A clicking or popping sensation in the knee is a frequently reported experience following a knee arthroplasty (TKA). This noise can be unsettling, but it is often a benign consequence of the joint’s mechanics. Unlike the natural knee, which uses soft cartilage for silent movement, the prosthetic knee is composed of hard bearing surfaces. Understanding the underlying reasons for this noise, whether from the implant’s design or the surrounding anatomy, can help alleviate concerns about the joint.
Component Interaction and Implant Design
The most common source of noise in a replaced knee is the direct physical interaction between the prosthetic components. A knee replacement consists of a metal component on the end of the femur, a metal tray on the tibia, and a polyethylene (plastic) spacer that fits between them. When these surfaces move rapidly across each other during certain actions, the movement can generate an audible click or tap.
This mechanical sound occurs because the hard surfaces of the implant do not offer the smooth, silent cushioning of natural cartilage. The sound frequently occurs when the metal femoral component articulates rapidly against the polyethylene liner as the knee straightens or bends. The slight separation and re-engagement of components as surrounding ligaments tension and relax can also produce a clicking sensation.
Specific implant designs, particularly posterior-stabilized (PS) knees, are inherently more prone to clicking sounds. These designs incorporate a mechanism known as a “post and cam” to substitute for the removed posterior cruciate ligament. The polyethylene post on the tibial insert interacts with a cam feature on the femoral component to prevent the knee from sliding too far forward or backward.
The engagement of this post and cam is a deliberate mechanical event designed to stabilize the joint during deep flexion, such as when climbing stairs or squatting. This interaction often results in a distinct sound or “clunk” as the components momentarily strike one another. If this mechanical click is painless and consistent, it is typically the sound of the engineered joint functioning as intended.
Soft Tissue Impingement
A clicking sensation can also arise from the surrounding biological tissues. The soft tissues outside the implant can sometimes get momentarily caught or snap over the smooth, rigid edges of the prosthetic components, creating noise. This is known as soft tissue impingement, a biomechanical issue rather than a structural failure of the device.
One common cause is the snapping of tendons or ligaments over the implant’s edges during movement transitions. For example, the popliteus tendon or the iliotibial band can become taut and momentarily catch on the lateral side of the femoral component, especially after revisions, resulting in a distinct snapping phenomenon. This type of noise is often felt during specific, repeatable movements as the tissue slides past a fixed point.
Excessive scar tissue, known as arthrofibrosis, can form around the joint capsule. If a fibrous nodule develops, it can interfere with the smooth motion of the implant, causing a distinct “clunk” or click when it is momentarily compressed or caught between the prosthetic surfaces. A specific condition called Patellar Clunk Syndrome occurs when a fibrous nodule forms beneath the kneecap and impinges on the femoral component, causing a painful click as the knee moves from flexion to extension.
The alignment of the kneecap, or patella, can also contribute to noise if it does not track smoothly over the femoral component’s groove. Patellar tracking issues can cause a grinding or clicking sensation, or crepitus, as the kneecap moves. This issue is related to the balance of the surrounding muscles and ligaments, which may need targeted physical therapy to improve alignment and reduce the noise.
Evaluating the Noise and When to Consult a Physician
A painless, consistent clicking sound, especially one that occurs only during deep bending or high-flexion activities, is considered normal for a knee replacement. For many patients, the noise is simply the characteristic sound of the metal and plastic surfaces operating without the dampening effect of natural cartilage. This benign noise does not indicate a problem with the implant or its function.
However, certain accompanying symptoms serve as red flags that warrant immediate consultation with an orthopedic physician. The most concerning symptom is a new onset of pain that accompanies the click or clunk. Pain combined with noise can signal issues like component malalignment, loosening of the implant from the bone, or soft tissue inflammation.
Signs Requiring Urgent Medical Attention
Other concerning signs include a sudden change in the quality or loudness of the noise, or the development of instability, where the knee feels like it is giving way. Signs of potential infection, such as swelling, warmth, redness, or fever, also require urgent medical attention. A severe, single loud “clunk” followed by an immediate loss of function or inability to bear weight could indicate a component fracture and requires emergency evaluation.
A physician will evaluate the noise through a physical examination to determine the movement that provokes it. They may use imaging tests, such as X-rays, to check the position and security of the prosthetic components and look for signs of loosening or wear. If the clicking is persistent and painful, the doctor will determine if the cause is a benign mechanical interaction or a problem requiring intervention.