Histology of the fascial planes: a systematic review of the microstructural foundations of regional anesthesia

Background: Fascia, once viewed as a passive structural tissue, is now recognized as a biologically active interface integral to musculoskeletal stability, force transmission, proprioception, and nociception. Despite the increasing clinical use of fascial plane blocks, the microanatomy of fasciae relevant to regional anesthesia remains poorly characterized. Understanding their histological features-including innervation, vascularization, and microstructure-is critical to optimizing anesthetic efficacy and elucidating mechanisms of pain and tissue response. Methods: This systematic review was prospectively registered on the Open Science Framework (reference: yk4ua, 19 September 2025) and reported according to PRISMA guidelines. MEDLINE, Embase, and Cochrane CENTRAL were searched from inception to September 2025 without language or date restrictions. Eligible studies included histological or microanatomical investigations of human fascial planes relevant to regional anesthesia (e.g., pectoral, thoracolumbar, abdominal, and fascia lata). Data were extracted independently by multiple reviewers, and study quality was assessed using the Anatomical Quality Assessment (AQUA) tool. Findings were synthesized qualitatively by anatomical region. Results: Seventeen studies met inclusion criteria, encompassing fasciae from the thoracic, abdominal, lumbar, and lower limb regions. Fasciae exhibited considerable structural heterogeneity but shared a multi-layered organization of dense and loose connective tissue laminae rich in type I collagen. The fascia lata and thoracolumbar fascia demonstrated highly ordered collagen fiber orientation, multilaminar organization, and dense innervation, whereas thinner fasciae (e.g., pectoral fascia) showed simpler single-layer structures with fewer neural and vascular elements. Hyaluronic acid content ranged from 29 to 35 µg/g, with fasciacytes identified as the principal secretory cells. Nerve fibers-often associated with vessels and collagen bundles-were consistently present across all deep fasciae, with regional variations in density and mechanoreceptor type. Pathological changes, such as thickening, increased vascularization, and inflammatory infiltration, were reported in chronic pain states. Conclusions: The fascia should be viewed as a dynamic, active tissue network rather than a passive sheath. Methodological limitations-including small sample sizes, regional heterogeneity, and histological artifacts-restrict current understanding. Future multimodal studies integrating histology, imaging, and biomechanics are warranted to clarify how fascial microstructure affects anesthetic diffusion, pain modulation, and postoperative recovery.