Seventy-five-year-old David Parker has had his fair share of knee troubles. The former semi-professional soccer player had both knees operated on in his teens and one again in his early 30s to keep him playing. “In those days, they would completely remove the cartilage if it was damaged,” he recalls. Each operation upped his chances of getting osteoarthritis, so after retiring from soccer, he was pleasantly surprised to escape any pain through middle age. Then, as if his knees were keeping count, aches and sharp jabs up his thigh appeared right as he turned 70.

At first, his doctors prescribed paracetamol and anti-inflammatories. But on a trip to Tokyo in November, Parker found himself on the wrong side of a crowd as he approached subway stairs. He weaved to dodge the rush but got caught wrong-footed. “My knee just gave way, and I fell down the stairs,” he says. “I couldn’t walk properly from then on. So, as soon as we got back, I rang a surgeon.” Within weeks, he was having both knees replaced.

Surgeons replace some 150,000 joints in Australia every year – a figure that’s been increasing by about 5 per cent a year since 2006. Most people opt for the operation in their late 60s. While the surgery can have remarkable results – an artificial joint can feel like a new lease on life to someone in severe pain – it’s not an option doctors take lightly. “In general, patients see joint replacements as an easy fix,” says Ian Harris, a professor of orthopaedic surgery. “And it’s not always. They don’t realise that it’s major surgery that has a significant risk of complications – and sometimes can be avoided.”

Why do our joints need replacing? At what point is surgery the best solution? And how can a man-made mechanism help us walk, bend and reach?

Why do joints stop working?

Joints, also called articulations, first appeared about 500 million years ago when animals developed segmented skeletons. Today, the human body has around 360. There are fixed joints, called sutures, between bones in the skull and cartilaginous joints, such as those between the ribs and the sternum, that allow for limited movement. Most of our joints, including knees, hips, shoulders and elbows, are synovial, made of a fluid-filled cavity and smooth cartilage that together cushion the joint and ensure flexibility.

Our joints can need replacing for many reasons. A fall can lead to a fractured hip in an elderly person – and replacing the joint might be much simpler and lead to less rehabilitation than reconstructing it. Or joints can become deformed at any age because of the autoimmune disease rheumatoid arthritis, which affects 2 per cent of Australians.

‘For every additional kilogram that a person carries, it’s four kilograms of excess load that goes across their knee.’

Professor David Hunter

In more than 90 per cent of joint replacements, however, the patient has osteoarthritis, which affects about 8 per cent of Australians. While there have been about 5350 ankle, 7700 elbow, and 1100 wrist replacements since 2006 in Australia, it’s the knees (78,000 a year), hips (58,000 a year) and shoulders (10,000 a year) that make up most joint replacement surgeries.

One of the common factors is age: 10 per cent of people aged 45 to 54 suffer from osteoarthritis, but that rises to 30 per cent in people older than 75.

“Essentially, it’s a failure of repair as we mature,” says David Hunter, a rheumatologist and professor at the University of Sydney. Hunter, one of the world’s leading osteoarthritis researchers, describes himself as “one of those weird people who finds it fascinating how people move”. “You know, I often walk along the footpath and watch the way a person uses their legs, how their knees and hips move, and try to understand mechanically what’s happening to their joints.”

Why are there so many replacements of the knee and hip in particular? “At least for hips and knees, it’s really a combination of the shape of the joint and the load that it undertakes,” says Hunter. It’s perhaps not surprising that 45 per cent of osteoarthritis in knees comes down to body weight. “For every additional kilogram that a person carries, it’s four kilograms of excess load that goes across their knee,” says Hunter.

Another fifth of cases of knee osteoarthritis are due to earlier injuries such as tears to ligaments, including to the anterior cruciate ligament (ACL) or to the meniscus, a disc of tough cartilage that distributes load across the knee. These injuries tend to crop up from activities that involve pivoting, such as netball, soccer, rugby and Australian rules. Running, despite perceptions to the contrary, doesn’t carry extra risk unless the knee is already injured or damaged.

The knee is “a finely balanced mechanism”, says surgeon Ian Harris. “So, just like any mechanical part, it relies on multiple things matching perfectly. Ligaments need to be balanced perfectly; the bones need to be aligned well. And everything needs to fit together. So if you’re even a little bit out of alignment, or you’ve had a break around the knee … that can lead to arthritis.”

