Knee replacement
Total or unicompartmental knee replacement resurfaces the damaged joint surfaces with metal and polyethylene components, providing reliable pain relief for end-stage knee arthritis.
📊 Over 100,000 knee replacements are performed annually in the UK, making it one of the most commonly performed elective operations. It is the most common orthopaedic procedure.
What is knee replacement?
Knee replacement surgery resurfaces the damaged articulating surfaces of the knee joint with metal and high-density polyethylene components. Total knee replacement (TKR) resurfaces all three compartments (medial, lateral, and patellofemoral). Unicompartmental knee replacement (UKR) resurfaces only one compartment - most commonly the medial - and is suitable for isolated single-compartment disease in patients with an intact anterior cruciate ligament.
Modern total knee replacements are designed to replicate the kinematics of the natural knee. Most designs are cruciate-retaining (CR), which preserves the posterior cruciate ligament, or posterior-stabilised (PS), which substitutes for the PCL using a cam and post mechanism. Rotating platform (RP) designs allow the polyethylene insert to rotate on the tibial tray, potentially reducing wear. The choice of design is influenced by bone quality, ligament integrity, and surgeon preference.
Robotic-assisted and computer-navigated knee replacement allows more precise component alignment, which may improve long-term implant survival. Oxford Phase 3 unicompartmental knee replacement (the most commonly used UKR in the UK) has excellent long-term outcomes with 95% implant survival at 15 years in the registry, with faster recovery and higher patient satisfaction than TKR in appropriately selected patients.
Common causes
- End-stage knee osteoarthritis (primary or secondary)
- Rheumatoid arthritis destroying the knee joint
- Post-traumatic arthritis after intra-articular fracture
- Failed previous knee surgery (osteotomy, partial replacement)
Who is at risk? Advanced age, obesity, severe deformity, and medical co-morbidities all influence the risk and outcome of knee replacement surgery.
Symptoms
Symptoms vary depending on the severity and duration of the condition. Common symptoms include:
- Severe knee pain limiting daily walking distance to less than 200-300 metres
- Pain at rest and at night
- Inability to perform basic daily activities without significant pain
- Deformity (varus or valgus) that is worsening
- Failed response to adequate non-operative management over 3-12 months
When to seek help: Knee replacement is considered when pain and functional limitation are severe, non-operative management has been fully tried, and the patient understands the realistic expectations including the recovery period and the limitations of the prosthesis.
How is it diagnosed?
Your surgeon will take a detailed history and examine the joint. The following investigations may be arranged to confirm the diagnosis:
- Weight-bearing X-rays - AP, lateral, and skyline views showing end-stage joint space loss
- CT scan - for pre-operative planning, particularly in cases of significant deformity
- MRI - to assess soft tissue structures if the diagnosis is uncertain
Knee replacement should not be offered purely on X-ray findings. The decision to operate requires both radiological evidence of advanced arthritis and significant clinical symptoms that have not responded to adequate non-operative treatment.
Treatment pathway
Treatment is tailored to the severity of the condition, your age, activity level, and overall health. Most conditions are treated in a stepwise fashion, starting with the least invasive options.
Unicompartmental knee replacement (UKR)
Resurfacing of one compartment (usually medial) through a smaller incision. Faster recovery than TKR. Higher patient satisfaction in appropriate candidates. Requires intact ACL and correctable deformity. 10-15% require conversion to TKR over 20 years.
Total knee replacement (TKR)
Resurfacing of all three knee compartment surfaces with a femoral component, tibial component, and polyethylene insert. Highly effective for severe tricompartmental disease. Over 90% implant survival at 15 years. Some patients report residual functional limitations despite a well-performed operation.
High tibial osteotomy (HTO)
Realignment of the knee by cutting and repositioning the tibia to shift weight away from the arthritic compartment. Delays the need for replacement in appropriately selected younger, active patients with isolated medial compartment disease and a correctable varus deformity.
Recovery
Recovery after total knee replacement requires a committed physiotherapy programme focusing on quadriceps strengthening and range of motion recovery. A good range of movement (over 90 degrees of flexion) before surgery predicts a better outcome. High-impact activities (running, contact sport, heavy manual work) are generally not recommended after knee replacement.
- After surgery: mobilise day 1:
- Walking without aids: 4-6 weeks
- Return to driving: 6-8 weeks
- Swimming and cycling: 8-12 weeks
- Full recovery: 6-12 months
What results can I expect?
Total knee replacement achieves significant pain relief in over 85% of patients. Approximately 15-20% of patients are dissatisfied despite a technically successful operation, most commonly due to residual pain or stiffness. Unicompartmental replacement has higher satisfaction rates in appropriately selected patients.
