Hip Resurfacing by John O'Hara
by John N O’Hara FRCS FRCSI MCh
April 22, 2010
NOTE: The Birmingham Hip Resurfacing was originally manufactured by the same company that is now manufacturing the Adept, so when he refers to Adept the design and metallurgy is the same for the BHR
The original or very first resurfacing that we know of was made of solid Teflon on both sides. It was designed by John Charnley, who was dissatisfied with the results and changed instead to develop metal on plastic hip replacements.
Although John Charnley’s experience with Teflon was poor, other surgeons in the sixties and seventies, such as Heinz Wagner in Nuremberg and Michael Freeman in London, experimented with metal on plastic resurfacing. This was little better than Teflon on Teflon resurfacing.
At the same time as the poor results of metal on plastic resurfacing were becoming apparent in the 1980s in Birmingham, the good long-term results of metal-on-metal hip replacements performed between about 1964 and 1978 were becoming apparent, as there were scores of patients in the clinic who had not been discharged and who had had prostheses inserted over 20 years earlier.
The Birmingham hip resurfacing and the Adept hip resurfacing uniquely reproduced the same metallurgy and the same bearing gap as was used in more than 20 metal-on-metal hip replacement patients in Birmingham, from whom we obtained prostheses after more than 20 years.
It should be understood at the outset that apart from these two brands of resurfacing, all other metal-on-metal resurfacing prostheses available today are not the same metallurgically as these in a variety of different ways or have a bearing gap of less than 200 microns, again a feature essential to the longevity of the prosthesis, as all of the patients with metal-on-metal hip replacements that we were able to look at had bearing gaps of 200 microns or more. The durum and ASR resurfacings both had dangerously narrow bearing gaps of 100 microns.
Again at the outset, it should be known that apart from the Birmingham hip resurfacing and the Adept hip resurfacing, other manufacturers product carry various fundamental design flaws in their metallurgy, which are convenient to overlook when resurfacings give less than optimal results. The long-lasting MoM hip replacements were all of “as-cast” high-carbon chrome-cobalt. These bearings have immensely hard carbide “icebergs” sitting on their surfaces. As they run-in the prominent carbides wear down again each other and in doing so produce a smooth carbide “glaze” on the surface. This takes about ten or twelve months to occur and patients who previously had a slightly stiff start-up for the first few steps find their hip has miraculously loosened, - and remains so permanently. At this point we find that the rate of metal wear, as measured by their urinary or whole blood metal levels, falls significantly and permanently.
The Wright Medical, “Conserve Plus”, has a hard on soft bearing which makes one bearing surface sacrificial. The Corin bearing has been double heat treated and hot isostatic pressed – unnecessary processes (but they save the company money) which cause the carbides to move away from the bearing surfaces, so failing to protect the surfaces in the longer term.
There are at least three established causes of disappointment in patients who have had metal-on-metal hip resurfacings.
Aseptic lymphocytic vasculitis associated lesions (ALVAL) is poorly understood and may well be an inflammatory response to metal particles, which are part of the normal wear, bedding in and wear process. It is possible that this allergy arises in some patients because the rate of initial metal shedding during bedding in and wear is higher than normal but this is not yet known for certain.
In Birmingham, with an experience of more than five thousand resurfacings, we are not aware of a single patient with this problem.
The second problem commonly cited is inflammatory pseudotumors. Again the aetiology of this is not well understood but essentially a destructive process begins in the soft tissues, which damages muscles and sometimes bone around the resurfacing.
Again in Birmingham, I am aware of only one patient in our extensive experience of hip resurfacings who has developed this problem and this patient has been investigated by the team in Oxford.
It is also important to understand that in large centres with big experiences of either the Birmingham hip resurfacing or the Adept hip resurfacing, the complications listed above are virtually unknown. In the Nuffield Orthopaedic Centre I am aware that the principal implant that they use is not a Birmingham hip resurfacing or an Adept hip resurfacing and that only a small minority of resurfacings in the Oxford area are using either of these two prostheses.
