Partial Knee Replacement: Who Is a Candidate — and Who Is Not

Orthopedic surgeons sometimes call it the one-finger test. Ask someone with early knee arthritis where it hurts, and many will press a single fingertip against the inner edge of the joint — right there, every stair, every golf swing. Ask someone with advanced arthritis, and they tend to sweep an open palm across the whole knee.

That small gesture matters more than most people realize. The knee is not one joint but three compartments sharing a roof, and arthritis often moves in room by room rather than all at once. When the damage truly stops at one compartment, a surgeon can resurface just that section and leave the healthy two-thirds of the knee — bone, cartilage, and all four major ligaments — untouched.

The catch is that this smaller operation is choosier about its patients than a total knee replacement. Roughly one in ten people with knee arthritis meets the classic criteria by some estimates. Here is how the selection actually works, and what the evidence says about the trade-offs.

What exactly is a partial knee replacement?

A partial knee replacement — surgeons say unicompartmental knee arthroplasty — resurfaces one of the knee’s three compartments instead of the whole joint. Picture the knee as a small apartment with three rooms: the medial compartment on the inner side, the lateral compartment on the outer side, and the patellofemoral compartment behind the kneecap. Osteoarthritis frequently starts in one room and stays there for years before spreading.

During the procedure, the surgeon removes the worn cartilage and a thin wafer of bone from the affected compartment only, then caps the surfaces with a metal implant separated by a durable plastic spacer. Everything else stays: the other two compartments, both cruciate ligaments, the kneecap, and most of the joint capsule. According to MedlinePlus, that preservation is the whole point — the knee keeps its native structures, so it moves and bends more like the knee you were born with.

The medial compartment is the workhorse of the operation. Because the inner side of the knee carries more load during walking, arthritis strikes there first far more often, and roughly nine out of ten partial replacements are medial. Lateral and kneecap-only replacements exist but are less common and demand even more careful patient selection.

One useful mental model: a total knee replacement is a full renovation; a partial is repairing the one room with water damage while the rest of the house stays livable. Whether that repair holds depends almost entirely on whether the damage really is confined to that one room.

Partial vs. total knee replacement: what's the real difference?

The two operations solve the same problem — bone grinding on bone — with different philosophies. A total knee replacement resurfaces all three compartments and removes the anterior cruciate ligament (and often the posterior cruciate as well); the implant itself takes over stability. A partial keeps the ligaments and asks them to keep doing their job.

The practical differences show up in the first weeks and the following decades, and they pull in opposite directions.

	Partial knee replacement	Total knee replacement
Compartments resurfaced	One of three	All three
Cruciate ligaments	Preserved	ACL removed; PCL often removed
Typical incision	Smaller (often 3–4 inches)	Larger (often around 8 inches)
Hospital stay	Often same day or one night	Commonly one to two nights
Early recovery	Generally faster, less blood loss	Slower; more soft tissue disturbed
How the knee feels	More like a natural knee for many	Reliable, but can feel mechanical
Long-term revision rate	Higher in joint registries	Lower

Notice the last two rows. The partial tends to win on feel and speed; the total tends to win on durability statistics. Neither is simply “better.” The Mayo Clinic and other major centers frame it as a matching exercise: the right operation is the one that fits the pattern of damage inside your specific knee, not the one with the more appealing brochure.

Who makes a good candidate for partial knee replacement?

The classic candidate profile is surprisingly specific, and every item on it exists because the operation fails more often when it is ignored.

• Arthritis limited to one compartment. Usually the medial side, confirmed on standing X-rays. Pain that lives in one spot — the one-finger test — is a promising sign.
• An intact anterior cruciate ligament. The ACL is the knee’s stabilizing seatbelt. A partial implant relies on it; without it, the implant experiences abnormal shear forces and tends to loosen or wear early.
• Good range of motion. Most surgeons want the knee to bend to at least about 90 degrees and to straighten nearly fully — a fixed bend of more than roughly 10 to 15 degrees is a red flag.
• A correctable, modest deformity. A mild bow-leg or knock-knee that the examiner can gently straighten suggests the ligaments are healthy. A rigid deformity suggests they are not.
• Pain that has outlasted honest nonsurgical care. Strengthening, activity modification, weight management, and other conservative measures should have had a real trial first, per NHS guidance on knee replacement generally.

