Knee replacement surgery removes damaged bone and cartilage from your knee joint and replaces them with an artificial joint made of metal and plastic. The procedure typically takes one to two hours, but the full process, from pre-surgical preparation to complete recovery, spans about a year. Here’s what each stage looks like.
Weeks Before Surgery: Getting Cleared
Preparation starts well before you arrive at the hospital. Most surgeons recommend a general health checkup about six months out so any underlying issues can be addressed in time. You’ll need blood work done within 30 to 90 days of surgery, and if you have heart concerns, an echocardiogram may be required. All medical clearances typically need to be finalized at least two weeks before your surgery date, or the procedure can be postponed.
One step that surprises many people: dental clearance. Bacteria from dental infections or procedures can travel through the bloodstream and infect a new joint. Any dental work should be completed and fully healed at least six weeks before surgery. The same six-week buffer applies to steroid injections in the knee being replaced. You’ll also have a nasal swab to screen for staph bacteria, since carriers have a higher risk of surgical infection.
If you take blood thinners, immunosuppressant medications, or steroids, your surgical team will give you specific instructions on when to stop them. Diabetic patients need their blood sugar well controlled, with a hemoglobin A1C at or below 7.0 and daily blood sugar readings at or below 150.
Anesthesia: Regional vs. General
Most knee replacements use regional anesthesia, typically a spinal block. A local anesthetic is injected into the lower spine, where it quickly numbs sensation and movement from the waist down. You’ll also receive sedation so you’re unconscious during the procedure, but you breathe on your own without a breathing tube. This approach leads to less grogginess, less nausea, reduced blood loss, and a lower risk of blood clots compared to general anesthesia.
General anesthesia is the alternative. It requires intubation and a ventilator to manage your breathing, and you’re more likely to wake up feeling groggy or nauseated, sometimes with a sore throat from the breathing tube. Your anesthesiologist will recommend the best option based on your health and anatomy.
What Happens During the Operation
Once you’re anesthetized, the surgical site is cleaned and any hair is clipped away. Your heart rate, blood pressure, breathing, and oxygen levels are monitored continuously throughout. The surgeon makes an incision over the front of the knee to access the joint.
From there, the damaged surfaces on three bones are reshaped. The lower end of the thighbone is cut and fitted with a curved metal cap, usually made from a cobalt-chromium alloy chosen for its hardness and wear resistance. The top of the shinbone is trimmed flat and capped with a metal tray, often titanium. Between these two metal surfaces, the surgeon places a smooth plastic spacer made of polyethylene, which acts as the new cartilage, allowing the joint to glide with very little friction. If the underside of the kneecap is also damaged, it gets a plastic button as well.
Most implants are cemented into place using a fast-setting bone cement. Some designs, particularly in younger or more active patients, use textured surfaces that encourage bone to grow directly into the implant over time. A newer option called oxidized zirconium uses a metal core with a ceramic-like surface that’s harder and smoother than standard metal, reducing friction against the plastic spacer.
Partial vs. Total Replacement
Not every worn-out knee needs a full replacement. An estimated 25 to 47 percent of people eligible for knee replacement have damage limited to just one section of the joint, making them candidates for a partial (unicompartmental) replacement. This procedure replaces only the damaged compartment, preserving healthy bone, cartilage, and ligaments on the unaffected side. The incision is smaller, recovery is generally faster, and the knee tends to feel more natural afterward. However, if arthritis is present in more than one compartment, a total replacement is the standard approach.
The First 24 Hours After Surgery
Physical therapy begins almost immediately, often within 24 hours. A therapist will help you stand with assistance, and you’ll start simple exercises right away. Ankle pumps, where you rhythmically flex your foot up and down, help prevent blood clots and reduce swelling. You’ll do these for two to three minutes at a time, several times an hour. Quad sets, where you tighten your thigh muscle and try to straighten your knee, help reactivate the muscles around the joint.
Whether you stay overnight or go home the same day depends on your overall health. Patients with well-controlled health, good mobility, and a reliable support person at home may qualify for same-day discharge. Those with kidney disease, serious heart or lung conditions, or a need for services like dialysis will typically stay longer. Either way, you’ll need someone at home to help for the first few days.
Recovery Milestones Over 12 Months
Recovery follows a fairly predictable arc. In the first six weeks, the goal is to bend your knee to about 90 degrees by week two, reaching roughly 110 degrees by week six. By the end of this phase, most people can walk short distances without a walker or cane. Your surgeon and physical therapist will likely recommend 20 to 30 minutes of targeted exercises daily (sometimes two to three times daily) plus 30-minute walks two to three times a day.
Between six and twelve weeks, you’ll walk longer distances comfortably and your knee bend often reaches 120 degrees or more. Most people hit major functional milestones, like returning to daily activities and driving, within three to six months. Full recovery, meaning stable strength and range of motion, typically takes six to twelve months. The knee continues to feel more natural over this entire period as swelling gradually resolves and the muscles around the joint rebuild.
Risks and Complication Rates
Knee replacement is one of the most common and successful elective surgeries, but it carries real risks. A large study published in the Canadian Journal of Surgery tracked complication rates across tens of thousands of procedures. Blood clots in the leg occurred in about 0.6 percent of patients, typically appearing around nine days after surgery. Joint infection developed in about 0.5 percent of cases, but often didn’t show up until roughly two months post-op, which is why surgeons emphasize infection precautions well beyond the initial healing period.
Within the first year, about 0.5 percent of knee replacements needed a revision surgery for any reason, including implant loosening, instability, or infection. These numbers are low, but they’re not zero, and certain factors like obesity, diabetes, and smoking increase the odds.
How Long Implants Last
Modern knee implants have an impressive track record. In a review of nearly 55,000 knee replacement patients cited by Harvard Health Publishing, only 3.9 percent needed a revision within 10 years. At the 20-year mark, that number rose to 10.3 percent, meaning roughly 9 out of 10 knee replacements were still functioning well two decades later. Advances in plastic spacer technology, particularly highly cross-linked polyethylene that resists wear far better than older materials, are expected to push those numbers even higher for implants placed today.