Most people arrive at this question having read that a partial knee replacement is the smaller, better operation. It is smaller. Whether it is better depends on things that are specific to you, and the honest version of the comparison includes a number that the enthusiastic version leaves out.
What each operation does.
The knee has three compartments: the inside, the outside, and the joint between the kneecap and the thigh bone. A total knee replacement resurfaces all of them and takes the cruciate ligaments with it, or at least the anterior cruciate.
A unicompartmental, or partial, knee replacement resurfaces one compartment and leaves the rest of the joint, including both cruciate ligaments, untouched. In the great majority of cases that compartment is the inside of the knee.
Who is a candidate.
Roughly one in four people coming to knee replacement is suitable, and the criteria are reasonably strict:
- Arthritis genuinely confined to one compartment, confirmed on a standing X-ray with the knee flexed rather than on a lying film.
- An intact anterior cruciate ligament.
- A deformity that corrects passively, meaning the leg can be straightened by hand.
- Reasonable range of movement, without a significant fixed flexion deformity.
- No inflammatory arthritis.
Significant patellofemoral arthritis with symptoms, and arthritis in two or three compartments, both rule it out.
What a partial does better.
These advantages are real and they are not marketing.
- It feels more like your own knee. Preserving both cruciates preserves the proprioception that tells your brain where the joint is, and patients consistently report a more natural-feeling knee.
- Faster recovery. Smaller operation, less blood loss, shorter stay, quicker return to function.
- Better range of movement on average.
- Lower rate of serious medical complications, which matters most in older or medically complex patients.
And the number the enthusiastic version leaves out.
Partial knee replacements are revised more often than total knee replacements. That is consistent in the Australian registry and in registries internationally, and the difference is not marginal.1
Two things explain most of it, and only one is a fault of the operation.
The first is genuine: the rest of the knee is still your own, and arthritis can progress in a compartment that was healthy at the time of surgery. That failure is not the implant wearing out; it is the disease continuing.
The second is a threshold effect. Revising a partial knee to a total knee is a relatively straightforward operation, considerably more so than revising a failed total knee. So surgeons revise a painful partial knee sooner than they would revise a total knee with similar symptoms, and the registry counts both as revisions. The bar is lower, so the number is higher.
Registry data also shows partial knee revision rates falling with surgeon volume more steeply than for total knee replacement, which is a fair question to ask of anyone offering you one.
Why age cuts both ways here.
In a younger patient, the partial knee's faster recovery and more natural feel are worth more, and the higher revision rate matters more, because there are more years for it to express itself. Those two pull in opposite directions and there is no formula that resolves them. See am I too young for a knee replacement.
In an older patient with single-compartment disease the case is more straightforward: the smaller operation with the lower medical complication rate, and a lifetime that the implant will very probably outlast.
How it is decided.
On the imaging and the examination, not on preference. A standing posteroanterior view with the knee flexed shows whether the other compartments are genuinely preserved, a skyline view shows the kneecap, and the examination establishes whether the deformity corrects and whether the cruciate is intact.
Where it is close, the decision is sometimes confirmed at the time of surgery, and you would be consented for both. That is not indecision; it is that the cartilage in the other compartments can be seen directly in theatre and not on any scan.
If you are told you are not a candidate, ask which criterion you fail. There will be a specific one.
References.
- Australian Orthopaedic Association National Joint Replacement Registry. Hip, Knee & Shoulder Arthroplasty Annual Report. Adelaide: AOA.
- Evans JT, Walker RW, Evans JP, Blom AW, Sayers A, Whitehouse MR. How long does a knee replacement last? A systematic review and meta-analysis of case series and national registry reports with more than 15 years of follow-up. Lancet. 2019;393(10172):655-663.
This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.