Let’s start with the simple version, because once you understand this, everything else falls into place.

Your knee is essentially made up of three sections, or compartments: the inner side, the outer side, and the kneecap area at the front. Arthritis doesn’t always affect all three. In a lot of people, it’s only in one compartment — and most commonly that’s the inner side.

This is the detail that changes everything. If the arthritis is isolated to just one part of the joint, it raises an obvious question: why would you replace the whole thing? Replacing a healthy outer compartment and a healthy kneecap joint, just because the inner side has worn, means a bigger operation, a longer recovery, and removing parts of your knee that were working perfectly well.

That’s exactly why finding out which compartment is affected is the single most useful piece of information you can have before any decision is made. It’s the difference between a targeted fix and an unnecessarily big one.

The best way to think about your options is as a ladder. You start at the bottom and only move up a rung if you genuinely have to.

Osteotomy. This is the one most people have never heard of. An osteotomy is where the surgeon slightly reshapes the bone above or below the knee to shift load and pressure off the damaged compartment. No part of the joint is actually replaced. By redistributing where your weight travels through the knee, it essentially buys time — often years. It’s best suited to younger, more active people where the arthritis has been caught early enough and is isolated to one compartment. For the right person, it can delay or even remove the need for a replacement altogether.

Half (partial) knee replacement. Also called a partial knee replacement, this is the next rung up. If the arthritis is contained to one compartment and an osteotomy isn’t the right fit, a half knee replaces only the damaged section and leaves everything else completely untouched. Because you’re preserving the rest of the natural joint, recovery is significantly quicker than a full replacement, and the knee tends to feel more like your own afterwards. For many people with single-compartment arthritis, this is the sweet spot.

Full knee replacement. This is the top of the ladder, and it’s absolutely the right option in the right circumstances — but usually only when arthritis has spread across the whole joint, or when the other two options have been tried and are no longer working. It’s worth knowing that joint replacements don’t last forever; they have a lifespan. The longer you can reasonably delay getting one, the better, because it preserves your options for the future. That’s precisely why exploring the first two rungs of the ladder matters so much.

So when someone asks do you need a full knee replacement, the honest answer is: only if the arthritis is widespread, or the less invasive options have genuinely been exhausted. For a lot of people, that simply isn’t the case yet.

This isn’t theory. In my clinics, we regularly see patients who’ve been told they need a full knee replacement. They come in for a second opinion, and it turns out the arthritis is only in one compartment — yet they were never told about an osteotomy or a half knee. They were told from the very start that the whole knee needed replacing.

When that happens, the patient’s entire path changes. Suddenly they’re looking at less surgery, a faster recovery, and crucially, preserved options for the future. They go from facing the biggest operation available to having a much smaller, more targeted one — or sometimes avoiding a replacement for years.

I want to be fair here: I’m not saying surgeons get this wrong on purpose. Different surgeons have different specialisms and preferences, and a full replacement is a reliable, well-established operation. But a second opinion costs you nothing, and it could save you from a far bigger operation than you actually need. When the stakes are this high, that’s an easy decision to make.

If you remember nothing else from this article, remember this. If you’ve been told you need a knee replacement, ask — or find out — whether the arthritis is in one compartment or across the whole joint. That single piece of information shapes everything that follows.

If it’s confined to one compartment, ask your surgeon directly about an osteotomy or a half knee replacement, and whether either is an option for you. And if you’re not getting a clear answer, get a second opinion. You owe it to yourself to understand all the options before you commit to the biggest one — because this is a decision that affects the rest of your life, and a joint you can’t get back.

One last thing worth saying: before any of this, make sure you’ve given proper physiotherapy and injection therapy a genuine go first. For a lot of people, the right conservative treatment can keep an arthritic knee comfortable and active for years before surgery of any kind needs to enter the conversation. Surgery — of any size — is best thought of as a tool for when those options have been properly explored.

Every knee and every person is different, so the right answer can only really come from a proper, individual assessment. But going into that conversation knowing the three options, and knowing to ask which compartment is affected, puts you in a far stronger position. So if you’ve been told you need a full knee replacement, make sure you’ve asked the one question that could change everything — before you commit to the biggest operation of all.

Joe Sharp
BSc (Hons) Physiotherapy