What Is Knee Resurfacing and Who Is a Candidate?

Knee resurfacing is a surgical procedure that replaces only the damaged cartilage surface of the knee rather than removing the entire joint, preserving healthy bone, ligaments, and the undamaged portions of the knee. The most common form is unicompartmental knee replacement, which treats arthritis confined to one section of the knee, though resurfacing can also involve two compartments or even small focal areas of cartilage loss. The procedure has gained popularity over the past two decades as an alternative to total knee replacement, particularly for younger or more active patients whose damage hasn’t spread across the whole joint.

How Resurfacing Differs from Total Knee Replacement

A total knee replacement strips away the surfaces of all three compartments of the knee (the inner side, the outer side, and the area behind the kneecap) and replaces them with metal and plastic components. Resurfacing takes a more conservative approach: only the worn-out compartment gets a new surface, while healthy cartilage, the cruciate ligaments, and the majority of bone stock stay intact. Think of it as patching a damaged section of flooring instead of ripping up the entire room.

The most common version is unicompartmental knee replacement, often called a partial knee replacement, which typically addresses the medial (inner) compartment because that is where osteoarthritis strikes most frequently. When two compartments are damaged but the third remains healthy, surgeons can perform a bicompartmental resurfacing, sometimes combining a unicompartmental implant on one side with a patellofemoral replacement behind the kneecap. These options allow selective replacement of the worn compartments and a customized fit of the smaller implants to the native knee anatomy.¹

There is also a much smaller category called focal resurfacing, which uses a metal cap implant to cover an isolated cartilage defect rather than resurfacing an entire compartment. This sits at the boundary between joint replacement and cartilage repair, and is typically reserved for younger patients with a localized problem rather than widespread arthritis.

Who Is a Good Candidate

The central requirement is that your arthritis stays in its lane. If cartilage loss is limited to one compartment and the ligaments holding the knee together (especially the anterior cruciate ligament) are intact, resurfacing is on the table. Once damage spreads across multiple compartments, or if the knee is significantly unstable or severely deformed, a total replacement becomes the more reliable choice.

For years, unicompartmental resurfacing was thought of as a procedure for older, less active patients. That picture has shifted. Partial knee replacement has grown increasingly popular among younger patients with high physical demands, including those who want to return to recreational sports.² The reasoning is straightforward: resurfacing preserves more of the knee’s original architecture, which means any future revision surgery has more bone and soft tissue to work with. For a forty-five-year-old who may need another operation in twenty or thirty years, that preservation matters.

Beyond the compartment pattern, surgeons evaluate several other factors before offering resurfacing:

Ligament integrity: The cruciate ligaments must be functional. A knee with a torn or absent ACL is generally not a candidate for unicompartmental replacement, because the implant relies on the ligament’s stabilizing role.
Deformity severity: Mild to moderate angular deformity (a slight bow-leg or knock-knee) can be corrected with resurfacing, but severe fixed deformity pushes toward total replacement.
Range of motion: Significant stiffness that restricts bending to less than about 90 degrees usually signals that the problem extends beyond one compartment.
Body weight: While obesity is not an absolute contraindication, it does increase mechanical stress on a smaller implant and is weighed against other risk factors.

Inflammatory arthritis, such as rheumatoid arthritis, generally disqualifies someone from resurfacing because the disease process tends to affect the entire joint lining rather than a single compartment.

Figuring Out Which Compartments Are Damaged

The imaging step is more important than it might seem, because the whole point of resurfacing rests on accurately mapping where the damage is. Standard standing X-rays are the starting point, but not all X-ray views are created equal. A weight-bearing flexion view (sometimes called the Rosenberg view) has been shown to outperform the conventional straight-leg X-ray at detecting cartilage loss on the inner side of the knee. Research comparing this view against what surgeons actually find during surgery has demonstrated excellent diagnostic performance, rivaling MRI at a fraction of the cost.³

MRI does provide more detail on soft tissue and cartilage, and it remains useful for evaluating ligament integrity or spotting problems the X-ray might miss. But its moderate sensitivity for symptomatic knee arthritis, combined with its cost and the risk of flagging incidental findings that may not correlate with symptoms, means it is not always necessary for every patient being evaluated for resurfacing.³ In practice, many surgeons use standing X-rays (including the Rosenberg view) as their primary decision tool and reserve MRI for cases where the clinical picture is unclear.

