Popping and clicking after knee surgery are common, and on their own they rarely mean the operation has failed. A new knee or a repaired joint moves differently from the old one, and its parts and healing tissues can make noise as they glide past each other. The sound itself tells you less than what comes with it: pain, swelling, or a knee that catches says far more than a pop does.
Where the Sound Usually Comes From
Gas bubbles explain much of the cracking. Moving or stretching a joint can form small bubbles in the fluid inside it, the same process behind knuckle cracking.1PubMed Central. Scruples over Speckles So a pop when you bend after sitting can be bubbles releasing rather than a structure breaking. Bubbles also show up in knees with long-standing arthritis and thick joint fluid.2PubMed Central. Scruples over Speckles A bubble-type crack often can’t be repeated right away, much like a knuckle, and returns after the knee has rested a while. A harmless pop typically comes with no pain and no swelling afterward, and you can usually bend and straighten fully.
Implant surfaces can tap and click too. A replacement swaps worn cartilage for metal and plastic, which have no soft cushioning layer, so the parts can click softly as they meet, especially in the first months while the muscles are still weak. Different implant designs give off measurably different sound patterns, and those sounds change with time since surgery.3Thieme Connect (The Journal of Knee Surgery). Acoustic emission studies of posterior stabilized and cruciate retaining knee arthroplasties
Attention matters as well. After an operation most people watch the knee closely, so sounds that were always there, even in the old knee, can feel like new events. Swelling and stiffness in early recovery change how the joint glides, so noise may be more noticeable in the first months and then settle as the swelling goes down.
How Common Noise Is After a Knee Replacement
About 27% of knees with a cruciate-retaining implant, a design that keeps one of your own ligaments, make some joint noise.4PubMed. Increased prevalence of tibiofemoral noises in posterior-stabilised implants compared to cruciate-retaining and ultracongruent implants in total knee arthroplasty: A systematic review and meta-analysis In the posterior-stabilized (PS) design, which adds a plastic post and cam to replace that ligament, the share is about 35%.5PubMed. Increased prevalence of tibiofemoral noises in posterior-stabilised implants compared to cruciate-retaining and ultracongruent implants in total knee arthroplasty: A systematic review and meta-analysis Noise alone was not reported as harming how well the implant works.
In the PS design, the post on the plastic insert meets the cam on the thighbone part as the knee bends. That’s an extra point of contact a cruciate-retaining knee doesn’t have.
These figures count any joint noise, including soft clicks and grinding the person barely notices, so they don’t describe loud or painful popping. The numbers describe groups of people, so they can’t tell you whether your particular knee is fine. Your symptoms alongside the sound do that.
To find out which implant you have, check your operation note or discharge papers, which usually name the design. You can also ask your surgeon’s office whether it is a PS design.
Clicking is a minor symptom in knees that do fail. Among replacements that went on to fail, only 7% had clicking or grinding as a warning symptom.6PubMed Central. Prodromes of failure in total knee arthroplasty Pain at 84% and swelling at 76% were by far the most common early signs of trouble.7PubMed Central. Prodromes of failure in total knee arthroplasty
The change from your normal pattern matters more than the sound itself. New noise that arrives along with pain is different from noise you have always had.
