Experimental and numerical analysis of soft tissue stiffness measurement using manual indentation device - significance of indentation geometry and soft tissue thickness

Background: Indentation techniques haves been applied to measure stiffness of human soft tissues. Tissue properties and geometry of the indentation instrument control the measured response. Methods: Mechanical roles of different soft tissues were characterized to understand the performance of the indentation instrument. An optimal instrument design was investigated. Experimental indentations in forearm of human subjects (N = 11) were conducted. Based on peripheral quantitative computed tomography imaging, a finite element (FE) model for indentation was created. The model response was matched with the experimental data. Results: Optimized values for the elastic modulus of skin and adipose tissue were 130.2 and 2.5 kPa, respectively. The simulated indentation response was 3.9 +/- 1.2 (mean +/- SD) and 4.9 +/- 2.0 times more sensitive to changes in the elastic modulus of the skin than to changes in the elastic modulus of adipose tissue and muscle, respectively. Skin thickness affected sensitivity of the instrument to detect changes in stiffness of the underlying tissues. Conclusion: Finite element modeling provides a feasible method to quantitatively evaluate the geometrical aspects and the sensitivity of an indentation measurement device. Systematically, the skin predominantly controlled the indentation response regardless of the indenter geometry or variations in the volume of different soft tissues.