So, how can you save your knees, which are the most likely joints to be replaced, according to the Australian Orthopaedic Association National Joint Replacement Registry?
To give his patients a realistic idea of what is achievable, the deputy director of the registry, Professor Richard de Steiger, asks them to do a simple test to demonstrate what extra weight does to the knee.
De Steiger, an orthopaedic surgeon, studied biomechanics at Oxford and says the force through the knee when a person squats can be up to three times their body weight.
“I tell my patients if they put on 12 kilograms, that’s equivalent to 36kg when they squat.
“I ask them to buy a dozen, one-litre bottles of mineral water and feel the weight. That’s 12kg.”
Were they to carry 36 bottles, they’ll begin to get a feeling of the force they are exerting on their knees during a squat.
So, what does extra loading do to a knee?
“It wears them out,” says de Steiger, who is the Epworth Victor Smorgon chair of surgery at the University of Melbourne.
“These are hinge joints that can rotate and are very susceptible to forces of increased weight.”
And it’s not just mechanical, because fat is not inert. It releases inflammatory chemicals that can contribute to joint destruction.
Ferida Felstead, 67, from Melbourne, who was obese for much of her adult life, well understands the effect of excess mass on the knees.
The retired secondary school assistant principal struggled to control her weight. But with a career, a family and a husband who worked in Bass Strait, there was never enough time for everything.
“Then, about 15 years ago, my knees started to give me trouble. They were painful and despite consulting doctors and physiotherapists and taking anti-inflammatories, nothing helped.”
When she consulted de Steiger, he found her knees in bad shape but he didn’t want to operate.
She believes she was probably too heavy and too young. Back then implants were likely to last 15 to 20 years, which meant she would need a new set in a procedure that would be more complicated.
So Felstead returned to her Balwyn home and took charge. She began walking. “Initially I couldn’t walk very far without getting puffed. The hills almost killed me.
“Eventually I was doing 40km to 45km a week. Although I was getting lighter, every step still hurt.”
Blocked out the pain
Motivated by the desire to maintain her independence and not be a burden on the family, she blocked out the pain.
Having lost almost 35kg – saving the equivalent of 105kg force when squatting – she went back to de Steiger.
This time he agreed to replace both knees.
Before her first operation last April, she did her research and felt ready. Although she never got “to meet” her implant before it was inserted, it’s now so much a part of her she forgets it’s there.
But she does get some unexpected reminders, as happened when she passed through airport security and the alarm went off.
Her second knee was replaced six weeks ago. “Earlier this week, I went to Westfield Doncaster and as I walked around it occurred to me that this was the first time in well over a decade that I had been able to walk without any pain at all,” she says.
“My gait, however, has changed and I’m having physiotherapy to get the biomechanics right.”
Felstead used to battle with stairs. “Now, I don’t exactly bound up them, but I do go up and down normally.
“I know, if I had not had this done, I would have been a bigger drain on society in the future than I am now. Considering my mother is almost 101, I may have a few years yet.”
Should she live long enough to need a fresh set, by then she may receive one of the smart implants now being developed.
With built-in sensors to monitor the forces acting upon them, they can indicate how much activity is negatively affecting their function so patients can make adjustments.
This should help them find their sweet spot of activity, say the researchers from the State University of New York at Binghamton.
Rather than using a battery that requires replacing, they are trying to power the implant from energy generated by friction between the joint components during motion.
Writing in the journal Smart Materials and Structures, they say this triboelectric energy could be enough to power the load sensors.
As the sensors disclose more about the demands placed on implants, so improvements in the design can be made.
But that hope is still confined to the laboratory.
The hope on the ground is that Australia develops the capacity either to meet or reduce the future national demand for joint replacements.
Watching your weight is not the only pathway to reducing the likelihood of an implant.
General exercise and remaining fit is important and so is targeted exercise therapy to build the musculature around the knees.
This can make them stronger, help to maintain appropriate co-ordination and reduce pain.
While 98 per cent of people who receive a new knee have it because of osteoarthritis, most people with osteoarthritis don’t need or get one.
Although it’s commonly believed osteoarthritis only affects the bone and the articular cartilage, many more joint structures are affected too.
High pain thresholds
Just having polished bone on bone is not reason enough to have a knee replacement.
“A knee replacement should be used as an end stage treatment for severe pain and reduced function,” says de Steiger.
“Our aim is to try to reduce the pain of arthritis to prevent a knee replacement. Many people have high pain thresholds and just do not need surgery despite their X-rays looking bad.”
End stage arthritis can be thought of as “total joint failure”, much as we think of heart or kidney failure.
Post-operatively, he says the joint never quite functions as it once did. The average 65-year-old may get back to 75 per cent to 85 per cent function but their knees will not be the same as when they were younger and arthritis free.
Myths about new knees abound. One is that it’s better to have the implant when younger as the body is stronger, recovers more quickly and better accommodates it.
De Steiger disagrees. He recommends a replacement only when symptoms warrant it.
He also debunks the notion that marathon running is bad for younger people. For those in their 30s doing marathons he says data linking excessive running and arthritis is not strong.
“The reason is that these runners are not obese. What they lose through impact loading of their joints, they probably gain in the effects over time because they are normal to underweight.”
While young people who sustain knee injuries through contact sport are predisposed to arthritis, he says that if they stop playing sport and put on weight as well, their risk of it increases further.
Rather, they should have the injury properly assessed, undergo appropriate rehabilitation, and carefully consider the timing of their return to sport in conjunction with their physiotherapist or treating doctor, to minimise the likelihood of re-injury.
He and Associate Professor Ilana Ackerman, from Monash University’s School of Public Health and Preventive Medicine, recently published a study on the likelihood of having a knee replacement after a knee injury.
“We found those with a sports-related knee injury were more than twice as likely to have a total knee replacement within 15 years – and that’s a relatively short time,” says Ackerman, a physiotherapist.
Fearful of exercise
If the injury was decades ago and the knees are now problematic, she says it’s best to keep weight down, keep fit and focus on tailored exercise therapy.
Although many people are fearful of exercise because they worry it may make their osteoarthritis worse, she says pain during exercise doesn’t equal damage.
But it doesn’t have to be endured either. “People should speak to their GP about pain control options to support them to exercise more comfortably with osteoarthritis.”
Their latest study raises awareness of the problems facing Australia and informs future healthcare resource planning policies.
In a decade, they showed more than 70 per cent of Australian adults are expected to be overweight or obese, creating the need for an additional 25,000 knee replacement surgeries costing an extra $521 million.
Published in BMC Musculoskeletal Disorders, their study provides a strong policy and public health argument for supporting weight-loss awareness campaigns and interventions.
Should you need a joint replacement, all orthopaedic surgeons voluntarily give their data to the registry so it is on the public record.
This registry is internationally recognised and in June its director, Professor Stephen Graves, and de Steiger will be honoured at a world meeting in Portugal.
Jill Margo is an adjunct associate professor at the University of NSW Sydney.