Robotic knee replacement: what it can and cannot do

What the robot does

A knee replacement has two jobs. The worn surfaces are removed, and the new parts are positioned so the leg is straight and the ligaments are equally tight with the knee bent and straight. The second job decides how the knee feels for the next twenty years.

In a conventional operation that positioning is done with metal cutting guides, set by measurement and by the surgeon's eye and hands. It works well, and most knee replacements in the world are still done this way.

Navigation adds a computer. Small markers are fixed to the thigh bone and shin bone, a camera tracks them, and a screen shows the alignment of the leg degree by degree while the guides are set. The surgeon still makes the cuts with ordinary instruments.

A robotic system goes a step further. The plan is made on the computer, with some systems from a CT scan taken beforehand, and a robotic arm or a handheld tool keeps the saw inside the planned boundaries. It will not cut where the plan says not to. The surgeon is operating throughout.

Where it helps

The research is consistent on one point: robotic and navigated knees are placed more accurately, and more consistently, than knees done with instruments alone. Fewer end up far from the alignment that was planned. That matters most in a few situations.

• A badly bowed or knock-kneed leg. The further the leg has bent, the harder it is to balance the ligaments by feel, and the more each degree counts.
• An old fracture or a plate in the thigh bone. Conventional instruments pass a rod up the inside of the thigh bone to find its axis. When a healed fracture or old metalwork is in the way, navigation and robots can find the axis without the rod.
• Partial knee replacement. The parts are small and the margin for error is smaller, and robots are used for partial knee replacement for that reason.

There may be a benefit in the first weeks as well. An early study from London, published in The Bone & Joint Journal in 2018, found that patients with robotic-arm knees had less pain in the first days and went home sooner than those with conventional knees. Not every study since has found the same, and where there is a difference it is in the early recovery.

Where the evidence is still thin

What has not yet been shown convincingly is that robotic knees feel better or last longer years later. The national joint registries, which follow hundreds of thousands of knees, have not settled it, partly because robotic surgery is young compared with the fifteen to twenty years a knee replacement should last. Better-aligned knees ought to wear more evenly. Whether they do is still being measured.

A few practical points before you choose a hospital because of a robot:

• The operation can take a little longer, particularly while a team is new to the system.
• The markers need small pins in the thigh and shin bones, a few centimetres from the knee. The pin sites heal like any small wound, and problems with them are rare.
• Some systems need a CT scan before surgery, which is an extra scan and some radiation.
• It adds to the cost of the operation. Ask for the difference in writing.
• None of it replaces the exercises. What you do in the first six weeks decides the bend you keep, robot or no robot.

My own work on this

I trained in robotic and navigation-assisted knee replacement during my arthroplasty fellowship at Lilavati, Sir HN Reliance and Breach Candy Hospitals in Mumbai, and my research since has been on exactly this question. I compared robotic and navigation-assisted knee replacement in knees with a severe inward bow, where accurate alignment is hardest to achieve by hand. The study has been submitted to The Bone & Joint Journal, and I will keep its numbers to myself until it is published.

What I tell patients in the meantime is this. For moderate arthritis in a fairly straight leg, a carefully done conventional replacement gives an excellent result, and I would not ask anyone to wait or travel for a robot for that knee. For a badly bowed leg, or a thigh bone with old metalwork in it, the technology earns its place. Which systems are available depends on the hospital, and at the consultation I will tell you which technique I would use for your knee, and why.

If you have already been offered robotic surgery elsewhere and are unsure, a second opinion before a joint replacement covers what to bring and what to ask.

Questions to ask about it

• Is this a robotic system or a navigation system? Both are good tools, they are not the same thing, and both get called robotic.
• How many has the surgeon done with this particular system?
• How would you do my knee if the robot were not available?
• What does the extra cost buy in my case?

Further reading

• Kayani B, et al. Robotic-arm assisted total knee arthroplasty is associated with improved early functional recovery and reduced time to hospital discharge compared with conventional jig-based total knee arthroplasty. The Bone & Joint Journal, 2018.
• NHS: Knee replacement. The operation and the recovery, whichever technique is used.

Common questions