Partial knee replacement removes only the damaged section of your knee joint

Partial knee replacement, also called unicompartmental knee arthroplasty or UKA, replaces only the worn cartilage and bone in one section of your knee—usually the inner (medial) compartment. Instead of replacing the entire joint surface like a total knee replacement does, the surgeon preserves the healthy parts of your knee and removes only what is damaged. This means less bone is cut away, fewer ligaments are disturbed, and more of your natural knee structure stays in place.

The procedure works best when arthritis or damage is confined to one compartment of the three that make up your knee joint. Your surgeon will assess whether your other compartments are healthy enough to support a partial replacement, and whether your ligaments are intact. If damage is spread across multiple areas or your ligaments are torn, a total replacement becomes necessary instead.

Key Takeaways

  • Partial knee replacement removes only the damaged compartment of your knee, leaving healthy bone and cartilage untouched.
  • The surgery requires less bone removal and causes less trauma to surrounding tissue than total knee replacement.
  • Recovery is often faster and the hospital stay shorter, though not all knees are suitable candidates.
  • Your surgeon will use imaging and physical examination to determine whether one compartment is damaged while the others remain healthy.
  • Long-term outcomes depend on whether arthritis develops in the other compartments over time.

How partial replacement differs from total knee replacement

In a total knee replacement, the surgeon removes the damaged surfaces from all three compartments of your knee—the inner, outer, and kneecap areas—and replaces them with metal and plastic components that cover the entire joint. In a partial replacement, only one compartment gets a new surface. The implant is smaller and the surgery is less invasive because less bone needs to be removed.

Because more of your natural knee remains, partial replacement patients often report better knee feel and more natural movement after recovery. Some people say their knee feels more like their original knee than after a total replacement. However, this advantage only holds if the other compartments stay healthy. If arthritis develops in an untouched compartment later, you may need a second surgery to convert to a total replacement.

Who is a candidate for partial knee replacement

Your surgeon will consider several factors to determine whether partial replacement is right for you. The damage must be limited to one compartment—usually confirmed by X-ray or MRI—while the other compartments show little or no arthritis. Your ligaments, especially the anterior cruciate ligament (ACL), must be intact and stable. If your ACL is torn, a partial replacement cannot work because the implant will not have the ligament support it needs.

Age is not a hard barrier, but younger patients are sometimes steered toward total replacement because they may outlive the partial implant and need revision surgery later. Weight matters too: surgeons are cautious about partial replacement in patients with obesity because extra stress on the smaller implant may cause it to fail sooner. Your activity level and overall health also factor in—the goal is to match the implant to your realistic demands on the knee.

What happens during the surgery

The surgeon makes a smaller incision than in total knee replacement, usually 3 to 4 inches long, to access the damaged compartment. After examining the joint to confirm the diagnosis, they remove the damaged cartilage and bone from that section only. A metal implant is cemented or press-fit into place on the femur (thighbone) and tibia (shinbone), and a plastic spacer is inserted between them to create a smooth gliding surface.

The procedure typically takes 60 to 90 minutes. You will receive general anesthesia or regional anesthesia (numbing from the waist down) depending on your health and your surgeon's preference. Because less tissue is disturbed, blood loss is usually less than in total replacement, and some patients donate their own blood beforehand to avoid needing a transfusion.

Recovery timeline and what to expect

Most people go home the same day or after one night in the hospital, whereas total knee replacement often requires a two-night stay. Pain and swelling are typically less severe because less bone and soft tissue were cut. Physical therapy usually begins within a few days, focusing on regaining motion and strength. Many patients report walking without a crutch or walker within two to three weeks, compared to four to six weeks for total replacement.

Full recovery takes about three to six months. During this time, you will work with a physical therapist to restore your knee's range of motion and rebuild the muscles around it. Returning to low-impact activities like walking, swimming, or cycling is usually possible within two to three months. High-impact activities like running or jumping may take longer, and some surgeons recommend avoiding them permanently to protect the implant.

Potential complications and revision surgery

Partial knee replacement carries the same general surgical risks as any joint surgery: infection, blood clots, and anesthesia reactions. Specific to this procedure is the risk that arthritis will develop in one of the other compartments you did not have replaced. Studies show this happens in roughly 10 to 20 percent of patients over 10 years, though the rate varies depending on your age, weight, and activity level at the time of surgery.

If arthritis spreads to another compartment, you will likely need revision surgery to convert to a total knee replacement. This second surgery is more complex than the original partial replacement because the surgeon must work around the existing implant. The good news is that revision is usually successful, and many surgeons consider it a reasonable trade-off for the better short-term outcomes of partial replacement. Some patients never develop arthritis in the other compartments and keep their partial replacement for life.

Implant materials and how long they last

Partial knee implants use the same basic materials as total knee implants: a metal alloy (usually cobalt-chromium or titanium) on the bone surfaces and a plastic (polyethylene) spacer between them. The metal components are cemented or press-fit to your bone, and the plastic insert slides on the metal to allow movement. These materials are designed to last 15 to 20 years or longer in most patients, though individual results vary.

Wear on the plastic insert is the most common reason an implant eventually fails. Over time, tiny particles of plastic can shed and trigger inflammation in the joint, leading to pain and loosening of the implant. This is why surgeons recommend avoiding high-impact activities and maintaining a healthy weight—both reduce stress on the plastic and slow wear. If the implant does wear out or loosen, revision surgery can replace it, though this is more involved than the original procedure.

Frequently Asked Questions

Can I have a partial knee replacement if I am overweight?

Your surgeon will assess your weight in relation to your overall health and the stress it places on the implant. Obesity increases the risk of implant failure and may make you a better candidate for total replacement instead. Weight loss before surgery, if time allows, can improve your outcomes and reduce complications.

Will I need physical therapy after partial knee replacement?

Yes. Physical therapy usually starts within days of surgery and continues for several weeks to months. A therapist will help you regain motion, reduce swelling, and rebuild strength. Skipping or cutting short therapy can slow your recovery and limit how well your knee functions long-term.

Can arthritis develop in the other parts of my knee after partial replacement?

Yes, it can. Studies show arthritis develops in the untouched compartments in roughly 10 to 20 percent of patients over 10 years. If this happens, you may need revision surgery to convert to a total replacement. This is a known risk your surgeon will discuss with you before the procedure.

How soon can I return to sports or running after partial knee replacement?

Low-impact activities like walking and swimming are usually possible within two to three months. Running and jumping are riskier because they place high stress on the implant and may speed up wear. Many surgeons recommend avoiding these activities permanently, though some patients do return to them with their surgeon's approval.

What is the difference between cemented and press-fit implants?

Cemented implants are glued to your bone with surgical cement, which hardens and locks them in place when ready. Press-fit implants rely on a tight mechanical fit and bone growth to find them over time. Both approaches work well; your surgeon will choose based on your bone quality and other factors.