Total Hip Replacement.

What do you need to know about Total Hip Replacement? (THR). Like anything else in life, look at history, because it tends to repeat itself and you may avoid making the same mistakes as others.

Early pioneers devised Hip Replacements made of various materials including acrylic cement, ceramics, metal and polyethylene. The first repeatably reliable & successful THR was devised and promoted by Sir John Charnley around 1960. This was a cemented device using a metal stem and head with a polyethylene socket (acetabular component). The Charnley Hip and similar devices dominated the field of THR from 1960's and different concepts were gradually abandoned. Unfortunately these early THRs all suffered from one common flaw that was impossible to get around: after being in place for several years, the plastic would wear and tiny particles would attack the interface between the cement and the bone. This would cause bone to resorb and a new disease was born: aseptic loosening.

Aseptic Loosening

This process begins when tiny particles are generated at the bearing couple of a joint replacement. Since most joint replacements use polyethylene (a type of plastic) as one of the bearing surfaces, and since this is usually the softer material, most of the tiny particles will be polyethylene. The ones that cause the most trouble are so small that their size is measured in microns (thousandths of a millimeter) or even nanometres (thousandth of a micron). Aseptic loosening can also be caused by tiny particles of cement, metal or ceramic. It is the size and number of particles that counts rather than the actual material of which they are made. A typical metal / polyethylene joint replacement will generate millions of these particles every year, even if functioning as designed. Little plastic will be worn away as the particles are so very small. The plastic wear will therefore not be detectable by ordinary X-rays. Everyone believes that all is well ...

The particles attract a cell type called a macrophage. These cells try to engulf and digest the particles that are seen as 'foreign' to the body, much as they would a bacterium. I.e. this is a natural and essential part of the human body's defense mechanism. Unfortunately plastic causes macrophages a fatal case of indigestion and these cells die, releasing enzymes and other chemicals. It is these enzymes and chemicals that cause the bone to be eaten away. If only a few macrophages die and release their chemicals, little harm is done. The problem is most joint replacements release millions or billions of particles so a corresponding number of macrophages may be attracted and die. It is all about numbers

Aseptic loosening eventually shows on X-rays as lines appearing around the prostheses or isolated cavities. See an example below.

When the problem was first observed affecting early joint relacements in the 1960's ,70's and 80's; it was initially poorly understood. Microscopes to see the tiny particles were not available. Many thought the problem was caused by acrylic bone cement and the term 'cement disease' was used. The North American market was therefore dominated by cementless designs in an effort to avoid the problem (a situation that remains!).

When cementless designs began to fail as well, this was labelled osteolysis. Osteolysis is the same basic process as aseptic loosening, just a slightly different pattern.

In summary, all joint replacements whether cemented or uncemented are at risk of failing as a result of aseptic loosening/osteolysis. This is especially true of designs using polyethylene as one of the bearing surfaces. Bad designs or poor technique can greatly accelerate the rate of particle production and therefore increase the risk.

Older patients tend to be less active and will generate fewer particles than a younger patient per annum because they will tend to use their joint replacement less. This places younger patients at greater risk of aseptic loosening. Good designs (cemented or uncemented) and good surgical technique, in contrast, will give survival rates of >95% at 18 to 20 years.

It took a long time for scientists to understand what was happening and why. We still do not know everything about this mysterious process! Further attempts to avoid aseptic loosening and osteolysis have led Orthopaedic surgeons to use materials such as titanium, hydroxyapatite, and newer polyethylenes. Ceramics have had a resurgence of interest after golfer Jack Nicklaus received such a device and his experience was used in a high profile media campaign in the USA . The field however remains in turmoil! Be very careful about what you read and believe, especially if it comes from a commercial source.

So what does the layman do to decide? It really depends on your age and activity levels. Age is relative, so the comments that follow should be seen in that light. If you are around 70 to 75 years of age, a standard, cemented, metal on polyethylene hip replacement such as the Charnley, Exeter, Stanmore, Muller or similar may give you good service for the rest of your lifetime. These prostheses represent 'tried and trusted' technology, and remain the 'Gold Standard'. Many more recent designs and systems may do an equally good job. You should possibly be more interested in the skill and reputation of your surgeon!

If you are younger (below 60 to 65 years of age) and especially if still very active, you need to think carefully about the prosthesis you will receive as well as the surgeon! Wherever possible, my preference in this group is to advise Hip Resurfacing rather than Hip Replacement. At some stage, have a look at the sections on Minimal Incision Hip Replacement and Myths, Facts and Fictions.