By: 14 January 2019
A look ahead to the future of hip replacement and hip resurfacing

Derek McMinn and Ronan Treacy give OPN an update on the hip replacement and hip resurfacing market

OPN: It has now been over eight years since we last talked. We are all aware of the significant change in relation to the world of metal-on-metal hip replacements and hip resurfacings, with several devices being withdrawn from the market or greatly restricted in their prescribed application. Can you both update us on your personal experiences to date?

RT: The Birmingham Hip Resurfacing®(BHR) is now over 21 years old, and both Derek and I have implanted this device since 1997. My personal results, published at a minimum follow up of ten years,demonstrated the survival for male patients was 98 per cent. Survival of the total cohort with aseptic revision as the endpoint was 95.5 per cent.The median modified Oxford hip score at ten years was 46 (48 being the best possible score), and the median University of California, Los Angeles score was 7.0.

Additionally, we subsequently published our experience in the
< 50 years age group up to 14 years post-operatively with no loss to follow up. Our overall cumulative survival of 94.1 per cent at 14 years was satisfying. When we looked at aseptic revision as the endpoint, the survival for 195 men with primary osteoarthritis was 100 per cent at both ten years and14 years. And in 109 women with primary osteoarthritis it was 96.1 per cent at10 years and 91.2 per cent at 14 years.

DM: In the excellent publication from Baylisset al, The Lancet, April 2017, the lifetime risk of revision following a total hip replacement in men aged 50–54 years is 30 per cent. We asked, can hip resurfacing work in this challenging group of patients? We reported our 12 to15-year implant survival assessment of a prospective single-surgeon series ofBHR, which focused on the earliest 1000 consecutive BHRs including 288 women(335 hips) and 598 men (665 hips) of all ages and diagnoses with no exclusions.When we looked at revision for any reason as the endpoint we found ourKaplan–Meier survival analysis to be 97.4 per cent and 95.8 per cent survival at 10 and 15 years, respectively. Radiological assessment at that time showed11 (3.5 per cent) femoral and 13 (4.1 per cent) acetabular radiolucencies which we believe may be because of stress shielding at the acetabular component to host bone interface. This publication shows that the performance of the BHR continues to be good at 12 to 15-year follow-up.

In the patients who are under the age of 50 years with a diagnosis ofPrimary Osteoarthritis, women have a 99 per cent implant survivorship at 20years, just ahead of men with a 98.5 per cent implant survivorship at 20 years. We found it to be very interesting that this group of young women performed so well when all previously published data had shown the inferiority of MoM resurfacing in women.

RT: The 14th National Joint Registry report(NJR) notes a significant reduction in the use of Hip Resurfacings (less than 1per cent) and subsequently the increase in the use of traditional hip replacements in the younger active patient groups. The NJR reports a higher revision rate for young patients having a total hip replacement.

Female patients under 55 years of age demonstrated a survivorship of86.5 per cent at 13 years. At 13 years the survivorship for the under 55 males across all bearing types is 90 per cent.

OPN: Why do you believe these noticeable differences in women occur?

RT: When the NJR compared their own data with a view to the best performing hip resurfacing procedures by brand, they note a slightly better survivorship of between 92 per cent and 91 per cent. Derek has identified some interesting findings in relation to female patients considering the potential effect of nickel sensitivity accumulated over time.

DM: The data collected at the McMinn Centre suggests a difference in the age-based subsets between men and women. As we have discussed, men in general perform well to 20 years with a metal-on-metal resurfacing. When we look at the women we noted in the group over the age of50, survivorship falls to 92.7 per cent at 20 years, a stark contrast to women under 50 years who demonstrate a 99 per cent survivorship at 20 years.

OPN: What do you think is the cause, Mr Treacy mentioned sensitivity issues?

DM: We were motivated to come up with an explanation of potential contributors to this outcome. Mr Joseph Daniel and the research team at the McMinn Centre set to work reviewing the causes of failure in all my female patients over the age of 50. There turned out to be 502 patients in this group. We had three infections and eight femoral head failures, but by far the biggest contributor were 15 metal reactions. Implant retrieval analysis showed an absence of high wear and edge wear in most cases. We had previously reported on this during 2012 at the American Academy of Orthopaedic Surgeons (AAOS). The McMinn team presented, ‘High incidence of nickel sensitisation at 10 years: A review of metal-on-metal arthroplasty.’ Pre-revision testing of metal-on-metal resurfacings for metal allergies revealed 75 per cent of these patients tested positive to nickel.

