Knee Popping, Clicking, Catching, and Grinding After ACL Surgery
You are a few weeks or months into your ACL recovery, and your knee is making sounds. A pop when you bend it. A click when you straighten it. A catching sensation going up the stairs. A grinding that you can almost feel as much as hear. If any of those descriptions sound familiar, you are not alone, and in most cases you are not in trouble.
Knee noises after anterior cruciate ligament, or ACL, reconstruction are extremely common and are one of the most frequent concerns patients bring to their physical therapists, athletic trainers, and surgeons during recovery. The sounds can be alarming, especially when you are already anxious about how your knee is healing. But understanding what is actually causing them, and knowing the specific signs that warrant concern, makes a significant difference in how you experience and manage this part of your recovery.
This article breaks down each type of knee sound and sensation, explains the most common causes, and gives you a clear framework for knowing when to let it go and when to pick up the phone.
Why Knee Sounds Are So Common After ACL Surgery
Before getting into specific sounds, it helps to understand why the knee is so noisy after surgery in the first place. ACL reconstruction is a significant procedure that alters the mechanical environment inside the knee joint. The surgeon drills tunnels through the femur (thighbone) and tibia (shinbone), places a graft, and fixes it with hardware. The joint then has to heal, adjust, and regain normal movement patterns around all of those changes.
In the weeks and months following surgery, several things are happening simultaneously inside the knee that can produce noise. Swelling changes the way joint surfaces move against each other. Scar tissue forms as part of the healing process and can create friction or tightness during movement. The surrounding muscles, particularly the quadriceps and hamstrings, are weakened and may not be controlling the movement of the kneecap and joint line as smoothly as they did before surgery. The graft itself is going through a biological remodeling process that changes its mechanical properties over time. All of these factors can produce sounds and sensations that feel new and unfamiliar.
The important thing to understand is that noise alone is not a reliable indicator of a problem. The question is not whether your knee is making a sound. The question is what the sound is accompanied by.
Knee Popping After ACL Surgery
A pop or crack in the knee after ACL surgery is one of the most common and most alarming sounds patients report. The first thing most people think when they feel or hear a pop is that something has torn again. In the vast majority of cases, that is not what has happened.
What Causes Knee Popping After ACL Surgery
The most benign cause of knee popping is the release of gas bubbles from the synovial fluid that lubricates the joint. This is the same mechanism that causes knuckle cracking. The joint fluid contains dissolved gases that can form small bubbles under pressure changes during movement, and when those bubbles collapse they produce an audible or felt pop. This is completely harmless.
Popping can also occur as scar tissue breaks down during rehabilitation. As physical therapy progresses and the knee regains range of motion, adhesions, which are areas where scar tissue has formed small internal attachments, can release suddenly. This often feels like a pop and may be briefly uncomfortable but is generally a sign that progress is happening rather than damage occurring. Athletic trainers working with patients in a school, collegiate, or sports medicine setting often observe this happening during rehabilitation sessions and can help distinguish a normal adhesion release from something that needs further evaluation.
A third common cause of post-surgical popping is the graft itself or the hardware used to fix it adjusting slightly as the knee moves through its range. This is particularly common in the early weeks after surgery before the graft has fully integrated.
When Knee Popping After ACL Surgery Warrants Concern
If a pop is accompanied by sudden pain, significant swelling that was not there before, or a feeling of instability or giving way, contact your surgeon. A painful pop with swelling and instability can indicate a graft problem and should be evaluated promptly rather than managed with a wait-and-see approach. If you are working with an athletic trainer, report the pop to them at your next session so they can assess the knee and determine whether escalation to the surgeon is appropriate.
Knee Clicking After ACL Surgery
Clicking is generally a softer, more repetitive sound than popping. Many patients describe hearing or feeling a click at the same point in a movement, such as a particular degree of knee flexion or extension, every time they perform that motion. This consistency is actually a clue that it is mechanical rather than serious.
