Introduction

The hip is a 'ball and socket' joint which can wear out at different points during a person's life. If an individual has problems early in life with conditions such as Dysplasia (mal-formation), Slipped Upper Femoral Epiphysis (growth plate injury), or Perthe's disease (loss of blood supply) then the joint can be prone to much earlier wear than those of their peers. I have indeed had to replace the hips of some patients in their 20's. Our aim as orthopaedic surgeons is to give the patient a total hip replacement which functions as near normally as possible, which is as resistant to dislocation as possible, which preserves as much bone as possible (in case further surgery is needed later in life) and which will last as long as possible. Increasingly, as surgical techniques and manufacturing technologies advance, we can achieve this with greater and greater success.

A Brief History of Total Hip Replacement

The first clinically successful total hip replacement (THR) was developed by Sir John Charnley in Wrightington, UK during the 1950's and 60's.

He went through a series of set-backs using materials which failed early (on the cup side), but eventually found success with High Density Polyethylene (plastic) which could withstand the half-to-one million cycles which it would be exposed to each year during it's lifespan. There are still some of Charnley's original hips still going strong today.

The problem however, was that the plastic cup was prone to wear in young or active patients. Also, the bone cut, or osteotomy used to gain access to the hip could occasionally fail to heal properly, giving some patients a limp.

Therefore with the passage of time different approaches to the hip have been developed, as have alternate 'bearings' (the moving surfaces of the joint), in an attempt to optimise function and longevity in these hips.

Types of Total Hip Replacement

Broadly, these can be broken down into the stem and the cup. These are further sub-divided into cemented and uncemented.

Traditonally, cemented hips have been reserved for older patients or those with somewhat weaker or less active bone. They do however have the benefit of very long term results, so that we can give good assurances as to how long each type is likely to last. The original Charnley stem was all in one piece, or 'monoblock', whereas now replacement stems tend to have separate heads and sometimes necks, so the optimal hip can be 'built' within the patient to suit each individual person's anatomy. The stems are also increasingly tapered and polished, a philosophy introduced by Professor Ling and Graham Gie in Exeter. Studies suggest this design transmits load to the bone more successfully and thereby maintains good bone strength.

Increasingly, uncemented designs are proving their worth however. Hip replacement practice in the US is now almost exclusively uncemented. Designs once again vary, and we initially used these stems in the younger and more active patients, whose bone we supposed would rapidly grow onto or into these devices. More recently however, it has been reported that these stems also give very good results in the more elderly population. The benefit of an uncemented stem is the 'biological' fixation which is constantly renewed and adapts as a patient's skeleton changes with age and health.

Another benefit of total hip replacement is that shorter stem designs can be used. We believe this to be a step forwards, especially in younger patients, because we use less of a patient's bone, and put the load through the bone as high up the femur as possible, thereby maintain strength through more of the bone. Modern designs are shown below.

(This young patient in their 20's had a previous fracture in a motorcycle crash, hence the plates and screws. His hip was irretrievably damaged, but we were able to give him a modern total hip replacement.)

Cups used in Total Hip Replacements

The cemented cup used in a total hip replacement is generally plastic and used in older patients, although in the last 2-3 years a process called 'cross-linking' has meant that these replacements may well now last much longer than previously, and may broaden the indications.

Uncemented cups have a surface, like their counter-parts on the stem side, which has grooves or pores onto or into which bone can grow. The really nice feature of uncemented shells is the ability to put different surfaces or 'bearings' into them. We can therefore use ceramic, metal or plastic surfaces, depending on the patient's activities and medical history. This has also allowed us to use larger heads which reduces dislocation rates and offers a better range of motion post operatively.

More on Total Hip Replacement

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