Reviewed by Dr Vishal Pai
A partial knee replacement resurfaces only the worn part of your knee and leaves the healthy parts and the ligaments alone. This page covers who it may suit, what the operation involves, recovery, results and risks.
At a Glance
- A partial (unicompartmental) knee replacement replaces only the part of the knee worn by arthritis, most often the inner (medial) side.
- It may suit you if arthritis is limited to one part of your knee, your ligaments work well and non-surgical treatment has not helped enough.
- It is a smaller operation than a total knee replacement, and the early recovery is usually quicker.
- In national registry data it is more likely to need further surgery than a total knee replacement, often because arthritis develops elsewhere in the knee.
- Dr Vishal Pai performs partial knee replacements with the Mako robotic arm, using the fixed-bearing Stryker Restoris MCK implant.
What is a partial knee replacement?
Once the cartilage in the knee joint wears out, the underlying bone is exposed. When bone rubs on bone, it can cause arthritic pain. A partial knee replacement can treat osteoarthritis that is confined to one part of the knee. The unaffected parts of the knee, including the ligaments, are left alone.
Your knee has three compartments:
- Medial compartment: on the inside of the knee.
- Lateral compartment: on the outside of the knee.
- Patellofemoral compartment: underneath the kneecap and the groove that it sits in.
If arthritis affects only one compartment, you may be suitable for a partial knee replacement. The most common type replaces the medial compartment.
Who might need a partial knee replacement?
Treatment for knee osteoarthritis starts without surgery. Exercise therapy, physiotherapy, weight loss where needed and simple pain relief help many people. Exercise programmes, whether aerobic, strengthening or a mix, are likely to improve pain and function. [1]
A knee replacement is considered when the pain is no longer controlled with these measures. The pain may limit how far you can walk, stop you doing activities you enjoy or wake you at night. A partial knee replacement may be possible if the arthritis affects only one compartment of your knee, your ligaments work normally, and you don’t have a significant deformity or stiffness.
On X-rays, about 4 in 10 arthritic knees look suitable for a partial replacement, but fewer meet all the clinical criteria once the ligaments, the other compartments and your symptoms are assessed. [2] [3] In general, people with rheumatoid arthritis or other types of inflammatory arthritis are not suitable, because the arthritis may affect all parts of the knee and cause the partial replacement to fail.
Dr Vishal Pai will arrange a focused series of knee X-rays and may organise X-rays from your hip to your ankle, called long-limb alignment views. These show which compartments are affected by arthritis and the overall alignment of your leg. If it is unclear whether other compartments are affected, he will arrange an MRI scan of your knee.
Partial, total or osteotomy?
A partial knee replacement often suits people who are slightly older and still active, with arthritis in only one compartment of the knee. If arthritis has spread to more than one compartment, or the ligaments don’t work well, a total knee replacement is usually the better option. For younger, active people with arthritis on the inner side of the knee and bow-legged alignment, knee realignment surgery (osteotomy) may be an alternative that keeps your own joint. Read more about the choice between partial and total knee replacement.
What is involved in partial knee replacement surgery?
The steps involved in partial knee replacement surgery include:
1. Preparation of the bony surfaces
Damaged cartilage within the affected compartment of the knee is removed along with a small amount of underlying bone. The cruciate ligaments (ACL and PCL) are preserved during surgery. Dr Vishal Pai uses the Mako robotic arm to plan and guide the bone cuts. He still controls every step of the operation, and the robotic arm is there for assistance only.
2. Implantation
The artificial components are then cemented into place. There are three types of partial knee replacement:
- Medial unicompartmental knee replacement: the inner side of the knee is replaced, using a metal tray in the tibia and a metal shell over the end of the femur. A polyethylene (plastic) spacer sits between them, allowing smooth movement.
- Lateral unicompartmental knee replacement: the outer side of the knee is replaced in the same way.
- Patellofemoral knee replacement: the kneecap (patella) and the groove it runs in (trochlea) are resurfaced with artificial components.
Dr Vishal Pai uses the Stryker Restoris MCK, a fixed-bearing implant in which the plastic spacer is secured to the tibial tray rather than sliding on it.
3. Alignment and testing
Once the components are in place, the alignment and stability of the knee are checked by moving it through its range of motion. Adjustments may be made to ensure proper fit and function. The operation takes about 1 to 1½ hours.
