Quantifying mechanical and morphological properties of plantar foot soft tissues: a systematic review of techniques, methods and their clinimetric properties

Plantar foot soft tissues play a crucial role in absorbing and distributing mechanical stresses during weight-bearing activities. Accurate quantification of their mechanical and morphological properties is essential for understanding load distribution, tissue integrity, and managing conditions like diabetic foot disease. However, the best technique to investigate these properties remains unknown. This systematic review evaluates the clinimetric attributes (i.e. reliability, validity, and repeatability) of existing techniques to identify those most suitable for research and clinical practice. Following PRISMA guidelines, we systematically searched MEDLINE, EMBASE and Web of Science for studies evaluating the mechanical or morphological properties of plantar foot soft tissues. Studies reporting reliability, validity, and/or repeatability of used techniques were evaluated and clinimetric outcomes were interpreted using published cut-off values. Finally, the methodological quality of studies was assessed using the COSMIN Risk of Bias tool. Of 4115 screened studies, 37 were included. Nine techniques to assess mechanical properties (e.g. elastography- and indentation-based techniques, durometer, optical coherence tomography) and six to assess morphological properties (e.g. CT, MRI, ultrasound) were identified. Reliability (29/37 studies) and repeatability (23/37) were frequently reported, while validity assessments were limited (4/37) likely due to the methodological complexity of in-vivo validity assessment, including the absence of an established reference standard for plantar soft tissue mechanics. Inconsistent statistical approaches (26/37) and doubtful blinding (26/37) were the most reported methodological limitations across studies. Only Shear Wave Elastography (SWE) and ultrasound had reliability and validity comprehensively investigated. Ultrasound demonstrated good-to-excellent reliability (ICC = 0.70–0.99) and moderate-to-strong validity (r = 0.5–1.0) using MRI and radiography as reference for measurement of morphological properties. Despite excellent intra-rater reliability (ICC > 0.90) and strong validity (R2 = 0.91) to assess mechanical properties, SWE exhibited systematic measurement bias, likely related to tissue anisotropy, probe pressure, and boundary condition effects. For both techniques, operator-dependent factors (i.e. probe positioning, pressure, inclination, and quantity of coupling gel applied) might influence measurement performance. SWE and ultrasound are currently supported by the most consistent and comprehensive clinimetric evidence among available techniques and emerged as most suitable techniques to quantify the mechanical and morphological properties of plantar foot soft tissues. Nevertheless, heterogeneity in study design, test conditions, and analysis methods limits inter-study and inter-technique comparisons, and highlight the need for more research into clinimetric properties, and into development of more robust and/or valid techniques. Standardized measurement protocols (e.g. probe positioning and pressure, coupling medium, foot positioning) and statistical reporting of clinimetric properties would substantially improve comparability across future studies with potential for future integration into quantitative monitoring approaches for plantar tissue health.