Bad Signs After ACL Surgery: When Should You Get Help?

By Emma - The Hip & Knee Physio

Pain, swelling, bruising and weakness are all common after ACL reconstruction. The difficult part is knowing when your symptoms are part of the expected recovery and when they may be a sign that something needs medical or rehabilitation review.

In brief: Seek urgent medical help after ACL surgery if you develop chest pain, difficulty breathing, significant calf or thigh swelling, or feel acutely unwell. Contact your surgeon promptly for fever, worsening wound redness or discharge, rapidly increasing knee pain or swelling, or a sudden major loss of movement. Persistent stiffness, recurrent swelling or failure to progress should also be reviewed rather than simply pushed through.

Your surgeon’s postoperative instructions take priority over any general information in this article. Procedures differ, particularly when ACL reconstruction is combined with meniscus repair, cartilage surgery or another ligament procedure.

I am Emma Glynn, an APA Titled Musculoskeletal Physiotherapist with more than 13 years of experience. I provide ACL rehabilitation in Camberwell for people recovering after reconstruction and for those concerned that their knee is not progressing as expected.

What symptoms are normal after ACL surgery?

Some pain and swelling are expected after an operation.

During the early phase, it is common to experience:

• swelling around the knee;

• bruising around the knee, thigh or lower leg;

• pain when moving or completing exercises;

• temporary difficulty bending the knee;

• weakness or difficulty activating the quadriceps;

• fatigue;

• disturbed sleep;

• an altered walking pattern;

• mild numbness around an incision or graft-harvest area.

These symptoms should generally show an improving pattern, even if progress is not perfectly linear.

A busier rehabilitation session, longer period on your feet or reduction in pain medication may temporarily increase symptoms. The important question is whether the knee settles again or continues becoming more painful, swollen or restricted.

For a broader guide to the normal early stages, read ACL Rehab: What to Expect in the First 12 Weeks After Surgery.

Call 000 for chest pain or difficulty breathing

Call Triple Zero on 000 if you develop chest pain, sudden shortness of breath, difficulty breathing, faintness or other symptoms suggesting a medical emergency.

This is particularly important if these symptoms occur alongside pain or swelling in one calf or thigh.

Surgery and reduced mobility can increase the risk of a deep vein thrombosis, which is a blood clot that most often develops in the leg. Part of a clot can travel to the lungs and cause a pulmonary embolism. Healthdirect advises calling 000 for chest pain or trouble breathing where a blood clot may be involved.

Do not wait for a routine physiotherapy or surgical appointment when you are experiencing emergency symptoms.

Seek immediate medical assessment for possible blood-clot symptoms

Contact a doctor immediately if one calf or thigh becomes:

• noticeably more swollen than the other;

• increasingly painful or tender;

• red or discoloured;

• unusually warm;

• tight or uncomfortable without a clear exercise-related explanation.

Some postoperative swelling can travel into the lower leg because of gravity. That does not mean every swollen calf is a blood clot.

Unfortunately, appearance alone cannot safely rule a clot in or out. A suspected deep vein thrombosis may require medical assessment and an ultrasound.

Contact your surgeon promptly if infection is possible

Infection after ACL reconstruction is uncommon, but it requires prompt medical attention.

Contact your surgeon or treating medical team if you develop:

• fever or chills;

• feeling increasingly unwell;

• worsening redness around the wound;

• pus, cloudy discharge or unpleasant-smelling fluid;

• a wound that is opening;

• increasing heat around the knee;

• rapidly worsening pain;

• increasing swelling and reduced movement rather than gradual improvement.

ACL reconstruction involves both surgical wounds and a graft within the knee. A deep joint infection may present with increasing pain, swelling, heat, restricted movement and sometimes fever. Not everyone with an infection will have every symptom.

Do not attempt to manage suspected infection by adding ice, doing fewer exercises and waiting several days to see what happens.

Contact the surgical team.

Rapidly increasing pain or swelling deserves review

The knee will usually be swollen after reconstruction. The concern is not the mere presence of fluid. It is an unexpected or progressively worsening pattern.

Contact your surgeon or physiotherapist when:

• swelling suddenly increases without an obvious reason;

• the knee becomes increasingly tense or difficult to move;

• pain is worsening rather than gradually settling;

• swelling repeatedly returns after relatively light activity;

• you are losing movement that you had previously regained;

• the knee becomes significantly hotter or redder.

