What is a hip replacement?
Forty years ago hip replacement surgery became a reproducibly successful treatment for arthritis. In terms of quality adjusted life years (QUALYS) it is the most successful treatment intervention across all disciplines of both surgery and medicine. Despite being such a well established procedure there are continuing innovations in order to obtain the holy grail of hip replacement, the everlasting prosthesis. Unfortunately some of these innovations are ill thought out and poorly researched before being introduced into clinical practice. An unsuspecting public are often beguiled by ill informed articles in daily papers. More than one of the major hip manufacturers has been quoted as saying they will advertise direct to the public, bypassing professional opinion and peer review, in order to promote their latest product. Given the volume of worldwide sales involved the implant companies have a lot at stake financially, they therefore adopt an aggressive stance in order to achieve sales.
This article on hip replacement surgery is written by Mr Robert Marston is a Consultant Orthopaedic Surgeon at St Mary's Hospital in Paddington, London.
The Swedish Hip Register was started in the mid-1970’s and rapidly adopted by the other Nordic countries so that orthopaedic surgeons now have well documented evidence of what works in hip replacements, giving good long term results, and what doesn’t.
Don’t be a Guinea Pig
So what remains controversial? At present the following alterations to standard hip replacement or technique are being debated. Patients need a well informed, dispassionate opinion by their surgeon or general practitioner.
1. Bearing surfaces and articular head size
2. Minimally invasive surgery
4. Computer assisted surgery
1. Bearing Surfaces and Femoral Head Size
Most recent research has concentrated on the bearing couple of the hip joint, the material which acts as the articulating surface of the ball and socket. The nature of the material which acts as the articulating surface of the ball and socket is critical to the final success of the implant. This traditionally has been a metal ball bearing on a high density polyethylene (HDPE) socket. Polyethylene wear particles are known to incite a macrophage (immune) response in some patients which leads to bone loss and eventual loosening of the implant. This tends to be more common in younger active patients. In view of this, articulating couples avoiding polyethylene have been developed such as ‘metal on metal’, ‘ceramic on ceramic’ and ‘ceramic on polyethylene’. All these combinations are commonly used in patients at present and are being appraised. New types of polyethylene are also being tested, for example, ones impregnated with vitamin E are in development. The presence of vitamin E is thought to make the polyethylene more resistant to wear by having a greater resistance to oxidative breakdown.
Metal on metal articulations are very hard wearing. However, there are some isolated reports of an accelerated form of inflammation (lymphocytic vasculatis), which causes pain and soft tissue erosion around the implants. It seems that it is likely that these problems are due to a hard metal bearing on a softer metal. However, this problem might be due to the way in which the implants are orientated, especially the pelvic socket or ‘acetabular component’ which can increase this risk. Ceramic on ceramic articulations are extremely hard wearing however there have been instances of catastrophic failure. Since the only material harder than ceramic is diamond it means that any future revisions must have a ceramic on ceramic bearing since it is impossible to rid the joint space of particulate ceramic material. In addition, some patients with ceramic bearings complain that their hip “squeaks” due to problems with surface lubrication in these types of bearings.
For many years the European standard head size was 28 mm. This was a compromise between stability (which increases with increasing head size) and wear of metal on polyethylene implants (which reduces with head size). Over the last ten years there has been a tendency to have larger bearing couples since this is thought to not only improve stability but also improve range of movements.
2. Minimally Invasive Surgery
Minimally invasive surgery started in the United States. Initially three separate 2 inch incisions were used, later modified to two separate 2 inch incisions. Surgeons using the technique in Europe realized that there was an unacceptably high incidence of nerve injury, femoral fracture and implant mal-position. In the UK and mainland Europe the ‘two incision’ technique has been abandoned, however, a single incision from the front of the thigh ‘anterior’ or single incision from the back ‘posterior’ approach has been developed with success. Those of us who are routinely using a minimally invasive approach feel that there are benefits to the patient in terms of reduced pain; blood loss and rehabilitation time, however, prospective randomised studies are yet to verify this. My own approach involves a 6 to 7.5 cm posterior incision whereby the sciatic nerve is seen and protected throughout the procedure. The main muscle (gluteus maximus) is split parallel to the line of its fibres. The only muscles that are actually cut are the short external rotators which are then sutured back together again. My patients usually have the operation in the morning; get up full weight bearing on the same afternoon with two crutches (full weight bearing) and use two sticks the following day. On their second postoperative day they are normally walking with one walking stick having succeeded walking up and down stairs. Patients stay in hospital for between two and four nights postoperatively. It is rare for patients to go home on pain relief any stronger than paracetamol.
