The development of magnesium-based materials for orthopaedic applications

Abstract

The currently used biomaterials for surgical implantation include stainless

steel, titanium and its alloys. However, due to the non-degradability and the

mismatch of the mechanical properties between these metallic implants and

human bone, there maybe a long-term adverse effect of inflammation or stress

shielding effect. This may lead to bone loss which brings with a higher risk of

implant failure. To avoid this problem, implants made of biodegradable

materials are the alternatives. Due to the poor mechanical properties of

biodegradable polymer especially for load-bearing area, biodegradable metal is

used instead. Magnesium is the potential candidate since it is degradable with

mechanical properties similar to human bone whilst magnesium ion is an

essential element to human bodies.

With the advantages of using magnesium for implantations, it can be

potentially used for fracture fixation implant and bone substitutes. However, its

rapid degradation and release of hydrogen gas may inhibit its use. Hence,

modification is required. In this project, plasma immersion ion implantation

and deposition (PIII&D) using aluminium oxide as the plasma source was

conducted on the magnesium alloys. The corrosion resistance properties of the

plasma-treated magnesium alloy were found to display significant

improvement in immersion test especially at early time points. The

plasma-treated sample was compatible with osteoblasts. Cells attached and

grew on the treated sample but not the untreated sample. The animal study

showed consistent results with the cell study, and there was a significant

increase in bone formation around the treated sample when compared to the

untreated sample.

The other potential application of magnesium is its usage as a bone

substitute. Due to the limitations of autografts and allografts, synthetic bone

substitutes are developed. The ideal bone substitutes should have similar

properties to those found with autografts. However, no such bone substitutes

presently exist; hence, a novel hybrid material is fabricated in this project

through the addition of magnesium granules into a biodegradable polymer

polycaprolactone (PCL). The immersion test showed that an apatite layer

composed of magnesium, calcium, phosphate and hydroxide was formed on the

hybrids but not on pure PCL, which suggested that the hybrids were

osteoinductive and osteoconductive. The compression test showed that the

mechanical properties were enhanced with the incorporation of magnesium

granules into pure PCL and were still maintained after 2 months of immersion.

Osteoblasts grew well on the PCL-Mg hybrids. The addition of smaller

amounts of magnesium granules (0.1g PCL-Mg) resulted in higher ALP

activity and up-regulation of different bone markers when compared to the

pure PCL. Finally, the animal studies showed that more new bone formation

was found around the 0.1g PCL-Mg hybrids especially at early time points,

which suggested that the healing time could be shortened.

In conclusion, fracture fixation implants and novel bone substitutes based

on magnesium were developed in this project. The aluminium oxide coating

was able to improve the corrosion resistance properties of magnesium alloy by

suppressing the release of magnesium ions. The PCL-Mg hybrids were found

to be biodegradable, biocompatible, osteoconductive, osteoinductive and

mechanically matched to human bone properties.