MEDICAL QUALITY CONTROL
1. METALS AND METAL ALLOYS
• Stainless steels, which are steels (maximum 1% carbon) with at least 12% chromium content by mass. It is this chromium content that makes the steel stainless.
The types of stainless steel used in the biomedical industry are so-called “martensitic” stainless steels (magnetic and heat-treated) for surgical instruments and so-called “austenitic” stainless steels (non-magnetic and with a nickel content between 10 and 14%) for surgical implants.
Stainless steel is the most common metal used in the medical field, particularly austenitic stainless steel 316L (X2CrNiMo17-12-2).
• Titanium and its alloys bring very beneficial characteristics in addition to biocompatibility. Their corrosion resistance is exceptional (higher than stainless steel) and they have better fatigue properties and better elasticity than cobalt-chromium alloys and stainless steels. The density of titanium is also a big factor, since its value is low for a metal (4.5 compared to about 8 for stainless steels).
The applications of titanium in the medical field are very vast: implants, osteosynthesis, orthopaedics, prosthesis, etc… A very common grade of titanium is TA6V (TiAl6V4).
• Cobalt-chromium alloys (stellites), which are cobalt alloys with chromium as the main alloying element. They frequently have a high molybdenum content and a low carbon content (non-magnetic alloys).
The corrosion resistance of these alloys is excellent. Their mechanical properties are equally remarkable (high hardness, toughness and wear resistance), giving these alloys higher rigidity than stainless steels and titanium.
CoCr alloys are mainly used for hip, knee or rachis prostheses, for osteosynthesis and for dental prosthesis.
• Alumina (Al2O3), is a pure, dense medical-grade bio-inert ceramic. It is mainly used for its good tribological properties, ageing and mechanical resistance. It is used for the tips of hip prosthesis and in dentistry (dental implants).
• Zirconia (ZrO2) has the same characteristics as alumina but with higher toughness, i.e. it has a better resistance to crack propagation.
• Hydroxyapatite (HAP) Ca10(PO4)6(OH)2 and tricalcium phosphate β (TCP) Ca3(PO4)2 are bioactive ceramics. These ceramics are porous, which makes them osteo-oconductive, i.e. they promote bone regrowth. They also have the advantage of being bioresorbable and are used for implants, orthopaedic surgery and dental fillings.
• Bioglass is a bioactive ceramic with a carbonated hydroxyapatite layer on its surface, which is chemically and structurally identical to the mineral phase of bone. A link can be made between the ceramic and bone, making osteoconduction and osteoproduction possible. Bioglasses are mainly composed of oxides, silicon (SiO2), sodium (Na2O), calcium (CaO) and phosphorus (P2O5).
The following polymers stand out:
• Functional polymers are used as friction surfaces (in addition to metals and ceramics), as anchoring materials for prosthesis (allowing better convalescence) and in ophthalmology, neurosurgery, cardiovascular or plastic surgery (catheters, drains, syringes, prosthesis).
• Resorbable polymers make it possible to avoid further surgery. They must have sufficient mechanical properties to ensure their functions and then be resorbed afterwards. They are therefore used as surgical cements, bone fillers, diaphyseal obturators, suture threads, etc.
These biomaterials are developed and used in such a way that they are not rejected by the host’s organism, do not contain toxic elements and offer precise mechanical characteristics that cope with the various constraints exerted by the environment.
In other words, they must comply with very demanding specifications in terms of physico-chemical properties, shaping, service life and/or deterioration, porosities, implantation or injection, etc.
It is for these same reasons that a multitude of tests must be carried out, some of which require metallographic preparation.
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