A sudden increase may reflect excessive load, bleeding into the joint, irritation from an associated procedure or a less common complication.

Symptoms alone cannot determine the cause.

If swelling is the main concern and you are several weeks into rehabilitation, read Why Is My Knee Still Swollen Six Weeks After ACL Surgery?.

Losing knee extension is not something to ignore

Restoring knee extension is an important early rehabilitation priority after ACL reconstruction.

Your knee may initially feel stiff and swollen, but the ability to straighten it should generally improve with appropriate rehabilitation.

Arrange a review if:

• the knee is becoming harder to straighten;

• progress has stopped for a sustained period;

• you previously had better extension and have lost it;

• the knee feels mechanically blocked;

• walking remains persistently bent-kneed;

• extension exercises repeatedly create substantial swelling or pain.

Persistent motion restriction can have several explanations, including joint swelling, pain, muscle inhibition or excessive scar-tissue formation.

Arthrofibrosis describes problematic postoperative stiffness associated with scar tissue. Evidence supports recognising and addressing significant motion loss early rather than ignoring it for months.

This does not mean every stiff knee has arthrofibrosis. It means a knee that is not regaining movement deserves assessment and a coordinated plan.

What if your knee will not bend?

Knee flexion commonly takes time to recover, especially when swelling is substantial or the surgery included a meniscus repair.

Your surgeon’s restrictions matter. Some procedures deliberately limit knee bending or weight-bearing during the early phase.

Arrange a review if:

• bending has stopped improving;

• movement is becoming more restricted;

• the knee feels blocked rather than merely tight;

• pain rises sharply when you attempt the prescribed range;

• your progress differs substantially from the surgeon’s protocol;

• you do not know whether you are permitted to push further.

More force is not always the answer.

An irritable knee may require better swelling control, a different exercise dose or clarification of surgical restrictions. Rehabilitation after ACL reconstruction is exercise-led, but the available evidence does not support one universal exercise dosage for every postoperative knee.

Is a pop after ACL surgery a bad sign?

A noise or sensation in the knee does not automatically mean the graft has ruptured.

Knees can click, crack or pop because of:

• swelling;

• changes in kneecap movement;

• scar tissue;

• tendons moving over surrounding structures;

• altered muscle control;

• normal joint cavitation.

A pop deserves prompt assessment when it occurs with:

• a new twisting injury or fall;

• sudden substantial pain;

• rapid swelling;

• loss of movement;

• a new feeling that the knee is giving way;

• inability to continue walking or exercising.

A clinician cannot confirm graft damage from the sound alone. The injury mechanism, swelling response, stability examination and sometimes imaging are needed.

New or repeated giving way should be assessed

Weakness can make the operated leg feel uncertain, particularly early in recovery.

Repeated episodes where the knee genuinely buckles or gives way are different.

Arrange a review if:

• the knee collapses during normal walking;

• instability is becoming more frequent;

• giving way begins after a new injury;

• instability occurs alongside swelling or pain;

• you no longer trust the knee during activities you previously managed.

Possible contributors include quadriceps weakness, poor control, pain inhibition, associated meniscus problems or graft-related instability.

The solution depends on the cause. Treating every episode as “just weak muscles” is no more sensible than assuming every wobble means the graft has failed.

New locking or catching needs attention

Occasional clicking is common and often harmless.

A knee that becomes mechanically locked or cannot fully bend or straighten should be medically assessed, particularly if the reconstruction included a meniscus injury or repair.

Contact your surgeon if:

• the knee becomes stuck in one position;

• you suddenly cannot straighten it;

• catching is painful and worsening;

• locking follows a new twist or fall;

• symptoms are accompanied by substantial swelling.

Do not force a genuinely locked knee through repeated exercises.

Persistent numbness or weakness may need review

Small areas of altered sensation around the surgical incisions or graft-harvest area can occur after ACL reconstruction.

Seek review if you notice:

• numbness spreading rather than remaining local;

• new weakness in the foot or ankle;

• difficulty lifting the foot;

• severe burning or electrical pain;

• increasing symptoms below the knee;

• changes in circulation, colour or temperature of the foot.

These symptoms do not automatically indicate permanent nerve damage, but they should not be assessed through an internet article and optimism.

When is swelling a rehabilitation problem rather than a surgical emergency?

Swelling that increases after activity but settles with sensible load modification is often a sign that the knee has been asked to do more than it can currently recover from.

