Age-Defying Implants

3 April 2008 (Last Updated April 3rd, 2008 18:30)

As living standards in the developed world continue to improve, so too does life expectancy. Lorrie Kelly talks to some experts about how the changing lifestyles of a rapidly ageing population have affected the development of joint replacement technology.

Age-Defying Implants
Advances in joint replacement technology can enable a new lease of life for older patients.

For all the benefits of better healthcare and more active lifestyles, deterioration of the body cannot be avoided – especially when considering weight-bearing joints. It is estimated that diseases of these joints affect more than 10% of the global population.

In the US alone, 50% of those over 65 have been diagnosed with some form of arthritis. Of these sufferers, only a handful achieves functional pain relief through currently available drug therapies. When pain affects a patient's overall quality of life, joint replacement surgery may become their only viable long-term option.

THE ORTHOPAEDIC IMPLANT MARKET

In 2006, worldwide sales of orthopaedic implants reached $28.6bn, with an average annual growth rate of 9%. Prior to the 1990s, orthopaedic implants, such as total hip replacement, achieved approximately ten functioning years before osteolysis would set in and ultimately lead to joint failure. This limitation restricted patient indication and most agreed that implants used in young and/or active individuals would potentially have a higher rate of failure.

"Early implants were designed primarily to relieve pain," explains Dr Jim Nevelos, franchise manager at orthopaedic implant specialist Stryker Europe. "Hip and knee replacements are among the most successful operations being carried out today. As implants have evolved, survivorship of implants has increased markedly to the point where, typically, more than 90% of implants are still functioning well and providing pain relief after ten years or more. "

"The majority of recent developments have been about improving function without compromising the longevity of the implant," he continues. "A lot of this development in hips has been based around improved bearing materials."

Worldwide knee implant sales were estimated to be $5.2bn during 2006, whereas hip implant sales were $4.5bn. Though hips have traditionally been the largest category, the global trend has seen knee replacements overtake hips.

"Future market projections are for knees to continue to grow briskly at 12 to 13% with hip replacements in the high single-digit range," says Nevelos. "With the ageing populations in the first world, and the improved designs and materials allowing for earlier intervention, continued growth [is expected] in all areas of joint replacement."

Orthopaedic implant manufacturers face two main challenges to remaining competitive:

  • To offer new solutions which represent a genuine improvement for the patient, surgeon and healthcare provider.
  • To be able to offer those solutions in a cost-effective way.

"As technology improves and treatments become available to more people, the costs of providing those treatments increase," says Nevelos. "Therefore, any product must not only have a strong clinical history or heritage and strong technical data to support it, it must also be able to demonstrate cost effectiveness."

CONTROVERSIAL MARKETING AND NEW TECHNIQUES

Fierce competition within the market has made direct-to-patient marketing (DTP) an attractive option for manufacturers. DTP marketing is more common in the US than in the EU due to regulations. This controversial practice has left some members of the medical community questioning whether advertisements raise unrealistic expectations among patients and place physicians in a difficult position when explaining why a specific product may not be the right choice.

"Raising awareness of new treatment options can benefit everyone," explains Nevelos. "However, promoting a treatment option which is suitable for a small percentage of the patient population can lead to confusion and disappointment.

"DTP marketing is best done when it speaks more broadly, [rather than] telling people they need a specific product. This promotes better understanding of treatment options and conversations between surgeons and patients. Trying to make one implant or implant type fit all people is not good enough."

"In 2006, worldwide sales of orthopaedic implants reached $28.6bn, with an average annual growth rate of 9%."

Procedures which shorten rehabilitation time for patients, as well as reduce pain and scarring, will play a pivotal role in implant design trends. "Shorter hospital stays also improve hospital efficiencies," says Nevelos.

"Efficiency will be a key area of development, which includes tools to improve alignment of prosthetic implants. However, all of these procedural developments must demonstrate satisfactory patient outcomes in terms of function and alignment.

"Surgical techniques, like implants, have evolved over time. The selection of a technique should always be done to optimise the outcome of the surgery. Some new techniques require extensive training for the surgeon, which is the key to implementing these methods," he concludes.

AGE EFFECTS IN THE UK

According to Dr. Keith Tucker, consultant orthopaedic surgeon and president of the British Hip Society, the past ten years have seen an increased fitness level among elderly patients. Subsequently, doctors have become better at assessing these patients to make sure they will survive total hip replacement (THR). The demand has increased and most surgeons do not refuse THR based on age alone. "My oldest patient was 102," says Tucker.

Anaesthetic techniques have evolved and there are greater numbers of anaesthetists who can safely take an elderly patient through surgery. But the complications of advanced age, such as post-operative stroke, myocardial infarction and confusion, remain issues. So too does a lack of post-operative support for the large number of individuals who live alone. "Many units lack rehab facilities for the post-op management of these patients who stay in hospital longer than our trusts would like," says Tucker.

Though the NHS remains under constant pressure to reduce costs, Tucker believes this will not translate into reducing the number of hip surgeries performed. Instead, shorter stays in hospital combined with stronger community support networks seem likely to be the way forward.

