The clicking or popping sound coming from a knee that has undergone total knee arthroplasty (TKA) is a common concern for patients following the procedure. During TKA, damaged bone and cartilage are removed and replaced with prosthetic components made of metal alloys and medical-grade plastic. This mechanical substitution is effective at relieving pain, but it introduces different physical dynamics than a natural joint. Hearing an audible noise from the knee, though startling, is a frequent post-surgical experience that usually indicates a normal interaction between the new materials.
Understanding the Mechanics of the Noise
A natural knee joint is silenced by soft, compliant structures, primarily articular cartilage and synovial fluid, which absorb shock and ensure smooth, quiet motion. The prosthetic knee, conversely, is composed of hard materials like cobalt-chromium alloy and ultra-high-molecular-weight polyethylene (UHMWPE). When these surfaces engage or disengage, they lack the natural dampening properties of a biological joint, making sounds more likely to be transmitted and heard.
The main components involved are the femoral component (metal), the tibial tray (metal), and the polyethylene insert that acts as the bearing surface between them. An abrupt change in movement or weight distribution can cause a sharp meeting of the metal and plastic, generating a sound wave perceived as a click or clunk. A high frequency of patient-perceived noise is reported after TKA, suggesting noise generation is common with certain implant designs. This mechanical reality means that some degree of noise is simply a byproduct of having an artificial joint.
Common, Non-Concerning Causes of Clicking
The most frequent reason for a knee replacement to click is soft tissue snapping. This occurs when a tendon or ligament, adapting to the new joint geometry, momentarily catches on the edge of a prosthetic component during flexion or extension. As the tissue slides over the implant edge, it rapidly returns to its normal position, creating a snapping or popping sound that is painless.
Another common source of noise is the interaction between the components, particularly the patella (kneecap) and the femoral component. The patella, which may or may not have a plastic “button” replacement, can tap against the metal femoral implant as swelling subsides and the joint gains movement. This tapping is often heard when walking down stairs or slopes, as the kneecap tracks in the trochlear groove of the implant. The slight shifting of the polyethylene insert within the metal tibial tray during motion can also contribute to audible sounds.
Early in recovery, muscle weakness surrounding the joint can lead to temporary clicking. Weakened quadriceps and other stabilizing muscles allow for less controlled motion, causing components to interact more sharply until strength is restored. Some modern implant designs, such as posterior-stabilized designs, may be inherently more prone to noise due to their specific geometry and the mechanism used to substitute for the removed posterior cruciate ligament. These benign causes often diminish or resolve as surrounding tissues heal and muscle strength returns, typically within the first three to six months post-surgery.
Warning Signs Indicating a Mechanical Problem
While most clicking is harmless, patients must be aware that certain characteristics or accompanying symptoms can signal a mechanical or biological issue requiring immediate consultation with the orthopedic surgeon. The primary red flag is persistent, new, or increasing pain accompanying the clicking. Painful clicking suggests the noise is not just soft tissue snapping, but rather an indication of an underlying problem like impingement or excessive component movement.
A sudden change in the type of sound, such as a quiet click turning into a loud clunk, grinding, or a grating sensation, warrants urgent attention. Grinding sounds (crepitus) can indicate increased friction between the patellar and femoral components, which may be related to wear or alignment issues. Instability is another serious symptom; if the knee feels like it is giving way or locking up along with the noise, it could indicate component malalignment or loosening.
Component loosening occurs when the bond between the implant and the bone or bone cement fails, allowing the prosthetic parts to shift and click against the bone or other components. This is often associated with the late onset of clicking, sometimes years after the initial surgery. Signs of infection, including increasing swelling, redness, warmth, or drainage around the incision site, should be treated as an urgent concern, even if the primary complaint is noise.
Treatment and Management Options
For most patients, managing benign clicking involves monitoring the symptom and adhering to a rehabilitation protocol. Consistent physical therapy is recommended, as strengthening the muscles around the knee improves joint stability and controls the movement that causes soft tissue snapping. Exercises focused on the quadriceps and hamstring muscles can help the knee components track more smoothly, potentially reducing the frequency of the noise.
If the clicking is determined to be non-painful and non-progressive, the standard recommendation is observation, as many benign noises resolve over time as the joint continues to adapt. If the sound is linked to a minor alignment issue or excessive scar tissue buildup, the surgeon may recommend specific adjustments to the physical therapy program or, in rare cases, a minor procedure to address the scar tissue.
Surgical intervention is rarely necessary solely for noise, but it becomes a consideration if the clicking is definitively linked to component loosening, severe pain, or instability that cannot be managed otherwise. In these instances, revision surgery may be required to address the underlying mechanical failure. Patients should report any noise to their orthopedic surgeon, particularly if it is associated with pain, swelling, or a decline in function, to ensure the implant is functioning correctly.