Scar Tissue Bands and the Kneecap Clunk
After a replacement, bands of fibrous tissue can form inside the joint and cause painful popping, catching, or grinding at the kneecap.8Journal of Bone and Joint Surgery. Intra-articular fibrous bands. Patellar complications after total knee replacement In one large series this happened in about 11 of 635 replacements, under 2%.9Journal of Bone and Joint Surgery. Intra-articular fibrous bands. Patellar complications after total knee replacement So painful kneecap popping is uncommon even though plain noise is common. Removing the bands through keyhole surgery cleared the symptoms in every case.10Journal of Bone and Joint Surgery. Intra-articular fibrous bands. Patellar complications after total knee replacement
A kneecap clunk is a specific version of this. A nodule of scar tissue forms where the kneecap meets the thigh tendon above it.11PubMed. Arthroscopic treatment of patellofemoral soft tissue impingement after posterior stabilized total knee arthroplasty As the knee bends, the nodule slips into a notch in the thigh part of the implant, and when you straighten, it pops back out with a felt clunk.12PubMed. Sonographic appearance of fibrous nodules in patellar clunk syndrome: a case report It’s linked mainly to PS implants,13PubMed Central. Patellar Clunk Syndrome Following Posterior Stabilized Total Knee Replacement: Report of Two Cases but the same kind of tissue growth can cause anything from mild grinding to a full clunk.14PubMed Central. Patellar Clunk Syndrome Following Posterior Stabilized Total Knee Replacement: Report of Two Cases
A clunk is a jump or catch you feel, not just a sound you hear. It often happens at the same point each time you straighten from a bend, such as rising from a chair, and it can hurt and make you hesitate on stairs. A click is lighter and usually painless. It’s usually noticed months after surgery as the scar tissue matures. It may slowly become more frequent and is often worse with deep bending. You tend to feel it at the kneecap or just above it rather than at the side or back.
The problem is usually fixable because the cause is a bit of extra soft tissue rather than a failed implant. The usual treatment is to remove the tissue, not replace the knee.
Tendons and Cartilage That Catch
A tendon on the outer back of the knee can snap and cause a pop on the outside of the joint.15PubMed. Snapping popliteus tendon syndrome. A cause of mechanical knee popping in athletes The snap often happens at a slight bend of about 20 to 40 degrees.16PubMed Central. An all-arthroscopic surgery technique for snapping popliteal tendon syndrome: A case report and literature review Repeated rubbing can thicken and inflame the tendon, especially with heavy activity.17PubMed Central. An all-arthroscopic surgery technique for snapping popliteal tendon syndrome: A case report and literature review After a replacement, the tendon can also get trapped between the plastic insert and the metal part.18Journal of Surgery. Arthroscopic Tenotomy Results in Complete Resolution of Snapping Popliteus Tendon Syndrome After Total Knee Arthroplasty-A Case Report Releasing the tendon through keyhole surgery ended the snapping in one reported case.19Journal of Surgery. Arthroscopic Tenotomy Results in Complete Resolution of Snapping Popliteus Tendon Syndrome After Total Knee Arthroplasty-A Case Report Outer-side snapping is felt at the side of the knee rather than the front, happens early in the bend rather than when straightening fully, and tends to be brought on by sports and repeated twisting, which helps separate it from kneecap trouble.
Catching or locking points to cartilage. After ACL reconstruction, a knee that sticks in one position until you wiggle it free is less normal than a click.20Mr Mark Webb, Consultant Orthopaedic Surgeon. Why Is My Knee Clicking After ACL Reconstruction? Causes, When to Worry, and Treatment It can mean a meniscus repair hasn’t healed, a new tear has formed, or a loose cartilage fragment is moving around.21Mr Mark Webb, Consultant Orthopaedic Surgeon. Why Is My Knee Clicking After ACL Reconstruction? Causes, When to Worry, and Treatment An unusually shaped or loose meniscus can also cause snapping on its own.22PubMed Central. Abnormal displacement of discoid lateral meniscus with snapping knee detected by full extension and deep flexion MRI: Report of two cases
A knee that sticks, especially with pain, can mean something inside the joint is physically in the way. An exam, and often an MRI, show whether a meniscus repair or a loose piece is the cause.
True locking means you cannot fully straighten the knee until it releases. A brief stiff feeling that eases with movement is different. Locking that keeps coming back should be checked soon.
Popping After ACL Surgery
When surgeons use a hamstring tendon for the new ligament, about 25% of patients feel a sudden painful pop or pull behind the thigh within 6 weeks.23BSM Foundation. Hamstring Tendon Autograft It’s often set off by a sharp bend, such as pulling a shoe off with the other foot.24BSM Foundation. Hamstring Tendon Autograft It is a minor muscle strain that usually settles within about a week,25BSM Foundation. Hamstring Tendon Autograft and it does not harm the result of the surgery.26BSM Foundation. Hamstring Tendon Autograft The sharpness usually fades over minutes to days into a sore, tight feeling rather than getting steadily worse.
