Knee replacement is a surgical procedure that removes damaged bone and cartilage from your knee joint and replaces them with artificial components made of metal and medical-grade plastic. It’s one of the most common and successful orthopedic surgeries performed today, with over 90% of implants still functioning well at the 20-year mark. Most people who get one have severe arthritis that hasn’t responded to other treatments.
Why People Get Knee Replacements
The most common reason is osteoarthritis that has worn away the cartilage cushioning the joint, leaving bone grinding against bone. But the surgery isn’t considered until you’ve tried other approaches first: physical therapy, anti-inflammatory medications, injections, bracing, and activity changes. When those stop controlling the pain, and imaging shows significant joint damage, replacement becomes a realistic option.
Surgeons evaluate three things together: how much pain you’re in, how well you can still function, and what your X-rays or other imaging show. Pain that wakes you at night, difficulty with everyday activities like walking or climbing stairs, and visible joint space narrowing on an X-ray form the typical picture of someone ready for surgery. Occasionally, a knee that’s progressively bowing inward or outward with increasing instability can justify surgery even when pain isn’t the primary complaint.
Total vs. Partial Replacement
Your knee has three compartments: the inner side, the outer side, and the area beneath the kneecap. In a total knee replacement, all three compartments are resurfaced. In a partial replacement, only the damaged compartment is addressed, leaving healthy bone, cartilage, and ligaments intact. A partial replacement is roughly one-third the scope of a total replacement.
Partial replacements tend to offer a quicker recovery, less post-surgical pain, and a knee that feels more natural afterward. They used to be reserved for patients over 60 with well-preserved ligaments and minimal deformity, but surgeons now perform them on younger patients whose damage is confined to a single compartment. The tradeoff is longevity. Total knee replacements last significantly longer, and patients who are good candidates for the full procedure report high satisfaction and better long-term functional scores.
What the Implant Looks Like
A knee replacement implant has three main parts, each designed to mimic the shape of the joint surfaces it replaces. The femoral component is a curved metal piece that caps the end of the thighbone. It’s typically made from a cobalt-chromium alloy or titanium alloy. The tibial component sits on top of the shinbone and consists of a flat metal platform topped with a cushion of ultra-durable polyethylene plastic. The patellar component is a small dome of the same polyethylene that resurfaces the underside of the kneecap.
The design is intentional: metal always glides against plastic, never metal on metal. This pairing creates smoother motion and reduces wear over time. Some newer implants use ceramics or ceramic-metal mixtures like oxidized zirconium, which can be a good option for patients with metal sensitivities.
How Long Implants Last
Modern knee replacements are remarkably durable. In a study tracking a common implant design over two decades, about 93% of replacements were still functioning at 15 years, and roughly 91% survived to the 20-year mark. When researchers looked specifically at mechanical loosening (the implant gradually detaching from the bone, which is the most common structural failure), 99% of implants were still solidly fixed at 15 years and 98% at 20 years.
For someone getting a knee replacement in their mid-60s or later, there’s a strong chance the implant will last the rest of their life. Younger patients may eventually need a revision surgery, where worn components are swapped out, but implant technology continues to improve with each generation.
Risks and Complications
Knee replacement is considered safe, but like any major surgery, it carries real risks. Blood clots in the deep veins of the leg are the most common concern, occurring in roughly 9 to 10% of patients in large studies, though many of these are small clots caught on screening and managed with blood thinners. Infection at the surgical site happens in about 1 to 2% of cases. Infections caught early can often be treated, but deep infections sometimes require additional surgery.
Other possible complications include stiffness that limits your range of motion, nerve injury causing numbness around the incision, and, rarely, fracture of the bone around the implant. The overall risk of a serious, life-threatening complication is low, but these numbers help explain why surgeons reserve the procedure for people whose quality of life has genuinely deteriorated.
Robotic-Assisted Surgery
Many hospitals now offer robotic-assisted knee replacement, where the surgeon uses a robotic arm guided by a 3D model of your knee to position cuts and implant placement with high precision. Early data shows some practical advantages: patients in one study required fewer pain medications after surgery, spent less time in the hospital, and needed fewer physical therapy sessions to reach their recovery goals.
At the one-year mark, however, patient-reported outcomes were essentially the same whether the surgery was done with robotic assistance or by traditional manual technique. Both groups reported similar pain relief, activity levels, and overall physical and mental health scores. Robotic assistance may smooth out the early recovery period, but it doesn’t appear to change the final result for most patients.
What Recovery Looks Like
Full recovery from a knee replacement takes about a year, but the trajectory is front-loaded. Most people can resume their usual daily activities within six weeks. You’ll start working with a physical therapist almost immediately after surgery, and the first six weeks of rehab are where the biggest gains happen.
A typical physical therapy schedule involves three sessions in the first week, ramping up to four sessions in the second week, then gradually tapering to two sessions per week by weeks five and six. Research suggests that completing at least 10 sessions within those first six weeks gets most patients to a good functional range of motion (around 110 degrees of knee bend, enough for most daily activities). Interestingly, more therapy isn’t always better. Patients who attended well beyond the prescribed 17 or so sessions didn’t report better outcomes, and those attending 23 or more sessions actually tended to have worse self-reported results, likely because they were the patients struggling most with recovery rather than because extra therapy caused harm.
Preparing Before Surgery
What you do in the weeks before surgery can influence your experience afterward. Prehabilitation programs typically involve simple home exercises done five days a week for several weeks leading up to surgery: ankle pumps, knee bending and straightening, straight-leg raises, and quadriceps strengthening. These exercises help you go into surgery with stronger muscles and better baseline function, which can make the early days of recovery less difficult.
The evidence on prehab is nuanced. Studies show that patients who do prehab exercises tend to have shorter hospital stays, though the difference isn’t always statistically significant. Pain and function scores at six months tend to be similar whether or not patients did prehab. One interesting finding: in a study of patients enrolled in a 12-week prehab program, nearly 20% improved enough that they decided to postpone surgery altogether. Even if prehab doesn’t dramatically change your six-month outcome, building strength and confidence before the procedure gives you a head start on the recovery process.