What Causes Knee Clicking After ACL Surgery
The most common cause of clicking after ACL surgery is the kneecap, or patella, not tracking perfectly through its groove on the front of the femur. When the muscles around the knee are weakened after surgery, particularly the quadriceps, the kneecap can shift slightly off its normal path during movement and produce a clicking sensation as it moves over the edges of the groove. This is called patellar maltracking and it improves significantly as quadriceps strength is rebuilt through rehabilitation.
Clicking can also come from tendons or ligaments around the knee snapping over bony prominences as they adjust to the changed mechanical environment after surgery. The iliotibial band along the outside of the knee and the tendons of the hamstring muscles along the inside are common sources of this type of click.
Scar tissue is another frequent contributor to clicking. As the body heals, tissue can form in areas of the knee that were disrupted during surgery, and as that tissue is stressed and moves with the joint, it can produce consistent clicking sensations.
A study published through PubMed found that roughly 65 percent of patients with post-surgical grinding and clicking experienced symptom resolution, and consistent physical therapy was the most reliable path to improvement across all groups studied. Athletic trainers play a meaningful role in this process, particularly in settings where they see patients daily and can monitor changes in clicking patterns, intensity, and pain response over time in ways that weekly physical therapy appointments sometimes cannot capture.
When Knee Clicking After ACL Surgery Warrants Concern
If clicking is consistently painful, if it is accompanied by swelling or catching, or if it seems to be getting worse rather than better over weeks of rehabilitation, bring it to your physical therapist, athletic trainer, or surgeon’s attention. Persistent painful clicking can sometimes indicate a meniscal issue, a hardware problem, or scar tissue that requires more aggressive management.
Knee Catching After ACL Surgery
Catching is different from clicking and popping in an important way. Catching feels like the knee briefly locks up, sticks, or gets stuck at a particular point in the movement before releasing. It can feel like something is physically blocking the motion. For many patients it is more alarming than noise because it affects the ability to move normally.
What Causes Knee Catching After ACL Surgery
Catching after ACL surgery is most commonly caused by scar tissue or adhesions inside the joint that interfere with smooth movement. When the knee heals after surgery, the body sometimes produces more scar tissue than necessary, and that tissue can partially block the joint space or get pinched between joint surfaces during movement.
Another common cause of catching is the graft itself in the early stages of recovery. If the graft is slightly prominent or has not yet fully integrated, it can occasionally create a catching sensation as the joint moves through certain positions. This typically resolves as healing progresses.
In some cases, catching can indicate a problem with the meniscus. If a meniscal tear was present alongside the ACL injury, or if the meniscus sustained any stress during the recovery process, loose or displaced meniscal tissue can cause a genuine mechanical catching or locking sensation. This is one reason catching deserves more attention than simple painless clicking.
Athletic trainers and physical therapists are often the first members of the care team to hear about catching because patients see them most frequently during rehabilitation. A study published through PubMed found that athletic trainers play an essential role within the healthcare team following ACL injury, with 99 percent of athletic trainers surveyed reporting they feel a strong obligation to support ACL patients through the rehabilitation process. That daily presence means they are often positioned to catch changes in symptoms, including new or worsening catching, before those changes escalate.
When Knee Catching After ACL Surgery Warrants Concern
Catching that is consistent, painful, or accompanied by the knee locking up fully so that you cannot complete a movement without manually repositioning the leg should be reported to your care team promptly. Significant catching with locking is less likely to resolve on its own than simple clicking and may require further evaluation, including imaging or in some cases arthroscopic intervention.
Knee Grinding After ACL Surgery
Grinding, sometimes described as crepitus, is a sensation of roughness or grating during knee movement. It is often felt more than heard, though in some cases it is audible as a coarse, sand-like sound. Grinding tends to be the most concerning of the four sensations because it is most closely associated with cartilage changes, which are worth taking seriously even when they are not immediately dangerous.
What Causes Knee Grinding After ACL Surgery
After ACL surgery, grinding in the short and medium term is often related to swelling, scar tissue, and the kneecap tracking issue described above. The same factors that cause clicking can produce a more textured, grinding quality when more tissue is involved or when the kneecap is moving more significantly off track.
However, grinding can also reflect changes in the articular cartilage, which is the smooth tissue that covers the ends of the bones and allows them to glide against each other. ACL injuries are associated with an increased risk of cartilage damage both at the time of injury and over the longer recovery timeline, and grinding that persists and is accompanied by pain may be a signal that the cartilage is being stressed in a way worth evaluating.
