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Case Study - Modern Materials in Hip Replacement Surgery

One of the inherent problems scientists face with prosthetics is trying to replace materials shaped by millions of years of evolution to exactly perform a particular task. For internal prosthetics such as knee- or hip-replacements there are many factors to consider to give maximum performance. Taking hip-replacements as an example, it is interesting to chart the history of developments leading to today's cutting edge metal, ceramic and plastic components.

Hip replacement X-ray
The hip joint is a classic ball and socket joint, with the ball, or femoral head, articulating inside the socket, or acetabulum cavity.

The surfaces of these are covered in a smooth cartilage which allow the ball to slide easily into the socket, and which absorb the stresses associated with movement. As time or disease erodes the cartilage, pain and discomfort are caused by the bone surfaces of the ball and socket rubbing against each other. In such cases replacement of the hip joint, known medically as interpositional arthroplasty, may be offered, where the ball is replaced by a metal or ceramic one, and the socket is reshaped and filled with a plastic, metal or ceramic coating.

The Mechanics of Hip Joint Replacement

The replacement of the hip involves work on both the ball and the socket part of the joint. The socket (acetabulum) is first reshaped to form a smooth surface into which a two part prosthetic acetabular is fitted, consisting of a liner and a shell. The shell is manufactured to fit exactly into the reshaped socket, will be cemented or screwed in place, and may be porous to allow bone ingrowth to further fix the joint. Common materials for the shell are porous coated alloys. The shell can also contain a locking mechanism to allow the liner to lock in place. The liner is commonly made out of ultra-high weight polyethylene designed to provide minimum friction with the femoral head. Depending on the materials used in other components, metal or ceramic liners can also be used.

As shown below, the femoral head prosthesis consists of the ball and the stem.

Austin Moore
The stem element is designed to be fitted inside the existing femur, which is often hollowed out to form an exact fit. As with the acetabulum shell, cement may or may not be used depending on the structural viability of the existing femur, and other factors as determined by the surgeon carrying out the procedure. The stem is commonly made from a strong alloy such as titanium or cobalt chromium. The ball element is designed to fit inside the liner, and is made out of either a metal of ceramic, with the aim of reducing friction and wear.

Problems with traditional materials

As with natural hip joints, prosthetics are prone to wear, but in such cases the foreign nature of the elements used can cause their own problems. Particulates that are abraded off of the inner surface of the socket into the body cause a foreign body reaction, known as phagocytosis; meaning that the body attempts to get rid of the foreign contaminant by digesting it using special cells called macrophages. Thus, the aim of research into modern materials for hip replacements is to reduce the rate of wear as much as possible to give a longer hip replacement life.

Zirconia Ceramic Femoral Head

Zirconia (zirconium oxide Zr02) ceramic has been used in orthopaedics since 1985. Zirconia is mixed with yttria (yttrium oxide, Y2O3) to stabilise its tetragonal crystal structure at room temperature. The tetragonal phase has the best mechanical properties, and so the powders used have been developed to maximise this phase in the finished article. The mechanical properties of the product are highly dependent on purity and activity of the powder, thus very fine, pure powders are required.

Performance of Zirconia Hip Replacements

One of the main strengths of Zirconia as a head (ball) component in hip replacements is its mean surface roughness or Ra-value. The Ra-value of a Cobalt-Chromium head is significantly higher than that of a Zirconia head, for example. The relatively low surface roughness of Zirconia contributes to very low wear in the acetabulum.

The most commonly used material for the cup is ultra-high density polyethylene. A study was made in which the ascension of the head into the polyethylene cup due to wear was recorded by taking x-rays immediately after implantation, and again some years later. Fifty four measurements were made after 5 years, 31 after 6 years, and 16 after 7 years. The results were impressive, the majority of patients showing no sign of wear in the cup at all.

Other benefits of using ceramics over metals for hip replacement parts are to do with the inherent biocompatibility of ceramic materials. Being highly oxidised, they are very inert and so the risk of any reactions with the host is negligible. Some Zirconia powders contain small traces of radioactive impurities such as hafnium oxide. These had been a source of concern in the past, but due to the purity of current powders, radioactivity of a finished head is minimal (about 100 Geiger counts per hour), compared to that of metal and even alumina heads.

This article is based on a case study developed under the supervision of Dr. Irene Turner of the University of Bath.



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