What Are the Disadvantages of Partial Knee Replacement?

Partial knee replacement, formally called unicompartmental knee arthroplasty (UKA), carries a roughly doubled revision rate compared to total knee replacement. Registry data across multiple countries put the overall revision rate at about 11% for partial knees versus about 5.6% for total knees.¹ That gap is the headline drawback, but it only begins to capture the full picture. The reasons partial knees fail, the diagnostic headaches they create, and the challenges of converting them to total replacements down the road all deserve a closer look.

The Revision Gap

The single most cited disadvantage of partial knee replacement is how much more often people end up back in surgery. In a meta-analysis pooling data from clinical studies and worldwide joint registries, partial knees were revised at roughly twice the rate of total knees.² That does not mean every partial knee is doomed, but if longevity without further surgery is your top priority, it is a meaningful trade-off against the faster recovery and more natural knee feel that partial replacements offer.

Part of the reason the revision rate looks so much worse is that “revision” for a partial knee can mean something less drastic than a revision of a total knee. A partial that wears out is often converted to a total replacement using mostly standard components, while revising a failed total knee may require specialized implants and more bone work. Still, no one wants another surgery, and the numbers clearly favor the total replacement for sheer durability.

How Partial Knees Fail

The failure modes of partial knee replacements are distinct from total knees and worth understanding because they influence who should get one and how closely it needs to be monitored. The most common reasons a partial knee gets revised break down into a few categories.

Arthritis progression: A partial knee only resurfaces one compartment of the joint, leaving the other compartments with their original cartilage. Over time, arthritis can advance in those untreated areas. In one long-term follow-up tracking 62 knees, arthritic progression in the non-resurfaced compartments was common and was the reason for two of the four revisions performed at an average of 12 years out.³ For lateral partial knees specifically, arthritis progression is the leading cause of revision, accounting for about 35% of cases.⁴
Aseptic loosening: The implant can gradually lose its bond with the bone without any infection being present. This is among the leading reasons for revision of partial knees overall.⁵ For lateral partial knees, it accounts for roughly 17% of revisions.⁶
Bearing dislocation: In mobile-bearing designs, the plastic insert can shift out of place. For lateral partial knees, this makes up about 14% of revisions.⁷
Polyethylene wear: The plastic liner wears down over years. When the implant functions normally, wear rates can be very low, but impingement or misalignment dramatically increases them.⁸

The combination of these failure modes means that a partial knee is essentially a more fragile system. It depends on the rest of the joint staying healthy, the implant staying firmly fixed, and the bearing behaving itself. Any one of those can go wrong independently, which is partly why the cumulative revision rate climbs higher than for a total knee.

Unexplained Pain Is Surprisingly Common

Perhaps the most frustrating disadvantage of partial knee replacement is persistent knee pain that cannot be clearly attributed to any specific problem. According to registry data from England and Wales, unexplained pain accounts for roughly 23% of partial knee revisions, compared to about 9% for total knees.⁹ An analysis of the same registry data found that the five-year rate of revision specifically for unexplained pain was 1.6% for partial knees and just 0.2% for total knees, putting the risk nearly seven times higher with the partial procedure.¹⁰

The sources of this pain are varied and sometimes hard to pin down. Possible culprits inside the joint include loose cement fragments, a torn meniscus in the remaining compartment, or subtle implant issues. Outside the joint, the pain might stem from hip or spine problems referring down to the knee, nerve disorders, or even complex regional pain syndrome.¹¹ The fact that the remaining natural compartments are still present creates more potential pain generators than a total knee, where the entire joint surface has been replaced.

For the patient, this can be deeply discouraging. Imaging might look fine, no infection is present, the implant seems well-positioned, and yet the knee hurts. Roughly 15 to 20 percent of patients with any type of knee replacement report dissatisfaction with the result,¹² and the higher rate of unexplained pain with partial knees likely contributes to that number.

