Page 177 - Clinical relevance of current materials for cranial implants

Effects of sterilization on the mechanical properties of PMMA
INTRODUCTION
Poly(methyl methacrylate) (PMMA) has been widely used in different fields of healthcare. It is used as bone cement for fixation of knee and hip implants in orthopedics, as the base of dental prosthesis, for cranial reconstruction in neurosurgery, and for many other medical devices.1 PMMA is light, radiolucent, cost efficient, and easy to use. However, it is associated with complications such as infection2. The exothermic polymerization of PMMA can cause burn injuries if applied directly onto tissues and there are indications that residual monomers are toxic to the body1.
The mechanical properties of personalized medical devices are essential for long-term survival. These properties may be affected by storage time, pre-treatment, sterilization and the location of the inserted medical device in the body. PMMA demonstrates increased flexibility in a liquid environment compared to a dry environment, and storage at 37°C makes PMMA less resistant to fracture than storage at 21°C3.
The most common sterilization methods for medical applications are ethylene oxide
gas (EtO), hydrogen peroxide gas plasma (HPGP), autoclavation, and γ-irradiation4.
These sterilization methods are important as PMMA-based medical devices are not
only prepared by powder and liquid mixing in the operating room, but pre-fabricated
3D-printed methacrylate-based materials and ex vivo polymerization are also used5-7.
The advantage of 3D-printing is a better control on the shape and material properties
of the medical device. Manufacturing the medical device before surgery reduces
surgical times and removes limitations to the environmental conditions during 9 polymerization, enabling optimizations that may lead to better clinical outcomes.
However, the device then needs to be sterilized, this presents a challenge to retain optimal material behavior.
The sterilization of PMMA powder is usually performed by γ-irradiation, except for Palacos, which is sterilized using EtO8. The liquid MMA monomer is sterilized through membrane filtration8-11. γ-irradiation of PMMA results in chain scission, detectable through a decrease in molecular weight9,11-13. This directly influences mechanical properties such as fracture toughness, fatigue, and flexural strength9,10,12.
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