Partial Knee Replacement

Partial knee replacement is a knee resurfacing operation in which only the compartment damaged by arthritis is replaced with an artificial joint surface while the unaffected parts of the knee are preserved.

The procedure is also called unicompartmental knee replacement, unicompartmental knee arthroplasty, UKA, UKR, or occasionally unicondylar knee replacement.

These terms are often used interchangeably when one tibiofemoral compartment is being replaced.

The knee is not one simple hinge. It contains three main compartments:

• the medial compartment, on the inner side of the knee;

• the lateral compartment, on the outer side;

• the patellofemoral compartment, between the kneecap and the front of the femur.

Most partial knee replacements involve the medial compartment because isolated medial-compartment osteoarthritis is more common than isolated lateral disease.

A lateral partial knee replacement can also be performed when arthritis is confined to the outside compartment.

Patellofemoral replacement technically replaces only part of the knee as well, but it is usually considered a separate procedure because the anatomy, implant design and indications differ.

AAOS describes unicompartmental knee replacement as resurfacing only the arthritic portion of the knee while preserving healthy cartilage, bone and the major ligaments.

That preservation is the main conceptual difference between partial and total knee replacement.

During a total knee replacement, the damaged surfaces across the knee are more extensively resurfaced. The anterior cruciate ligament is generally removed in a conventional total knee replacement, and depending on implant design the posterior cruciate ligament may also be sacrificed or substituted.

During a well-selected partial knee replacement, the unaffected compartments remain intact and the cruciate ligaments are usually preserved.

This can allow the knee to maintain more of its natural movement and proprioception.

Many patients therefore describe a successful partial replacement as feeling more like their own knee.

AAOS specifically notes that preservation of healthy bone, cartilage and ligaments may lead to better range of motion and a more natural-feeling knee compared with total knee replacement.

However, the procedure should not be viewed as a “small knee replacement” that can be given to anyone with arthritis.

Patient selection is fundamental.

If arthritis is widespread across two or three compartments, replacing only one side may leave the patient with substantial pain.

Similarly, severe ligament insufficiency or major uncorrectable deformity can make a partial implant inappropriate.

For patients with isolated medial-compartment osteoarthritis who are suitable for either option, NICE specifically recommends offering a choice between partial and total knee replacement and discussing the advantages and disadvantages of each procedure.

That recommendation is important because there is no universal answer to the question “Is partial knee replacement better than total knee replacement?”

For one patient, preserving the healthy compartments and achieving a quicker recovery may make partial replacement particularly attractive.

For another patient with broader arthritis or unstable ligaments, a total knee replacement may provide a more appropriate solution.

Partial knee replacement has evolved significantly.

Older eligibility rules often restricted the procedure by age, weight or activity level. Modern practice focuses more heavily on the actual anatomy of the knee, where the arthritis is located, ligament function, alignment and the patient's expectations.

AAOS notes that modern unicompartmental implants can demonstrate excellent medium- and long-term survivorship in both younger and older appropriately selected patients.

Modern imaging, instrumentation, cementless implants, computer navigation and robotic-assisted partial knee replacement have also expanded the technical options available to surgeons.

None of these technologies changes the central principle:

Only the damaged compartment should be replaced.

Understanding Knee Osteoarthritis Before Partial Knee Replacement

Osteoarthritis develops when articular cartilage—the smooth tissue covering the ends of the bones—breaks down.

Healthy cartilage allows the femur and tibia to glide with very little friction.

As cartilage deteriorates, the joint space narrows.

Eventually, patients can develop areas where the protective cartilage is almost completely lost.

This can produce:

• knee pain;

• stiffness;

• swelling;

• reduced walking distance;

• difficulty using stairs;

• pain standing from a chair;

• reduced ability to exercise;

• night pain;

• deformity.

Importantly, osteoarthritis does not always progress evenly across the knee.

Some patients have severe bone-on-bone arthritis on the medial side while the lateral compartment and patellofemoral joint remain comparatively healthy.

That pattern creates the possibility of partial knee replacement.

AAOS identifies advanced osteoarthritis limited to one compartment as the core indication for unicompartmental replacement.

The purpose is not simply to treat an X-ray.

A patient needs clinically meaningful symptoms that correspond to the diseased compartment.

Someone with dramatic radiographic arthritis but little pain may not need surgery.

Conversely, a patient with severe disabling pain, loss of function and isolated bone-on-bone disease despite appropriate conservative treatment may be a strong candidate.

