Knee Hemiarthroplasty (Partial Knee Replacement)

Knee hemiarthroplasty, more commonly called unicompartmental knee arthroplasty (UKA) or partial knee replacement, replaces only the damaged compartment of the knee while leaving the healthy bone, cartilage, and ligaments intact. Unlike a total knee replacement, which resurfaces all three compartments of the joint, a partial replacement targets just the worn-out section, whether that is the inner (medial) side, the outer (lateral) side, or the groove where the kneecap tracks. The procedure has been around since the 1970s, has gone through periods of enthusiasm and skepticism, and has matured into a well-studied option with specific advantages and trade-offs compared with a full replacement.

What Counts as Knee Hemiarthroplasty

The knee has three compartments: medial (inner), lateral (outer), and patellofemoral (the kneecap groove). Arthritis can hit one of these while leaving the others relatively unscathed. When that happens, replacing only the affected compartment makes intuitive sense. The two main forms of knee hemiarthroplasty are unicompartmental knee arthroplasty (UKA), which addresses either the medial or lateral compartment, and patellofemoral arthroplasty (PFA), which resurfaces the kneecap joint. In selected patients with isolated medial or patellofemoral arthritis, these partial replacements can produce successful outcomes.¹ Medial UKA is by far the most common variant, since medial compartment arthritis is the most frequent pattern of isolated knee wear.

Patellofemoral arthroplasty has a somewhat different set of indications. Candidates typically have arthritis limited to the kneecap groove, with symptoms that have not responded to at least three months of conservative treatment, and imaging that shows a normal joint space in the tibiofemoral (main weight-bearing) compartments.² Second-generation onlay patellofemoral implants have shown safe and acceptable functional results and survivorship at short and mid-term follow-up, making them a reasonable alternative to total knee replacement in appropriately selected patients.³

Why Preserve the Rest of the Knee

The strongest argument for a partial replacement is biomechanical. In a cadaver study comparing unicompartmental and total knee implants, the total replacement significantly altered knee kinematics, while the unicompartmental replacement preserved normal motion patterns.⁴ Keeping the cruciate ligaments and the healthy compartments intact means the knee still moves, rotates, and stabilizes itself more like a natural joint. For patients, that translates to a knee that feels more normal and allows a broader range of activities.

This biomechanical advantage shows up in patient satisfaction data. Using the Forgotten Joint Score, a measure of how often patients are aware of their artificial joint during daily life, UKA patients scored higher than total knee replacement patients, suggesting the partial replacement felt more like a natural knee.⁵ In a head-to-head comparison of patients with the same pattern of arthritis, about three-quarters of UKA patients reported being very satisfied, compared with roughly six in ten total knee replacement patients. UKA patients also reported higher quality of life.⁶

Who Is a Good Candidate

Not everyone with knee arthritis qualifies for a partial replacement. The arthritis must be confined to one compartment, and the ligaments need to be functionally intact. Imaging plays a critical role in confirming this. MRI has proven significantly more accurate than stress X-rays for evaluating whether the cartilage in the opposite compartment is healthy enough. In one study, the specificity of MRI for selecting UKA candidates was above 94%, while stress X-rays had a specificity of only about 6%, meaning many patients would be wrongly deemed suitable based on stress films alone.⁷

Body weight is a more nuanced consideration. Two meta-analyses have examined how obesity affects UKA outcomes, and both reached the same broad conclusion: obese patients face a higher risk of revision and somewhat lower functional scores, but obesity alone is not a firm reason to deny someone the surgery. One meta-analysis found that patients with a BMI over 30 had about a 50% higher risk of needing a revision for non-infection-related causes.⁸ The other similarly found higher revision likelihood for obese patients, though both groups still showed meaningful improvements in knee function scores after surgery.⁹ Age, interestingly, does not appear to affect functional outcomes after UKA in a meaningful way.¹⁰

Recovery and Return to Activity

One of the most appealing features of partial knee replacement is the lighter surgical burden. When researchers compared bilateral simultaneous UKAs (both knees at once) with a single total knee replacement, the partial replacement group had less blood loss, fewer transfusions, shorter hospital stays, fewer complications, and better clinical outcomes.¹¹ The smaller incision, less bone removal, and preserved ligaments all contribute to a faster return to normal function.

For people who want to stay physically active, the numbers are encouraging. A systematic review found that return-to-activity rates after UKA ranged from 87% to 98%, and sport participation only dropped by a few percentage points compared with pre-symptom levels.¹² By comparison, some reports put the return-to-sport rate after total knee replacement at around 63%.¹³ Activities like golf, hiking, cycling, and doubles tennis are commonly resumed. High-impact sports are a more individual decision, and surgeons vary in what they recommend.

