A replaced knee can click, and the literature says how often and when it matters.

A replaced knee has surfaces that a natural knee does not

In a total knee replacement, a metal component on the thigh bone moves against a polyethylene (plastic) insert on the shin bone. Those surfaces do not have the cartilage and meniscus of a natural knee, and they can be felt and heard. AAOS OrthoInfo puts it plainly: most people feel or hear some clicking of the metal and plastic when bending or walking, and this is normal ¹.

Researchers list several mechanisms. Gas can be released from the joint fluid, tendons or other soft tissue can snap over bone or implant edges, and the bearing surfaces themselves can generate sound. In the largest cohort the bearing surface was judged the most likely source, because noise rates varied with implant design ².

The kneecap is a separate source. Scar tissue can form under the quadriceps tendon and catch on the femoral component as the knee straightens, producing a distinct clunk ³.

Noise is common in the published cohorts

In a cohort of 2,056 knees, 27% of patients reported grinding, popping, or clicking from the operated knee in the previous 30 days. Noise was more common after total knee replacement (29%) than after partial replacement (21%) ².

Design mattered in that study. Rotating-platform designs (45%) and posterior-stabilized designs (41%) were noisier than cruciate-retaining designs (23%) ². A 2023 cohort found about 40% of patients reported noise, with no difference between robotic-assisted and conventional surgery ⁴.

Noise tends to appear early. The kneecap clunk pattern, for instance, was reported at a mean of about seven months after surgery in a systematic review ³.

Whether noise affects satisfaction is where studies disagree

The evidence does not point one way. In the 2,056-knee cohort, patients who reported noise also reported more difficulty rising from a chair (38% versus 25%). They also reported more limping, swelling, and stiffness than those without noise ².

In the 2023 cohort, about 70% of patients were not dissatisfied about their noise. Those with noise still scored lower on the Forgotten Joint Score (45.5 versus 66.1), a measure of how aware a person is of the joint in daily life ⁴.

A series of 295 knees found only a weak correlation between noise and joint awareness, and concluded that noise had a limited effect on outcomes ⁵. A fair reading of the three is that noise alone is common and often tolerated, and that noise together with other symptoms is what tends to lower scores.

Patellar clunk is a specific, treatable pattern

Patellar clunk syndrome comes from a fibrous nodule that forms under the quadriceps tendon. It causes pain at the front of the knee and a painful catching as the knee moves from bent to straight, ending in an audible clunk ³. The reported incidence ranges from 0% to 20% depending on implant design, and it is associated almost exclusively with posterior-stabilized designs ³.

When it persists, arthroscopic removal of the nodule has high reported satisfaction and improvement in pain and function ³. Whether a given knee has this pattern is a matter for the surgeon's exam and imaging.

The noises and symptoms that are not routine

A painless click in a stable knee is what the cohorts above describe as common. The literature treats the following differently:

• Pain that arrives with the noise, or catching and locking, rather than a sound on its own ³.
• A knee that gives way, especially going down stairs, or that cannot be trusted on uneven ground. These are the presenting symptoms of instability, which accounted for 17.4% of single-stage revision knee replacements in the UK National Joint Registry ⁶.
• Swelling that keeps coming back. A recurrent effusion is reported in 70% to 85% of patients with flexion instability ⁶.
• A new limp, or new difficulty rising from a chair, since both travelled with noise in the largest cohort ².
• Fever above 100°F, chills, increasing redness or drainage at the wound, or pain that increases both at rest and with activity. AAOS lists these as possible signs of infection ¹.

When to call your surgical team

Call the same day for the infection signs above ¹. Call too after a fall or twist that leaves the knee painful, swollen, or unwilling to take weight. A noise that is new after an injury belongs in that call.

Bring the rest to your next visit, and be specific: when the sound happens, whether it hurts, and whether the knee swells or gives way afterwards. Your surgeon decides from your exam, imaging, and history whether a noise needs an X-ray, a look at kneecap tracking, or nothing more than time.

• Which implant design do I have, and is noise expected with it? Noise rates differed by design in the largest cohort ².
• Is my kneecap tracking as expected on the X-ray?
• Which change in this noise would you want to hear about before the next visit?