A half knee replacement removes and replaces only the damaged part of your knee
A half knee replacement, also called a unicompartmental or partial knee replacement, replaces only the worn cartilage and bone in one section of your knee joint. Your knee has three compartments — the inner (medial), outer (lateral), and kneecap (patellofemoral) — and a half replacement addresses damage in just one. The surgeon removes the damaged surfaces and caps them with metal and plastic implants, leaving the rest of your knee untouched.
This is different from a total knee replacement, which replaces all three compartments. A half replacement is an option only when arthritis or injury has damaged one compartment while the others remain healthy. Your surgeon will use imaging and physical examination to confirm that the other two compartments have no significant wear before recommending this procedure.
The surgery takes about 60 to 90 minutes, and most people go home the same day or after one night in the hospital. Recovery is typically faster than total replacement because less bone is removed and fewer ligaments are disturbed.
Key Takeaways
- A half knee replacement replaces only one of the three compartments in your knee, leaving healthy sections intact.
- You are a candidate only if arthritis or damage is confined to a single compartment and the other two are healthy.
- Recovery from half replacement is usually faster than from total replacement, with many people returning to light activity within 6 to 8 weeks.
- A half replacement may eventually require conversion to a total replacement if arthritis develops in the remaining compartments over time.
Who is a candidate for half knee replacement
Your surgeon will recommend a half replacement only if your imaging shows damage in one compartment and healthy cartilage in the other two. Age alone does not disqualify you — some surgeons perform half replacements on patients in their 50s, while others prefer to wait until patients are older. The key factor is the pattern of wear, not your age.
You also need good alignment in your knee. If your leg is severely bowed inward or outward, a half replacement may not distribute weight evenly across the implant, and a total replacement becomes the safer choice. Your surgeon will measure the angles of your leg during the evaluation to determine this.
Patients with inflammatory arthritis (such as rheumatoid arthritis) are usually not candidates for half replacement because the disease tends to affect all compartments over time. Half replacement works best for people with osteoarthritis confined to one area.
How a half replacement compares to total replacement
| Factor | Half Replacement | Total Replacement |
|---|---|---|
| Compartments replaced | One of three | All three |
| Bone removed | Less | More |
| Ligaments affected | Fewer | More |
| Surgery time | 60–90 minutes | 90–120 minutes |
| Hospital stay | Same day or one night | One to two nights |
| Recovery to light activity | 6–8 weeks | 8–12 weeks |
| Risk of future surgery | Higher (arthritis may develop in other compartments) | Lower (all compartments replaced) |
A half replacement preserves more of your natural knee structure, which some surgeons believe may make future surgery easier if needed. However, because only one compartment is replaced, arthritis can still develop in the untouched sections. Studies show that 10 to 15 percent of people who have a half replacement eventually need a total replacement within 10 years, though this varies based on age and activity level.
Total replacement is more invasive but addresses all the damage at once, reducing the chance you will need another surgery on that knee. The choice between the two depends on your age, the pattern of your arthritis, your activity level, and your surgeon's experience with half replacements.
Recovery timeline after half knee replacement
Most people begin physical therapy the day after surgery or within a few days. The first two weeks focus on reducing swelling and regaining the ability to straighten and bend your knee. You will use crutches or a walker during this time and ice the knee regularly.
By week 4 to 6, many people can walk without crutches and begin gentle strengthening exercises. By 8 to 12 weeks, most return to light activities like walking, swimming, or stationary cycling. Return to higher-impact activities like running or sports typically takes 4 to 6 months and depends on your surgeon's clearance and your physical therapy progress.
Pain and swelling can persist for several months after surgery. Stiffness is common in the first few weeks but usually improves with consistent physical therapy. Some people report that their knee feels unstable or gives way during the first few months — this usually resolves as the muscles around the knee strengthen.
Potential complications and long-term outcomes
Infection, blood clots, and nerve or blood vessel injury are rare but possible after any knee surgery. Stiffness that does not improve with physical therapy occurs in a small percentage of cases and may require manipulation under anesthesia (a procedure to break up scar tissue while you are asleep).
The implant itself can wear out or loosen over time, though modern materials last longer than older designs. If this happens, revision surgery (replacement of the implant) may be needed, typically 15 to 20 years after the original procedure.
The most common long-term issue is the development of arthritis in one of the untouched compartments. This happens in some people within 5 to 10 years and may eventually require conversion to a total replacement. Your activity level and weight affect how quickly this occurs — higher impact activities and heavier body weight increase the risk.
What to expect during the surgical procedure
Your surgeon will make a small incision (usually 3 to 4 inches) on the front of your knee, typically on the inner side. The damaged cartilage and bone are removed from the affected compartment, and the surfaces are shaped to fit the implant. The metal and plastic components are then secured in place, usually with bone cement.
Your surgeon will test the knee's movement and alignment before closing the incision. The entire procedure takes about 60 to 90 minutes. You will be under general anesthesia or regional anesthesia (numbing from the waist down) during the surgery.
Some surgeons use computer-assisted navigation or robotic guidance to improve the precision of implant placement, though this is not standard at all hospitals. Ask your surgeon whether these tools are available and whether they recommend them for your case.
Questions to ask your surgeon before half knee replacement
Ask whether your arthritis pattern truly qualifies you for a half replacement or whether a total replacement would be safer given your age and activity level. Ask about your surgeon's experience with half replacements — surgeons who perform many of these procedures tend to have better outcomes than those who do them rarely.
Ask what the timeline is for returning to your specific activities, whether physical therapy will be in-clinic or at home, and what signs of complications you should watch for after discharge. Ask whether your surgeon uses computer navigation or robotic information and whether it is included in the cost.
Ask about the implant brand and materials used, and request information about how long that particular implant typically lasts. Ask what happens if arthritis develops in the other compartments — whether conversion to total replacement is straightforward and whether the original implant can stay in place.
Frequently Asked Questions
Can I have a half knee replacement on both knees at the same time?
Some surgeons will replace both knees in one surgery if both meet the criteria for half replacement and you are healthy enough for a longer procedure. Others prefer to do one knee at a time, spacing them weeks or months apart. Discuss your surgeon's approach during your consultation.
Will I set off metal detectors after half knee replacement?
The implant may trigger some metal detectors, though modern airport scanners are designed to allow orthopedic implants. Bring your implant card (your surgeon will provide one) to show security. You can also request a pat-down instead of walking through the detector.
Can I play sports after half knee replacement?
Low-impact sports like golf, swimming, and cycling are usually possible after full recovery. High-impact sports like running, basketball, or tennis are generally not recommended because they increase wear on the implant and the remaining natural cartilage. Your surgeon will give you specific guidance based on your implant and recovery.
What if arthritis develops in the other compartments after my half replacement?
If arthritis develops in the untouched compartments, your surgeon can convert the half replacement to a total replacement. The original implant is usually removed and replaced with a full prosthesis. This second surgery is more complex than the original half replacement but is a standard procedure.
How long does a half knee replacement implant last?
Modern implants typically last 15 to 20 years, though some last longer. The lifespan depends on the implant material, your weight, your activity level, and how well you follow post-surgery guidelines. Your surgeon can tell you the expected lifespan of the specific implant they plan to use.