Materials

Why would you want to know this?

The overwhelming number of patients who ask me on a daily basis what their knee, hip or shoulder implants are made of suggests there is a pressing desire to know what kind of implant is in them. I get the sense that, at least for the local population here in Singapore, they need to be assured that an implant in them will not rust. If that is all that concerns you then read no further, all but a few designs are made of titanium, cobalt chrome, ceramic or polyethylene and these do not rust. The remaining implants are made of steel which is medical grade and they do not rust. Some do corrode but this is an electrical phenomenon (see below) and is not rust per se. In twenty years of practise I do not recall having to remove an implant due to rust.

Some patients might want to know about the material of their implant because it somehow suggests the quality of the implant. I would assure you , however, that the material used in an implant has less to do with the quality of the material than of the design philosophy subscribed to in the implementation of the implant. So, for example, if titanium is being used for a hip implant then it has probably been chosen because it is more ductile and therefore would do better if inserted in an uncemented fashion.

Implants in arthroplasty surgery are generally made of the following materials:
1. Titanium
2. Cobalt chrome
3. Steel
4. Polyethylene
5. PMMA (poly-methyl methacrylate)
5. Hydroxyapatite
6. Tantalum
7. Ceramics (like zirconia and alumina)

The materials have specific properties which make them ideally suited to certain applications but clearly not every material is ideally suited to all applications.

Titanium
In 1952 Per-Ingvar Brånemark discovered the osteointegration of Cambridge designed ‘rabbit ear chamber’ for use in the rabbit femur. This yielded the idea that titanium with its high biocompatibilty might have value in implant design. Titanium was first used in the early fifties in plate fixation of fractures and soon after in the Sivash hip in Russia. As a metal therefore, titanium enjoys a long history of use in humans. Its main qualities tat make it suited to joint replacements are its pliability, ductility or low Young's modulus. This means that when put into the femur as a hip replacement, it is able to bend in bone making it better tolerated. Coupled with the fact that bone actually grows onto titanium (osteoconduction) this makes this material ideally suited to uncemented implants.  It is used in virtually all joint replacement strategies usually on the point of the implant in contact with bone. Metal allergy is improbable with this metal but being relatively soft it does fracture in poor designs(eg. when the neck of a hip replacement is too long).

Cobalt chrome
For the articulating surface, like the ball in a hip joint or the condyle in a knee joint, a harder material which can be polished to a shine and provide a smooth articulating surface is required. This is usually made of cobalt chrome. The human body is filled with salts in bodily fluids and when two dissimilar metals like some grades of steel and cobalt chrome are in contact, they can set up an electrical couple and cause corrosion - this is not rust which is oxidation of iron. Neither of these phenomenon are common in joint replacements as designs nowadays do generally account for the phenomenon.

Steel
The use of steel in implants pre-dates scientific testing of implants. If it were the case many of the early failures would be used to prevent its use today. Fortuitously however some implants (eg. Exeter) have been around for 40 years with good clinical history and they are made of a special steel known as Orthinox. It is the only other metal that can be polished and is hard enough to function as an articulating surface.

Polyethylene
This is similar to tupperware plastic and is considered the gold standard articulating surface. Attempts at hardening the material by creating highly cross-linked polethylene are presently being evaluated. However, there may be issues with these new materials being so brittle that they crack. Particles of polyethylene are known to cause the body's defense system to attack and reject implants causing loosening.

PMMA
This is bone cement. It is produced as a dry powder mixed with a liquid to make a putty to stick implants in the body. Variants of this material may carry antibiotics. The main side effect of the material is that large amounts of cement pressurised into a closed canal can cause severe hypotension during surgery. Previous concerns of cement particles causing implant loosening are presently believed to be unfounded or at least exaggerated. This so called "cement bone disease" has been more predictably ascribed to polyethylene.

Hydroxyapatite
In some implants this is sprayed on. This allows bone to grow onto the implant and fix it in place. In the past some designs resulted in delamination or pealing away and loosening of the implant. Presently however the technology to secure the hydroxyapatite has been seen to be more robust.

Tantalum
This material relatively new in the market has remarkable osteoconductive properties. They are used as alternatives to titanium or hydroxyapatite to allow bone to grow onto implants and keep them secured.

Ceramics
This is the material that floor tiles are made off. They are smooth, hard and offer the best wear rates at one particle per year compared to 1000 in polyethylene. The main drawback is that in the early designs there was a tendency for ceramics to fracture. This has been circumvented in present designs. In addition squeaking hips can result but with better understanding of design it is believed that a more horizontal acetabular cup with thicker metal backing can stop this. It can be distressing to watch but the author, who has been using them since 2005 with the newer methods of implementation has not experienced this side effect. Squeaking hips occur with metal on metal implants as well and seem to be a function of hard on hard articulating surfaces where the acetabular component is too vertical. They are the preferred articulating surface in metal allergic patients. 

The squeaking hip