Your Surgeon’s Caseload Matters More Than You’d Think

Partial knee replacement is a technically demanding procedure, and the outcomes vary dramatically depending on how many of these operations your surgeon performs each year. A database analysis from Michigan found that high-volume surgeons had a five-year revision rate of about 4.3%, compared to roughly 5.2% for medium-volume and about 7.2% for low-volume surgeons.¹³ That gap between low and high volume is considerable; a low-volume surgeon’s patient faces nearly 70% greater relative risk of needing a redo within five years.

An analysis of failure rates and contributing factors confirmed similar findings, showing that surgeons performing fewer than 25 partial knees per year had a nearly doubled hazard of aseptic revision compared to higher-volume operators.¹⁴ The same study found that uncemented implants carried a higher loosening risk than cemented ones, which adds another technical variable that experienced surgeons may navigate more successfully.

This surgeon-volume effect is a genuine practical disadvantage of the procedure. Total knee replacements are performed far more frequently and are more “forgiving” in surgical technique, meaning the learning curve is gentler and outcomes are more consistent across different surgeon experience levels. If you live in an area where the nearest high-volume partial knee surgeon is hours away, the operation’s appeal diminishes.

Implant Design Trade-offs

Partial knee implants come in two main bearing types: fixed and mobile. Fixed-bearing designs lock the plastic liner to the metal tray on the tibia. Mobile-bearing designs allow the liner to glide, theoretically distributing forces more evenly and reducing wear. But that freedom of movement comes with a catch: the bearing can dislocate.

A pooled analysis of mobile-bearing partial knees reported an overall dislocation rate of about 2.4%, though the rate was considerably higher in some populations, reaching about 4.5% in South Korean cohorts compared to less than 1% elsewhere.¹⁵ More troubling, once a dislocation occurs, the chance of it happening again is high, with a re-dislocation rate of about 32%.¹⁶

Bearing design has evolved to address this. In one series, switching from a symmetrical bearing to an anatomically shaped one dropped the dislocation rate from about 9.6% to roughly 1.1%.¹⁷ That is a massive improvement, but the risk does not disappear entirely, and it remains a failure mode that simply does not exist with fixed-bearing designs or with total knee replacements.

Polyethylene wear in the liner is another concern. A retrieval study examining 47 Oxford bearings found that when the device functioned normally, the wear rate was minuscule, about 0.01 mm per year. But when extra-articular impingement was present, wear jumped to 0.05 mm per year. With intra-articular impingement or non-congruent movement, it rose to 0.12 mm per year.¹⁸ Wear itself is a secondary pathway to loosening, because wear debris triggers bone resorption around the implant. This means that subtle surgical errors in implant positioning can create a slow-motion failure that takes years to become symptomatic.

Periprosthetic Fractures and Component Sizing

A fracture through the bone around or beneath the implant is uncommon but serious. In a study of over 1,800 partial knee cases, about 1% experienced a tibial plateau fracture.¹⁹ The risk was linked to implant placement and sizing: a shorter distance between the implant’s keel and the edge of the bone, a steeper tibial slope, and the use of the smallest available implant size all increased fracture likelihood.²⁰

The first three months after surgery appear to be a critical window. Temporary post-operative bone loss and local changes in how forces pass through the tibia make the bone more vulnerable during this period.²¹ Patients with smaller bones, which correlates with smaller body size, are at particular risk because the available implant sizes may not leave enough bone around the component’s edges.

Tibial component sizing creates a different kind of headache too. If the metal tray on the tibia overhangs the bone edge by more than about 2 mm, it starts to press on the medial collateral ligament, which can cause pain on the inner side of the knee. A cadaveric study found that loads on the ligament nearly doubled going from 2 mm of overhang to 4 mm.²² On the other hand, sizing the tray too small (underhang) can leave unsupported bone at the edge, potentially contributing to loosening.²³ The margin for error is slim, and getting it wrong in either direction creates problems that may not show up until months or years later.