Your rehabilitation may need reassessment when:

• each session produces a larger reaction;

• swelling remains elevated into the next day;

• you repeatedly lose movement after exercise;

• your quadriceps becomes harder to activate;

• walking worsens after training;

• you are progressing exercises because of the calendar rather than knee response;

• your program has not changed despite several weeks of limited progress.

The Aspetar ACL rehabilitation guideline identifies exercise as the main component of rehabilitation while recognising that pain, swelling and motion restrictions affect when and how exercise can progress.

A rehabilitation review may involve changing:

• exercise resistance;

• range of movement;

• the number of sets and repetitions;

• walking volume;

• gym frequency;

• cycling or running volume;

• recovery between harder sessions;

• expectations for the current phase.

If you are unsure whether your current program matches your knee, learn more about ACL rehabilitation in Camberwell.

Who should you contact?

Call 000

Call 000 for:

• chest pain;

• sudden shortness of breath;

• difficulty breathing;

• collapse or fainting;

• symptoms that feel immediately life-threatening.

Contact your surgeon or urgent medical care

Seek prompt medical assessment for:

• signs of a blood clot;

• fever or feeling acutely unwell;

• worsening wound redness or discharge;

• rapidly increasing knee pain or swelling;

• a wound that opens;

• a genuinely locked knee;

• major new trauma;

• a sudden significant loss of movement.

Contact your physiotherapist

A physiotherapy review may be appropriate for:

• swelling that repeatedly follows rehabilitation;

• persistent difficulty regaining extension or flexion;

• poor quadriceps activation;

• a worsening limp;

• exercise progression that feels unclear;

• continued weakness or low confidence;

• a knee that is not meeting expected functional milestones.

Where symptoms could reflect a surgical complication, your physiotherapist should communicate with or refer you back to the surgeon.

What happens during an ACL rehabilitation check-in?

An ACL check-in may include assessment of:

• swelling;

• knee extension and flexion;

• quadriceps activation;

• walking;

• pain behaviour;

• current exercises;

• adherence to surgical restrictions;

• strength and single-leg control where appropriate;

• how the knee responds later that day and the following morning.

The purpose is to work out whether:

• your progress is reasonably consistent with the operation;

• your exercise dose needs changing;

• an important milestone has been missed;

• another knee structure may be contributing;

• your surgeon should review the knee.

It is not to provide false reassurance or diagnose a postoperative complication beyond physiotherapy scope.

What this means for you

Some discomfort, swelling, weakness and stiffness are expected after ACL surgery.

The pattern matters more than one isolated symptom.

Seek help when symptoms are:

• rapidly worsening;

• accompanied by fever or feeling unwell;

• affecting your calf, breathing or chest;

• associated with wound discharge;

• causing a sudden major loss of movement;

• following a new injury;

• repeatedly preventing rehabilitation progress.

If the problem is not urgent but your knee remains swollen, stiff or difficult to progress, do not simply perform more exercises and hope enthusiasm eventually defeats physiology.

You can book an ACL rehabilitation assessment in Camberwell to review your movement, swelling, strength and current rehabilitation plan.

For guidance on later-stage progression, read ACL Rehab Timelines That Actually Matter.

Emma

References

Ekhtiari, S., Horner, N. S., de Sa, D., Simunovic, N., Hirschmann, M. T., Ogilvie, R., & Ayeni, O. R. (2017). Arthrofibrosis after ACL reconstruction is best treated in a step-wise approach with early recognition and intervention: A systematic review. Knee Surgery, Sports Traumatology, Arthroscopy, 25(12), 3929–3937. https://doi.org/10.1007/s00167-017-4482-1

Healthdirect Australia. (2025). Deep vein thrombosis.

Kotsifaki, R., Korakakis, V., King, E., Barbosa, O., Maree, D., Pantouveris, M., Bjerregaard, A., Luomajoki, J., Wilhelmsen, J., & Whiteley, R. (2023). Aspetar clinical practice guideline on rehabilitation after anterior cruciate ligament reconstruction. British Journal of Sports Medicine, 57(9), 500–514. https://doi.org/10.1136/bjsports-2022-106158

NHS. (2025). Complications of ACL surgery.

Medical disclaimer

This article provides general educational information and is not a substitute for the postoperative instructions provided by your surgeon or treating medical team. Contact your surgeon or seek urgent medical care if symptoms are severe, rapidly worsening or suggest infection, a blood clot or another surgical complication. Call Triple Zero on 000 for chest pain, difficulty breathing or a medical emergency.