Biomechanical characterisation of soft tissue for transtibial prosthetics
Creators: name: Zani, Lorenzo
Publisher: name: Loughborough University
Year: 2022
Subjects: Mechanical engineering not elsewhere classified

description: The loss of a limb has tremendous consequences in the life of a person. Transtibial amputation is the most common case of major amputation. A postoperative prosthesis is considered the most effective rehabilitation aid for the patient. Replacing the lost limb offers the chance to reacquire mobility and independence. The prosthetic socket main function is to transfer effectively mechanical load from the skeleton to the artificial limb. The greatest challenge in doing so is to avoid overloads on soft tissue and ensure comfort so that the amputee can wear the prosthesis throughout the day not compromising the already reduced quality of life. Unfortunately, the manufacturing of prosthetics is a labour-intense and timeconsuming process. It highly depends on the prosthetist skills and experience not always leading to reliable results. Discomfort and soft tissue damage are the main reasons for an unsuccessful prosthesis. This problem is exacerbated by notstandardised clinical practice and arbitrarily prosthesis revision. To reduce inconsistencies in the prosthetics manufacturing process and subjectivity in lower limb load-bearing assessment, a more rigorous and quantitative approach is needed. This study aims to address the challenge by developing a framework that combines experimental testing, computer simulation, design, and non-traditional manufacturing. The first step is the acquisition of mechanical and morphological information using a bespoke testing rig coupled with a rigorous testing protocol. Axisymmetric finite-element (FE) simulations are then employed to simulate experimental testing and identify optimised coefficients of soft tissue mechanical model (formulated as non-linear hyperelastic) via an inverse data fitting method. The optimised material coefficients are then fed to a 3D subject-specific FE model simulating the hydrocasting process on the residual limb of a transtibial amputee. Finally, a prosthetic socket prototype is designed based on the deformed surface of the virtual residuum and eventually built via additive manufacturing. The rationale behind this approach is to create a prosthetic socket that provides uniform distributed pressure on the stump below the discomfort threshold which offers enhanced prosthetic suspension compared to alternative designs that discriminate between tolerant and sensitive areas. Moreover, this data-driven approach is based on quantifiable measurements on the residual limb that leads to repeatable results regardless of the manual skills of the healthcare professionals involved in the procedure. When compared to the traditional manufacturing process, the working time needed to build a prosthetic socket following the approach suggested by this study reduces from several days to just half a day removing the need for frequent patient visits. This corresponds to the minimum time required to carry out the experimental assessment of the residual limb while most of the in-silico tasks are automatized as well as the actual socket manufacturing. This approach applied to a real-case scenario demonstrates the importance of biomechanical characterisation of residual soft tissue and its potential in changing clinical practice.