Although often proclaimed as a new development, hip ‘resurfacing’ is in fact the first commonly accepted type of hip replacement. It was originally developed by Smith Peterson in the United States and remained the gold standard of joint replacement until it was overtaken by the reproducibly reliable results of the Charnley Hip in the early sixties. Over the intervening decades a variety of surgeons around the world have tried to make the technique work in the long term. The most recent reincarnation of hip resurfacing was developed in Birmingham. Articles often give the misleading impression that it isn’t a total hip replacement. It is a disingenuous marketing ploy to suggest that a procedure that replaces the bearing surface of the femur and acetabulum is anything but a hip replacement. Whilst a cap is placed over the worn head of the femur, and therefore the femoral head itself is not removed, most surgeons would agree that to do a resurfacing properly requires a much greater surgical exposure. The idea that it is “minimally invasive” is in my view grossly misleading and untrue. Granted a small amount of spongy (cancellous) bone is not removed in the upper end of the bone (femoral metaphysic), however, the subsequent surgical dissection is much more involved. Of more concern is the fact that there are no long term studies of the latest resurfacing methods. The longest follow up from Birmingham at the present time is eight years. Previous published studies show an increasing failure rate, probably due to a local lack of blood supply (avascularity) of the femoral neck and head, from eight years onwards. It remains to be seen whether the Birmingham hip avoids the mid term failure that has accompanied all previous “resurfacing” techniques.
4. Computer Assisted Orthopaedic Surgery (CAOS)
There is a movement among certain orthopaedic surgeons to suggest that the gold standard of surgery is that performed with computer navigation. . The adaptation of techniques that have been previously used in precise positioning (stereotactic) neurosurgery have been adopted for use in the knee, and more recently the hip. In the knee it certainly has been shown that there is greater accuracy of alignment of components, however, this has not corresponded with any studies to show improved outcome in terms of pain, function or longevity of the replaced joint.
Recently computer navigation has been used for hip replacement. Undoubtedly the correct inclination and angle of the acetabular component is vital. Incorrect positioning of implants can lead to instability, increased wear or loosening. Interest in the use of computer navigation has been particularly focused on metal on metal bearings. Whilst papers have shown that patients with problems following metal on metal hips more commonly have unacceptably high inclination angles or degrees of ante-version, none of these papers discuss findings or procedures at surgery such as removal of anterior or posterior bone growths (osteophytes), which are a cause of impingement of the components themselves.
My choice of hip replacement is based on the evidence of the Scandinavian Hip Register and my own experience of working in orthopaedics since 1988. In addition I get referred patients with hips requiring revision and therefore have first hand knowledge of the types of hips that fail unacceptably early.
Based on that, my choice is an un-cemented metal on metal hip replacement in patients under 70 with the exception of women with advanced osteoporosis. Over the age of 70 I suggest that patients have a cemented total hip replacement with a ceramic head articulating on a high density polyethylene socket. Because the components are cemented into what is normally softer bone at this age there is immediate reliable ‘osseo’ integration of the prosthesis into the patient’s bone. Patients who have neurological conditions or previous stroke are given a constrained socket or a larger head size since they have an increased risk of dislocation. Finally some patients present with abnormal anatomy due to developmental dysplasia of the hip, previous surgery or trauma. Sometimes the abnormal bony anatomy dictates that they require a CAD CAM (Custom Aided Design Custom Aided Manufactured Hip). This is a bespoke hip which specifically recreates the correct anatomy specific to the patient.
Decision making in surgery should be based on ‘risk’ versus ‘benefit’ analysis. Whilst new technology is often promoted as being superior the benefits are often hypothetical and the risks unknown. Although “new” makes column inches - patients should be aware that it does not always equate with “better”.
Read the Hip Fact Sheet
Mr Robert Marston is a Consultant Orthopaedic Surgeon at St Mary's Hospital in Paddington, London. He works privately at The Hospital of St John & St Elizabeth where he also runs an emergency trauma unit for elderly patients. His areas of special interests are in primary and revision hip and knee arthroplasty, minimally invasive hip and knee replacement and lower limb trauma. Mr Marston's philosophy is to treat patients as efficiently as possible in order to minimise their pain and inconvenience following surgery for arthritis or trauma. He refuses to compromise patients' long term outcome by putting them on the cutting edge of unproven technology.
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