"Our population is getting older and many more elderly patients are living alone," he says. "They need their new hips to maintain their independence. If anything, we will probably carry out operations earlier in the disease process. In most units, the policy is to undertake the surgery only when all else has failed."

NEW MATERIALS FOR JOINT REPLACEMENT

Dr. William Bargar, a private practice orthopaedic surgeon in Sacramento, California, US, believes the development of alternative bearing surfaces has drastically changed how surgeons practise joint replacement, especially in the hip and, to a lesser degree, in the knee. There are currently three options available: improved polyethylene bearing surface, or highly cross-linked polyethylene; ceramic; and metal.

"These new implant materials have made a significant impact as far as who we can offer joint replacement to and what restrictions we put on our patients," says Bargar. "With the improved polyethylene and alternative bearings, we are not expecting the prosthetics to fail because, in the lab, these materials are ten to 20 times better in terms of wear.

"Whereas the old material would last about ten years in a young active person, the new material should last 20–30+ years in these patients. Where we used to tell younger people with arthritis to 'just put up with it' because the failure rate was too high, now we say to them, if you've got severe arthritis, we'll do it. We feel as though we have something that could last, if not their lifetime, at least a good portion of it."

Each of the alternative bearings offers a potential risk. Highly cross-linked polyethylene is weakened by its manufacturing process, making it more subject to cracking. Though some companies have continued to work on this problem and claim their new processes have addressed the issue, it remains a concern.

"Whereas the old material would last about ten years in a young active person, the new material should last 20–30+ years in these patients."

"Many of us are apprehensive about the "metal ion" that metal-on-metal surfaces create within the body," Bargar explains. "Whenever a metal-on-metal implant is used, there are chromium and cobalt ions that circulate through the body and build up at multiples of what normal body levels would otherwise be.

"Although there has been no definite link with cancer, it is a concern with long-term exposure. As patients get older, their kidney function declines. Since this is the main pathway for the body to clear excess chromium and cobalt from the blood stream, it may become an issue in the future."

Ceramic-on-ceramic may be the solution to these concerns. "The bearing by-products are not reactive so you don't get toxic build-ups of bio-inert wear particles," says Bargar. "This material wears better than the rest. However, it is brittle and there have been some fractures. Manufacturing is addressing that, but the incidences can be one in 20,000 of ceramic-on-ceramic cracking or breaking."

But these concepts only apply to hip replacement, as Bargar explains. "The reason why these things have happened for the hip and not the knee is because the knee is a more difficult bearing surface. The knee is more complicated – it slides, glides and rotates. So the engineering concept of what the knee has to do places more stress on the bearing materials." Materials such as highly cross-linked polyethylene are more wear resistant but can weaken and are therefore not applicable for knee replacement. Moderately cross-linked polyethylene may have potential, but metal-on-metal or ceramic-on-ceramic are not ideal surfaces.

"Some of the designs we've seen from the UK, the pioneer for these implants, include mobile bearing knees," Bargar says. "Instead of two surfaces rubbing together, there is a piece of polyethylene moving in the knee. This allows designers to spread the load over a bigger area and lower the stresses on the polyethylene."

These particular designs were developed over 20 years ago, but recently have been refined so that most problems have been eliminated. Mobile bearing knees have increased in use in the US over the last five to ten years and considered much more for younger patients.

FURTHER DEVELOPMENTS AND METHODOLOGY

There has also been recent emphasis on unicompartmental knee replacements. In the knee there are three compartments – the lateral, medial and behind the knee cap. When a total knee replacement is carried out, all of these are resurfaced. However, partial knee replacements, or unicompartmental knee replacements, are seeing resurgence. Though the procedure has been around for many years, they can now be performed through smaller incisions, helping patients to get back to a functioning level more quickly.

Another benefit of unicompartmental replacements is, if they fail, they can be converted to total knee replacement. UK statistics show that almost 30% of knee replacements are unicompartmental, whereas in the US the rate is nearer 10%. "But the number here is growing," says Bargar. "We are looking at the UK and thinking this is an area to consider because we can broaden the indications."

Over the last few years, focus in orthopaedic surgery has shifted away from the technology of implants and is concentrating more on how implants are put in. With the increased use of minimally invasive surgeries, surgical method is now affecting the design of implants. Smaller incisions mean that standard implants can no longer be used, or are more difficult to put in. But new implant designs will allow physicians to perform these types of surgeries more easily.

"A lot of technology is involved in how we do these things in a less invasive way," says Bargar. "Different instruments were developed, such as computer-aided surgery (CAS). One of the problems you run into with minimally invasive surgeries is that you cannot see what you are doing, so you are more prone to error in placing the implant."

"Another benefit of uni-compartmental replacements is, if they fail, they can be converted to total knee replacement."

An important part of CAS is the computer navigation system which guides the surgeon through the body to perform complex procedures. Bargar believes that robotics will become an integral part in the future of implant surgeries, aiding surgeons to perform tasks more accurately.

"The whole reason robots were introduced into manufacturing was to control quality and variation," he explains. "Surgeons are human and have good days and bad days, therefore you will have variation. You don't really want variation when placing implants."

The focus for the past few decades has been the technology of the implant and the materials used to produce them. But the future is concentrating on just how implants are put in, and how to improve operations, recovery time and patient quality of life.