Is a robotic partial knee replacement better?
Robotic-arm assistance is designed to help the surgeon place the implant accurately. Whether this leads to better long-term results is not yet clear. Studies so far show similar function with and without the robotic arm. Some suggest fewer people need further surgery after a robotic partial replacement, but few robotic knees have been followed for long. [4] [5] Dr Vishal Pai uses robotic assistance because he values the accuracy it gives in planning and bone cuts. This is his preference, not a guarantee of a better result. Read more about robotic knee replacement.
How long does it take to recover from a partial knee replacement?
Recovery is usually quicker than after a total knee replacement, and people go home from hospital sooner. [6] [7] Physiotherapy continues after you go home, and Dr Vishal Pai works closely with local physiotherapists to monitor your rehabilitation. Avoid resting with a pillow under your knee. Swelling in and around the knee can last up to 6 months. As a rough guide:
- 1 to 3 daysYou will usually go home from hospital.
- 2 weeksYour wound is checked and your stitches are removed at your first follow-up appointment. After this visit, you can shower without a dressing.
- 4 to 6 weeksMany people with an office-based job are able to return to work.
- 6 weeksBy this stage, Dr Vishal Pai expects most of your movement to have returned. Most people no longer need strong pain relief or crutches, and you may return to driving.
- 6 to 12 weeksPeople in physical or heavy jobs can usually return to work.
- 12 to 18 monthsMost people reach their final result in this period, and your knee usually feels more natural by this time.
What results can you expect?
Partial and total knee replacements both relieve arthritis pain well. In a large trial, people were slightly better 2 years after a partial replacement: the knee bent a little further and felt more natural, although the difference in overall knee scores was small. [8]
The Australian joint registry records almost every knee replacement done in the country. About 89 in 100 partial knee replacements for osteoarthritis have not needed revision at 10 years, and about 82 in 100 at 15 years, compared with about 95 in 100 total knee replacements at 10 years. [5] For the implant Dr Vishal Pai uses, the Restoris MCK (a fixed-bearing medial partial knee placed with the Mako robotic arm), about 96 in 100 had not been revised at 5 years. There are no 10-year figures for it yet. [5] These figures are averages for all patients and surgeons, not one surgeon’s results.
Registry data show that results are better when surgeons do partial knee replacements regularly. [9] [10] Dr Vishal Pai regularly performs partial knee replacements.
Most people return to sport or exercise after a partial knee replacement, mainly lower-impact activities such as walking, cycling, swimming, golf and doubles tennis. More people return to sport than after a total knee replacement, although fewer go back to high-impact sport. [11] [12]
What are the risks and complications?
Every operation carries risks, and Dr Vishal Pai goes through them with you before you decide. The main risks of a partial knee replacement are:
- Infection, in less than 1 in 200 people
- Blood clots in the leg
- Stiffness, which occasionally needs a manipulation under anaesthetic if the knee is not bending or straightening by 6 to 8 weeks
- Numbness beside the scar
- Arthritis developing in another part of the knee, which is the most common reason for further surgery
- Loosening of the implant
Some people have ongoing pain even when the knee replacement is working as planned. Rarely, the bone fractures during surgery, the knee feels unstable, or the artery, vein or nerve behind the knee is injured. If a partial knee replacement needs revision, it is usually converted to a total knee replacement. This is more complex than a first total knee replacement. Results are often similar, although some studies report slightly poorer function. [13] [14] You will receive a written information sheet with the full list of risks.
Where does Dr Vishal Pai consult and operate?
Dr Vishal Pai consults at Sandringham Orthopaedic Specialists, Suite 1, 60 Bay Road, Sandringham. He performs partial knee replacements at Calvary Holmesglen Private Hospital, 490 South Road, Moorabbin, in Melbourne’s south-east. Patients come from across Bayside, Glen Eira and the Mornington Peninsula, and some travel from regional Victoria.
References
Show all 14 referencesHide references
- Yan L et al. 2025. Comparative efficacy and safety of exercise modalities in knee osteoarthritis: systematic review and network meta-analysis. The BMJ.
- Klasan A et al. 2021. Measuring appropriate need for unicompartmental knee arthroplasty: results of the MANUKA study. Knee Surgery, Sports Traumatology, Arthroscopy.
