This study highlights the interconnectedness of the fascial bands. These bands not only intertwined and connected with other fascial bands at different depths, but they also merged with the epimysium immediately beneath the subcutaneous layer and ascended superficially to intersect with the skin's dermis. As confirmed by multiple 2D images and 3D reconstructed renditions, the SQ fascial bands formed a coherent network of fibers that ensured various layers of the skin structurally and mechanically linked. These finding corroborates what others have reported in the literature. In a cadaveric study of the extremities, trunk, anterior chest wall, and neck, Abu-Hijleh MF
[3] observed interconnecting and merging fascial membranes within the subcutaneous layer at all locations studied. Johnson D
[7] and Nash
[8] also observed fascial bands radiating toward the skin from deeper subcutaneous/epimysial layers to form adherent junctions with the dermis. These junctions termed “skin ligaments” were identified in the abdomen and the extremities far more frequently than what is commonly perceived as limited to certain locations (i.e. Zygomatic ligament in the cheek and Coopers ligament in the breast)
[7]. This ubiquitous fascial network likely serves to maintain structural cohesiveness between the skin and the underlying muscle and bones. Without the tensile resilience afforded by the collagen fibers and without the structural junctions to serve as anchors, skin would otherwise be prone to disengaging and completely detaching from the body. Importantly, this structural arrangement also ensures that the skin and its structural components are not mechanically isolated. Substantial muscular movement or large skin displacements would generate mechanical strain along the fascial bands that can theoretically span well beyond a focal location and thus affect fibroblast mechanotransduction and even nerve activity at a large spatial scale. Within this framework, the fascial network may conceivably form a body-wide network that not only helps mediate mechanical forces but also cellular and nervous activities as well
[9].
Subcutaneous blood vessels were also extensively integrated in this fascial network. In our ultrasound images at all three locations, SQ fascial bands were found to compartmentalize and encase the blood vessels (). At the spatial resolution of our ultrasound, the vessel wall appeared inseparable from the fascial bands, particularly at the most superficial and deep aspect of the vessel. Past authors have equated this structural arrangement with the “Egyptian eye” and reported such morphologies at both the saphenous vein
[10] and the cephalic vein
[11]. For both these veins, the blood vessels were bounded by subcutaneous fascial bands superficially and by perimuscular fascia at a deeper level. Our images have identified similar arrangements, although subcutaneous vessels were at times bound by subcutaneous fascial bands on the deeper aspect of the vessel as well. From a clinical perspective, these fascial bands help prevent excessive dilatation of the veins and account for why veins constrained by fascia are less prone to develop varicosities relative to tributary veins without fascia sheathing
[12]. Given the fact that fascial bands are contiguous with both skin and muscles, they may transmit mechanical forces arising from muscle contraction or from skin shear movement to the vessel wall and thus help modify blood flow in a manner that is beneficial to the affected limb. Moreover, the variable structural arrangements between the vein and fascial bands seen in may be strategically formed in such a way that only specific segments of the vein are affected by mechanical stresses.
Quantitative analyses of the ultrasound images revealed that the total SQ fascial band area was greatest at the calf and thigh and smallest at the arm. This difference may be attributed to the greater mechanical forces typically generated at the lower extremities. Daily ambulation and the need to chronically sustain the weight of the body are two conditions that likely facilitate a sustained increase in fascial band content. Future studies may consider evaluating how daily activities or weight training would affect the total fascial content in the subcutaneous layer, and whether greater total fascial band area would be identified in more active and possibly heavier individuals.
