Designing Orthopaedic Implants based on Magnesium-Polylactic Acid Composites

Abstract

Magnesium has attracted much attention in recent years for its potential as an orthopaedic implant material. It has many properties that make it appropriate for this application, such as: a similar elastic modulus to bone, and biodegradability within the body. The latter means that magnesium implants will not require a second operation to be removed from the body, saving patients from follow up surgery. In body conditions however, it has a high corrosion rate, meaning that the implant may degrade before the wound has healed sufficiently. This high corrosion rate also results in the evolution of hydrogen gas and increases in pH level, both of which are detrimental to the healing process. This thesis first uses a polymer-biomineral composite coating system in an attempt to improve magnesium’s corrosion resistance. It was seen that these coatings provide some resistance to corrosion, however due to gas evolution, the coatings blister and crack, allowing corrosion to increase. The use of polymer-biomineral composite coatings also decreases the adhesion strength compared to a polymer only coating – due to poor bonding between the biomineral layer and the magnesium substrate. The adhesion strength of the coating was seen to correlate with the time until cracking failure (during the corrosion test); therefore it was concluded that the decreased adhesion strength of the composite coatings lead to cracking failure occurring earlier during the corrosion test, indicating worse performance. A different approach was then attempted, using a magnesium-PLA (polylactic acid) interpenetrating network composite (where both components are mixed and interconnected in 3- dimensional space) - a novel approach that has not been tested before. It is thought that this type of composite may allow constant, slow degradation, rather than the abrupt change in corrosion rate seen with the coating. A production method was developed, using a salt casting technique to produce porous magnesium, which was then infiltrated with molten PLA using injection moulding. The yield strength of the composite was lower than bulk magnesium (71±2 MPa, compared to 113±10 MPa) due to the inferior strength of the PLA. The composite was also seen to undergo faster corrosion, although this was attributed to degradation of PLA into lactic acid, decreasing pH and therefore accelerating corrosion, which was confirmed by pH monitoring. Corrosion of the composite is seen to occur coherently, from the outside inwards, meaning that the polymer component has prevented corrosion fluid from permeating the entire structure. This is an encouraging result as it indicates that a polymer can be incorporated into the magnesium structure without exposing the entire porous area to corrosion. This means that with correct choice of polymer and other parameters, there is potential to control the degradation rate of the implant material.