Towards the design of hierarchical biomaterials to replace cartilage: development and validation of a localised cartilage model

Allen, Piers C H (2023). Towards the design of hierarchical biomaterials to replace cartilage: development and validation of a localised cartilage model. University of Birmingham. Ph.D.

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Abstract

Osteoarthritis (OA) is cited as a key factor for replacement and repair procedures in synovial joints. Osteoarthritis includes damage of the joint’s articular cartilage (AC) and sclerosis of its subchondral bone. Better computational and physical models of articular cartilage and its subchondral bone has the potential to enable novel solutions to OA, such as better materials or constructs for replacement. A limitation of many current models is that they ignore the dynamic environment within which articular cartilage is found: specifically, material models often ignore that articular cartilage is not loaded to a state of equilibrium. Indeed, there is scope to exploit additive manufacture (AM) to better mimic the geometric profiles of articular cartilage on bone on a macro and micro scale. This thesis aims to develop the knowledge that is lacking around cartilage’s material characterisation and utilise this to further the research into computational and physical AC models. This has been divided into three main components: material model, AM: macro, AM: micro.
Python and Matlab code was used to develop a program that aided in the mechanical modelling of hyper-viscoelastic materials, in our case AC, through the use of finite element analysis (FEA). Human femoral head dynamic mechanical analysis (DMA) data was used as the initial and validation set. The FEA outputs of storage and loss modulus were used as the inputs to a Prony series minimisation problem. Through the use of a cyclic generation and automatic analysis of simulated models, a genetic algorithm was used to minimize the Prony series solution and provide the optimal material parameters for AC. The fitness evaluation for each generation of the genetic algorithm was performed on the simulation results of each set of models, to allow the models to be the driving source of the parameter value choice.
Macro and micro scale analysis was performed on bovine cartilage samples but using a combination of AM, DMA, and Micro-CT. For macro scale, AM and DMA were exploited to produce a dynamic test set-up used to establish the effect that joint contour has on cartilage material response. Contours that were evaluated were representative of the geometry seen on the tibial plateau. Differences in energy dissipation and viscoelastic properties were highlighted between contours representing the tibial components and the current testing procedures.
The third development was in two parts with the micro scale analysis of cartilage geometry changes pre and post dynamic loading. DMA was applied to osteochondral core samples from bovine humeral heads with micro-CT imaging performed before and after compression, providing micro-scale values of the porosity of the tissue. Statistical significance was not identified pre and post CT, however, distribution in trabecular geometry showed there was a marked change. These values were then used as insight into the AM of prototype replacements tissues utilising PLA as the chosen biomaterial. The AM prototypes were imaged with micro-CT loaded under DMA with all outputs evaluated against tissue values. The findings were then combined to produce a complete osteochondral core with geometric and material response metrics similar to that of the original tissue samples.
Ultimately, the work presented in this thesis has implications for the continued development of cartilage modelling and furthers the aim to create replacement structures and biomaterials which are designed for the dynamic environment within which articular cartilage is found.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):