According to our quantitative analysis, the total fascial band area in the calf was statistically similar to that of the thigh, despite the thigh's thicker SQ zones (greater SQ fat layer). This decreased fascial band densities at the thigh was manifested by a comparatively greater number of fascial bands but an overall reduced fascial band thickness and was revealed by the fragmented and curved appearance of echogenic bands in panel T3 in contrast to fewer yet thicker bands seen in the calf images (, Panel C1–C3). This inverse relationship between the average SQ fascial band thickness and SQ zone thickness was observed across body sites (including the arm) and across individuals as well (p<0.0001, ). Interestingly, a statistically significant correlation between band number and SQ zone thickness was not observed when comparisons were made across individuals (p
=
0.2543, ). Why these patterns exist remains unclear, but one may speculate that thinner SQ layers would sustain greater amounts of mechanical (both shear and lateral) forces per volume and thus require thicker and more cohesive fascial bands. A thicker SQ layer, on the other hand, should dissipate such forces over its full depth and thus may not require the thicker, cohesive collagenous bands. The fact that band thickness was significantly correlated with SQ zone thickness and not total band number suggests that the SQ tissue preferentially alters the band's
thickness rather than to change the
number of bands to respond to these hypothetical mechanical forces. This, however, cannot be conclusively established without more microscopic imaging techniques, larger sample size, or more prospective data (e.g. temporal response to changes in mechanical force).
Paradoxically, in our study, increased average fascial band number was associated with an increase in fascial band thickness (). A reasonable functional explanation for this finding cannot be given, although it may reflect regional differences. The thigh, in particular, appears to have increased band thickness as the band number is increased while the other two regions did not.
To investigate the possible functional significance of the SQ fascial bands, we investigated the anisotropy of the SQ zone using a spatial coherence measure. In material science, anisotropy typically indicates a directional preference and implies the ability of a material to handle mechanical forces along a specific axis. In biological tissue, anisotropy exists either to facilitate a functional role or to adapt to persistent mechanical forces. Muscles, for instance, are characterized by high anisotropy ensuring that it is optimized for generating tensile force along a specific axis; bones are usually able to withstand greater tensile forces along the longitudinal axis compared to the transverse direction. Collagenous fibers, similarly, possess anisotropy and respond to mechanical force by aligning along the primary direction of tension. In a recent in vitro experiment involving collagen and fibroblasts, strong collagen fiber alignment and densification occurred in response to applied strains greater than 5%
[13]. Fiber alignment was permanently imprinted when the material was cyclically stretched up to strains of 15%. Our in vivo analysis of SQ zone coherence suggests that, SQ anisotropy is significantly associated with SQ zone thickness and body site. Reduced SQ zone thickness was associated with increased SQ anisotropy and implies that mechanical stress is distributed across the subcutaneous tissue. Thinner subcutaneous tissues are more likely to concentrate the mechanical forces and thus are associated with increased spatial anisotropy. This multivariable analysis also reveals that the calf is associated with greater anisotropies and thus theoretically more likely to experience mechanical stress than the arm or thigh.
This study has a number of limitations. First, it involved a limited number of patients, evaluated three specific body locations, and did not have sufficient power to evaluate the effects of gender, age or ethnicity. Future studies may consider evaluating other body segments and other locations within the extremity of a larger, more diverse study cohort. Second, the study relied on static ultrasound images for characterizing the SQ fascial system. Functional, dynamic measures were not obtained. Third, spatial anisotropy is only a surrogate marker of mechanical force and cannot be interpreted as a direct measure of stress. Finally, the ultrasound device itself has technical limitations. Ultrasonography relies on heterogenic impedance of acoustic waves traveling axially from skin to deeper layers. Vertical structures poorly reflect acoustic waves traveling in an axial (vertical) trajectory and thus are not well characterized on the ultrasound images. This will generate an intrinsic anisotropy within the image that should be considered prior to interpreting the data.
Despite these limitations, this study has a number of advantages that adds to the existing, albeit limited, SQ fascia literature. In vivo ultrasonography was used to quantify and characterize the SQ fascial bands – a method that is more likely to be valid to real-life physiology than cadaveric-dissection studies. In addition, novel techniques incorporating principal component analysis, three dimensional reconstruction, and spatial coherence measures were incorporated in this study to better reveal the morphological features of the SQ fascial bands. These methods and techniques collectively have revealed that SQ Fascia is an interconnected network that likely transmits mechanical forces over large spatial scales and across various tissues. The exact functional significance of this behavior remains unclear and awaits additional study.
