<scp>2D</scp> and <scp>3D</scp> microstructural analysis of the iliotibial tract

The fascial system has gained recognition for its integral role in connecting skin, superficial and deep fasciae, and underlying muscles. However, consensus on its microstructure depending on its topography remains elusive as well as its implications in clinical practices, such as reconstructive surgery and physiotherapy techniques. This study focuses on the iliotibial tract (ITT) implicated in the iliotibial band syndrome. The goal is to describe microstructural characteristics using classical 2D histology and cryogenic contrast-enhanced microcomputed tomography (cryo-CECT) such as the total thickness, number of layers, layer thickness, fibre orientation and tortuosity, according to the specific topography. The total thickness of the ITT varied across topographic regions, with the superior part being on average thicker but non-significantly different from the other regions. The inferior part showed heterogeneity, with the anterior region (AI) being the thinnest and the posterior one (PI) the thickest. The ITT exhibited 1-3 layers, with no significant differences among regions. Most commonly, it consisted of two layers, except for the antero-superior (AS) and antero-middle (AM) regions, which sometimes had only one layer. The posterior regions frequently had 2 or 3 layers, with the PI region having the highest mean (2.7 layers). The intermediate layer was the thickest one, varying from the AI region (0.368 mm ± 0.114) to the PI region (0.640 mm ± 0.305). The superficial layer showed regional variability, with the AS region being the thinnest. The deep layer appeared thinner than the superficial one. Fibre orientation analysis indicated that the intermediate layer mainly consisted of oblique longitudinal fibres, orientated downward and forward, while the superficial and deep layers had transversal or oblique transversal fibres. Cryo-CECT 3D observations confirmed these findings, revealing distinct orientations for different layers. Fibre tortuosity exhibited differences based on orientation. Transversal fibres (>65°) were significantly less tortuous than longitudinal fibres (<25°) and oblique intermediate fibres (25°-65°), aligning with 3D plot observations. This quantitative study highlights various microstructural characteristics of the ITT, offering insights into its regional variations. The analysis accuracy is increased due to the novel technology of cryo-CECT which emerges as a valuable tool for precise assessment of 3D fibre orientation and tortuosity. These findings contribute to a deeper understanding of the ITT structure, useful in clinical practices, such as reconstructive surgery and physiotherapy, and future research endeavours.