To avoid the shoe-removal strain, sit down and use your hands to take shoes off in the early weeks. Avoid sudden forceful bending of the knee against resistance. Ice and rest usually ease the pulled feeling.
A graft-site strain is felt in the back of the thigh, not inside the knee. An injury inside the knee more often brings swelling and a feeling of instability, and a knee that gives way after a pop needs prompt review.
Many people with ACL reconstruction also notice soft clicks or grinding as they regain movement, especially in the first months. When painless and without swelling, these are generally expected as the knee recovers strength and control. A click that comes with swelling, pain, or a feeling that the knee will give way is different, and worth reporting.
Signs Your Popping Needs a Doctor
- Pain: pain with the popping, the leading early sign in replacements that failed.
- Swelling: swelling that keeps returning.27PubMed Central. Prodromes of failure in total knee arthroplasty
- Catching or locking: the knee sticking in one position.28Mr Mark Webb, Consultant Orthopaedic Surgeon. Why Is My Knee Clicking After ACL Reconstruction? Causes, When to Worry, and Treatment
- Changing shape: the knee becoming increasingly bowed or knock-kneed.29PubMed Central. Prodromes of failure in total knee arthroplasty
- Giving way: a feeling the knee can’t hold you.30PubMed Central. Prodromes of failure in total knee arthroplasty
A single sign doesn’t prove a problem, but pain or swelling that keeps returning, or a knee that gives way, is a reason to call the surgeon’s office soon rather than wait. Sudden severe pain, a hot red knee, or fever are different and should be seen urgently, since they can mean infection or another acute problem.
X-rays pointed to the problem in 91% of replacements that failed, often by showing loosening or plastic wear.31PubMed Central. Prodromes of failure in total knee arthroplasty In one group of painful, snapping replaced knees, the causes included loose cement or scar bands, and all were fixed by recementing or band release.32PubMed. Management of knee pain and stiffness after total knee arthroplasty An ultrasound taken while you bend the knee can show what is snapping as it happens.33AJR Am J Roentgenol / PubMed Central. Snapping knee: imaging findings with an emphasis on dynamic sonography
Keyhole surgery uses small cuts and a camera so the surgeon can see inside the joint. Scar bands or a trapped tendon can be released, and recovery is usually much shorter than after the first operation.
To prepare for the appointment, note which movements trigger the pop and at what bend. Record a short video of the sound or movement on your phone, and write down any pain, swelling, or episodes of locking and when they happen. Bring the name of your implant if you have it.
Keep Up Rehab When Your Knee Clicks
Harder rehab sessions can bring on clicking or rubbing sensations along with extra soreness.34PubMed Central. Symptom experience of patients after total knee arthroplasty in China: a longitudinal qualitative study These symptoms are usually temporary,35PubMed Central. Symptom experience of patients after total knee arthroplasty in China: a longitudinal qualitative study but many patients read them as damage and cut back their exercise.36PubMed Central. Symptom experience of patients after total knee arthroplasty in China: a longitudinal qualitative study People with lasting clicking often worry about loose parts or poor surgery.37PubMed Central. Patient perspectives of pain and function after knee replacement: a systematic review and meta-synthesis of qualitative studies Asking your surgeon early can ease the worry before it changes how you exercise.38PubMed Central. Patients’ experiences of discontentment one year after total knee arthroplasty- a qualitative study
A rehab click comes with a session but fades afterward, without sharp pain or new swelling. Sharp pain, a catch that stops the movement, or swelling that stays up the next day suggests the load was too much or something else is going on.
You can adjust without stopping. Scale back intensity if swelling rises after a session, then build up again. Painless clicks are no reason to skip exercises, but stop and call your team if an exercise causes sharp pain or catching. Tell your physiotherapist about the noise so they can adjust the plan. Kneecap pain is not caused by one weak thigh muscle alone,39Truemeds. Patellofemoral pain syndrome so rehab works best when it targets strength and control across the whole leg. Many people notice the noise less as the muscles regain strength over months, though a replacement may always click a little.