- Loh K et al. 2025. Wear patterns of compartments in varus knee osteoarthritis among an Asian population: a cross-sectional study with radiographic and intraoperative analysis. International Orthopaedics.
- Banger M et al. 2021. Robotic arm-assisted versus conventional medial unicompartmental knee arthroplasty: five-year clinical outcomes of a randomized controlled trial. The Bone & Joint Journal.
- Lewis P et al. 2025. Hip, Knee and Shoulder Arthroplasty: 2025 Annual Report. Australian Orthopaedic Association National Joint Replacement Registry.
- Wilson H et al. 2019. Patient relevant outcomes of unicompartmental versus total knee replacement: systematic review and meta-analysis. The BMJ.
- Knifsund J et al. 2021. Functional results of total-knee arthroplasty versus medial unicompartmental arthroplasty: two-year results of a randomised, assessor-blinded multicentre trial. BMJ Open.
- Mortensen JF et al. 2026. Medial unicompartmental versus total knee arthroplasty in the treatment of isolated anteromedial knee osteoarthritis. The Journal of Bone and Joint Surgery.
- Liddle A et al. 2015. Optimal usage of unicompartmental knee arthroplasty: a study of 41,986 cases from the National Joint Registry for England and Wales. The Bone & Joint Journal.
- Mohammad H et al. 2022. The influence of surgeon caseload and usage on the long-term outcomes of mobile-bearing unicompartmental knee arthroplasty: an analysis of data from the National Joint Registry for England, Wales, Northern Ireland, and the Isle of Man. The Journal of Arthroplasty.
- Cerasoli T et al. 2026. Sport participation after unicompartmental knee arthroplasty: high return rates independent of implant design or technique, a systematic review and meta-analysis. Journal of Experimental Orthopaedics.
- Braz R et al. 2026. High rate of return to low-impact physical activity or sports after total and unicompartmental knee arthroplasty: a systematic review with meta-analysis. Knee Surgery, Sports Traumatology, Arthroscopy.
- Levy KH et al. 2023. Unicompartmental knee arthroplasty revised to total knee arthroplasty versus primary total knee arthroplasty: a meta-analysis of matched studies. The Knee.
- Lee JK et al. 2018. Inferior outcome of revision of unicompartmental knee arthroplasty to total knee arthroplasty compared with primary total knee arthroplasty: systematic review and meta-analysis. Knee Surgery, Sports Traumatology, Arthroscopy.
Frequently asked questions
Does a partial knee replacement feel like a normal knee?
Many people feel that a partial knee replacement is closer to a natural knee than a total knee replacement, because the ligaments and the healthy parts of the knee are kept. [8] Even so, it may not feel completely normal. The knee usually feels more natural by about a year after the operation.
Am I too young for a partial knee replacement?
There is no strict age limit. In the registry, younger people have higher revision rates, partly because they are more active and live with the implant for longer. [5] For some younger, active people with bow-legged alignment, knee realignment surgery (osteotomy) is an option to discuss with Dr Vishal Pai.
What is a kneecap (patellofemoral) replacement?
A patellofemoral replacement resurfaces the back of the kneecap and the groove it runs in. It is used much less often than a medial partial knee replacement. In the registry, about 10 in 100 had been revised by 5 years, mostly because arthritis developed in the rest of the knee. [5]
Will I be able to kneel?
Kneeling will not damage the knee replacement. However, many people find it difficult to kneel for several months after the operation, as the scar can be sensitive. Kneeling usually becomes easier with time. A foam knee pad is recommended while kneeling.
Will my knee look different after a partial knee replacement?
If arthritis has made your knee look bowed, it may look a little straighter afterwards. A partial knee replacement aims to restore the alignment you had before the arthritis started, so it does not fully straighten a bow-legged knee.
When should I seek help after surgery?
Contact Dr Vishal Pai’s rooms or your GP, or go to your nearest emergency department, if you notice:
- Increasing swelling, redness or warmth around the wound
- A temperature above 38°C, or chills
- Drainage from the wound after 72 hours
- Pain or swelling in your calf
- Loss of movement after a fall, or your knee bending less than before
Call 000 if you have chest pain or shortness of breath.
Next step
Talk through your options
If you are weighing up a partial knee replacement, Dr Vishal Pai can examine your knee, review your scans and explain all your options, including treatment without surgery.