Orthopaedic evaluation

Before recommending a partial knee replacement, the surgeon should establish that the symptoms, physical examination and imaging all point to disease in the same compartment.

The assessment normally begins with a detailed history.

The surgeon asks:

• where the knee hurts;

• how long symptoms have been present;

• whether pain occurs with walking or at rest;

• whether stairs are difficult;

• whether the knee gives way;

• whether there is locking or catching;

• which treatments have already been attempted;

• what activities the patient wants to regain.

The location of pain matters particularly in partial replacement.

Medial disease typically causes inner-knee pain.

Lateral disease typically causes outer-knee pain.

Diffuse pain does not automatically exclude partial replacement, but it requires greater caution.

Physical examination

The surgeon evaluates:

• gait;

• standing alignment;

• knee swelling;

• tenderness;

• range of motion;

• deformity;

• ligament stability;

• hip movement;

• circulation;

• nerve function.

The knee may also be examined under gentle stress to determine whether the deformity is correctable.

X-rays

Weight-bearing X-rays are fundamental.

They help the surgeon identify:

• joint-space loss;

• bone-on-bone arthritis;

• osteophytes;

• alignment;

• disease in other compartments.

Different views may include standing anteroposterior, lateral, skyline or Rosenberg-type views.

Some surgeons obtain stress X-rays to assess the other compartments and ligament function.

MRI

MRI is not required for every partial knee replacement candidate.

X-rays and clinical examination often provide enough information.

However, MRI may be useful in selected patients when the diagnosis is unclear or when the surgeon needs more information about cartilage, menisci or ligaments.

AAOS notes that MRI may be used to evaluate the cartilage in the relevant compartment.

Nonsurgical treatment first

Partial knee replacement is typically considered after appropriate nonoperative treatments no longer control symptoms.

Depending on the patient, previous treatment may include:

• activity modification;

• weight management where relevant;

• structured exercise;

• physiotherapy;

• topical or oral pain medicines;

• anti-inflammatory medication when appropriate;

• walking aids;

• selected injections.

The decision for surgery is not based on how many injections a patient has received.

It is based on the combination of symptoms, structural disease and failure of reasonable nonsurgical care.

AAOS describes arthroplasty as an option when arthritis pain causes substantial disability despite nonsurgical treatment.

Medical optimization

Preoperative assessment may include:

• blood count;

• kidney function;

• blood glucose;

• coagulation testing when indicated;

• ECG;

• anaesthetic review;

• infection screening according to hospital protocol.

Diabetes, anaemia, cardiovascular disease and other significant conditions should be managed as well as reasonably possible before elective surgery.

Smoking should also be addressed because nicotine can interfere with wound healing and increase surgical risk.

Medication review

Patients should provide a complete list of:

• anticoagulants;

• antiplatelet medicines;

• diabetes medicines;

• steroids;

• supplements;

• prescription medications.

No important medication should be stopped without instructions from the treating medical team.

Preparing the home

Patients should arrange practical support before surgery.

Helpful preparations include:

• clearing loose rugs;

• placing frequently used items within easy reach;

• arranging help with shopping;

• installing a stable shower aid if necessary;

• preparing comfortable seating;

• arranging transportation;

• obtaining the prescribed walking aid.

For international patients, the hotel or accommodation should have safe access, elevators where required and minimal unnecessary stairs during the first days.

How is partial knee replacement done?

During partial knee replacement surgery, the surgeon removes damaged cartilage and a small amount of bone from the arthritic compartment, resurfaces it with metal components and places a polyethylene insert between them while preserving the healthy parts of the knee.

AAOS describes three fundamental stages: preparing the bone, positioning the metal implants and inserting a plastic spacer to create a smooth gliding surface.

Step 1 — Anaesthesia

The procedure may be performed under spinal or general anaesthesia.

Regional nerve blocks or periarticular local anaesthetic injections can be used as part of multimodal pain management.

The objective is effective pain control while reducing reliance on opioids where possible.

Step 2 — Surgical exposure

The surgeon makes an incision over the front or slightly to the side of the knee.

Partial knee replacement generally requires a smaller exposure than traditional total knee replacement because only one compartment is being treated.

However, incision length varies according to anatomy, implant system and surgeon technique.

“Minimally invasive” should not be understood as surgery performed through a tiny opening at any cost.

Accurate implant position and protection of the soft tissues are more important than the cosmetic length of the incision.

Step 3 — Confirming suitability

This step is especially important.

Once the joint is exposed, the surgeon inspects the other compartments and confirms that the arthritis is genuinely localized.