This work led us to explore other nickel-containing implants as a comparison. We performed a cross-sectional assessment of metal hypersensitivity in four individual groups; patients with BHRs (10 year min), historical controls with no metal exposure, preoperative patients with no metal exposure and patients with metal-on-polyethylene cemented Exeter total hip replacements (10 year min). Our results were fascinating. Those individuals with no metallic devices at all exhibited a 24 per cent sensitivity to nickel, those with metal-on-metal resurfacings 52 per cent, but the surprise for us was the metal-on-polyethylene total hip replacement group who demonstrated 57 per cent sensitivity. The long-term impact of these findings needs to be explored further.

OPN: The hip resurfacing surgeon community is relatively small and close-knit, is there any combined resurfacing registry data that can support the continued use of the hip resurfacing procedure?

RT: Yes of course, Catherine Van Der Straeten who is the Head of the Institute for Health Research and Innovation at GhentUniversity Hospital, has drawn together the largest international hip resurfacing database in the world. She has collected data from 27 centres and13 countries in patients having all types of hip resurfacing under the age of50 years at surgery. She presented her findings most recently at the ISTA 2017meeting in Seoul, Korea. In all, she reported on some 11,386 metal-on-metal hip resurfacings of various designs. Selfishly, we are only concerned with the performance of our own design, and so Catherine kindly demonstrated the results for the BHR. She looked at the results for patients <=50 years of age with a diagnosis of osteoarthritis, the number of BHR’s in her data set was 4,056 implants. When she looked at the male and female results in isolation, she reported 3,187 male patients with a Kaplan–Meier survivorship of 96.2 per cent to 20.1 years. In female patients the numbers were lower, at 869 implants who demonstrated an 86.8 per cent Kaplan–Meier survivorship at 19.5 years. We remain encouraged by the performance of our design.

OPN: The obvious question is what is next for Hip Resurfacing, will we see a resurgence?

DM: Despite the wonderful results of the BHR outlined above, for a variety of reasons MoM resurfacing is on its way out.

OPN: Where do you see the hip resurfacing market going, will metal-on-metal make a comeback?

RT: When you consider our hip resurfacing design in isolation, I truly believe the orthopaedic community, and the wider industry in general, threw the baby out with the bath water. We left our active females high and dry and drove them into the shadows to seek alternative treatments. Many men were given unnecessary THRs, and once again the procedure was demonised by the poor outcomes and association with unsatisfactory designs.We do not see metal-on-metal making a comeback in our lifetime.

OPN: How about alternative materials?

DM: As you will know, our friend and colleague ProfessorJustin Cobb’s well publicised Ceramic on Ceramic H1 Hip Resurfacing Study is under way. We are pleased to have another keen hip resurfacing addict attempting to keep the technique alive.

In our revision experience we have seen a number of cases of severe acetabular stress shielding behind the BHR cup at long- term follow up. Given that alumina ceramic is twice as stiff as cobalt chrome, we do not believe that ceramic cups are the way forward. We are much more attracted to the idea of titanium porous coated cross-linked polyethylene cups, which have a stiffness closer to bone.  

OPN: Back in 2010 you ruled outPolyethylene as a suitable material for use in Hip Resurfacing, you stated, andI quote directly from the article here; “Ceramic-on-ceramic was a non-starter;the components were too thick for a conservative arthroplasty and brittleness and fracture was an issue. Polyethylene had been tried but the volumetric wear associated with the necessarily large head for resurfacing resulted in the Pseudo tumors of the day – osteolysis – and the five-year survivorship was less than 50 per cent”. What has changed your opinion of this material?

DM: At the time of our first interview, highly crosslinked polyethylene had yet to establish itself as the bearing material of choice for use in primary total hip replacements.

Very importantly, simulator studies have demonstrated that the wear of highly crosslinked polyethylene is very low with all femoral head sizes. This is in sharp contrast to conventional polyethylene, where increasing head size leads to increasing wear volume of polyethylene.

Once we were convinced of highly crosslinked polyethylene’s potential viability in the resurfacing setting, we set off to solve the next problem.

OPN: Interesting, what problem are you referring to?

DM: Uncemented total hip replacement cups use a porous coated metal shell and a highly crosslinked polyethylene liner. The thickness of this construct is totally unsuitable for acetabular bone conservation in hip resurfacing. We had to get a porous titanium surface directly on to the crosslinked polyethylene without the use of a metal shell. This development took seven years with various groups of engineers and considerable expense.

RT: One of the key benefits of hip resurfacing, as discussed earlier, is to preserve acetabular bone. Over-reaming of the acetabulum to accommodate a thick component is unacceptable, much in the same way as reducing the articulation thickness with polyethylene materials below 3mm
would be a concern for many of us. Derek managed to find a way around these issues.

OPN: What do you believe to be the optimal thickness of components for hip resurfacing?

