Why does my knee replacement click or knock, and is it normal?

One of the most common questions in the post-operative period after knee replacement is some version of: "my knee is making a noise — is that normal?" The short answer, in the great majority of cases, is yes. Noises and sensations after knee replacement are very common and almost always benign. Understanding why they happen takes most of the alarm out of the experience.

In the vast majority of cases, clicking, clunking, and other noises after knee replacement are entirely normal and not a sign that anything is wrong.

Why does a knee replacement make noises?

The explanation comes down to materials. A natural knee is cushioned by cartilage — a smooth, hydrated surface that absorbs and transmits load almost silently. A knee replacement replaces the joint surfaces with a metal femoral component, a metal tibial tray, and a plastic (polyethylene) bearing insert that sits between them. These materials are excellent at what they do, but they move against each other differently to cartilage-on-cartilage. Metal and plastic in motion, particularly under load or during deep knee movements, can produce clicking, clunking, or snapping sounds that are entirely mechanical in origin and entirely normal.

This is not a sign that something is loose, or wearing out, or wrong. It is simply the acoustic signature of a different set of materials doing the job that cartilage used to do.

General clicking and snapping

A variety of clicking and snapping sensations can occur during everyday knee movements — going up or down stairs, rising from a chair, bending the knee deeply, or walking on uneven ground. These are most common in the early months after surgery, when swelling in the soft tissues around the knee alters the way tendons and other structures slide over the implant.

As swelling reduces over the first three to six months and the soft tissues adapt to the new joint, many of these noises settle of their own accord. Some persist long-term without ever causing a problem. If the knee is comfortable and working well, clicking alone is not a cause for concern.

The patellofemoral clunk

The most well-known — and well-named — noise after knee replacement is the patellofemoral clunk. This is a distinct clunk felt (and sometimes heard) when the knee is extended from a flexed position: classically when standing up from a low chair or when the leg straightens during walking up stairs. It can feel quite pronounced and often alarms patients when they first notice it.

The patellofemoral clunk occurs when a small nodule of fibrous tissue that forms in a particular part of the knee — specifically in the notch at the front of the femoral component — catches and then releases as the knee straightens. Some implant designs are more prone to this than others, and the phenomenon has been one of the drivers of improvements in femoral component design over the past two decades.

In most cases, the patellofemoral clunk is not painful and resolves with time and physiotherapy as the fibrous tissue softens and the movement pattern normalises. In a smaller number of cases, the fibrous nodule is large enough to cause consistent discomfort and interfere with function. If this is the case, a minor procedure called an arthroscopic debridement — a keyhole procedure to remove the fibrous tissue — is effective. This is a straightforward procedure and, where it is needed, delivers very reliable relief.

When should I be concerned?

The key distinction is between a knee that makes noises but functions well and feels comfortable, and a knee where the noises are accompanied by other symptoms. The following warrant assessment:

Increased pain. If a clicking or clunking noise is associated with pain — particularly if the pain is new, worsening, or different in character to previous discomfort — this should be assessed. A comfortable knee that clicks is almost always benign. A painful knee that clicks is a different matter.

New or increasing swelling. Swelling is expected in the early weeks after knee replacement and normally settles gradually. New swelling appearing months or years after a previously settled knee replacement — particularly if associated with warmth or redness — needs assessment. Infection can present in this way, and early identification is important.

A change in the character of the noise. A patient who has had a consistent, predictable click for two years and notices it suddenly changing in character or becoming more pronounced should mention it at their next review.

A sensation of instability. Knee replacement should feel stable during walking and activity. A feeling that the knee is giving way or feels unstable under load is a symptom that needs assessment, regardless of whether it is accompanied by noise.

Pain with weight bearing that is different to what you have experienced before. Implant loosening — where the bond between the implant and bone has failed — typically produces a deep-seated aching or pain specifically with weight bearing. This is different from the more common soft-tissue discomfort of the early post-operative period.

What should I do?

If you have a knee replacement that clicks or makes noises but is comfortable and functioning well, there is no reason to be concerned. This is a normal part of living with a replacement joint, and no action is needed.

If the noise is new, has changed in character, or is accompanied by any of the symptoms described above — pain, swelling, instability, or altered gait — contact the rooms for a review. Do not wait for a scheduled appointment if symptoms are changing; it is always better to check in early.

As with all aspects of post-operative care, individual assessment is always more informative than general guidance. This article reflects common scenarios, but every patient and every knee is different.