There is a pattern here worth naming: candidacy is less about who you are — age, occupation, ambition — and more about what the inside of the knee looks like. Two 58-year-old recreational hikers with identical pain scores can get opposite recommendations because one has a pristine lateral compartment and a taut ACL, and the other does not. This is why the workup matters as much as the operation.

Who is not a candidate — and why the rules exist

Every exclusion criterion is a failure mode someone learned the hard way. The major ones:

• Inflammatory arthritis. Rheumatoid arthritis and related autoimmune conditions attack the synovial lining of the entire joint, all three compartments at once. Replacing one room while the disease works on the other two is a losing strategy, and this remains a near-absolute contraindication.
• Arthritis in more than one compartment. If the lateral side or the kneecap surface already shows significant wear, a partial simply relocates the pain rather than resolving it. This is the single most common reason people are steered toward a total.
• A deficient or torn ACL. Without that ligament, the femur slides abnormally on the tibia with every step, and the implant’s plastic bearing wears unevenly. Most surgeons will not implant a standard partial into an ACL-deficient knee.
• Significant stiffness or fixed deformity. A knee that cannot straighten, or a bow-leg that cannot be passively corrected, signals contracted ligaments and disease beyond one compartment’s borders.
• Prior conditions that compromise bone or alignment — certain fractures, osteonecrosis extending beyond one compartment, or prior surgery that altered the joint’s mechanics — may also tip the decision.

Hearing “you’re not a candidate” can feel like a door closing. Reframe it: the criteria exist to protect you from an operation with a high chance of a second operation. A well-matched total knee replacement is a far better outcome than a poorly matched partial.

Does age or weight automatically rule you out?

For decades, the textbook answer was rigid: partial knees were reserved for patients over 60, under a certain body weight, with sedentary lifestyles. Those rules came from a single influential paper in the late 1980s, and the field has been quietly walking them back ever since.

Here is what the evidence actually shows now. Age matters less than activity pattern and anatomy. Younger patients do face higher lifetime revision odds — simple arithmetic, since a knee implanted at 50 must survive more years and more steps than one implanted at 72 — but that is true of total knees too, and some surgeons prefer a partial in younger patients precisely because it preserves bone for a future revision. Older adults, meanwhile, often benefit from the partial’s gentler recovery and lower rates of medical complications; several large registry analyses have found this advantage most pronounced in patients over 75.

Weight is similarly nuanced. Higher body weight increases mechanical load on any implant and modestly raises surgical risks across all joint replacement, but modern studies have not consistently shown that body mass alone dooms a partial knee. Most contemporary surgeons treat weight as one factor in a shared, individualized conversation rather than a bright-line cutoff — and framing it any other way does patients a disservice.

The honest summary: no single number on a chart should decide this for you. The condition of the ACL and the other two compartments carries far more predictive weight than a birthday or a scale reading.

Is it worth doing a partial knee replacement?

For the right knee, yes — and the reasons are concrete, not cosmetic.

Because the operation disturbs less tissue, early recovery is measurably easier. Blood loss is lower, hospital stays are shorter (same-day discharge is increasingly routine), and people typically reach walking, driving, and daily-life milestones weeks sooner than after a total knee. Large database studies have also found lower rates of serious medical complications — blood clots, infection, the need for transfusion — in the weeks after partial versus total replacement.

