The In Vitro and Vivo Validation of a New Ultrasound Method to Quantify Thoracolumbar Fascia Deformation

<b>Background:</b> A new method for quantifying thoracolumbar fascia deformation (TLFD) and its shear capacity has been introduced, and its reliability for discriminating patients with low back pain (LBP) from healthy controls has been demonstrated in a recent paper. The aim of this study was to investigate the method in terms of criterion validity. <b>Methods:</b> First, the concurrent validity of the TLFD ultrasound measurement method (TLFD_US) was tested in vitro, using a custom-made tissue sliding device that mimics tissue shearing and generates ground truth data. Second, ultrasound images and videos of TLFD were acquired from 10 acute LBP patients and 10 healthy controls by a blinded assessor. In vivo, the concurrent validity of TLFD_US and speckle tracking analysis was then tested. Third, the contribution of the surrounding tissue layers of the erector spinae muscle and dermis to TLFD was calculated using multiple linear regression. <b>Results:</b> The in vitro concurrent validity between TLFD_US and ground truth was excellent (ICC = 0.99; <i>p</i> < 0.001). In vivo, the concurrent validity between TLFD_US and speckle tracking analysis was large (r = 0.701; <i>p</i> < 0.001). Multiple linear regression revealed a large effect regarding the relationship between dermis shear and TLFD (R² = 0.353; <i>p</i> = 0.01). <b>Conclusions:</b> TLFD_US showed excellent criterion validity. Its suitability for capturing morphological parameters of the thoracolumbar fascia is further reinforced.