Using Skin-Mounted Microphones for Reconstructing in Vivo Interskeletal Forces: A Feasibility Study
This study investigates the feasibility of using a non-invasive external measurement technique utilizing microphones on the surface of human skin to enable characterization of forces transferred through bone structures during natural motion. While the measurement of forces within the skeleton during human movement is of great interest to researchers and clinicians alike, the requirement for non-invasive sensors does not allow for direct measurement of these forces. The conventional inverse dynamical method of determining internal bone-on-bone forces in biomechanics can be limiting due to the necessity of performing measurements in a laboratory environment and the reliance on a link-segment model, which tends to propagate error. The novel method investigated in this study involves the measurement of pressure waves that propagate through human soft tissue during dynamic loading of the skeletal frame. This research uses a simplified anatomical test specimen consisting of a hollow aluminum bar cast in ballistic gelatin, representing a femur and the surrounding soft tissues, to experimentally examine the feasibility of this new measurement technique. In these tests, an impact force representing interskeletal forces is applied to the bar with an impact hammer and surface-mounted electret microphones measure the resulting pressure waves transferred to the surface of the ballistic gel. Feasibility of the measurement technique was determined by applying least squares regression fits to measured acoustic autospectral data treated as a function of impact force characteristics in the time-domain. The acoustic autospectral amplitudes and energy in frequency bands were found to be highly correlated with both the peak impact force and impulse. Ultimately, results show that the measurement technique is feasible, thus providing a motivation for the development of more advanced inverse methods utilizing this measurement technique.