A bone scan on a knee replacement shows areas of increased bone activity around the implant, helping your doctor determine whether pain or other symptoms are caused by a loose component, infection, stress fracture, or simply normal post-surgical healing. The scan works by tracking where your body is actively building or repairing bone tissue, which makes it especially useful for spotting problems that X-rays can miss early on.
How the Scan Works
A small amount of radioactive tracer is injected into a vein in your hand or arm. This tracer binds to the mineral component of bone, and it concentrates most heavily in areas where new bone is being formed or repaired. The key factor isn’t just blood flow to the area. The tracer accumulates based on how much bone-building activity is happening, so regions under stress, healing from surgery, or responding to infection light up brighter than quiet, healthy bone.
After the injection, you’ll wait two to four hours for the tracer to circulate and settle into your bones. During that time, you’ll be encouraged to drink water and empty your bladder to flush out tracer your body hasn’t absorbed. The scan itself involves lying still on a table while a camera passes over your knee, capturing images that can take up to an hour.
If your doctor orders a three-phase bone scan, you’ll have images taken at three distinct points: immediately during the injection (showing blood flow), within the first few minutes (showing soft tissue inflammation), and again three to five hours later (showing bone metabolism). Each phase reveals different information about what’s happening around the implant.
Normal Findings After Surgery
This is a critical point many people don’t realize: a bone scan can look abnormal for years after a knee replacement, even when nothing is wrong. After total knee replacement, 60% or more of the thighbone components and up to 90% of the shinbone components show increased tracer uptake for at least a year in patients with no symptoms whatsoever. That activity reflects normal bone remodeling as your skeleton adapts to the implant.
In fact, a bone scan may remain positive for up to five years after total knee replacement due to this ongoing physiological remodeling. This is significantly longer than after a hip replacement, which typically normalizes within about two years. Because of this extended timeline, interpreting a bone scan after knee surgery requires careful comparison with the timing of your operation. A “hot” scan at 18 months post-surgery could be completely normal.
Signs of a Loose Implant
One of the main reasons doctors order a bone scan on a knee replacement is to check for aseptic loosening, where the implant loses its secure bond with the surrounding bone without any infection present. This typically appears as diffuse tracer uptake spread around the implant. One early warning sign is increased activity at the tip of the prosthesis, particularly when it shows up a year or more after surgery.
Sequential bone scans that show increasing tracer uptake over time also suggest loosening. However, this finding isn’t definitive on its own, because there’s wide variability in how tracer uptake changes even in patients who have no problems. Intense focal uptake appearing more than six months after surgery points toward loosening or infection, but false-positive rates can run as high as 72%. A bone scan is better at ruling problems out than confirming them. A negative scan is strong evidence that the implant is not the source of your pain, and your doctor should look elsewhere.
Detecting Infection Around the Implant
Periprosthetic joint infection is a serious complication, and the three-phase bone scan is highly sensitive for catching it. Research shows a sensitivity of about 93% for knee infections, meaning it picks up the vast majority of cases. Its negative predictive value ranges from 96% to 100%, so if the scan is clean, infection is extremely unlikely.
The catch is specificity. Only about 56% of positive results actually indicate infection. The scan lights up for loosening, wear debris, stress reactions, and normal healing in nearly the same way it does for infection. Bone breakdown caused by plastic wear particles from the implant’s bearing surface can produce scan appearances that closely mimic infection, both visually and clinically.
This is why a positive bone scan for suspected infection often leads to a second, more targeted scan. A labeled white blood cell scan uses your own white blood cells, tagged with a different radioactive marker, and re-injects them. Infection draws white blood cells to the site, so increased uptake on this scan points more specifically toward infection rather than mechanical loosening. Combining this with a bone marrow scan further sharpens the diagnosis: when the two scans show mismatched patterns of uptake, infection becomes the likely cause.
Other Problems the Scan Can Reveal
Beyond loosening and infection, a bone scan can identify several other issues around a knee replacement:
- Stress fractures appear as a focused, intense line or spindle-shaped area of activity along the bone near the prosthesis. These can develop when the implant changes how force is distributed through the bone.
- Patellar problems show up as increased uptake in the kneecap, which can indicate stress across the front of the joint, early loosening of a patellar component, or impending fracture.
- Heterotopic ossification, where bone forms abnormally in the soft tissues around the joint, also causes increased tracer uptake because the tracer binds to any actively forming bone, not just bone in its expected location.
What a Bone Scan Cannot Tell You
The biggest limitation is that a bone scan shows that something is happening around the implant, but it often can’t tell you exactly what. The pattern of tracer distribution is not specific enough to reliably distinguish loosening from infection from wear-related bone loss. All of these conditions cause increased bone turnover, and all light up on the scan in overlapping ways.
Think of the bone scan as a screening tool. A negative result carries real diagnostic weight: it strongly suggests the implant and surrounding bone are not the problem. A positive result, on the other hand, opens the door to further testing. Your doctor may follow up with more specialized nuclear imaging, blood tests for infection markers, or joint fluid analysis to narrow down the cause. The addition of SPECT/CT imaging, which combines the bone scan’s metabolic information with a CT scan’s structural detail, can reduce false positives and help pinpoint exactly where the problem is located.