultrasound image ROI. Red and blue indicate positive (toward the right) and negative (toward the left) shear strain respectively. (B) and (C) respectively correspond to cumulative tissue displacement and shear strain at the end of one flexion cycle of the motorized table. D: Diagram illustrating positive and negative shear strains which represent sliding or deformation of an object in different directions. The shear component is obtained by taking the gradient of lateral displacement (Ux) along the positive axial direction (+y). The x-y coordinates are defined corresponding to the ultrasound imaging configuration (see axes in Figure 2).

Figure 6

Thoracolumbar shear strain in human subjects with and without LBP. Thoracolumbar shear strain was ~20% lower in human subjects with chronic LBP compared with No-LBP. *indicates p < .01. N = 121 subjects. Error bars represent standard errors.