How Long the Implant Lasts

Implant survival is where the picture gets more complicated. Medium and long-term studies suggest survival greater than 95% at ten years for medial UKA performed for osteoarthritis or osteonecrosis.¹⁴ But registry data, which captures all surgeons rather than just expert centers, tells a less rosy story. Finnish registry data over 27 years showed UKA survivorship of about 89% at five years, 81% at ten years, and 70% at fifteen years, compared with 96%, 93%, and 89% for total knee replacements.¹⁵

That gap deserves context. UKA is technically demanding, and outcomes depend heavily on patient selection and surgical precision. Expert centers with high volumes consistently report survivorship in the mid-90s at a decade, while low-volume surgeons drive up the national averages for revision. This is not just a quality concern; it shapes the economics of the procedure, as we will see.

What Goes Wrong and Why

When a partial knee replacement fails, the most common culprit is the progression of arthritis into the compartments that were left alone. In one long-term follow-up study, arthritic progression in the non-replaced compartments was common on X-rays, though only a small number of patients developed symptoms severe enough to warrant revision.¹⁶ For lateral UKAs specifically, disease progression in the remaining compartments was also the leading reason for conversion to total knee replacement.¹⁷

Periprosthetic tibial fracture is another concern, particularly with certain implant designs and patient profiles. Risk factors include low bone mineral density, advanced age, female sex, and a tibial component that is too small for the bone it sits on. Undersized tibial components roughly triple the fracture risk.¹⁸ Laboratory and clinical studies have identified excessive bone removal during surgery and an overly tight press-fit as additional mechanical contributors to fracture.¹⁹

Fixed-Bearing Versus Mobile-Bearing Implants

UKA implants come in two main designs. In a fixed-bearing implant, the plastic liner is locked to the metal tibial tray. In a mobile-bearing implant, the liner can rotate and slide on the tray. The theoretical appeal of mobile bearings is that they distribute forces more evenly and reduce wear, since the plastic surface conforms more closely to the metal femoral component.

In practice, a meta-analysis found no significant differences between the two designs for arthritis progression, aseptic loosening, persistent pain, or overall reoperation rates. Where they did differ was in their specific failure modes: mobile bearings had a higher risk of bearing dislocation (the liner slipping out of position), while fixed bearings had a higher rate of polyethylene wear over time.²⁰ Bearing dislocations tended to happen early (within the first year), while polyethylene wear was a much later phenomenon, emerging around eight years after surgery. Interestingly, patients with fixed-bearing UKAs reported higher Forgotten Joint Scores than those with mobile bearings, suggesting the fixed-bearing knee felt more natural in daily life.²¹

At a material level, the basic physics of polyethylene wear supports the idea that more contact area means less wear. Experimental testing showed that larger contact surfaces produced significantly lower wear rates than smaller ones, with the reduction linked to milder wear processes at the microscopic level.²² Both designs aim for this in different ways, and neither has emerged as clearly superior across all outcomes.

Cemented Versus Cementless Fixation

How the metal components are attached to bone has become one of the more active debates in UKA surgery. Traditional cemented fixation uses bone cement to glue the implant in place. Cementless designs rely on a roughened or coated surface that encourages bone to grow directly into the implant.

National registry data from England and Wales comparing the two approaches over ten years found that cementless UKAs had a significantly lower overall revision rate, with ten-year survival of 93% versus 90% for cemented implants. The risk of revision for aseptic loosening was less than half in the cementless group, and revisions for unexplained pain and bone resorption also dropped significantly.²³ There is a trade-off, however: periprosthetic fractures were roughly three times more common with cementless fixation, likely because of the press-fit technique needed to achieve initial stability.

A five-year clinical comparison found that cementless implants produced significantly better pain scores and functional outcomes, even when controlling for changes in surgical skill over time. The researchers compared early cementless cases with contemporaneous late cemented cases, and the cementless group still came out ahead, suggesting the advantage was in the implant type rather than the learning curve.²⁴ The current trajectory in UKA practice is moving toward cementless fixation, though cemented implants remain widely used and well proven.

Robotic-Assisted Partial Knee Replacement

Precision matters enormously in UKA because the implant must sit within a narrow alignment window. A degree or two of error in component positioning can change how the knee loads, potentially accelerating wear in the remaining compartments or loosening the implant. Robotic-assisted platforms aim to tighten that precision.

