The objectives of this research are: (1) to develop advanced nonlinear, porohyperviscoelastic models accurately characterizing the biomechanical response of human soft tissues to externally applied loads; and (2) to develop enhanced, nonlinear, 3-D finite element computational software, capable of inputting and using the resulting biomechanical models, in "anatomically" meshed FE models of the respective subjects' torsos, and/or (residual) limbs to precisely compute the stresses and strains generated in the subjects' tissues as a function of the respective device designs and the loading applied.
The modeling methods, measurement instrumentation, and FE computational analysis methods and software developed in this project will be directly useable in numerous applications involving soft tissue load bearing. As such, the devices and procedures developed will contribute to the advancement of the science of tissue biomechanics, and lead to significant improvements in the quality of the fit, comfort, and function attained in load bearing devices designed and manufactured using the techniques developed.

I. Tissue Characterization
II. MRI images for knee geometry
III. 3D FEM modeling
IV. Comparison of FEM and experimental results

I. Characterization of Residual Limb Tissue

Measurement of trans-tibial amputee residual limb tissue mechanical properties with the our servo controlled force/position feedback indentor.

Amputee residual limb (popliteal) tissue creep response to ten sequentially applied step force loads of increasing magnitude measured over 160 sec. To characterize bulk residual limb tissue steady state response, the compressive, creep response steady state measurements for each subject were fit, under a weighted least mean square error criterion, to 2nd order Ogden elastomeric material models. To characterize bulk residual limb tissue transient response, the initial, transient portion of each subject's creep response measurements were fit under a weighted least mean square error criterion to a third order weighted exponential model. back to top

II MRI images for geometric properties
MRI was used to obtain the geometries of the bones and soft tissues

BK MRI images while wearing prosthetic socket

 back to top

III. 3D FEM modeling

Finite element (FE) models were generated from the digitized, integrated optical and magnetic resonance images of the subjects' limbs. 4 node membrane elements were used to model the limb integument, 8 node hexahedral elements to model the underlying bulk soft tissues. back to top

IV. Comparison of FEM and experimental results

To determine the prosthetics loads typically applied to trans-tibial amputees' residual limbs, three representative subjects (one with conventionally classified "firm", one with "average", and one with "soft" durometer tissues) were selected, and their habituated, (reasonably) "well-fitting" prostheses were instrumented with two VA-Tekscan 1360 element P-Scan transducers and 960 element F-Scan transducers.

Pressure Map from instrumented socket.

The resulting FE models were then validated by comparing FEA predicted limb tissue displacements and limb surface stresses with actual MRI measured tissue displacements and P-Scan measured interface loads for each subject in his/her respective habituated socket/prosthesis with specified loads applied, as shown.

back to top