Knee replacement - what to expect
In numbers
Typical activity timelines for this condition. These are approximate and vary considerably between patients. Always follow the specific guidance given by your surgeon and physiotherapist.
| Activity | Typical timeline | Notes |
|---|---|---|
| Walk with aids | Day of surgery | Patients usually stand and take steps with crutches or a frame the day of surgery or the next day.[2] |
| Off aids | 4-6 weeks | Most patients are off crutches and walking unaided by 4-6 weeks, transitioning to a stick if helpful.[2] |
| Drive | 4-6 weeks | When you can react and brake safely, bend the knee adequately, and walk comfortably. Inform your insurer.[5] |
| Return to work | 4-12 weeks | Desk work: 4-6 weeks. Standing or manual work: 8-12 weeks. Heavy manual work: 12 weeks or more.[2] |
| Swim | After wound has healed | Usually around 4-6 weeks once the wound is fully healed. Excellent low-impact rehab.[2] |
| Cycle | 6-8 weeks | Once knee bend is sufficient (usually 90-100 degrees). Stationary bike first, then road cycling.[2] |
| Return to sport | 3-6 months | Walking, golf, doubles tennis, swimming, and cycling are typically encouraged. Running and jumping sports are usually discouraged.[4] |
| Full final outcome | 12 months | Most pain relief is felt early but functional improvement and the final outcome continue through the first year.[3] |
Common concerns from patients with this condition or recovering from treatment, and whether they are expected or worth mentioning to your team.
Your questions, answered
Plain-English answers to the things people most often ask, grounded in UK clinical guidance. Tap a question to open it.
About thisWhen is a knee replacement the right choice?
A knee replacement is considered when osteoarthritis is causing severe pain and stiffness that limits your daily life, and exercise, weight management, pain relief and injections are no longer giving enough benefit. It is an elective operation, so the timing is your decision, guided by how much the knee is holding you back.
The operationWhat actually happens in the operation?
The worn surfaces of the knee are removed and resurfaced with metal and a hard-wearing plastic spacer, so the joint glides smoothly again. Some people are suitable for a partial (unicompartmental) replacement, which only resurfaces the damaged side of the knee.
AnaestheticWhat type of anaesthetic will I have?
Knee replacements are commonly done under a spinal anaesthetic, often combined with sedation and a nerve block to keep the knee comfortable afterwards. A general anaesthetic is also an option. Your anaesthetist will talk through what suits you.
Pain after surgeryHow painful is it afterwards?
There is real discomfort in the first couple of weeks, and the knee is often swollen and warm for longer than that. Pain is managed with regular medication, ice and elevation, and it steadily improves. Getting the knee moving early actually helps the pain settle.
RecoveryWhen can I walk, drive and get back to normal?
Most people are walking with aids the same or next day and progress to a stick over a few weeks. Driving is usually possible at around four to six weeks once you can control the car safely. The bulk of recovery takes about three months, with improvements continuing for up to a year.
How long it lastsHow long will the new knee last?
Modern knee replacements are very durable, and the majority are still working well at fifteen to twenty years or more. Longevity depends on factors such as age, weight and activity, and your surgeon can give you a personalised view.
PreparingHow can I prepare for the best result?
Going in as fit and strong as possible helps recovery, so keep up gentle exercise, stop smoking if you can, and manage your weight and any long-term conditions. Sorting out home support and equipment in advance makes the first weeks much easier.
When to worryWhat signs need urgent attention?
Contact your team urgently for spreading redness, wound discharge, fever, or a calf that is swollen, hot or tender. Sudden chest pain or breathlessness is an emergency. These can be signs of infection or a blood clot and should not be left.
- 1.National Joint Registry annual report (NJR), hip and knee implant data
- 2.NHS: Knee replacement
- 3.NICE NG157: Joint replacement (primary): hip, knee and shoulder
- 4.British Association for Surgery of the Knee: Patient information
- 5.Royal College of Surgeons of England: Recovering from surgery, driving guidance
References are to UK clinical guidance and patient information from recognised organisations. This page is for general information and does not replace personalised advice from your own clinical team.
Preparing for surgery?
Read our step-by-step guide - what to expect before, during, and after your procedure.
How is it diagnosed?
- ✓Weight-bearing X-rays - AP, lateral, and skyline views showing end-stage joint space loss
- ✓CT scan - for pre-operative planning, particularly in cases of significant deformity
- ✓MRI - to assess soft tissue structures if the diagnosis is uncertain
Recovery milestones
- ●After surgery: mobilise day 1:
- ●Walking without aids: 4-6 weeks
- ●Return to driving: 6-8 weeks
- ●Swimming and cycling: 8-12 weeks
- ●Full recovery: 6-12 months