A meta-analysis comparing robotic-assisted UKA with conventional manual surgery found that conventionally performed UKAs had roughly 58% higher total revision rates.²⁵ The failure patterns also differed: limb malalignment was a major cause of revision in conventional surgery but was essentially absent as a reason for revision in the robotic group. This aligns with evidence that robotic systems improve alignment accuracy and reduce early revision rates, particularly for surgeons who do not perform a high volume of UKAs.²⁶

Whether robotic assistance is necessary for high-volume UKA surgeons is less clear. Experienced surgeons already achieve results that rival robotic precision, and the added cost and operative time of the robot may not be justified in their hands. But for the broader surgical community, where most surgeons perform relatively few UKAs per year, robotic guidance appears to meaningfully close the gap between average and expert outcomes.

What Happens If a Partial Replacement Fails

One of the historical selling points of UKA was that if it wore out, converting to a total knee replacement would be straightforward, essentially like doing a primary total knee on a relatively preserved joint. The reality is more nuanced. Revision of a UKA to a total knee replacement can usually be accomplished with standard primary total knee components rather than the more constrained revision-specific implants needed for a failed total knee.²⁷ That is a genuine advantage.

However, a systematic review cautioned against viewing UKA as a simple time-buying procedure. Several studies found that the functional outcomes after converting a failed UKA to a total knee were not as good as those after a first-time total knee replacement, and some revisions proved more technically challenging than expected.²⁸ The takeaway is that while revision is feasible and generally less complex than revising a failed total knee, patients and surgeons should not treat UKA as a disposable stepping stone to an inevitable total replacement. The goal should be getting the right operation the first time.

The Cost Question

Partial knee replacement is consistently less expensive than total knee replacement, and the savings extend well beyond the operating room. A major randomized trial in the UK, known as TOPKAT, found that beyond one year after surgery, UKA was cost-effective compared with total knee replacement, producing greater health benefits measured in quality-adjusted life years at lower overall healthcare costs.²⁹

But here is where surgeon volume becomes economically important. A population-level cost-effectiveness study using English and Welsh registry data found that when UKA was performed by surgeons who used the procedure for at least 10% of their knee replacements, it was unambiguously cost-saving and health-improving compared with total knee replacement across all age and gender groups. When performed by surgeons with very low usage rates (below 10%), UKA was no longer expected to produce better health outcomes, and total knee replacement became the more cost-effective choice.³⁰ The procedure itself is sound, but the economics only work when the surgeon has enough experience with it.

Partial Knee Replacement with a Torn ACL

A missing or torn anterior cruciate ligament (ACL) has traditionally been considered a contraindication to UKA, because the ACL helps stabilize the knee and prevent abnormal loading of the implant. But a growing body of evidence supports combining ACL reconstruction with UKA in younger, active patients who have both ligament damage and isolated medial compartment arthritis.

A systematic review concluded that simultaneous ACL reconstruction and UKA is safe, with effective improvement in functional and clinical outcomes.³¹ A ten-year follow-up study comparing this combined procedure to total knee replacement found comparable functional scores between the two groups, with the combined approach offering the additional benefits of less invasiveness, less blood loss, and better knee kinematics.³² A separate study comparing two different implant designs used alongside ACL reconstruction found significant clinical improvements in both groups with no major differences between them at final follow-up.³³

This combined approach is still not mainstream, and it requires a surgeon comfortable with both procedures. But for younger patients who would otherwise face a total knee replacement decades before the typical age, it offers a way to preserve healthy tissue while addressing both the instability and the arthritis.

The Historical Arc of the Procedure

Knee hemiarthroplasty has had a rocky ride through orthopedic history. Early designs in the 1970s and 1980s suffered from poor patient selection, crude instrumentation, and implant designs that did not replicate normal knee anatomy well. Revision rates were high, and many surgeons abandoned the procedure in favor of total knee replacement, which had become increasingly reliable. The concept of interpositional hemiarthroplasty, where a spacer is placed between the joint surfaces without formally resurfacing the bone, was explored, fell out of favor, and has seen renewed interest in certain niche applications.³⁴

What changed was a combination of better implant design, improved surgical instrumentation including minimally invasive approaches, stricter patient selection criteria, and ultimately robotic assistance. Modern UKA is a fundamentally different operation from what was performed in the 1980s. The gap between expert-center results (95%+ ten-year survival) and population-wide registry data (closer to 80-90% at ten years) reflects that the procedure’s success still depends heavily on who performs it and on whom. When both sides of that equation are right, knee hemiarthroplasty delivers outcomes that total replacement struggles to match: a more natural-feeling joint, faster recovery, better sport participation, and lower cost. When either side is wrong, the results suffer in ways that total knee replacement, a more forgiving operation technically, does not.