‘If you have an immediate family member who has hip osteoarthritis, you’re four times more likely than the general population to have it.’

Professor David Hunter

For the hip, about 50 per cent of our chance of having arthritis is influenced by genetics. “If you have an immediate family member who has hip osteoarthritis, you’re four times more likely than the general population to have it,” says Hunter. Why people get arthritic shoulders is less well understood, although genetics and, again, injuries such as fractures, dislocations and tears in the rotator cuff (the tendons that wrap around the top and back of the shoulder to hold the ball in the socket and allow movements such as raising the arm) are known to play a role.

When seeking treatment, patients might be told their joint is “bone on bone” – with no cartilage left. But while the cartilage is often degraded in severe cases of osteoarthritis, Hunter says it’s “an innocent bystander” and rarely a cause. “Osteoarthritis can start in any of the joint tissues,” he says. “It could be the bone; it could be as a result of ligament damage; it could come directly from the lining – the synovium – it could come from the meniscus within the knee and, theoretically, it could start in the cartilage, but that’s much less frequent. Because all of the tissues ‘talk’ to one another, that affects the other tissues.”

In this sense, osteoarthritis should be understood “as a whole-joint disease”, not just worn-out cartilage. In fact, there aren’t pain receptors in the cartilage, so it should not be the focus of someone’s treatment.

When’s the right time to operate?

Anna Mathieson can still recall the pain in her right knee after crashing onto ice while skiing at Perisher in her 20s. “You think you’re invincible, and nothing you do is ever going to impact what happens when you’re old,” she says. She never determined the exact nature of the injury, but constant aches and a family history of joint replacements prompted her to see a rheumatologist in her late 50s – which is when she learned she had osteoarthritis in her right knee with zero cartilage, leaving her femur and tibia bones rubbing together when she walked.

‘With a knee, it’s a funny joint because you can have quite severe arthritis according to an X-ray ... and their symptoms are relatively mild.’

Professor Sam Adie

A challenge for doctors is that a patient’s pain levels are not always reflected in X-rays. “It’s a bit different in some joints, like, say, for the hip, there appears to be more of a correlation,” says Sam Adie, a professor of orthopaedic surgery at UNSW, “as in, the worse it looks on a scan or an X-ray, the worse that patient’s symptoms are. But with a knee, it’s a funny joint because you can have quite severe arthritis according to an X-ray – we’re talking complete eradication of cartilage and bone-on-bone appearance – and then they present to you, and their symptoms are relatively mild. They function quite well, and they’re able to do pretty much anything that they want to do.”

David Hunter actually has osteoarthritis in his knees, back and hands. In fact, he also lost his maternal grandmother in his youth – “probably the closest person to me growing up” – after she had a knee replacement due to the condition and died a week later with an infection. “She wasn’t offered any other treatment prior to the surgery,” he says. “I think in the back of my mind, a lot of the time, she’s kind of motivating what I do.”

To manage his osteoarthritis, Hunter keeps his weight down and has cut out high-impact exercise. “I wasn’t looking after myself as well as I could. I was working too hard, not living a particularly balanced life.” International guidelines for treating osteoarthritis encourage people to strengthen muscles around the affected joint, increase physical activity, lose weight and become educated about the disease.

The Australian Commission on Safety and Quality in Health Care’s standards say nine out of 10 people can manage osteoarthritis without needing knee replacement surgery. This is why the word “wear” in the cartilage is a bugbear for Hunter. “‘Wear’ suggests that activity is a bad thing. And the more activities that you do, and the more steps you take during your lifetime, is adverse for your joint health. But a joint itself is what we call a trophic organ – the more stimulus it gets, or the more physical activity it gets, that stimulates repair.”

Anna Mathieson consulted a surgeon, and they decided to try other treatments too: two years of pain medication, physiotherapy, low-impact exercise, platelet-rich plasma therapy (where blood is drawn from the patient and injected back into the knee under ultrasound to encourage healing) and a hospital trial of several different shoes. But the pain in her knee remained unbearable, every step like “walking on jagged gravel barefoot”.

‘For young people, it’s really important to try to maximise the service life of their own joints before they eventually get replaced.’

