Accuracy and reliability testing of a portable soft tissue indentor

We have designed, built, and tested a portable indentor device that allows us to determine force/displacement (F/D) measurements on soft tissue in a clinical or research setting. The indentor system consists of a load cell mounted on a three-dimensional measurement device (Metrecom). The output of the load cell and the Metrecom are recorded and analyzed by software running on a notebook computer. The displacement calibration of the Metrecom gave an average error = 0.005 mm [standard deviation (SD) = 0.062)]. The force calibration of the load cell resulted in an average error = 0.022 N (SD = 0.049) and a linearity of 1.0062 (R² = 0.9998). The indentor device was tested on six different human soft tissues by two different investigators. The interreliabilities and intrareliabilities were 0.99 [interclass correlation (ICC)] indicating that the results were repeatable by more than one investigator. F/D measurements from indentor testing on two materials were comparable to values measured using an Instron device (5.34 versus. 5.52 N/mm, and 0.98 versus 1.04 N/mm). The device was used to measure the soft tissue characteristics on the plantar surface of the foot of one subject. These data were used to calculate the effective Young's modulus for the tissue using equations derived by Zheng et al. [1] and indicated a wide range of values dependent upon the portion of the F/D curve used. All results indicate data from this portable indentor device are reliable, accurate, and sensitive enough to identify mechanical properties of human tissues.