Although the functional aspects of implanted materials can be anticipated fairly reliably, it is very often difficult to anticipate how well they will be tolerated clinically. For materials of any kind there are two aspects which have to be taken into account. They are:
Adhesion involves all the problems of using cements and adhesives, the role of which is to transmit and distribute the stresses over the largest area of contact possible. This adhesion problem is far from being resolved satisfactorily from the practical point of view and there is still plenty of scope for the researchers to investigate. Should a prosthesis be cemented, screwed, or introduced with force, hoping that its irregular surface will allow the bone to grow again and for the prosthesis to be fixed into the bone? More and more surgeons are currently abandoning these latter methods because of the frequency of painful failed fixations requiring surgery to be repeated (6–8 % on average after 12 months). Cement has its drawbacks but according to the current state of knowledge seems to be the best compromise for fixing material into bone.

I would like to dwell on this problem of metal corrosion for a few moments. Some metallic materials are very resistant to generalized corrosion. This is the case for Vitallium, stainless steels, or alloys based on titanium, but they are still vulnerable to corrosion if pitted, the risk of which increases with contact friction which leads to breaks in the protective passive layer. It is also necessary to take into account the simultaneous action of the corrosive environment on the prostheses and the mechanical stresses to which they are subjected. This results in the risk of corrosion under stress, and corrosion due to fatigue which can lead to the appearance of weak points with the risk of breakage. Another well-known case of corrosion is galvanic corrosion caused by placing two different metals in contact with each other in a conducting liquid which then behave like an electric battery.

When there is corrosion, metal ions pass into the body. Therefore, some studies have shown that for austenitic stainless steel osteosynthesis plates, 9.1 mg of the alloy passed into the body 2 years after having been implanted. That is to say that there is a release of iron, nickel, and chromium in an equal proportion to that of the composition of the alloy. For example, in an individual who had had intramedullary pinning of the tibia, after 18 years he was found to have a nickel concentration in his serum, urine, hair, and nails which was up to 18 times the normal concentration, almost the same level as is found in workers in the nickel industry.

More generally, the implantation of foreign material, and particularly a metallic material, always has consequences for the surrounding biological environment. It was even possible to demonstrate a transformation of the proteins left in contact with nickel, in particular by electron transfer at the metal–electrolyte interface.

The problems listed above therefore require the practitioner to know the mechanical and chemical properties of the materials to be implanted without, of course, forgetting the sterilization conditions which can alter certain materials (such as gamma rays on plastics, ethylene dioxide absorbed by certain materials then released producing toxic reactions).

If the surgeon cannot check all the properties of the material he uses by appropriate tests, he has to rely on the manufacturer’s literature to make his choice. But if he knows the properties that he can expect for a given application, the dialog will be more to the point.

That is the current direction in the area of French orthopedics.

First, where steels are concerned, the introduction of alloys leads to a spectacular improvement in oxidation. Molybdenum plays an essential role in resistance to corrosion caused by pitting.

Chromium also plays an essential role from the point of view of corrosion. Indeed, exposed to the air or to an oxidizing environment, chromium allows a very thin, invisible film of chromium oxide to form – this is called the passivation phenomenon. A minimum chromium content of 12 % is necessary to give steel its stainless properties.

Other elements can be added; this is true for nickel which, when in a proportion of 10–14 %, makes it possible to obtain an improvement in mechanical performance without leading to brittleness.

Steel with a high carbon content is therefore suitable for temporary surgical implants (osteosynthesis plates, intramedullary nails) because of its malleability and its stainless properties. But its poor prolonged resistance to corrosion means that it has to be removed after a few years.

Alloys based on cobalt–chromium are shaped by microfusion or casting, which is less good mechanically, and only very rarely has it been possible to make forgeable alloys, owing to considerable additions of molybdenum, tungsten, and nickel.

Although these materials have a resistance to corrosion and a breaking load which is better than stainless steel, their elastic limit is very close to the breaking load, which prevents any possibility of permanent deformation. And, as their resistance to fatigue is low, a significant breakage rate has been seen for femoral implants.