Types of Hip Replacement
There are many different types of hip replacement or hip arthroplasty, allowing orthopaedic surgeons to choose the correct technique and hip prosthesis for the patient's particular clinical presentation. The main hip replacement causes remain osteoarthritis but different hips are available for rheumatoid arthritic patients and trauma. The hip arthroplasty definition is the replacement of the two sides of the arthritic joint with artificial parts, in this case a metal thigh component and a metal or plastic cup.
Hip pain is the main indication for hip replacements with the commonest reason being degenerative joint disease or osteoarthritis. Other reasons for a replacement hip are avascular necrosis of the femoral head, hip dysplasia (congenital abnormality of the hip), rheumatoid arthritis, ankylosing spondylitis and fractured neck of femur or broken hip.
With the steady increase in the elderly population in western industrialised societies there will be a continuing rise in hip replacement surgery and hip replacement costs although the costs of knee replacement are likely to surpass hip replacement if they have not already done so. The choice from the types hip replacement prosthesis available has important consequences for future expense and surgical management.
Total Hip Replacement
Of the major types of hip replacement joints the best long term results are from the Exeter, the Stanmore and the Charnley hip prosthesis types. All have published results over many years which proves their reliability and durability and that they will have predictable results in future patients. These are all cemented designs which use a tapering femoral component and a polyethylene cup. This video shows the femoral or thigh component of an Exeter hip.
Ceramic Hip Replacements
As hip replacement has become more and more successful the technique has been gradually applied to patients in younger age groups, leading to questions about durability over time. An average hip replacement may last 15 years to 25 years which is a great achievement, but younger people will live much longer and are known to wear their hip replacements out much more quickly than older people.
Revision hip replacement, when the hip is redone, can be performed but is more complex and more risky than initial or primary replacement, leading to somewhat poorer results overall. The aim is for the replacement to last longer than the patient does!
Ceramic hip replacement has been developed to resist the physically stresses put upon hips by younger people and to wear less quickly. Regular steel and plastic hips only wear the plastic socket by about one millimetre a year, but this generates huge numbers of small plastic particles which are suspected of contributing to loosening of the implants with time.
A ceramic hip implant is very smooth and very hard, with a much smaller rate of wear than the metal on plastic designs. However, it is not yet known whether they will have better results in the long term as studies have not been following them for long enough yet.
Cementless Hip Replacement
Cement-less types of total hip replacements are used for younger, more active patients but there are concerns as to whether the bone really does grow into the design effectively and about the relatively high incidence of thigh pain. Revision surgery is much easier than in the cemented technique. Hip replacement methods vary considerably and surgeons usually favour one method over the others.
Hip resurfacing is a newer technique with good results over ten years or more and differs in that very much less bone is removed during the procedure. The design minimised the risk of dislocation and allow people to get back to normal activities that might not be advisable for total hip replacement.
This technique is promoted as particularly for younger, more active patients and still retains the possibility of doing a hip replacement easily in the future. However the results of this technique may still not reach the gold standard of cemented hip replacement.
Understanding what goes on during the development of hip arthritis and then during a hip operation can be difficult so watching a hip replacement video can be very useful to get a visual idea of what is happening. Understanding more of what is going on can help the patient participate in hip replacement exercises with the physiotherapist and improve their rehabilitation
The top (or bottom) line
What matters in choosing from the hip prosthesis types is the presence or absence of published long-term results. Any good replacement design will have results published following the patients over five, ten and more years. The “survival” (ie where the replacement remains in place, functioning well) is recorded, the level of complications such as infection and loosening, the frequency of revision (redoing of the operation) and the quality of people’s lives.
If you have a hip replacement which has good published long-term results, you can be sure you are getting a proven technology, performed by staff who are very familiar with the technique.
The Exeter, Charnley and Stanmore hip replacements have long-term results in the UK, along with others. The Exeter design will soon be implanted for the 500,000th time! Ask your surgeon, he or she should be able to give you a detailed answer about the success of the particular design they have in mind.
Fixing the replacement to the bone
Hip replacements are fixed by mechanically jamming into the bone, either with or without cement in-between. Without cement the metal or plastic is fixed by jamming it firmly into the bone, although as the thigh bone is curved and the stem of a replacement straight, the fit is not perfect and there are areas of greater and lesser contact and pressure.
Cement does not bond to the bone like glue but rather fills in all the gaps and is pressured into the bone structure.
Design of the prosthesis
Design has evolved from surgical experience, with Philip Wiles in 1938 covering both the surfaces of the socket and ball with metal. McKee in 1940 pursued the same approach, but only 50% were successful. In 1948 he used the Thompson prosthesis with a metal socket attached to the bone with cement. This is the first real hip replacement.