Paul Smith

Age is another factor to consider. The rate of needing a revision surgery after a joint replacement increases by half a percentage each year, says Paul Smith, clinical director of the National Joint Replacement Registry. For example, after 10 years, there is a 5 per cent chance of needing additional surgery. “It’s a balance between long-term risk and the degree of disability someone has,” he says. “For young people, it’s really important to try to maximise the service life of their own joints before they eventually get replaced.”

Adie defers half of the prospective knee replacements he sees because the patient hasn’t undergone enough low-impact exercises or lost weight. “The ideal patient for me, as a surgeon, is someone who has been managed by their GP for a little while. And the patient has obviously tried all of that, they failed all the non-operative management, and they’ve still got a problem with their symptoms, and it’s impacting their life.”

Mathieson tried to avoid surgery for as long as possible, but for her, “none of those [lifestyle] things, at the end of the day, worked”. With her right knee replaced, though, she became aware of pain in her left – and had it, too, replaced two years later. Then she began to feel pain in her back and pelvis, which turned out to be osteoarthritis in her hip – which was replaced when she was 66.

What does the surgery involve?

Ivory implants with nickel-plated screws went into the hips, knees and elbows of the first joint replacement patients in the late 19th century. German surgeon Themistocles Gluck was attempting to mend joints in patients with tuberculosis that had spread, but many had their prostheses removed due to complications. In 1925, American surgeon Marius Smith-Petersen implanted the first partial hip replacement, moulded out of glass, but it shattered when its recipient walked. In 1938, British surgeon Philip Wiles used stainless steel bolted and screwed to the bone – the first total hip replacement – but the attachment came loose.

Today, many joint replacements use materials that prevent wear, such as plastic on metal or ceramic. Surgeons have learned from mistakes: ceramic-on-ceramic replacements led to squeaking noises in a small number of patients in the early 2000s, and in 2010, the DePuy’s ASR hip implant was recalled because the metal-on-metal device elevated cobalt and chromium levels in the blood. Although ankle surgery is the fastest-growing replacement due to improved technology, there have been hiccups: in April 2024, the Therapeutic Goods Administration warned of a high level of revision surgery of 28.5 per cent with the Hintermann Series H3 ankle prosthesis.

The operations themselves are not keyhole affairs ... They involve exposing and removing parts or all of the damaged joint and resurfacing and reshaping bone and cartilage.

Sizing up the right prosthesis for a person’s anatomy “is a bit like going to the shoe shop”, Smith tells us, which includes bespoke options. “If someone needs to have a more specialised type of implant, and these things exist – obviously, everyone’s not built the same – then you have to plan for those.” The fledgling technology of 3D printing prostheses to match a patient’s anatomy was first used in Australia in 2022, although its long-term efficacy remains to be seen.

The operations themselves are not keyhole affairs – they’re “quite an invasive, gruesome surgery”, says Sam Adie. They involve exposing and removing parts or all of the damaged joint and resurfacing and reshaping bone and cartilage. Smith, who has done 5000 hip replacements, describes the surgery as a “complex dance”. “There are defined steps to that dance to get it right and ensure that the components are well fixed and in a good position, and with a safe and stable range of movement.” Robotic tools such as a computer guidance system that shows surgeons exactly where to cut are increasingly assisting, but the jury is still out on whether this improves outcomes.

The knee, a hinge joint, is far more complex than the hip: the femur rolls and glides over the tibia in a process called translation, which lessens strain. Knees are made up of the medial, lateral and kneecap compartments, and surgeons sometimes need to replace only one of these if arthritis isn’t widespread. For a total knee replacement, the prosthesis has metal implants that go into the femur and tibia and are either cemented into the bone or the bone grows into the implant to secure it. A lining made of crosshatched polyethylene plastic sits between the two implants to act as cartilage.

The extent of arthritis in the hip – a ball and socket joint – also determines how much of the joint gets replaced. The hip, a ball and socket joint, is fairly simple, says Smith, “you’re really rotating around the centre of the ball, the centre of the circle”. “I guess the advantage of the hip is that it’s also some very powerful muscles, which activate it. It always helps if you’ve got a serious amount of functional grunt to make a new thing work.” A full replacement often uses a ceramic or super-hardened alloy metal ball joining into a plastic socket.

