Validation and reliability of mechanical stiffness assessment tools in multilayered polyurethane phantom models of healthy and diabetic plantar soft tissues

Plantar soft tissue stiffness plays a crucial role in the development of diabetic foot complications, but in vivo assessments are constrained by anatomical variability and probe-induced measurement artifacts. To develop anatomically layered polyurethane phantom models mimicking healthy and diabetic plantar soft tissues and evaluate the reliability and concurrent validity of four mechanical stiffness assessment tools, [MyotonPRO, Shore Durometer, IndentoPRO, and Tissue Compliance Meter (TCM)] against Shear Wave Elastography (SWE). Six regional phantom models (calcaneus, midfoot, forefoot) with skin, fat pad, fascia, and muscle layers were fabricated. SWE was performed in no-contact mode to eliminate surface compression, improving measurement consistency. A total of 162 configurations were tested under blinded and randomized conditions. Intra- and inter-rater reliability was assessed using ICCs; concurrent validity was evaluated via correlation and regression analyses with SWE. All devices demonstrated excellent reliability (ICC range: 0.88-0.99). SWE-derived stiffness was significantly higher in diabetic models, especially in the calcaneal and midfoot regions (p < 0.001). IndentoPRO showed the highest correlation with SWE (r = 0.91), followed by MyotonPRO (r = 0.87), TCM (r = 0.85), and Durometer (r = 0.78). TCM exhibited the highest predictive value (R2 = 0.502) and most consistent performance across diabetic regions. The developed phantom models offer a standardized platform for evaluating stiffness assessment tools. While SWE remains the reference standard, mechanical devices, particularly TCM and IndentoPRO, demonstrated valid and reproducible performance. This phantom-based approach holds promise for supporting medical device development, regulatory validation, and preclinical testing in diabetic foot biomechanics.