Sir John Charnley is the man who will always be associated with the development of the first really successful hip replacement. His initial effort with resurfacing the joint surfaces with Teflon sheeting failed. He then reduced the head size and adopted a cup made of high-density polyethylene and this design forms the basis of all modern designs.
The McKee Farrar prosthesis gave good results for thirteen years but produced a sludge of metallic material and tended to loosen. The metal head/polyethylene socket design took over, such as the Charnley and the Muller.
Initially the complications were fracture of the stem of the femoral component and loosening of the components without infection. Both of these problems are related to the effectiveness of the cement surrounding the components. Insufficient cement support can lead to stem fracture and if the thickness of the cement is less than 5mm then loosening visible on x-ray is increased.
To counteract this the stem material was altered to a stronger alloy and the design changed. Cementing technique was also refined and these two improvements were referred to as ‘second generation’ technique. This includes thorough cleaning of the prepared bone and pressurising cement into the bone structure.
Harris in Boston (1982) reviewed a group of hip replacements done with second generation techniques. At just over 11 years the femoral component had an incidence of loosening of 3%, 3% of components had migrated in the bone, 24% showed x-ray signs of loosening around at least 50% of the cement, 7% showed (?)endosteal bone loss. The second generation cementing technique seems to be beneficial in all cases but the alteration in stem types was not advantageous at times.
In 1986 research in Boston compared the same replacement with first and second generation cementing techniques. In the replacements done with first generation technique there was a higher number of migrating components and defects in the cement mantle. In Exeter in 1988 research showed a significantly reduced amount of socket migration and four times fewer revisions using second generation cementing techniques.
There are advantages to second generation cementing techniques but they may be outweighed by variations in stem design. Cementing technique buys time, puts off the onset of loosening and reduces the rates of revision but cannot compensate for poor stem design in younger patients, who have their prostheses for longer periods than 15 years.
Cementless hip replacement
The main problems with cemented hip replacements, loosening and loss of bone stock, have impelled a new look at replacement without cement. The aim has been to encourage bone growth up to and sometimes into the implant, but this can best occur when the implant and the bone are very closely placed together.
Without the cement there are gaps between the bone and the implant due to the differing shapes of the femoral canal and the metal implant, sizes varying but up to 2.2mm. Even with the special coatings developed for the implants to encourage bony ingrowth there is likely to be only partial success.
A study in 1992 of cemented and uncemented hip replacements showed the uncemented stems had a 10% revision rate by five years, and suffered from thigh pain in 28% of patients, loosening in 33% and bony absorption in 67% of cases. This makes this type of replacement perhaps less useful in young active males, the exact group for whom it was thought to be most helpful.
Due to the differences in elasticity between the metal, plastic and bone it is inevitable that some movement takes places in uncemented implants. There is no cement to help transfer the load across from the implant to the bone.
How surgeons can tell a hip replacement is failing or likely to
Apart from the reports of the patients the main source of data on which the surgeons rely is the x-rays taken soon after operation and during the follow-up period in the years that follow.
Danger signs that all is not well on x-rays close to the operation time are the appearance of radiolucent lines between the cement and the bone, a varus orientation of the hip and a deficiency of the cement surround.
Later x-ray findings which make the likelihood of revision much higher are the radiolucent lines as above, a loosening of the lock between the stem and the cement, and the tendency for the artificial components to migrate.
Other danger signs are a tendency for the hip to migrate into a varus position, absorption of bone appearing at the area of the cut femoral shaft and the gradual disappearance of bone from within the shaft of the femur. Migration of the components and the development of radiolucent lines are both unfavourable signs and hip replacement techniques are aimed at minimising these.
Looking at successful and failing hip replacements
It useful to try and separate the role of the surgeon’s technique from the long-term stresses the person puts on the artificial joint. In one study of femoral stem implants which had survived more than five years several types of prosthesis were looked at, including muller and exeter varieties.
The most important factors which the surgeon could influence were, in order, the adequacy of the cement mantle around the upper third of the femoral stem, the soundness of the interface between cement and bone, the geometrical design of the prosthesis and the orientation in which the prosthesis had been inserted.
The factors correlating most closely with the need for revision surgery were, in order, subsidence at the interface between the cement and the bone, resorption of bone at the cut femoral surface, radiolucent lines between the cement and the bone, migration of the stem into varus and migration of the stem distally.
A similar study involving revision of the cup components surviving for ten years showed that those cups requiring revision had their anatomical axis medialised in 79% of cases compared to 9% of cases in cups surviving for over 14 years.
Finally, the choice of components
Choosing the right components seems to be related to the geometrical design in the femoral stem but not in the cup. The femoral stem performs better in cemented cases, but this does not seem to matter in the cup. The Exeter stem has the best results for fixation.
However good the components, they have to be implanted with good surgical technique as well, or results could be poor. In the cups, getting the orientation as close to the normal axis of rotation is the main factor in ensuring a long life in the component.