When a Partial Knee Needs to Become a Total

One of the selling points of partial knee replacement is that “you can always convert it to a total knee later.” That is true, but the conversion is not as simple as a first-time total knee. A study comparing partial-to-total conversions with primary total knee operations found that conversion cases took significantly longer, about 103 minutes versus 73 minutes on average. They also required extra hardware far more often: tibial augments (metal wedges to fill bone gaps) were needed in 38% of conversions compared to essentially zero percent of primary totals, and stems were used in 56% of conversions versus 3% of primary totals.²⁴

The central problem is bone loss. Removing the old partial knee implant often leaves defects in the tibia that a standard total knee implant cannot simply sit on top of. The thicker the polyethylene insert that was in place, the more bone typically needs to be removed, and surgeons need to be ready for that going into the revision.²⁵ None of this makes conversion impossible or inherently high-risk, but the promise of an easy future upgrade is overly optimistic. You end up with something closer in complexity to a revision total knee than a primary one.

Diagnostic Challenges After Surgery

Following up on a partial knee with imaging can be confusing even for experienced clinicians. On X-rays, narrow lines between the implant and bone (called radiolucent lines) are common after partial knee replacement. These look concerning because similar lines around a total knee implant often indicate loosening. With partial knees, though, many of these lines are benign.

A study of Oxford partial knees found that well-defined, narrow radiolucent lines were common and were not associated with pain or poor outcomes. The authors concluded that these “physiological” lines should be ignored when trying to figure out why a partial knee hurts.²⁶ The problem is distinguishing benign lines from pathological ones that actually do signal loosening. It requires experience and often serial imaging over time to tell the difference. For the patient, this ambiguity can mean repeated imaging, ongoing uncertainty, and sometimes unnecessary revision surgery when a clinician misreads a normal finding as a failure.

Medial Versus Lateral Partial Knees

Most partial knee replacements address the medial (inner) compartment, because that is where arthritis most commonly develops. Lateral (outer) compartment partial knees are less common, and they come with their own set of challenges. The lateral side of the knee has more natural looseness and different biomechanics, making the procedure technically more demanding. Many surgeons are simply less accustomed to performing lateral partial knees, which compounds the difficulty.²⁷

The failure patterns differ between the two sides as well. On the medial side, the most common reason for revision is aseptic loosening, followed by arthritis progression. On the lateral side, arthritis progression leads, followed by loosening and then bearing dislocation.²⁸ This means the disadvantages of the procedure shift in emphasis depending on which compartment is involved. A patient with isolated lateral compartment arthritis considering a partial knee needs to weigh a different set of risks than someone with medial disease.

Age and Activity Expectations

Partial knee replacement is sometimes recommended for younger, more active patients precisely because it preserves bone stock and more closely mimics a natural knee. But younger patients face a built-in disadvantage: more years of use mean more opportunity for failure. A systematic review looking at knee arthroplasty in young patients found that while knee function typically improves compared to before surgery, those improvements often do not translate into a return to the activity levels or quality of life patients hope for. This population faces both a higher and an earlier risk of needing revision compared to older patients.²⁹

That creates a difficult calculation. A 55-year-old with single-compartment arthritis might reasonably expect a partial knee to last 15 years or more with a high-volume surgeon, but the odds of eventually needing a second procedure are meaningfully higher than with a total knee. The trade-off is essentially faster recovery and better early function now, in exchange for greater uncertainty later. Whether that trade-off makes sense depends heavily on how much physical demand you intend to place on the knee, how close your surgeon is to the high end of the volume spectrum, and how much the idea of a potential second surgery bothers you.

The Role of Uncemented Implants

One technical detail worth knowing about if you are considering a partial knee: the way the implant is fixed to the bone matters for longevity. Cemented implants are bonded to the bone with surgical cement at the time of the operation. Uncemented implants rely on the bone growing into a textured or coated surface over the first weeks after surgery. The analysis of failure rates mentioned earlier found that uncemented partial knee implants had a roughly 38% higher risk of aseptic loosening compared to cemented ones.³⁰ This does not mean uncemented designs are bad across the board; newer cementless fixation technologies continue to evolve. But it does mean that not all partial knees are equal, and the fixation method is worth a direct conversation with your surgeon.