Then there is the feel. Because both cruciate ligaments remain, the knee retains its natural front-to-back stability and proprioception — the body’s internal sense of where the joint is in space. Many patients describe a well-functioning partial as “forgetting” the knee entirely, and studies of patients who have one of each (a partial in one knee, a total in the other) often find a preference for the partial side. That is a striking endorsement, though worth reading with the caveat that these are selected patients.

Function follows. People with partial knees frequently return to low-impact sport — cycling, swimming, golf, doubles tennis, hiking — at high rates, and often faster than their total-knee counterparts.

So the value proposition is real. What it is not is free. The price of the smaller operation is a higher long-term revision rate, which deserves its own unflinching section — because a decision this consequential should be made with both columns of the ledger visible.

What is the disadvantage of partial knee replacement?

Three, honestly stated.

First, higher revision rates. National joint registries in the UK, Australia, and Scandinavia — which track hundreds of thousands of knees over decades — consistently show that partial knees are revised roughly two to three times more often than total knees over ten to fifteen years. In round numbers, about 90 percent or more of partials are still functioning at ten years, versus roughly 95 percent or more of totals. Some of that gap reflects biology (arthritis progressing in the untouched compartments), some reflects a lower threshold to revise a partial, and some reflects surgical experience — surgeons and centers that do many partials get markedly better results than those that do a few.

Second, the arthritis can move. The implant fixes one compartment; it does not stop osteoarthritis elsewhere. Progression in the lateral or kneecap compartment is the leading reason partials get converted to totals. Careful patient selection shrinks this risk substantially, which is exactly why the candidacy criteria are strict.

Third, the margin for error is smaller. A partial’s success is more sensitive to implant positioning and to selection judgment than a total’s. This is a legitimate reason to ask any surgeon, without embarrassment, how many of these they perform in a typical year.

None of this makes the partial a gamble. It makes it a trade: an easier recovery and a more natural knee now, in exchange for a somewhat higher chance of a second operation later. Reasonable people weigh that trade differently — which is precisely why it should be their call, made with real numbers.

How do surgeons decide? Inside the candidacy workup

The decision usually comes together from three sources of evidence, and no single one settles it.

The conversation and the exam. Where does it hurt — one fingertip’s worth of territory, or the whole knee? Pain localized to the inner joint line points toward the medial compartment; diffuse pain, or pain concentrated behind the kneecap when descending stairs, points elsewhere. The examiner then checks range of motion, tests the ACL with specific maneuvers, and gently stresses the knee to see whether any bow-leg deformity corrects — a passively correctable deformity implies intact ligaments.

Standing X-rays. These are taken with weight on the leg, because cartilage loss only reveals itself under load; a lying-down X-ray can look deceptively healthy. Surgeons look for “bone-on-bone” narrowing confined to one compartment and a well-preserved joint space in the others. Special angled and stress views help assess the kneecap compartment and ligament integrity.

Sometimes MRI. When X-rays are ambiguous — early disease, suspected damage in a second compartment, uncertainty about the ACL — an MRI can map cartilage and ligaments in detail. It is not routine for everyone.

Worth knowing: the final verdict is occasionally rendered in the operating room. Surgeons directly inspect the other compartments before committing, and most will have discussed a contingency plan with you in advance — consent for a total knee if the joint’s interior tells a different story than the images did. Ask about that plan explicitly; good surgeons welcome the question.

What actually happens on surgery day

Less than most people fear, and faster than most expect.

The operation itself typically runs one to two hours. Anesthesia is usually spinal (numb from the waist down, often with light sedation) or general; many centers add a local nerve block around the knee that blunts the first day or two of soreness. The incision is small by joint-replacement standards — often three to four inches along the front-inner knee, roughly half the length used for a total.

Inside, the sequence is methodical: the surgeon confirms the other compartments are healthy, removes the damaged cartilage and a thin layer of bone from the affected compartment, sizes and positions the metal components on the ends of the femur and tibia, and seats the plastic bearing between them. Precision here — a matter of millimeters and single degrees of angle — is what the implant’s lifespan hinges on.