The shoulder, another ball-and-socket joint, has the greatest range of motion of any joint. “The ball is very round, and the socket of the shoulder is not the same curvature. And so the ball spins on the socket surface as well as rolling on the socket surface,” says surgeon David Gill, an assistant clinical director for the upper limbs at the National Joint Registry. “That allows the extended range of movements that we get in a shoulder that we don’t get in a hip and knee.”

‘The main reasons joint replacements go wrong is: they are too long, they’re too short, they dislocate, they’re too stiff, the joint doesn’t move properly, they’re still painful, they’re unstable ...’

Professor Ian Harris

Some shoulder replacements are “anatomic”: they reflect the shoulder joint we’re all born with. But the most common is a total reverse shoulder replacement, where the ball and socket are flipped: a prosthetic ball is inserted on the shoulder blade (scapula), and a socket is implanted on the upper arm bone (humerus). This allows the shoulder to work without rotator cuff muscles. The surgery is for people who have damage to these muscles or who are at risk of them deteriorating. Still, says Gill, “everything comes with consequences”. “In general terms, a reverse replacement gives someone less rotation. Especially behind their backs.”

Of course, no surgery is risk-free. “The main reasons joint replacements go wrong,” says Ian Harris, “is: they are too long, they’re too short, they dislocate, they’re too stiff, the joint doesn’t move properly, they’re still painful, they’re unstable, they get infected, they come loose, and they break, and cause breaks in the bones around them.”

Although only a fraction of cases require revision surgery, infection makes up about 25 per cent of those (the highest proportion being in the first year). If bacteria get into the bloodstream, a joint replacement can be a haven for it to flourish because, being artificial, it is not protected by the immune system. In serious cases, the whole prosthesis needs to be replaced. (This is, Harris reminds us, elective surgery: “Nobody has to have a joint replacement.” )

How easy is it to recover from the procedure?

David Parker woke up in intensive care to feel his two artificial knees. “I can’t really say I was in a huge amount of pain at any time. I was always waiting for it to come,” he tells us. All up, he spent 10 days at Epworth Geelong, including a five-day stay in inpatient rehabilitation. A full recovery from a joint replacement can take up to a year, depending on the procedure and the level of strength, function and pain the person had before the operation.

Each patient receives an individual program tailored to their needs and goals, says Hannah Teasdale, who oversees the team that managed Parker’s rehab. Exercises for a total knee replacement can start with bed-based straightening exercises and sitting over the edge of the bed to get as much bend as possible, through to functional and strengthening exercises, walking and use of the exercise bike.

Bending his knee again was the biggest challenge for Parker. “A lot of the exercises were aimed at getting a bigger, more flexible angle and also building up thigh muscles.” After returning home, he did two hydrotherapy and physiotherapy sessions a week and exercises every day. When it comes to recovery, says Teasdale, “every patient is different and there’s no magic wand”. Once they leave hospital, patients need to commit to many weeks of exercise at home to get the best outcomes.

Will the joint ever feel the same? Hips tend to have the best outcome, but knees usually don’t ever feel exactly as they did before, says Smith. “People who have joint replacements are usually disabled by terrible arthritic pain, so they’re having relief from pain and restoration of a chunk of their life. The price of this is you have a prosthetic part in you.” Parker says his knees do feel slightly unusual. “I don’t feel as though I’ve got someone else’s legs or knees, but it’s a strange feeling that everything’s not quite right.” The fall in Tokyo hasn’t held him back from globetrotting: “We have a big trip coming in May; we are doing a cruise around Norway. So that was the aim, to get myself fit for that. We don’t do hikes and all that kind of stuff, but just to get around cities and enjoy life.”

For Anna Mathieson, now 70, three surgeries in the past decade have paid her back in improving her quality of life. “I have no pain whatsoever. I can bend my knee up better than I ever could before I had my knees done. I tell people, ‘It’s been the best thing I’ve ever done’. I do Pilates, and I ride my bike.” The biggest change? “Well, now I just make sure that I walk. And I’m just very conscious of staying fit. I took walking for granted. They say that’s what happens when it’s taken away, and now I’m just fortunate.”

Get fascinating insights and explanations on the world’s most perplexing topics. Sign up for our weekly Explainer newsletter.