What Happens During Surgery

Unicompartmental resurfacing is typically performed through a smaller incision than total knee replacement, which translates to less soft-tissue disruption. Because the ligaments and the undamaged compartments are left alone, the surgery itself is generally shorter and involves less blood loss. The smaller footprint of the operation is one reason many centers now perform it as a same-day procedure.

Studies examining rapid discharge protocols for unicompartmental replacement have reported same-day discharge rates above 70 percent, and in well-developed programs, above 80 percent.⁴⁵ Going home the same day requires a coordinated protocol that includes pain management planning before surgery, physical therapy evaluation on the day, and clear criteria for safe discharge. Not everyone qualifies; patients with significant medical conditions, those who live alone without help, or people who develop nausea or dizziness after anesthesia may spend a night in the hospital.

Robotic Assistance and Implant Precision

Robotic-assisted surgery has become increasingly common in knee resurfacing and replacement. The technology helps the surgeon position implants more accurately by providing real-time feedback on bone cuts and alignment. A cadaveric study comparing robotic-assisted technique with conventional instruments found that the robotic group had roughly half the alignment error for bone cuts in several planes, and fewer cases where the alignment was off by more than three degrees.⁶

Clinical studies have backed this up. One series examining imageless robotic-assisted surgery reported an overall implant placement accuracy rate of about 95 percent and an average gap-balancing error of less than half a millimeter.⁷ Another study validating image-based robotic technique against postoperative full-leg X-rays confirmed that intraoperative alignment targets were reliably reproduced on weight-bearing radiographs, with no meaningful soft-tissue laxity affecting the result.⁸

Whether this improved precision translates into better long-term clinical outcomes for every patient remains an active area of research. Alignment accuracy matters most when it prevents outlier implant positioning, which is associated with early failure. For resurfacing in particular, where the implant footprint is smaller and the margin for error is tighter, precise component placement is especially relevant.

How Resurfacing Feels Compared to Total Replacement

One of the more compelling arguments for resurfacing is how the knee feels afterward. Researchers use a metric called the Forgotten Joint Score, which asks patients how often they are aware of their replaced joint during everyday activities. A higher score means the joint draws less attention, which is the goal. Studies consistently find that patients with unicompartmental replacement score higher than those with total replacement. One prospective study found this difference to be significant, with the authors attributing it to the greater preservation of soft tissue and bone in unicompartmental surgery.⁹ A separate cross-sectional study reported mean Forgotten Joint Scores of about 65 for unicompartmental patients versus 50 for total knee patients.¹⁰

The gait data tells a similar story. A meta-analysis comparing walking characteristics after unicompartmental and total knee replacement found that unicompartmental patients walked faster, had longer stride lengths, and demonstrated knee movement patterns closer to a natural knee, including better knee extension and flexion during walking.¹¹ These are not dramatic differences visible to the naked eye, but they add up to a knee that moves more like the original equipment.

None of this means total replacement is a poor operation. For patients with widespread arthritis, total replacement is the right procedure, and outcomes are excellent. The point is that when someone genuinely qualifies for resurfacing, the preserved ligaments and retained compartments give the reconstructed knee a mechanical advantage.

Focal Resurfacing for Isolated Cartilage Defects

Not every damaged knee needs a compartment-wide implant. Some patients, often younger and more active, have a single cartilage defect from an injury or from a condition called osteochondritis dissecans. For these localized problems, a small metal cap can be press-fit over the defect to restore a smooth surface without replacing the entire compartment.

