What is partial knee replacement?
The knee has three compartments: the inner (medial) side, the outer (lateral) side, and the joint between the kneecap and the thigh bone. In partial knee replacement, also called unicompartmental knee arthroplasty or UKA, only the worn compartment is resurfaced with metal and plastic components. The rest of the knee is left in place. In a total knee replacement, the surfaces of the whole joint are replaced.
Who might be a candidate?
Partial replacement is an option only when arthritis is confined to one part of the knee. Many people with knee arthritis have wear in more than one compartment, and for them total knee replacement is the usual choice. In the Australian national registry’s lifetime-risk analysis, partial replacements were about 1 in 15 of the primary knee replacements for osteoarthritis.1
Whether it fits you depends on your symptoms, an exam of the knee, standing X-rays and sometimes advanced imaging. Dr. Stepanyan will explain which operation he recommends and why. Before surgery, the same non-surgical steps apply as for any knee arthritis, which are covered on the total knee replacement page.
How does it compare with total knee replacement?
The best evidence comes from TOPKAT, a randomized trial in the United Kingdom of 528 patients with arthritis limited to one compartment of the knee. Half were assigned to partial and half to total knee replacement. At 10 years, knee function scores did not differ significantly between the two groups, and about 5% of each group had needed a revision.2
The researchers found no significant difference in clinical outcomes, reoperations or revisions, and they favored continued use of partial replacement in appropriate patients. These results apply to people chosen for the trial, not to every knee.2
How long does it last?
Registry data can estimate the lifetime risk of revision: the chance that a knee replacement will eventually need revision surgery, given your age at the time of the operation and how long you are likely to live. The Australian registry modelled this for 53,322 patients (average age 66) who had a modern partial knee replacement for osteoarthritis.1
| Age at surgery | All patients | Men | Women |
|---|---|---|---|
| 46–50 | 63.4% | 59.1% | 68.3% |
| 51–55 | 55.3% | 50.5% | 60.9% |
| 56–60 | 46.3% | 41.9% | 51.8% |
| 61–65 | 37.2% | 33.3% | 42.2% |
| 66–70 | 29.2% | 26.0% | 33.4% |
| 71–75 | 21.1% | 18.9% | 24.0% |
| 76–80 | 14.6% | 13.4% | 16.1% |
| 81–85 | 8.4% | 8.5% | 8.4% |
| 86+ | 3.9% | 4.3% | 3.5% |
Younger patients have a higher estimated lifetime risk mainly because they have more years ahead in which a revision could become necessary. Women had a higher estimated risk than men at every age through 76–80. Ten years after surgery, an estimated 84% of patients aged 46–50, 89% of those aged 61–65 and 92% of those aged 71–75 had not needed a revision.1
For comparison, the same registry estimated lifetime revision risk after total knee replacement at about 31% for ages 46–50, 16% at 61–65 and 9% at 71–75. The registry describes its figures as descriptive estimates that do not test for differences between groups, and the patients who have each operation differ, so these numbers do not show that one operation is better than the other. They are population estimates, with more uncertainty beyond the follow-up observed so far, and they cannot predict what will happen to any one person.1
What are the risks?
Partial knee replacement carries the general risks of knee surgery, including infection, blood clots, stiffness, persistent pain, and loosening or wear of the implant over the years. In the Australian registry, the most common reasons for revision after a partial knee replacement for osteoarthritis were progression of arthritis in the rest of the knee, loosening and pain, and infection was a less common reason.1
What does recovery look like?
Recovery after any knee replacement follows the same general stages: getting up and walking early, regaining motion first, then building strength. The typical timeline is on the total knee replacement page. Your own plan from Dr. Stepanyan and your therapist may differ.
Should I get a second opinion?
If you have been told you need a total knee replacement and want to know whether a partial replacement could be an option, or the other way around, a second opinion is reasonable. Request a visit and bring your X-rays and any MRI.
Sources
- Australian Orthopaedic Association National Joint Replacement Registry. Hip, Knee & Shoulder Arthroplasty: 2026 Annual Report. Special chapter: Modelling the Lifetime Risk of Revision (data to 31 December 2025). Registry report
- Beard D, et al. Assessing clinical and cost effectiveness of total versus partial knee replacement (TOPKAT): 10-year follow-up of a multicentre, randomised controlled trial. Lancet Rheumatol. 2025. Journal article