Characterising patient specific soft tissue deformation in the residual limb under compressive loads

Author: Mahamandige Kaushini Mendis

Mendis, Mahamandige Kaushini, 2021 Characterising patient specific soft tissue deformation in the residual limb under compressive loads, Flinders University, College of Science and Engineering

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A prosthetic socket can be considered as the primary link between the prosthesis and the residual limb in a lower limb amputated patient. Within the first two years post-amputation, the residual limb soft tissues tend to change in the aspects of shape, volume, composition, and sensitivity due to environmental, mechanical, and surgical factors. As these changes could occur within a single day, or within several months, a single mechanical design is not capable of interpreting the high rate of change of the stump. Thus, patients tend to wear temporary sockets within this time period. Patients wear a permanent socket at a later stage following amputation when the stump reaches its maturity. Even though the matured residual limb continues to change due to fluid movements and temperature variations, a single mechanical design can cope up with the low rate of change of the stump. The project considered the patient-specific soft tissue deformations at the matured stage of the residual limb under compressive loads.

The elevated stresses and strains caused by imperfect permanent socket fittings contribute to a range of short- and long-term complications and affect patient satisfaction in physical, mental, and financial aspects. The project intends to improve the current socket designing and manufacturing process, which depends on prosthetists’ experience and unreliable feedback from patients towards an engineering-driven framework to benefit the patient’s health, healthcare system and the economy.

Biomechanical modelling has been used over the past two decades to model the residual limb soft tissue mechanics in order to eliminate the ill-fitting sockets. The efficacy and fidelity of biomechanical models require accurate information on the geometry and material properties of soft tissues. The main limitation of the previous work was the consideration of literature reported material properties; most of the researchers were leaned towards assuming linear, elastic, isotropic and homogenous material properties for the residual limb soft tissues. Hence, the project’s aim was to estimate patient specific soft tissue deformations of the residual limb employing medical image data acquired while performing quasi-static loading experiments, which could assist in estimating patient specific material properties in the future.

The project considered the magnetic resonance imaging (MRI) scans related to three patients where each patient had to undergo a series of uniaxial quasi-static compression tests. The MRI scans were obtained using an MRI compatible sphygmomanometer (Cone Instruments, USA) which covered the patient’s residual limb at 0 , 30 , 60 and 100 pressures, respectively. The MRI scans were segmented through Simpleware ScanIP (Synopsys, California, USA) to obtain surface and solid geometries of the uncompressed and the compressed states of the residual limb.

Rigid iterative closest point (ICP) algorithms were performed to register the uncompressed to each of the compressed surface geometries of the residual limb. Non-rigid ICP was then used to calculate soft tissue deformation at the entire residual limb surface for each of the compressed states. The area in contact with the sphygmomanometer was extracted to derive the force applied at each compressed state. Finally, the patient specific soft tissue deformations of the residual limb were characterized by fitting piecewise linear models to force-deformation data acquired for each patient. All data processing were performed using MATLAB (MathWorks, Massachusetts, USA). The force-deflection plots indicated unique shapes for each patient confirming the discrepancies between the soft tissue deformations between patients. The obtained soft tissue deformations could aid substantially in biomechanical modelling of residual limb soft tissues. The study anticipates developing accurate computational models of the mechanical contact between the residual limb and encasing socket. Such models can aid current socket design and customization process in an effort to reduce the risk of tissue injury, to lessen the time, materials and workforce required for design and fabrication of a best-fit prosthetic socket, and to improve patient satisfaction rates. The study anticipates employing more patients in the future to understand the variability within the patient population.

Keywords: Biomechanical modelling, Transtibial amputation, Soft tissue deformation

Subject: Engineering thesis

Thesis type: Masters
Completed: 2021
School: College of Science and Engineering
Supervisor: Dr. Rami Al Dirini