DM: Our combined experience with metal-on-metal hip resurfacing spans close to three decades, this experience has demonstrated that a cup 6mm OD/ID difference is optimal for metal-on-metal. We aim to replicate this with our new design of metal-on-crosslinked polyethylene resurfacing as it offers us the ability to maximise polyethylene thickness while maintaining valuable acetabular bone stock.

OPN: How about the angle of articulation, what are your thoughts on this, we remember several references to this and edge loading during the metal-on-metal era?

RT: Clear understanding of this is critical incomponent design, and particularly relevant in hip resurfacing. We know what worked with MoM resurfacing and what added to the early failure of other designs when this issue was not fully appreciated. It would not be sensible to be creative with the design of a new resurfacing acetabular component in relation to its angle of articulation or its ease of insertion.

OPN: What about the femoral head, do you see any need for an alternative design or material here?

DM: Our experience with the cemented BHR femoral component has been rewarding over the years, we maintain the use of bone cement in this application is reliable and reproducible. However, we are conscious of some reports of over penetration of bone cement with our original design, as well as the stem being disproportionate in volume in the smaller sizes. Our new design of femoral component addresses both these issues.

OPN: You mention you have implanted some custom components, can you expand a little on your experiences with these?

RT: These devices have been employed in several useful settings where the use of a traditional total hip replacement would not be optimal in our practice, or in many cases accepted by the patients. Of interest is the cases of revision of failed metal-on-metal resurfacings and the use in patients with proven nickel sensitivity. We hope to publish on some specific case examples next year.

DM: To monitor this unproven concept as closely as we could,we took the precaution to embed RSA marker beads in the implants, and to seek consent from a small group of recipients to be part of our post implantation vigilance by undergoing RSA analysis as part of the follow up. All these patients are made fully aware of the unproven nature of these custom-made resurfacing devices, and the fact that they are not CE marked. They sign a specific consent form acknowledging these facts.

OPN: Is polyethylene hip resurfacing something we will be seeing on the UK market in the near future?

RT: Given the frequency of these custom-based referrals, we found ourselves disappointing several patients and interested surgeons with our inability to supply the custom implants more freely. We are being very circumspect with our introduction of this new technology.

DM: We have just over three years of experience with the custom metal-on-crosslinked polyethylene resurfacing. Clinical and radiographic results are excellent and there have been no revisions.

RT: Being aware of the guidance provided by the MHRA on the use of custom-made orthopaedic implants, coupled with the high production costs of such components, led us to the conclusion that the only viable route for us would be one of developing our own commercial route to progress a highly crosslinked polyethylene hip resurfacing system, in much the same way we achieved with the BHR during the MMT era.

OPN: Will we see a reappearance of MMT?

RT: Unfortunately not, that name and associated products were rounded up in the well-known acquisition by S&N in2004.

DM: We turned our focus to Jointmedica Ltd, a company which had been formed shortly after MMT was acquired. At that time its attention was directed towards the development and distribution of the Birmingham KneeReplacement (BKR) which I am now proud to say holds an ODEP 7A rating and outstanding survivorship and PROMS scores. With Jointmedica as a vehicle, we recruited a new Managing Director, changed the focus of the business and embarked on pulling together the infrastructure required to take on this new product development in an increasingly rigorous regulatory environment.

RT: During the first two years of this new direction, Jointmedica has taken residence in a modest facility, grown the staffing compliment to include some old friends from our past business successes and, importantly, we have interfaced closely with the BeyondCompliance Group to ensure the safest possible introduction and evaluation of this technology.

OPN: How is Jointmedica funded?

DM: The Jointmedica team has managed to secure project funding from the INNOVATE UK organisation. This serves two purposes in that a rigorous review of the company’s business plan and financial information has been undertaken and approved by the INNOVATE UK professionals, along with providing Jointmedica with a substantial contribution to the finances required to draw this project together. Jointmedica remains a privately-owned company,however, the renewed vigour of the organisation has resulted in additionalGrant-based activities to support its continued expansion along with additional development projects.

OPN: When do you expect the product to be commercially available in the UK?

RT: This is a difficult question to answer definitively. We and Jointmedica are aware of the heightened scrutiny surrounding the introduction of this class of orthopaedic implant and associated instruments. There can be no shortcuts to this process. Hip resurfacing history is tainted by numerous copycat devices that were rushed to market claiming equivalence, and in some cases, improvements to our original design. This should not happen again. We feel our experience of success in this discipline, coupled with the team at Jointmedica, holds the best recipe for our future success with this device.

DM: We will keep everyone at OPN up to date with our progress along the way.

OPN: Thank you, we look forward to following your progress.

® Birmingham Hip Resurfacing is a registered Trademark of Smith & Nephew Ltd