Then comes the part that surprises people: walking. Most patients are up on the new knee within hours, with a walker or crutches and a physical therapist alongside. Immediate weight-bearing is not bravado; early movement improves circulation, reduces clot risk, and speeds the return of muscle control. Cleveland-style rapid-recovery protocols have made same-day discharge common for healthy patients, and one overnight stay is typical for the rest.

You will go home with a short list of jobs: specific exercises several times daily, ice and elevation, a walking schedule that grows each day, and clear instructions on wound care and warning signs. The first two weeks are genuinely work. They are also where the partial’s recovery advantage is most visible.

How long does it take to fully recover from partial knee replacement?

Faster than a total knee, but “fully” still means months, not weeks. A realistic timeline for an uncomplicated recovery looks like this:

• Days 1–3: Walking with a walker or crutches; managing swelling with ice and elevation; starting bending and straightening exercises immediately.
• Weeks 1–2: Graduating from walker to cane or nothing, as balance allows. Many people are walking short distances unaided by the two-week mark.
• Weeks 2–4: Desk-based work often resumes here. Driving typically returns around three to four weeks — later if the operated knee is the driving leg — and only once you are off sedating pain medication and your care team clears you.
• Weeks 4–6: Most everyday activities feel normal: errands, longer walks, stairs without a handrail death-grip. Physically demanding jobs need longer.
• Months 3–4: Strength, stamina, and confidence approach their new baseline. Low-impact sport is usually back by now.
• Up to a year: Subtle gains — the last of the swelling, warmth around the knee, that final few degrees of bend — continue quietly.

Two honest caveats. Physical therapy attendance and home-exercise consistency predict outcomes better than almost anything else you control; the implant provides the surface, but your quadriceps provide the function. And timelines vary legitimately with age, baseline fitness, and job demands, so treat these numbers as medians, not promises. If your recovery is running slower, that alone is not failure — but it is worth a conversation with your surgical team rather than silent worry.

What can't you do after a partial knee replacement?

The forbidden list is shorter than people expect, and the reasoning behind it is mechanical, not arbitrary.

The plastic bearing inside the implant wears the way tire tread does: gradually, in proportion to load and repetition. High-impact, high-repetition activities — distance running on pavement, basketball, competitive singles court sports, repetitive jumping — multiply that wear and are generally discouraged for the life of the implant. This is a durability argument, not a fragility one; the knee will not shatter if you jog across a parking lot in the rain.

What is enthusiastically encouraged is nearly everything else: brisk walking, hiking, cycling, swimming, golf, doubles tennis, elliptical training, strength work, dancing. Experienced skiers often return to groomed slopes with their surgeon’s blessing. The pattern is consistent — smooth, controlled loading is good for the knee and for you; repetitive pounding is the enemy.

Kneeling deserves its own paragraph, because it is the question patients actually care about and brochures dodge. Many people find kneeling on a replaced knee uncomfortable — pressure over the scar and altered sensation in the skin are the usual culprits — but studies and surgeon consensus agree that kneeling does not damage the implant. Some people kneel comfortably with a cushion; some never enjoy it again. Discomfort here is a nuisance, not a warning sign.

In the early weeks, temporary restrictions apply: no driving until cleared, no soaking the incision until healed, no twisting pivots while the muscles are still relearning their job. Those expire. The high-impact caution is the only rule that stays.

How long does a partial knee replacement last?

The most trustworthy numbers come from national joint registries, which follow every implant in entire countries rather than the polished results of single centers. Their verdict: roughly 90 percent or more of partial knees are still in place and functioning at ten years, and around 70 to 80 percent at twenty to twenty-five years in well-selected patients. Individual high-volume centers report better figures still.

Three variables move those numbers meaningfully.

Selection. Knees that truly met the criteria — single-compartment disease, intact ACL, correctable alignment — dramatically outperform borderline cases. Most “early failures” in the literature are really selection failures in disguise.