The condition of the:

• opposite tibiofemoral compartment;

• patellofemoral joint;

• ACL;

• collateral ligaments;

• cartilage

is assessed.

AAOS notes that if intraoperative inspection shows the patient is not suitable for a partial replacement, a total knee replacement may be performed instead when that possibility has been discussed and consented to before surgery.

Patients should know about this possibility in advance.

Step 4 — Removing damaged surfaces

Special cutting guides or robotic/navigation systems are used to remove a controlled amount of damaged bone and cartilage.

For a medial unicompartmental replacement, the procedure generally resurfaces:

• the medial femoral condyle;

• the corresponding medial tibial plateau.

The lateral side, patellofemoral structures and cruciate ligaments are preserved when healthy.

Step 5 — Trial components

Temporary trial implants may be inserted.

The surgeon checks:

• implant size;

• flexion and extension gaps;

• stability;

• alignment;

• range of movement;

• soft-tissue balance.

Overcorrection should be avoided.

The goal is generally not to force the knee into an artificial perfectly straight alignment if doing so would overload the healthy compartment.

Instead, the surgeon aims to restore appropriate mechanics for that individual knee.

Step 6 — Implant fixation

Partial knee replacement components are usually made from metal alloys with a polyethylene bearing surface.

Fixation can be:

Cemented:
Bone cement secures the implant to the prepared bone.

Cementless:
A porous implant is press-fit against bone, allowing bone to grow onto or into the surface.

Both methods are used in modern practice.

A 2025 systematic review concluded that cementless unicompartmental knee arthroplasty demonstrated favorable safety and clinical outcomes at mid-term follow-up, although implant choice should remain individualized.

Step 7 — Polyethylene bearing

A highly durable plastic bearing is positioned between the femoral and tibial components.

Depending on implant system, the bearing may be:

• fixed-bearing;

• mobile-bearing.

Each design has specific technical considerations.

Neither should be advertised as universally superior for every patient.

Step 8 — Final assessment

The knee is moved through flexion and extension.

The surgeon checks:

• stability;

• bearing tracking;

• alignment;

• implant position;

• range of movement.

The wound is irrigated and closed.

A sterile dressing is placed.

Medial Partial Knee Replacement

The medial partial knee replacement is the most common type of unicompartmental knee arthroplasty.

It replaces the damaged surfaces on the inside half of the tibiofemoral joint.

The procedure is particularly suited to anteromedial osteoarthritis where disease is concentrated in the medial compartment and the other major structures remain functional.

The healthy lateral compartment remains untouched.

The cruciate ligaments are usually maintained.

That is one reason patients may retain more normal knee kinematics than after a conventional total replacement.

Lateral Partial Knee Replacement

A lateral partial knee replacement replaces the outer compartment.

Isolated lateral-compartment arthritis is less common and has different biomechanics from medial disease.

The procedure therefore requires particular technical experience.

Modern research suggests that carefully selected lateral UKA can achieve durable results.

A 2025 study with a mean follow-up of 14.5 years reported 94.7% implant survivorship in a cohort of lateral unicompartmental replacements, although the study involved a specialized population and should not be interpreted as a universal individual survival estimate.

Robotic Partial Knee Replacement

What is robotic partial knee replacement?

Robotic partial knee replacement uses computer planning and a robotic-assisted cutting system to help the surgeon position the implant and prepare the bone according to a patient-specific plan.

The robot does not independently perform surgery.

The orthopaedic surgeon remains in control.

AAOS describes robotic joint replacement as using 3D planning and computer-assisted instruments that can provide feedback while bone is prepared and components are positioned.

Some platforms require a preoperative CT scan.

Others create the map intraoperatively without CT.

Potential advantages

Robotic systems may improve:

• bone-cut precision;

• implant positioning;

• reproducibility;

• alignment control;

• planning of component size;

• soft-tissue balance.

These advantages can be especially relevant in partial knee replacement because the operation depends on integrating a prosthetic compartment with the patient's remaining natural knee.

Does robotic partial knee replacement produce better outcomes?

The answer is more nuanced.

Robotics can improve technical precision, but better radiographic precision does not automatically mean that every patient has less pain or a longer-lasting implant.

A 2025 systematic review found that image-based robotic UKA produced fewer radiological alignment outliers than imageless systems but did not demonstrate a difference in revision rates.

More recent meta-analytic evidence similarly suggests that some robotic platforms improve component-placement precision while short-term pain, range of motion and patient-reported functional outcomes can remain comparable to conventional surgery.