It is also worth noting that grinding in the months and years after ACL reconstruction can be related to early arthritic changes in the joint. ACL injuries are known to be associated with an elevated lifetime risk of knee osteoarthritis, and while this is a long-term concern rather than an acute one, it is part of the honest picture of ACL recovery that patients deserve to understand.
When Knee Grinding After ACL Surgery Warrants Concern
Grinding that is painful, that has been getting progressively worse over several weeks, or that is accompanied by swelling or significant stiffness should be discussed with your surgeon. Painless grinding that is improving or stable and not limiting your rehabilitation progress is generally less urgent but still worth mentioning at your next appointment with your physical therapist, athletic trainer, or surgeon.
What Your Care Team Can Do About Knee Sounds After ACL Surgery
The good news is that all four of these sensations, when they are problematic, respond well to intervention. Your care team includes your surgeon, your physical therapist, and your athletic trainer if you have access to one. Each member plays a distinct role. Your surgeon is responsible for evaluating structural causes and making decisions about further intervention. Your physical therapist designs and progresses your rehabilitation program. Your athletic trainer, particularly if you are a student or competitive athlete with daily access to one, monitors you most frequently, can assess changes in your symptoms in real time, and serves as an important communication bridge between you and the rest of your care team.
Physical Therapy and Athletic Training Interventions
Physical therapy is the foundation of management for post-surgical knee sounds. Targeted quadriceps and hamstring strengthening reduces patellar maltracking. Manual therapy and specific range of motion work can break down adhesions and reduce catching. Proprioception exercises help the knee move more smoothly and efficiently by improving neuromuscular control. Athletic trainers in school and sport settings often supplement formal physical therapy appointments with daily monitoring, targeted exercises, and hands-on assessment that keeps rehabilitation moving forward between sessions.
When Further Medical Intervention Is Needed
For scar tissue that is significantly limiting motion or causing persistent mechanical symptoms, a procedure called manipulation under anesthesia can break down adhesions more aggressively. For catching or locking that does not respond to physical therapy, a return to the operating room for an arthroscopic procedure to remove scar tissue or address a meniscal problem is sometimes necessary, though this is not the most common outcome.
The most important thing you can do right now is be specific with your care team about what you are experiencing. Tell them which sensation you are feeling, where in the range of motion it occurs, whether it is painful, and whether it is getting better or worse. That level of detail gives your physical therapist, athletic trainer, and surgeon the information they need to identify the cause and adjust your treatment plan accordingly.
Navigating unexpected symptoms during ACL recovery is not just a physical challenge. It is an emotional one too. The team at ACL Support has put together a thorough resource on the mental and emotional challenges of ACL injury and recovery at aclsupport.com/acl-injury-recovery-mental-emotional-issues that is worth reading alongside the physical management information here.
The Bottom Line on Knee Popping After ACL Surgery
Knee popping, clicking, catching, and grinding after ACL surgery are common, and in most cases they are expected parts of a joint that is healing and reorganizing itself after a significant procedure. Painless noise during rehabilitation is rarely cause for alarm. Sounds and sensations that are accompanied by pain, swelling, instability, or functional limitation deserve prompt attention from your care team.
The vast majority of patients who experience post-surgical knee sounds see meaningful improvement as their rehabilitation progresses and their strength returns. Whether you are working with a physical therapist, an athletic trainer, or both, keep the lines of communication open and be honest about what you are feeling. Your knee is working hard to heal. Give it the attention and support it needs, and most of what you are hearing will quiet down as the weeks go by.
For more information on the full emotional and mental health side of ACL recovery, visit aclsupport.com/acl-injury-recovery-mental-emotional-issues
Medical Disclaimer: The content in this article is intended for general informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified physician, orthopedic surgeon, athletic trainer, or other licensed healthcare provider with any questions you may have regarding a medical condition or treatment plan. ACL Support does not provide medical advice, and nothing in this article should be interpreted as a recommendation for any specific individual, treatment, or course of action.