Biomechanical characterisation of MG-based implants for bone regeneration using X-ray computed tomography

Student thesis: Doctoral Thesis

Abstract

Bone is an organ able to regenerate completely through a very efficient healing process, but high-energy trauma, disease, tumour resection or osteomyelitis represent extreme bone healing conditions that impair its regenerative capacity, leading to critical-sized defects and consequently to non-unions. Current research focuses on the restoration or replacement of irreversibly damaged bone tissue by implanting biodegradable materials that can mechanically maintain the bone tissue integrity for the first two months and then gradually degrade allowing cell invasion and blood vessel formation (angiogenesis), required for the bone healing process. Among the variety of biomaterials under investigation, magnesium (Mg) and its alloys represent a promising compromise between biocompatibility, bone regeneration ability and appropriate mechanical properties.
This PhD project aims to combine in situ high-resolution X-ray computed tomography (XCT) mechanics, digital volume correlation and electron microscopy to investigate the mechanical, morphological and biological properties of an innovative Mg-based scaffold, manufactured by crucible melt extraction and liquid-phase sintering, to be used as a regenerative treatment for critical-sized bone defect.
Inappropriate corrosion rate remains the main challenge for Mg-based materials and its incidence on local morphological and mechanical properties is yet to be fully defined. The mechanical and morphological properties of open-porous scaffolds made of the sintered Mg-based fibres were first characterised in a non-corrosive environment (i.e. air) and compared with those of trabecular bone tissue. A multi-scale approach was employed to comprehend the influence of Mg-based material properties on the mechanical performance of the scaffold, in order to establish a correlation between its structure and mechanical behaviour. In vitro corrosion tests followed by in situ mechanical testing were conducted on coated Mg-based scaffolds to assess their corrosion resistance and behaviour, and how this affected their mechanical integrity. In vivo implantation in a dog mandible model were also performed to evaluate bone ingrowth and degree of mineralization, osteointegration and osteoconduction capabilities. The mechanical performance of the newly formed bone-Mg system was also investigated during the first four months post-implantation.
Mg-based porous scaffolds tested in air exhibited a highly ductile behaviour without global failure, while trabecular bone developed microcracks for similar strain levels. This behaviour could be explained by the alloys highly connected porous network, which contributed to a more efficient load transfer. After immersion in Hank’s balanced solution (HBSS), the fluoride coating protected the scaffold from severe degradation, resulting in a relatively low in vitro corrosion rate and preservation of the mechanical integrity compared to uncoated alloys. Finally, in vivo implantation showed evidence of angiogenesis and bone ingrowth into the porous alloy, enabling the Mg-bone system to gain sufficient mechanical strength to support complete tissue healing.
In conclusion, the results suggest the mechanical and biological suitability of innovative Mg-based implants that preserve the mechanical integrity of the injured site while promoting bone ingrowth, suitable for the treatment for critical-sized bone defects.
Date of Award2022
Original languageEnglish
SupervisorGianluca Tozzi (Supervisor) & Frank Witte (Supervisor)

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