Robotic technology should therefore be described as a surgical tool, not as a guarantee of a better result.

An experienced surgeon using a conventional technique may produce an excellent result.

A robot cannot compensate for incorrect patient selection.

Partial knee replacement is increasingly performed with short hospital stays.

AAOS notes that patients may either return home after surgery or stay overnight.

Whether same-day discharge is appropriate depends on:

• age;

• general health;

• pain control;

• walking safety;

• home support;

• anaesthetic recovery;

• medical conditions.

After surgery, patients spend time in the recovery area.

Clinical staff monitor:

• blood pressure;

• heart rate;

• oxygen levels;

• wound dressing;

• circulation;

• sensation;

• pain;

• nausea.

Pain management is typically multimodal.

This may include:

• paracetamol/acetaminophen;

• anti-inflammatory drugs when safe;

• local anaesthetic;

• peripheral nerve blocks;

• limited opioid treatment when necessary.

Early mobilization is encouraged.

Patients usually stand and begin walking within hours of surgery with help from physiotherapists or nursing staff.

AAOS states that weight-bearing generally begins immediately after partial knee replacement, although a walker, cane or crutches may be needed initially.

Before discharge, the patient should be able to:

• walk safely with the recommended aid;

• transfer in and out of bed;

• use stairs when required;

• understand medication instructions;

• understand wound care;

• perform prescribed exercises;

• recognize warning signs.

Recovery after partial knee replacement is generally faster than after total knee replacement because less bone and soft tissue are disturbed, but full recovery still takes time.

Many patients can perform routine activities progressively over the first 4–6 weeks, while strength, endurance and knee confidence may continue improving for several months.

AAOS states that many partial knee replacement patients resume routine activities of daily living by about six weeks.

The NHS specifically notes that partial knee replacement recovery should generally be shorter than recovery from total knee replacement.

Recovery is not identical for every patient.

Factors include:

• preoperative fitness;

• age;

• muscle strength;

• pain;

• swelling;

• surgical technique;

• implant;

• other medical conditions;

• rehabilitation consistency.

Physiotherapy After Partial Knee Replacement

Rehabilitation has several objectives:

1. restore full extension;

2. progressively restore flexion;

3. reduce swelling;

4. strengthen the quadriceps;

5. restore walking mechanics;

6. rebuild balance;

7. regain functional independence.

Partial knee replacement can recover quickly enough that patients sometimes underestimate the importance of rehabilitation.

Feeling less pain does not necessarily mean that strength has fully returned.

Quadriceps weakness can persist after any knee arthroplasty.

The rehabilitation programme therefore progresses according to function rather than only the appearance of the incision.

Returning to Work

Return to employment varies considerably.

Desk work

Some patients return after approximately 2–4 weeks, particularly when they can work from home.

Others need 4–6 weeks.

Standing jobs

Jobs involving prolonged standing may require approximately 4–8 weeks or longer.

Physical occupations

Construction, nursing, warehouse work, heavy lifting and other physically demanding jobs may require substantially more recovery.

The surgeon should determine work restrictions based on the individual's duties.

Driving

Driving after a partial knee replacement depends on:

• which knee was replaced;

• whether the car is manual or automatic;

• pain;

• reaction time;

• medication;

• ability to perform an emergency stop.

The NHS indicates that driving may sometimes restart from around three weeks after partial knee replacement, provided the patient is medically fit.

This should never be interpreted as automatic permission to drive at exactly three weeks.

Flying After Partial Knee Replacement

Flying shortly after joint replacement deserves specific planning.

Surgery and immobility both increase venous thrombosis risk.

Long-haul travel can add another period of prolonged sitting.

International patients should ask the surgeon:

• when they may safely fly;

• whether thrombosis prophylaxis is required;

• whether compression stockings are recommended;

• how often to walk during the flight;

• whether medical documentation is required.

Patients should not assume that being medically fit for hospital discharge means they are immediately fit for a long international flight.

For appropriately selected patients, partial knee replacement can produce substantial pain relief and improvement in function.

AAOS states that most properly selected patients have good results. It also notes advantages that can include quicker recovery, less postoperative pain, less blood loss and a lower risk of some complications compared with total knee replacement.

Recent comparative evidence provides additional context.

A 2025 meta-analysis of randomized trials comparing unicompartmental and total knee arthroplasty for unicompartmental osteoarthritis found broadly similar clinically meaningful outcomes overall. Partial replacement was associated with fewer complications and shorter hospital stays, although differences in several functional scores were too small to clearly exceed clinically important thresholds.

