Aponeurosis

Detailed descriptions of the complex anatomy of lumbar paravertebral muscles and definitions regarding the spatial distribution of MFI on axial MRI are limited [37 –40 (link)]. Published images demonstrating investigators’ definition of ROI for these muscles predominantly depict the lower lumbar levels, with limited identification of separate muscles. Further, descriptions lack details towards acknowledging the complex three-dimensional structure that produces a changing spatial relationship observed across lumbar segmental levels. The lumbar paravertebral muscles typically examined in such studies include: multifidus (MF) as the largest lumbar spinotransverse muscles; erector spinae (ES) including lumbar longissimus and iliocostalis; and less frequently, psoas (including major and minor), and quadratus lumborum (see Fig. 1). This paper intentionally focuses on MF and ES as these are presumed to have the greatest clinical significance. However, other paravertebral muscles exist in the lumbar spine (e.g. the lumbar interspinales and intertransversarii, and thoracic semispinalis), yet they are generally not mentioned in descriptive investigations. This may relate to a lack of image resolution with available sequences, making it challenging to accurately delineate individual muscles from adjacent structures, and it therefore remains unclear how they should be treated when defining ROIs.Fig. 1

Axial E12 plastinated sections (a, c) and schematic illustrations (b, d) at approximately L1 (a, b) and L4 (c, d) highlighting anatomical structures at these vertebral levels. b, d Dotted lines and shading, Green - psoas major muscle; Blue – quadratus lumborum muscle; Purple – erector spinae muscles; Red – spinotransverse muscles. b round white dotted regions (bilateral) denote 12th rib. d square dotted box surrounds enlarged inset; round dotted circle indicates morphological feature of interest (ILB fatty ‘tent’). Legend: A – aorta; ES – erector spinae muscles; ESA – erector spinae aponeurosis; ILB – iliocostalis – longissimus boundary and indentation; ISL – interspinous ligament; IT – intertransversarii muscle; IVC – inferior vena cava; K – kidney; L – liver; P – psoas major muscle; QL – quadratus lumborum muscle; SAF – superior articular facet; SP – spinous process; SPC – spinal canal; SPT – spinotransverse muscle group; ZJ – zygapophysial joint

Our proposed method outlined in the results section, provides a foundational solution for the problem of how to measure muscles traversing the lumbar spine, and includes suggestions on operational characteristics for acquiring MR images. While we offer this starting point for a common methodology to facilitate accurate definition of lumbar muscle ROI, we are cognisant that the method is not a definitive end-point on ‘how to’. We hope that with time and new research findings these methods will be modified, expanded, and refined.

VL MT was assessed by the same investigator from images obtained in vivo at rest using B‐mode ultrasonography (Mylab 25; Esaote Biomedica, Genova, Italy), with a 50 mm, 7.5 MHz, linear‐array probe. MT has previously been assessed by placing the ultrasound probe transversally in relation to the limb and evaluated as the perpendicular distance between the skeletal muscle interfaces.20 Longitudinal ultrasound scans (ie, with the probe aligned with the fascicle plane) have also been used to detect changes in muscle size and growth as well as skeletal muscle architecture.21, 22, 23In this study, resting ultrasound images were taken at 50% of femur length, applying the same reference point used for the MRI scanning. The participant was resting supine on an examination bed with the knee in full extension (ie, anatomical zero).24 The transducer was placed longitudinally to the thigh along the mid‐sagittal axis of the VL, and carefully aligned to the fascicle plane to clearly visualize fascicles on the ultrasound screen. The experienced operator was careful in applying as little pressure as possible when placing the probe on the skin. Three images were acquired and stored for offline analysis. VL MT was measured as the distance between superficial and deep aponeuroses, in the proximal, central, and distal portions of the acquired image22, 23 (Figure 2), using the image analysis software ImageJ 1.42q (National Institutes of Health, USA). The mean of the three measures was calculated for statistical analysis.
The reliability of this ultrasound technique has been previously validated by cadaver anatomical inspection.25 Moreover, previous studies assessed the reliability of in vivo measurements of fascicle length26 and pennation angle.27 In this study, the interday reliability of MT was also assessed. All subjects were tested on two different days before the start of the training period. Volunteers were tested at the same hour of the day, and a permanent marker was used to trace the ultrasound probe contours in order to ensure that MT was assessed at the same VL site on both days. All images were collected and digitally analyzed by the same operator.

The paw diameter of each animal was measured before any treatment using a caliper, representing the baseline value (0 h); then, each animal received orally (1 mL/100 g, b.w.), using a gavage probe, the corresponding treatment. One hour after this treatment, each rat received (exception for the animals in the neutral control group) an injection of 2.5% formalin under the plantar aponeurosis of the left hind paw using a 1 mL syringe, then the diameter of the paw was measured 1 h, 2 h, 4 h, 8 h, and 24 h after formalin injection [41 (link)]. The extent of edema was assessed by determining the percentage increase in the volume of the sole of the paw [4 (link)]. $\begin{array}{c}\mathrm{I}\mathrm{n}\mathrm{c}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{s}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{P}\mathrm{a}\mathrm{w} \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\left(\mathrm{I}\mathrm{P}\mathrm{V}\right)=\frac{\mathrm{P}\mathrm{a}\mathrm{w} \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{T}-\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{P}\mathrm{a}\mathrm{w} \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{P}\mathrm{a}\mathrm{w}\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}}\mathrm{X}100\text{,}\\ \mathrm{P}\mathrm{e}\mathrm{r}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{I}\mathrm{n}\mathrm{h}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{\left(\mathrm{I}\mathrm{P}\mathrm{V}\right)\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}–\left(\mathrm{I}\mathrm{P}\mathrm{V}\right)\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}}{\left(\mathrm{I}\mathrm{P}\mathrm{V}\right)\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}\mathrm{X}100\text{.}\end{array}$

All sonograms were analysed off-line with Image J version 1.52 software (National Institute of Health, Bethesda, MD, USA). Images were first calibrated to the known field of view (10-cm), then for each image a fascicle of interest was identified. Finally, muscle thickness, pennation angle, observed FL and distance between fascicle end-point and super-fascial aponeurosis were measured three times within each image, to enable complete FL estimation using a previously established reliable linear equation [36 (link)].