Surgeon and center volume. Registry analyses show a consistent dose-response relationship: surgeons performing a healthy annual volume of partials have substantially lower revision rates than occasional users of the technique. Asking a surgeon “how many of these do you do per year?” is not rude; it is due diligence, and the evidence says it matters.

What you feed it. Body weight and activity pattern determine cumulative load on the bearing over decades. A partial in a 150-pound cyclist lives a different life than the same implant under a 280-pound roofer.

And if it does wear out or the arthritis advances? Conversion to a total knee replacement is the standard path. Because the partial removed so little bone, that conversion is usually more straightforward than revising a failed total — one of the quieter arguments for the partial in younger patients. It is still a second operation, with somewhat less predictable results than a first-time total, which is why the durability conversation belongs up front.

Not a candidate? Here's what the evidence supports instead

Being turned down for a partial is a redirection, not a dead end, and the alternatives have real evidence behind them.

Structured exercise and physical therapy. This is the most underrated treatment in orthopedics. Strengthening the quadriceps and hips measurably reduces arthritis pain and improves function — counterintuitive to people who assume a worn joint needs rest, but supported by decades of trials. A knee with strong surrounding muscle absorbs load that would otherwise grind through cartilage.

Weight management, where it applies. The mechanics are stark: each pound of body weight translates to roughly four pounds of force across the knee with every step. Modest, sustainable loss — even 5 to 10 percent of body weight — has been shown to reduce knee pain meaningfully. This is physics working in your favor, and it deserves compassionate support, not judgment.

Activity substitution. Trading pavement running for cycling or pool work keeps cardiovascular fitness without the impact bill.

Braces, walking aids, and footwear. Unloader braces can shift force away from a single worn compartment; evidence is mixed but some patients get genuine relief, and the downside is minimal.

Injection therapies. Several exist, with varying and honestly debated evidence; your clinician can walk through which, if any, fits your situation.

Total knee replacement. If arthritis spans the joint, this remains one of the most successful operations in modern medicine, with strong long-term satisfaction and durability data. “Not a partial candidate” very often means “an excellent total candidate.”

When should you see a doctor about knee pain?

Two lists here, and both matter.

Before any surgery — see a clinician if:

• Knee pain has persisted for more than a few weeks despite rest, activity modification, and over-the-counter measures your pharmacist or clinician recommends;
• Pain regularly wakes you at night or limits how far you can walk;
• The knee locks, buckles, or gives way — mechanical symptoms that suggest something structural;
• You notice visible deformity, or one knee looks increasingly bowed compared with the other;
• You cannot bear weight after an injury, or the joint swells suddenly and dramatically.

Seek care urgently — same day — if: the knee is hot, red, swollen, and accompanied by fever or feeling generally unwell. A joint infection is an emergency, replaced or not.

After a partial knee replacement, contact your surgical team promptly for: increasing redness, warmth, or drainage from the incision; fever; calf pain or swelling in either leg (possible blood clot); or pain that escalates rather than eases week over week. Chest pain or sudden shortness of breath after any surgery warrants emergency care immediately — call emergency services rather than waiting for a callback.

One more scenario deserves mention: the partial knee that was doing well and then, months or years later, starts hurting again. Do not assume it is “just aging.” New pain in a previously comfortable replaced knee — whether from loosening, wear, or arthritis progressing next door — is exactly what follow-up appointments exist to catch, and it is far easier to address early than late.

Frequently asked questions

What is the disadvantage of partial knee replacement?

The main disadvantage is a higher long-term revision rate — registry data show partial knees are revised roughly two to three times more often than total knees over ten to fifteen years. Arthritis can also progress in the untreated compartments, which is the leading reason partials are later converted to totals. Results are more sensitive to surgical precision and patient selection, which is why choosing an experienced surgeon and meeting the candidacy criteria matter so much.