A randomized trial comparing these focal metal implants with biological cartilage repair procedures found that the metal resurfacing group achieved significant improvement in all outcome measures about 75 percent of the time, compared with roughly 53 percent for the biological group. The biological procedures required a longer rehabilitation period, though they tended to produce better outcomes in younger patients. Focal resurfacing allowed patients to bear full weight earlier.¹²

This creates an interesting clinical trade-off. A younger patient with decades of activity ahead might benefit more from a biological repair that, if successful, regenerates cartilage tissue. An older or moderately active patient who needs a predictable recovery and quick return to function might be better served by a focal metal implant. The decision depends heavily on the patient’s age, activity goals, defect size, and willingness to commit to a longer rehabilitation if a biological approach is chosen.

How Long Do Resurfacing Implants Last

Longevity data for resurfacing implants varies depending on the type. For unicompartmental knee replacement, national joint registries generally report ten-year survival rates in the range of 90 percent or higher when performed by experienced surgeons in well-selected patients. Focal resurfacing implants, which address smaller cartilage defects rather than an entire compartment, have shown somewhat lower longevity. A nationwide Danish registry study of focal resurfacing implants reported revision-free survival of about 95 percent at one year, 84 percent at five years, and 80 percent at ten years.¹³ The median time to revision in that study was two years, suggesting that implants that fail tend to fail relatively early rather than wearing out gradually over time.

When a resurfacing implant does fail, the most common next step is conversion to a total knee replacement. Because resurfacing preserves the bulk of the bone and the ligaments, this revision surgery is generally less complex than revising a failed total replacement. This is one of the key strategic advantages of resurfacing in younger patients: it keeps the door open for a straightforward total replacement later in life if needed.

Cost and the Surgeon Experience Factor

Resurfacing tends to cost less than total replacement. A large UK randomized trial (the TOPKAT trial) comparing partial and total knee replacement for medial arthritis found that beyond the first year, partial replacement was associated with greater health benefits and lower healthcare costs, reflecting both the cheaper index surgery and reduced downstream healthcare use.¹⁴

A population-based cost-effectiveness analysis using English and Welsh registry data reached a similar conclusion: unicompartmental replacement was both less expensive and produced better quality-of-life outcomes than total replacement across all age and gender groups when performed by surgeons who used the procedure regularly.¹⁵ The biggest savings appeared in men over seventy-five, while the biggest quality-of-life gains showed up in women over seventy-five.

Here is the catch, and it is a significant one: surgeon experience dramatically changes the equation. That same analysis found that when unicompartmental replacement was performed by surgeons who used the procedure for at least 10 percent of their knee replacements, it was unambiguously cost-saving and health-improving. When performed by low-volume surgeons (those who used it for less than 10 percent of cases), unicompartmental replacement no longer produced better health outcomes, and total replacement became the more cost-effective choice.¹⁵ This is one of the most important things to know if you are considering resurfacing: the surgeon’s familiarity with the procedure influences your outcome at least as much as the procedure itself. Asking your surgeon how often they perform partial replacements is a reasonable and important question.

Why Resurfacing Remains Underused

Despite favorable outcomes data and cost advantages, unicompartmental knee replacement still accounts for a minority of knee replacements in most countries. Several factors contribute. Many orthopedic training programs devote more time to total knee replacement, leaving surgeons less comfortable with partial replacement technique. The narrower indications mean surgeons see fewer eligible patients, which in turn limits their volume and confidence. And registry data showing higher revision rates for unicompartmental replacement compared with total replacement (which reflect, in part, the ease of revising a partial to a total and the lower threshold for offering revision) can make the procedure look worse on paper than it performs in practice for appropriately selected patients.

The evolution of resurfacing over the past three decades has moved from early designs with mixed results toward modern minimally invasive techniques and refined patient selection criteria that have substantially improved outcomes.¹⁶ Robotic assistance is accelerating this trend by making precise implant positioning more reproducible, potentially reducing the learning curve that has historically limited wider adoption. Whether these technological advances will close the gap between how often resurfacing could be offered and how often it actually is remains to be seen, but the direction of the evidence favors broader use in well-selected patients treated by surgeons with adequate experience.