Investigation of a heirarchical collagen scaffold/3D printed lattice structure designed for use in total knee replacements
Aseptic loosening is a common cause of late-stage failure in total knee replacement (TKR) surgery. This multifactorial condition is exacerbated by stress shielding and micromotion at the bone/implant interface. Stress shielding can be reduced using implant materials with a similar modulus to that of the surrounding bone, such as Ultra High Molecular Weight Polyethylene (UHMWPE). Micromotion is reduced by improved implant osseointegration. A new design for the tibial component of a prosthesis based on macromolecular materials was therefore proposed. A fused deposition modelling (FDM) 3D printed poly ether ketone ketone (PEKK) lattice structure formed the load bearing component of the construct. This contained a lyophilised collagen scaffold filling the lattice voids, to encourage osseointegration. To understand the effects of printing parameters on the structure and properties of prepared samples, the bonds formed between FDM printed rasters were explored. Samples printed from poly lactic acid (PLA) and PEKK polymers were investigated. X-ray microtomography provided information on structural defects (including porosity) and tensile testing was used to investigate the relationships between printing parameters and the ultimate tensile strength (UTS) and modulus. The rasters of dumbbell specimens were aligned perpendicular to the applied load, such that the inter-raster bonds were strained during testing. Increased print speed and raster width were associated with reduced porosity and increased UTS and modulus in both PLA and PEKK. Based on these findings, optimised printing parameter sets were defined. Next, a lattice design was developed to match the compressive modulus of cancellous bone. Based on a combination of theoretical modelling and experimental testing, the effects of lattice volume fraction and geometry on compressive modulus were determined for a range of different lattice morphologies. A sheet diamond lattice morphology exhibited the highest modulus over a range of volume fractions and was taken forward for further experiments. A method of modulating local lattice volume fraction based on medical computed tomography (CT) data was developed. A modulus-matched tibial component derived from 3D printed PEKK sheet diamond lattices was designed using CT data and mechanical testing data. Having investigated the mechanical properties of the load-bearing component of the proposed construct, focus then moved to production and characterisation of the collagen component of the design. Lyophilisation was used to create highly porous collagen structures, which were characterised using X-ray microtomography. A method was developed to accurately evaluate pore size, avoiding artefacts that had previously provided erroneous measurements. Collagen scaffolds were produced in the absence and presence of the polymer lattice and it was found that the average pore size increased from 168 micro m to 229 micro m due, in part, to poor interfacial bonding between the collagen and the lattice struts. To address this issue, two different approaches to improve collagen scaffold/polymer lattice interfacial bonding were investigated. First, a range of different polymer surface topographies were produced using FDM printing. Polymer surfaces with increased topographical roughness were found to exhibit a collagen/polymer interfacial strength, in the dry state, of up to 28.1 kPa, compared with 5.5 kPa for less rough polymer surfaces. The architectural properties of these interfaces were investigated using micro-CT, which demonstrated the presence of interlocking collagen fibrils and polymer. However, it was found that when the interfacial strength of samples was tested in the hydrated state (to mimic the situation in-vivo) there was 18-fold reduction in interfacial strength. Next the effects of collagen crosslinking and the addition of hydroxyapatite (HA) into the collagen scaffold (to create a closer chemical analogue with bone) were explored. Collagen crosslinking (using 5:2:3.3 EDC:NHS:collagen) resulted in a 4 fold increase in the interfacial bond strength while the addition of 20 wt.% HA to collagen scaffolds by blending doubled the interfacial strength as compared with pure collagen scaffolds. Cellular response to the constructs was assessed using MG-63 osteosarcoma cell culture. Cell attachment was increased on collagen-PEKK constructs compared with pure collagen scaffolds. Cell proliferation, migration and metabolism were similar on the construct and pure collagen scaffolds. Increased osteoblast-specific metabolic activity was observed on the collagen component of collagen/PEKK samples compared with the PEKK component, indicating that the collagen component might be more biologically favourable than the PEKK component, in vitro. The work in this thesis demonstrates the mechanical and biological potential of a hierarchical collagen scaffold/3D printed PEKK lattice construct for application in TKR tibial components. The results offer important new insights on methods to improve the longevity of TKRs.
3D printing, collagen, orthopaedics, PEKK, total knee replacement
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