Systematic methodology for the design of a flexible keel for energy-storing prosthetic feet

Energy storage and stress-strain characteristics of a prosthetic foot: a priori design and analysis with experiments

This work proposes an experimentally validated numerical approach for a systematic a priori evaluation of the energy storage and stress-strain characteristics of a prosthetic foot during the stance phase of walking. Boundary conditions replicating the rocker based inverted pendulum model were incorporated. The mechanically complex Ottobock Solid Ankle Cushioned Heel (SACH) foot was opted as the test device. A non-linear finite element (FE) model of the device was developed inclusive of its sources of non-linearity. A custom rig was fabricated to investigate the prosthetic foot's mechanics in a gait analysis facility and experimentally validate the finite element approach. The numerical and experimental outcomes showed fair agreement, with the centre of pressure at the foot-floor interface deviating by 1.56 mm at the instance of highest strain. The magnitude and the distribution of the energy stored and a series of stress and strain parameters were analyzed for the test device using the proposed approach. The novel methodology proposed may act as an effective tool for the design, analysis, and prescription of energy storage and return prosthetic feet.

Biomechanical design framework for prosthetic feet: Experimentally validated non-linear finite element procedure

Performance evaluation of prosthetic feet during their design is typically performed experimentally, which may be time and cost intensive. This work presents a first-of-its-kind application of a numerical procedure for the a priori determination of various stance phase biomechanical parameters of a prosthetic foot, such as its roll-over characteristics, centre of pressure trajectory, ankle flexion moment arm and ankle range of motion, to aid in its design. The numerical procedure is based on finite element analysis, which includes geometric, material and contact non-linearity. Boundary conditions emulating the rocker-based inverted pendulum model were employed to evaluate the biomechanical parameters. The finite element model was validated by employing an inverted pendulum-based apparatus using the structurally complex Ottobock Solid Ankle Cushioned Heel (SACH) prosthetic foot as the test device. A comparison of the numerical and experimental results showed low magnitude of errors. For example, the percentage error of the radius of curvature of the roll-over shape was ~0.1%. The differences found appear to be clinically insignificant, which substantiates the reliability of the model. The proposed numerical model can be employed to obtain detailed a priori insights into the biomechanical parameters influencing a prosthetic foot's characteristics during gait, which can better inform the design, analysis and prescription of prosthetic feet.

Design of a Biomimetic SACH Foot: An Experimentally Verified Finite Element Approach

The Solid Ankle Cushioned Heel (SACH) foot is a commonly prescribed prosthetic foot for the rehabilitation of lower limb amputees. From the viewpoint of its biomechanical performance, the foot is known to cause drop-off effect and asymmetry in amputee gait. Therefore, the objective of this work is to improvise the effective foot length ratio (EFLR) and the progression of the centre of pressure (CoP) of the SACH foot by providing a novel design approach that utilizes finite element analysis. Boundary conditions employed for evaluating the roll-over characteristics of prosthetic feet were numerically incorporated in this work. The non-linear mechanical behavior of the foot was included with the incorporation of large deformation, a hyperelastic material model and the Augmented Lagrangian contact formulation. Outcomes from the simulations were experimentally verified using an inverted pendulum-like apparatus, thereby substantiating the numerical approach. The design process of the SACH foot involved the modification of the elastic modulus of its components for enhancing the parameters of interest. Results obtained presented a 5.07% increase in the EFLR and a 9.29% increase in the anteroposterior progression of the CoP, which may improve amputee stability. The design solution presented may support the large user base of the SACH foot towards achieving enhanced gait characteristics during ambulation. Moreover, this work successfully demonstrates a novel design procedure for a prosthetic foot through an effective numerical implementation.