Knee replacement surgery removes damaged bone and cartilage from your knee joint and replaces those surfaces with metal and plastic components that recreate the joint’s natural movement. It’s one of the most common and successful orthopedic procedures performed today, with about 90% of implants still functioning well at the 20-year mark. Here’s what actually happens before, during, and after the operation.
Why Knee Replacement Becomes Necessary
Knee replacement is typically the final option after other treatments have stopped working. Most people who reach this point have tried anti-inflammatory medications, cortisone injections, lubricating injections, physical therapy, or even prior surgeries without lasting improvement. The surgery becomes a serious consideration when you have moderate or severe knee pain at rest (day or night), chronic swelling that won’t resolve with medication or rest, or visible deformity where the knee bows inward or outward.
Osteoarthritis is the most common reason for the procedure. Over time, the cartilage cushioning the ends of your bones wears away until bone grinds against bone. Rheumatoid arthritis and post-traumatic arthritis from old injuries can cause the same kind of damage. There are no absolute age or weight restrictions for the surgery.
What Happens During the Procedure
The operation typically takes one to two hours. After anesthesia, the surgeon makes an incision down the front of the knee and moves the kneecap aside to access the joint. From there, the procedure follows a precise sequence of reshaping and resurfacing.
First, the surgeon removes the damaged surfaces from three bones: the bottom end of the thighbone (femur), the top of the shinbone (tibia), and the underside of the kneecap (patella). Only a thin layer of bone is cut away, just enough to make room for the implant components. The cuts are made at exact angles so the new surfaces will align properly and distribute your weight evenly.
Then the implant goes on in three pieces. A metal cap covers the reshaped end of the thighbone, curving to match the bone’s natural rounded shape. A flat metal plate sits on top of the shinbone, and a smooth plastic spacer is placed between the two metal surfaces. This spacer acts as the new cartilage, allowing the joint to glide with minimal friction. A third, smaller component resurfaces the underside of the kneecap so it tracks smoothly against the thighbone implant.
Before locking everything in place, the surgeon tests the knee through its full range of motion. The ligaments and soft tissues surrounding the joint need to be properly balanced so the knee feels stable when it bends and straightens. If the tension is uneven, the surgeon makes small adjustments until the joint moves naturally.
What the Implant Is Made Of
The metal components are made from cobalt-chromium or titanium alloys, both chosen because the body tolerates them well without triggering an immune reaction. The plastic spacer between them is ultra-high molecular weight polyethylene, an extremely durable material engineered to withstand decades of repetitive bending and weight-bearing. Some newer implants also use ceramics or zirconium alloys to further reduce wear.
Most implants are cemented in place using a fast-setting surgical bone cement that bonds the metal directly to the prepared bone surface. Some newer designs use a cementless approach: the implant surface has a porous, rough texture that encourages your bone to grow directly into it over time, creating a biological bond. A 10-year study comparing the two methods found equivalent patient-reported outcomes and survival rates, though cemented fixation showed a slightly higher rate of loosening over time. Many surgeons now use a hybrid approach, cementing one component while allowing the other to bond naturally.
Robotic-Assisted Surgery
An increasing number of knee replacements are performed with robotic assistance. The surgeon still does the operation, but works with a robotic arm that uses pre-operative imaging to create a 3D model of your specific knee anatomy. During surgery, the system helps position implant components in all three planes to closely replicate your natural joint line and limb alignment.
The practical advantage is precision. Robotic systems can balance the gap between the thighbone and shinbone components within 1 to 2 millimeters, which helps the surgeon avoid releasing intact ligaments and soft tissue structures that would otherwise need to be cut to achieve proper balance manually. This doesn’t necessarily mean better long-term outcomes for every patient, but it gives the surgeon finer control over the fit.
Recovery Timeline
Recovery starts fast. Most people stand with assistance within 24 hours of surgery and take their first walk with a walker or crutches shortly after. Hospital discharge typically happens once you can demonstrate safe, independent mobility, often within one to three days.
The first six weeks are the most intensive recovery period. You’ll use a walker or cane for short, frequent walks, gradually increasing distance. Physical therapy focuses heavily on regaining range of motion: the typical target is bending the knee to 90 degrees by two weeks and 110 degrees by six weeks. By the end of this phase, many people can walk short distances without any assistive device.
Between six and twelve weeks, you’ll transition off the cane entirely and walk longer distances comfortably. Range of motion often reaches 120 degrees or more during this window. Most people return to driving, light work, and daily activities during this period. Full recovery, where the knee feels “normal” and strength is fully restored, generally takes six months to a year, though the joint continues to improve subtly beyond that.
Risks and Complications
Knee replacement is a major surgery, and while most people do very well, complications do occur at low but measurable rates. Blood clots in the leg veins develop in about 0.6% of patients, typically around nine days after surgery. Blood thinners and early movement are standard preventive measures. Joint infection is the complication surgeons worry about most: it occurs in roughly 0.5% of cases, often showing up about two months after surgery, though it can appear much later. Stiffness that limits range of motion, nerve injury, and implant loosening are also possible but uncommon.
How Long the Implant Lasts
Modern knee implants are remarkably durable. Data from large national registries tracking hundreds of thousands of patients shows that 93% of total knee replacements are still intact at 15 years. At 20 years, 90% are still going strong. Research published by the UK’s National Institute for Health and Care Research found that more than 80% of total knee replacements can last 25 years.
When an implant does eventually wear out, the plastic spacer is usually the first component to degrade. A second surgery, called a revision, can replace worn components, though it’s a more complex procedure than the original. Factors that influence how long your implant lasts include body weight, activity level, and how well the components were aligned during the initial surgery.