What does the AOANJRR say about partial knee replacement?

When patients are considering a partial knee replacement, one of the most useful pieces of information available comes not from any single surgeon or single study, but from the collective experience of every joint replacement performed in Australia. That data is captured by the Australian Orthopaedic Association National Joint Replacement Registry — the AOANJRR — and it is one of the most valuable tools we have for understanding how different procedures and implants perform over time.

What is the AOANJRR?

The AOANJRR is a national database that has been collecting data on joint replacement surgery in Australia since 1999. Every hip and knee replacement performed in the country is reported to the registry, along with information about the implant used, the patient's characteristics, and — critically — whether the implant was ever revised. This gives an almost complete picture of how joint replacements perform at a population level, making the AOANJRR one of the largest and most respected joint registries in the world.

When a surgeon or a patient asks "how long will this implant last?" or "how does this procedure compare to the alternative?", the AOANJRR is where the most reliable Australian answer comes from.

What is a partial knee replacement?

A partial knee replacement — also called a unicompartmental knee replacement, or UKR — resurfaces only the damaged compartment of the knee rather than replacing the entire joint surface. The knee has three compartments: the medial (inner) side, the lateral (outer) side, and the patellofemoral (front) compartment. The medial compartment is by far the most commonly affected by osteoarthritis, and medial UKR is the most frequently performed type. Lateral UKR and patellofemoral replacement are less common but are also tracked in the registry.

What does the revision rate data show?

The AOANJRR tracks revision rates — that is, how often a joint replacement needs to be reoperated on to correct a problem. This is the primary measure of implant survival used in registry analyses.

For unicompartmental knee replacement, the registry consistently shows revision rates in the range of approximately 9 to 10 per cent at ten years. For total knee replacement (TKR), the ten-year revision rate is approximately 5 per cent. On the surface, this appears to suggest that partial knee replacement performs less well — but the comparison is not quite as straightforward as it first looks.

Why the comparison with TKR is more nuanced

The higher revision rate for UKR compared to TKR does not mean the partial replacement is an inferior choice for the right patient. Several important factors complicate a direct comparison.

Different patient populations. Patients selected for partial knee replacement tend to be younger and more active than those who receive total knee replacement. Younger, more active patients place higher demands on their implants and have a longer follow-up period during which a revision could occur. This alone accounts for a meaningful proportion of the difference in revision rates.

Different revision thresholds. Because conversion from UKR to TKR is usually a relatively straightforward operation — much simpler than revising a failed TKR — surgeons and patients may have a lower threshold for proceeding with revision when problems arise. This may inflate the apparent revision rate for UKR compared to TKR, where the bar for reoperation tends to be higher.

What "success" means. Patients who do well with a partial knee replacement consistently report that the knee feels more natural — because the intact compartments, including the cruciate ligaments, are preserved. Patient-reported outcomes for well-selected UKR patients are often excellent. The revision rate alone does not capture this.

A higher revision rate does not automatically mean a worse outcome. For the right patient, a well-performed partial knee replacement can feel more natural than a total replacement.

Trends in UKR in Australia

The registry shows that UKR volumes in Australia have been growing steadily, as patient selection has improved and the procedure has become better understood. Early in the registry's history, higher revision rates partly reflected a broader selection of patients — including some who were not ideal candidates. As selection criteria have tightened over time, outcomes have improved.

This is an important lesson from registry data more broadly: the procedure itself is only part of the story. Patient selection is at least as important.

What are the most common reasons for revision?

The AOANJRR data shows that the most common reasons for revising a unicompartmental knee replacement are disease progression (the development of osteoarthritis in the compartments that were not originally replaced), aseptic loosening of one or both components, and pain without a clearly identifiable mechanical cause. Infection, while always a concern with any implanted device, is a less frequent cause of revision in UKR than in TKR.

Disease progression is a particularly important concept: UKR assumes that the other compartments of the knee are healthy at the time of surgery, and that they will remain so. In some patients, OA progresses to affect the other compartments over time, making conversion to TKR necessary. This is not a failure of the original procedure — it is the natural history of the underlying disease in that individual.

What does this mean for patients considering UKR?

The registry data supports a clear message: a partial knee replacement, in the right patient with the right indication, is a very good operation. The key is patient selection. The patients who do best are those with genuinely isolated single-compartment disease, a functioning ACL, a body weight that is not excessively elevated, and realistic expectations about the procedure.

The data does not support using UKR indiscriminately as an alternative to TKR in patients with multi-compartmental disease. But for those who genuinely meet the criteria, the registry data — in the context of the broader literature — supports it as a sound and often excellent choice.

Discussing the registry data in the context of your individual situation is an important part of any consultation for knee replacement. The numbers are a starting point, not the whole story.