Partial (Unicompartmental) Knee Replacement

Here's something many patients don't realize: not every arthritic knee needs the whole joint replaced. Your knee has three separate compartments, and sometimes arthritis wears out only one of them. When that's the case, a partial — or unicompartmental — knee replacement can resurface just the worn part and leave the rest of your knee, including your own ligaments, untouched. For the right candidate, it's an elegant solution. The operative phrase, though, is right candidate.

What a partial knee replacement is

Think of the knee as having three compartments: the inner (medial), the outer (lateral), and the area behind the kneecap. In some patients, arthritis wears out just one of these — most often the inner (medial) compartment — while the others stay healthy.

A partial knee replacement resurfaces only that one worn compartment, leaving the healthy cartilage, bone, and ligaments in place. A total knee replacement, by contrast, resurfaces the entire joint. Both are excellent operations — they simply solve different versions of the same problem.

Partial vs. total: the key difference

The decision between partial and total isn't about which is “better” in the abstract — it's about where your arthritis actually is.

• Partial knee replacement — for the subset of patients whose arthritis is confined to a single compartment. It preserves the rest of the knee.
• Total knee replacement — the right choice when arthritis involves more of the joint. This is the case for most patients, which is why total knee replacement is the more common, more universally applicable operation.

Who's a candidate?

Candidacy is the heart of the matter. A partial knee tends to be an option when:

Your surgeon assesses this with an exam and imaging — and sometimes the final confirmation comes during surgery, when the other compartments can be inspected directly. If they're more worn than expected, the plan may shift to a total knee. For help recognizing when any knee replacement is worth considering, see signs you may need a knee replacement.

What a partial knee can't fix

Most of my indications follow from one fact about the operation, and it is worth understanding because it explains all of them: a partial resurfaces the knee. It does not rebalance it.

In a medial unicompartmental replacement I am removing the worn cartilage and taking a very small shave off each side of that one compartment. That is the whole operation. Which means:

So anything that needs the knee rebalanced or realigned sits outside what the operation can do. That is why the following are not candidates:

A flexion contracture

A knee that will not fully straighten. I cannot correct that with a partial, because correcting it means adjusting the gaps and that is not part of this operation. Loss of extension is covered in knee stiffness.

Varus beyond about 5 to 10 degrees

I tell patients honestly that I either cannot correct their bowing with a partial or the correction will be very minimal. Past roughly 5 to 10 degrees, that is not good enough — more on alignment in bow legs and knock knees.

Instability

Very difficult to address with a partial, for the same reason — the operation does not touch what is producing the instability. If your knee gives way, that is worth sorting out first; see knee giving way.

ACL or other ligament injury

Not indicated. This includes a pattern I look for specifically on imaging: posterior wear, which suggests the ACL is not doing its job even if nothing else has flagged it.

Wear in another compartment

A medial unicompartmental replacement addresses the medial compartment and nothing else. So meaningful patellofemoral wear behind the kneecap, or lateral compartment wear, is problematic — and severe wear anywhere outside the compartment being replaced rules it out.

Why I'm strict about this

I hold tighter indications for a partial knee than some surgeons do, and I want to explain the reasoning rather than just apply it.

The reason is what happens when the selection is wrong. The two most common reasons a partial knee needs to be revised are:

Both are largely a selection problem. A patient who did not really meet the criteria has an operation, does reasonably well for a while, and then comes back needing the total knee they would have been better served by in the first place.

Benefits and honest trade-offs

For a well-selected candidate, a partial knee offers real advantages — and it's only fair to weigh them against the trade-offs.

Potential benefits:

• A smaller operation — less bone and tissue removed, and often a smaller incision.
• A more natural-feeling knee — preserving the healthy compartments and your own ligaments helps the knee move and feel more like your own.
• Often a quicker recovery — and frequently performed as outpatient (same-day) surgery.

Honest considerations:

• It only works for the right knee — single-compartment arthritis with intact ligaments. Most patients don't fit this picture.
• Arthritis can progress in the compartments that weren't replaced.
• It may later need to become a total — a partial can be converted to a total knee replacement if arthritis advances elsewhere.

Recovery

Because a partial knee is a smaller, tissue-sparing operation, recovery is often quicker than a total knee replacement — though, as always, it varies from person to person and your surgeon and physical therapist guide the specifics. The fundamentals are the same as any knee recovery: gentle early motion, steady progress, and walking as the foundation. You can see the broad arc in the knee recovery timeline, what the recovery exercises involve, and — reassuringly — how durable modern knee replacements are.

What patients commonly tell me

Partial knee replacement comes up in a recognizable way:

The instinct to “keep as much of my own knee as possible” is a good one — and when your arthritis allows it, a partial knee does exactly that. When it doesn't, a total knee is the honest, better answer.

From Dr. Harb

If there's a theme to how I approach partial knee replacement, it's honesty about candidacy. A partial knee in the right patient is one of the most satisfying operations I do — a smaller surgery, a fast recovery, and a knee that feels natural. But a partial forced onto a knee that needed a total leads to disappointment, so I won't do that.

If you're weighing knee replacement, it helps to understand knee osteoarthritis, the nonsurgical options that come first, and the signs it may be time. Whether the answer turns out to be partial or total, the path starts with understanding your own knee.