Is it worth doing a partial knee replacement?

For a well-selected knee, evidence says yes. Recovery is faster, blood loss and early complication rates are lower, and because both cruciate ligaments are preserved, the knee tends to feel more natural — patients who have one of each often prefer the partial side. The trade-off is a somewhat higher chance of needing a second operation later. Whether that trade is worth it is a personal decision best made with your surgeon and real numbers.

How long does it take to fully recover from partial knee replacement?

Most people are walking unaided within about two weeks, back to desk work in two to three weeks, driving around three to four weeks once cleared, and doing most daily activities comfortably by six weeks. Strength and stamina typically approach their new baseline at three to four months, with subtle improvements in swelling and flexibility continuing up to a year. Consistent physical therapy is the single biggest factor you control.

What can't you do after a partial knee replacement?

The lasting restriction is high-impact, repetitive activity — distance running on hard surfaces, basketball, and repetitive jumping accelerate wear on the implant’s plastic bearing. Nearly everything else is encouraged: walking, hiking, cycling, swimming, golf, doubles tennis, and strength training. Kneeling may feel uncomfortable because of scar sensitivity, but it does not harm the implant. Early restrictions like no driving apply only until your care team clears you.

Who is the ideal candidate for a partial knee replacement?

The ideal candidate has osteoarthritis confined to a single knee compartment (usually the inner side), an intact anterior cruciate ligament, at least about 90 degrees of knee bend, a mild deformity that can be passively corrected, and pain that persists despite a genuine trial of nonsurgical care. Pain localized to one spot — the point-with-one-finger pattern — is a promising sign, but standing X-rays and a hands-on exam make the final call.

Who should not have a partial knee replacement?

People with inflammatory arthritis such as rheumatoid disease are not candidates, because the condition affects the entire joint. The operation is also inappropriate when arthritis involves more than one compartment, when the anterior cruciate ligament is torn or deficient, or when the knee is significantly stiff or has a fixed deformity that cannot be passively corrected. In these situations, a total knee replacement usually offers a far more reliable result.

Does a partial knee replacement feel more natural than a total?

Many patients say yes, and the mechanism is plausible: a partial preserves both cruciate ligaments, so the knee retains its native stability and joint-position sense. Studies of patients with a partial in one knee and a total in the other frequently find a preference for the partial side. That said, a well-done total knee also produces high satisfaction, and “more natural” does not mean identical to a healthy knee — some awareness of the implant is common with either.

Can arthritis spread to the rest of the knee after a partial replacement?

Yes — the implant treats one compartment but does not stop osteoarthritis from progressing in the other two, and this progression is the most common reason a partial is later converted to a total knee replacement. Careful candidate selection substantially reduces the risk, since knees with genuinely healthy remaining compartments tend to stay that way for years. Maintaining muscle strength and a healthy body weight helps protect the untreated compartments.

Am I too young or too old for a partial knee replacement?

Probably neither. The old rule reserving partials for patients over 60 has been largely abandoned; the condition of the ligaments and remaining compartments predicts success far better than age. Younger patients face higher lifetime revision odds simply because the implant must last longer, while older adults often benefit most from the gentler recovery and lower medical complication rates. Discuss your individual anatomy and activity goals rather than fixating on a birthday.

What happens if a partial knee replacement fails?

The standard solution is conversion to a total knee replacement. Because a partial removes very little bone, this conversion is generally more straightforward than revising a failed total knee — one reason some surgeons favor partials in younger patients. Outcomes after conversion are usually good, though slightly less predictable than a first-time total knee. Warning signs of a failing partial include new or worsening pain, swelling, or instability in a previously comfortable knee.

References

• Partial knee replacement — MedlinePlus Medical Encyclopedia
• Knee Replacement — MedlinePlus
• Knee replacement — NHS

This article is for general information only and is not a substitute for professional medical advice. Please consult a qualified doctor about your individual situation.