HIP REPLACEMENT IN YOUNG ADULTS
Hip replacement is one of the most successful operations and in majority of the patients the quality of life is tremendously improved. However, a hip replacement is a mechanical device and like any mechanical device will not last forever.
In a conventional hip replacement, the ball and socket of the hip joint is replaced by a metal on poly bearing surface (metal ball and plastic cup). It has been shown that the plastic cup wears with use and sheds minute plastic particles (micron sizes). These particles evoke a foreign body type tissue reaction which leads to gradual loosening of the hip replacement. This may eventually lead to the patient requiring revision surgery which is more complex than a first time hip replacement. In short, the conventional plastic cup is the main weak link in a hip replacement. Despite this, the conventional types of hip replacement work well in most patients with reasonably sedentary lifestyles.
However, increasing numbers of young and active patients are being offered hip replacement who wants to maintain their active lifestyle even after a hip replacement. There are three major factors which require special consideration in this group of patients.
Higher activity level: The young and active patients have much higher expectations of functional outcome from their hip replacements (work related activity as well as recreational activities). Wear of a mechanical joint is dependent on its usage. This is not dissimilar to the car tyres wearing out much faster in a car which is clocking 30,000 miles per year compared to somebody using their car for mainly school runs. While it is easy to change the car tyre easily, revision hip replacement is a much more complex procedure! Studies have shown that the activity level in a very active person undergoing a hip replacement can be 40 times more than the activity level of a sedentary person undergoing a hip replacement. The conventional plastic cup is likely to wear earlier like the car tyres in a high mileage car.
fig 1 Poly wear in a cememented cup with gross loosing due to osteolysis (inset shows probable original ball positions before polywear).
fig 2 Poly wear in an uncemented cup with gross loosening due to osteolysis. Retrieved components during revision surgery showed that the metal ball has not only worn out through the plastic, but also through the titanium metal cup (inset shows probable original ball position before poly wear.
As the main weak link seems to be the plastic component of the cup, material scientists and implant manufacturing companies have focused their efforts on developing materials (so called “alternative bearings”) which have better wear properties and hopefully may last longer even in young and active patients.
Better stability and increased range of movement: One of the issues in young and active patients is the fact that they expect to maintain their active lifestyle even after a hip replacement. Dislocation of a hip replacement is a known complication. The stability of the hip and the range of movement which can be achieved following a hip replacement depend on a number of factors including patient factors, surgeon factors and design factors. The two main design features which can have an influence are the “size of the ball” and “the head neck ratio”. The larger the ball size, the more stable is the hip replacement. The greater the head to neck ratio, theoretically there may be better range of movement.
Due to the material properties of conventional plastic, Sir John Charnley (the pioneer of hip replacement), chose 22.25 mm metal ball and matching plastic cup as the choice in the design choice in his hip replacement (Charnely hip replacement). Many surgeons have developed further designs with conventional plastic cups and metal balls which generally have a 28 mm metal ball and a corresponding matching plastic cup. However, 28 mm is still much smaller than the size of the natural ball (femoral head) of the hip joint (which varies generally from 40 mm to 54 mm). Further increase in the ball size beyond 28 mm when metal ball is coupled with conventional plastic cup has been shown to increase the wear rate. The “alternative bearings” allow bigger size ball (generally 36 mm with some limitation based on the size of the cup although slightly larger size balls are now being introduced by some implant companies for larger cups). The metal on metal bearings allowed even larger sizes “anatomical size ball” to be used (same size as the ball of the natural hip joint).
Bone preservation: Although improvement in materials have been made, it is still likely that further revision surgery will be required. Revision surgery is more complex than primary hip replacement and it may be useful to preserve bone stock as much as possible during the primary procedure. One of the techniques of bone preserving surgery which become popular about a decade ago was hip resurfacing. In this technique the ball of the hip joint is not completely removed and instead the ball is resurfaced to accept a metal cap which is usually cemented. The bearing surface was made up of metal on metal. This concept of resurfacing was not new and in fact when surgeons started hip replacement in very early days they tried to mimic natural hip. However, the early designs were made of plastic (polyethylene) and they were worn out very quickly and had very high failure rate. With improvement in the metallurgy and machining, hip resurfacing became metal on metal bearing designs. This also allowed large anatomical size balls to be used with very little risk of dislocation. The hope was that it would allow young and active patients to pursue their active lifestyle with little concern of dislocation.
What are the “alternative bearings”?
There are three main type of alternative bearings –
Cross Linked Poly
Ceramics: Ceramic components are now made of very strong ceramics. They have extremely high wear resistance and good lubrication properties. Very rarely they can fracture or squeak.
Metal on Metal
While all the three materials have shown better resistant to wear compared to conventional metal on plastic bearing (both in lab studies and short term clinical studies), they do not have long term clinical results (being modern!). In addition, like any modern invention the benefits come with some additional problems. The alternative bearing materials are more brittle than the conventional plastic and tolerate less optimal placement of the components less well than conventional plastic cups. While every surgeon tries to place the components in the correct position, it is very difficult to be precise and accurate in clinical practice as there is considerable variation in the patient’s morphology and anatomy of the hip joint and the difficulty in judging the position in a deep seated joint like the hip joint.
While the ceramic components can break, they are now very rare with the improvement in material properties and manufacturing process. Cross linked plastic components can also break rarely. Both metal and ceramic components can also occasionally result in squeaking.
While metal on metal bearings were considered as one of the better options and surgeons started using them widely in young patients, over the next few years, reports of high early failure rate particularly with certain implant designs started appearing. In addition to specific designs, factors like gender, size of the ball, component positioning were found to influence failure rate.
In April 2010 MHRA (Medicines and Healthcare products Regulatory Authority) issued an alert to the high failure rate of metal on metal hips and advised monitoring of patients with metal on metal hips with metal ion levels in the blood.
Over the ensuing few years, metal on metal bearings including resurfacings gradually fell out of favour and is done very infrequently.
Current view: As the metal on metal hip replacement have gone out of favour, the most common bearings used in young adults are either ceramic on cross-linked poly or ceramic on ceramic types as they have better wear properties.