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Total Hip Replacement

Hip replacement is a highly successful operation, relieving pain and restoring function for hundreds of thousands of patients worldwide every year. Success is judged by pain relief, how well the new joint works and its longevity; patients expect to get back to normal activities including sports; surgeons hope the hip will last forever! We need to be able to reliably fix the components to the bone, with a low wear bearing. Although we have been doing hip replacements since the early 1960’s, we still don’t really know what works best. All hip and knee replacements are now entered into a National Joint Registry (NJR) and this is finally beginning to point towards some answers.

For a long time it was thought that hip replacements failed because the bone cement used to fix the parts to the bone was crumbling and many new types of hip were introduced in an attempt to overcome this. We now know the real problem is wear debris from the moving part, which damages the bone around the artificial hip. Great efforts have gone into improving the bearing surfaces, including the introduction of improved polyethylene and harder bearings including metal on metal, which became popular with the hip resurfacing. Metal on metal bearings have a lower wear rate, but the NJR is showing that some designs have a higher early failure rate than other joints and one metal on metal bearing has even been withdrawn from the market.

The NJR is showing a trend towards cemented hip replacements having a lower early failure rate than hips put in without cement and hip resurfacing procedures. The literature shows that the use of bone cement produces reliable fixation of the new hip to the bone. For the hip to function well and be stable we need to restore the correct anatomy, allowing the muscles to work normally and get the leg the right length. Cement gives the surgeon more control over the position of the new hip components in the bone.

We recommend the cemented Exeter hip system which includes an extensive range of sizes, which helps get the mechanics right and has been demonstrated to produce outstanding results over more than 30 years, with numerous satisfied patients. Modern bearing technology will only extend the longevity of this type of hip.

What is Total Hip Replacement?

The hip joint consists of a socket (acetabulum), which is part of the pelvic bone and a ball, which is formed by the head of the femur bone.  The bones are lined by smooth, lubricated cartilage.  As the cartilage wears away arthritis develops and this causes pain.

Both parts of the joint are replaced in total hip replacement. The head of the femur is removed, the socket is prepared and a new socket implanted.  The femur is the prepared and a stem placed down the canal of the thighbone, with a new ball placed on top. Which articulates with the new socket.

There are a number of different approaches to the hip.  There has been a vogue for smaller incisions, but we think that this increases the risk of complications, such as fracture of the bone, or unequal leg length.  The most important thing is that the surgeon gets a good exposure of the hip and is able to repair the muscles and tendons back onto the bone at the end of the operation, to ensure that the hip works properly.  We make an incision at the back of the hip in the buttock about 15cm long, which would normally be covered by underwear or a modest swimming costume!

The components are either fixed to the bone with cement, or the surface of the implant is treated to encourage bone to grow onto it.  Both techniques are usually successful in fixing the hip to the bone.  Sometimes one part, usually the socket, is put in without cement and the other component cemented.  This is known as a ‘hybrid’.

As well as fixing the new hip to the bone it is also important to reproduce the normal anatomy and mechanics of the joint, to make sure the leg is the right length and the joint functions well, to allow the patient to return to full activities.  The surgeon needs to implant the hip very accurately to achieve this.

The moving part (the bearing) is extremely important, as it is the wear rate, which is most likely to determine the long-term performance and survival of the hip.  Traditionally metal against polyethylene bearings have been used.   The wear rate can be reduced by using hard bearings, such as metal on metal or ceramic on ceramic, but these bearing surfaces may cause other problems.  Great improvements have been made in polyethylene and very low wear polyethylenes are now available.

With so much variety, it is very clear that your surgeon has to be relied upon to choose appropriate implants and to implant them to the highest standard. Surgical experience and skill is of paramount importance to the success both in the short and long term following total hip replacement. You should be able to discuss with your surgeon his or her choice of implant and how it is to be implanted should you so wish. The advantages and disadvantages of the various implants and techniques will now be discussed. 

Cemented Total Hip Replacement

This is the “gold standard” hip replacement, originally developed by Sir John Charnley in Wrightington in the early 1960’s, against which all other hip replacements are judged. The Charnley, Exeter and Stanmore hip systems  have the longest and most established track records and with these implants we can expect a successful, reliable and durable hip replacement up to and even beyond 30 years. For the vast majority of patients requiring hip replacement cemented hip replacements are ideal and there should always be good reasons for choosing something else in their place. Implantation of cemented components is reproducible, relatively safe and associated with relatively few complications. The use of cement gives the surgeon very fine control over the position into which the implant is placed.  Cemented components are compatible with all of the modern day bearing surfaces and as long as they are inserted with a high level of skill they will work very well for a very long time.

Cemented cups have been associated with higher rates of loosening in some studies, particularly in active patients.   New wear resistant polyethylene acetabular components are now available, with systems for pressurising cement into the bone.  We think this is likely to improve the long term results for cemented acetabular components and we recommend a fully cemented hip for all patients over 65, where this type of hip is likely to represent a lifelong solution.

Cementless Total Hip Replacement

In very young or very active patients, cement fixation, especially in the socket, may fail with the passage of time. It was originally thought that this problem was something to do with the cement, so cementless systems were developed.  Occasionally, the bone of the hip is so deformed or abnormal that cement fixation is not possible.

Cementless components are usually made of titanium or cobalt-chrome, often coated with porous surfaces or hydroxyapatite (a bone mineral) to encourage bone to grow onto the metal. It is important to fit the implant tightly into the bone and if it is stable, over the next few weeks the bone grows on to the implant, as it becomes “part of the body.” During this period protected weight-bearing using crutches is often recommended.  Once the bone has grown on to the cementless implant, the likelihood of it ever coming loose is extremely small.

It is more difficult to precisely control the position of the implant in the bone and we think it is more difficult to get the leg length right with these systems.  In addition implanting cementless components can have other complications.  The need to obtain a tight fit in the bone means that fracture of the bone is about ten times more common with cementless when compared to cemented implants.

We very rarely use cementless stems for primary operations, but recommend a cementless socket for patients under 65, where a cemented socket may fail, requiring revision surgery.  Cementless sockets also give different bearing options, which may be advantageous in younger patients.

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Published with the kind permission of Mr Simon Bridle

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