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 (malformation), Slipped Upper Femoral Epiphysis (growth plate injury), or Perthes disease (loss of blood supply) then the joint can be prone to early wear. An orthopaedic surgeon’s aim 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. As surgical techniques and manufacturing technologies advance, this can be achieved with increasing success. Indeed, from a position a few years ago where a patient with a hip replacement would expect around 10 years of life from the joint, the implants I use have a 97.8% success rate at 10 years- an incredible improvement over short time span (National Joint Registry figures, Corail Pinnacle, Ceramic on XL polyethylene).
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 1950s and 60s.
He went through a series of setbacks 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 going strong today.
The problem, however, was the plastic cup which 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 divided 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 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. Hip replacement practice in the US is now almost exclusively uncemented. Designs once again vary, and these stems were initially used 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 more elderly people. 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 forward, 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. This maintains strength through more of the bone.
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 two to three 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 counterparts on the stem side, which has grooves or pores onto or into which bone can grow. The 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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