More importantly, the 2026 ten-year TOPKAT randomized trial provides long-term comparative evidence.

In its ten-year analysis, partial and total knee replacement showed similar clinical outcomes and broadly similar reoperation and revision rates. In the randomized treatment-allocation analysis, revision occurred in approximately 5% of each group, and economic analysis favored partial knee replacement.

Those results are highly relevant because earlier observational registries frequently reported higher revision rates for partial replacement.

The apparent discrepancy illustrates an important point:

Outcomes depend strongly on correct patient selection and surgeon experience.

Does Partial Knee Replacement Feel More Natural?

Some patients report greater joint awareness after total knee replacement than after partial replacement.

Because UKA preserves:

• more native bone;

• the opposite compartment;

• the cruciate ligaments;

• more normal knee geometry,

movement can remain closer to native mechanics.

A 2025 systematic review and meta-analysis reported more favorable “natural-feeling” joint measures after unicompartmental arthroplasty than total knee arthroplasty.

That does not mean every partial replacement feels completely normal.

Patients may still notice:

• stiffness;

• numbness;

• clicking;

• mild swelling;

• weather-related awareness;

• weakness.

How Long Does a Partial Knee Replacement Last?

There is no fixed expiration date.

Implant survival depends on:

• implant design;

• fixation;

• surgeon technique;

• alignment;

• activity;

• weight;

• bone quality;

• arthritis progression;

• patient selection.

Modern implants can function for many years.

One registry-based study of more than 14,000 matched Oxford unicompartmental replacements reported approximately 90% ten-year survival for cemented implants and 93% for cementless implants.

A 2025 long-term study in younger active patients reported an implant survival of approximately 86.7% at 15 years for the specific cemented mobile-bearing system studied.

These numbers should not be turned into a promise that an individual implant will last exactly 10, 15 or 20 years.

The NHS notes more broadly that knee replacements can function for approximately 25 years in some patients, but longevity varies according to implant type and individual circumstances.

Partial Knee Replacement vs Total Knee Replacement

This comparison should be especially prominent for SEO and GEO because many patients search this question directly.

NICE states that patients with isolated medial-compartment osteoarthritis who are eligible for both approaches should be offered a choice between partial and total replacement after discussion of the risks and benefits.

The TOPKAT ten-year results support the idea that neither option should automatically be considered universally superior.

Why Might Someone Choose Partial Replacement?

Potential advantages include:

Faster early recovery

Less extensive surgery generally means a quicker initial rehabilitation.

Less bone removal

Healthy bone is preserved.

That may be useful if revision surgery becomes necessary later.

Preservation of ligaments

Retaining natural ligament function can help preserve more physiological movement.

Less blood loss

AAOS identifies lower blood loss as an advantage compared with total replacement.

Lower risk of some early complications

AAOS notes lower risks of infection and blood clots among recognized advantages in appropriately selected patients.

More natural-feeling knee

Many patients report improved joint awareness and natural movement.

Why Might Total Knee Replacement Be Better?

Total knee replacement is generally more appropriate when osteoarthritis is not truly isolated.

Potential reasons include:

• disease across multiple compartments;

• extensive patellofemoral arthritis;

• significant ligament instability;

• major deformity;

• severe stiffness;

• inflammatory arthritis affecting the entire joint.

Replacing only one compartment in such a knee could leave substantial untreated disease.

Partial knee implants contain a femoral component and a tibial component separated by polyethylene.

Materials commonly include:

• cobalt-chromium alloys;

• titanium-based materials in certain components;

• ultra-high-molecular-weight polyethylene.

Implant systems vary in:

• geometry;

• fixation;

• bearing design;

• instrumentation.

Fixed-bearing UKA

The polyethylene insert remains fixed to the tibial component.

Mobile-bearing UKA

The polyethylene bearing can move within defined limits between the components.

Both systems have successful long-term clinical histories.

Implant choice should reflect the surgeon's technique and the patient's anatomy rather than branding alone.

Cemented vs Cementless Partial Knee Replacement

Cemented

Polymethylmethacrylate bone cement is used to fix the metal components.

It provides immediate fixation.

Cementless

The components contain porous surfaces designed to allow biological bone integration.

Potential theoretical advantages include reduced cement-related failure and biological fixation.

A 2025 systematic review found cementless UKA to be a safe and effective option in available mid-term evidence, but it does not establish that cementless fixation is best for every patient.

Bone quality, implant design and surgeon experience remain important.