I am also aware the initial batch of the problem patients in Oxford that drew this problem to the knowledge of the Oxford surgeons were operated upon by inexperienced, unsupervised juniors on Monday mornings while the boss was recovering from Sunday night. Once the surgeons in Oxford made this known, they attracted all other dissatisfied patients. AT no point have the Oxford group properly analysed the alignment of their prostheses, glossing over it and assuring us that their alignment was “OK” –but with the unaided eye, you can see that they aren’t.
Inaccurate sizing and implantation is the third problem that has arisen with resurfacing.
For the last 12 years it has been my practice to size the resurfacing component from the functioning diameter of the femoral head, whereas misguided advice suggesting that the component should be sized from the narrowest part of the femoral neck, is still commonly heeded.
In women, just as their bodies have a better formed waist than men, the femoral neck diameter is narrower, relative to the head diameter (they have a better head-neck ratio, so explaining how young girls can get one of their feet in their mouth) and thus it is very possible for a surgeon referencing the head size off the neck to put in a component smaller than is ideal. For instance, on one “leading” surgeon’s website the average resurfacing femoral component diameter is 4.6 mm narrower than the original femoral head diameter. (It should be 3mm larger than the subchondral outline, as the functioning diameter has hyaline articular cartilage all around the outside) For every 2 mm loss of diameter the patient loses approximately 8 degrees of flexion movement, as well as losses of movement in other directions. This loss becomes more critical, the smaller the femoral neck. It can thus be easily seen why women and patients with smaller femoral components are dissatisfied with their operations, as the loss of movement as a result of inadvertent under sizing can mean that even 6 or 12 months later they cannot get their knees to their chests or get to their toe nails. You wouldn’t go down to a tyre shop and have them put a new smaller/ lower profile one of your wheels! (Or would you??)
A recent work in the JBJS Am by our own Callum McBride has been shown that where the patient is female and where the femoral head size is less than 46 mm that there is a slightly increased risk of femoral neck fracture. However, it should be known that the cohort where this thesis arose from was necessarily undersized.
We have also learnt in recent years that the accurate implantation of the socket component is critical, not just to the stability of the hip and to the correct range of movement but also to reducing the wear on the bearing. If the head component is smaller than originally and the socket component is correspondingly reduced, the patient loses range of movements as indicated above and having hit the end point of movements, the ball is jacked, under significant muscular pressure, out of the socket causing edge wear at the back. When the hip moves forwards and backwards, ideally a fluid film should form between the two bearing components. If the range of movement comes to a sudden end because of under sizing or of socket mal-orientation, the fluid film is not as likely to form and the bearing is likely to wear more. It is true that the larger the ball is the easier it is to achieve a fluid film but it should be known that a small but significant number of the patients who had metal-on-metal hip replacements extracted for analysis after death had bearing sizes of only 38 mm and had minimal wear.
There are also patients (including 12 of my own), where the socket of the correct size has been implanted without enough anteversion, or forward tilt, to facilitate flexion – getting the knee up to the chest. All of the patients, when I have changed the socket for one better orientated, have enjoyed an improved range of movement and have eliminated their pain.
The failure on the femoral side (4% in my first 1000 cases with poor BHR instrumentation) has been overcome with the notchproof (Medial Reference) jig is 0% in 420 consecutive cases. The natural failure rate of a correctly implanted socket in the first 1000 is 1 in 300 and O% in the 420 with better acetabular instrumentation
In summary, it can be seen from the above that there have been some problems across the country and across the world with metal-on-metal resurfacing. These problems originate from failures of manufacturers to heed the lessons of history, in terms of bearing design, bearing gap and bearing wear: the number one hip simulator is the human body and it is only the human body that will eventually sort out the good and bad designs.
Accuracy of sizing and implantation is of cardinal importance and it is highly regrettable that manufacturers have, with the exception of the Adept prosthesis, completely failed to address these problems. It is clear that in an experienced surgeon’s hands, an accurately sized, accurately implanted, correctly designed prosthesis has a service life of more than 10 years in 95% of patients. One would hope that for the future other manufacturers would not wantonly tarnish the reputation of resurfacing by continuing to raise adverse publicity whilst ignoring the huge majority of patients that have enjoyed a new lease of life following this operation.
J N O’Hara FRCS FRCSI MCh
Consultant Orthopaedic Surgeon