Sudden knee pain usually starts after a twist, awkward landing, direct blow, or sharp increase in load. If you are asking why has my knee suddenly started hurting, the cause is often an acute injury involving the ligaments, meniscus, kneecap, cartilage, tendon, or bone around the joint.

This page is one of our knee pain and injury FAQs. Some sudden knee injuries are relatively minor, while others need urgent assessment. A clear diagnosis matters because an knee ligament injury, meniscus tear, fracture, or kneecap dislocation can look similar in the first few hours.

Pay close attention to how the pain started, where it hurts, whether swelling appeared quickly, and whether your knee locks, gives way, or feels unsafe to walk on. Those details help your physiotherapist or doctor work out whether you have a soft tissue injury, a joint surface problem, or something more serious.

Common early clues include:

• a pop or tear sensation during twisting or pivoting
• rapid swelling within a few hours
• pain with weight-bearing, stairs, or bending
• locking, catching, or giving way
• difficulty fully straightening the knee

Many people first notice sudden knee pain during a movement that feels slightly off, followed by discomfort, swelling, or instability.

What causes sudden knee pain?

Sudden knee pain is most often caused by an acute injury rather than gradual wear and tear. Twisting, pivoting, landing awkwardly, kneeling, falling, or taking a direct hit can overload the ligaments, meniscus, cartilage, kneecap, or surrounding muscles and tendons.

Many sporting injuries sit within the broader sports knee injuries cluster, especially if the pain began during running, football, netball, basketball, skiing, or gym training. Acute overload can also happen outside sport when you slip, misstep on stairs, or twist while carrying weight. Treatment may include physiotherapy treatment options based on the structure involved and the severity of the injury.

Which knee injuries commonly start suddenly?

Several conditions can cause sudden knee pain. The most likely diagnosis depends on the mechanism of injury, swelling pattern, pain location, and whether your knee feels unstable, stuck, or too painful to load.

ACL injury

An ACL injury often happens during a pivot, sidestep, or awkward landing. People commonly report a pop, rapid swelling, and difficulty trusting the knee. Instability is a major clue, especially in pivoting sports.

Meniscus tear

A meniscus tear may occur with twisting, deep bending, or a loaded squat. Joint line pain, delayed swelling, clicking, catching, and difficulty straightening the knee are common. Some tears settle well with rehabilitation, while others need further review.

Collateral ligament or other knee ligament injury

A medial collateral ligament (MCL) tear often follows a force to the outside of the knee or a valgus twist. The knee may feel sore, unstable, and painful on the inside. Other ligament injuries can also occur depending on the direction of force.

Patellar dislocation or kneecap instability

If the kneecap shifts or dislocates, the pain is often immediate and dramatic. Swelling, loss of confidence, and pain around the front or outer side of the knee are common. Some people describe the kneecap as moving out and back in again.

Tibial plateau fracture or bony injury

A fracture is less common, but it must be considered after a high-force fall, collision, or awkward landing. Inability to bear weight, marked swelling, severe pain, and joint-line tenderness raise concern. For general public guidance on knee injuries and urgent care signs, Healthdirect provides a useful overview of knee injuries.

Juvenile osteochondritis dissecans

In adolescents, sudden knee pain sometimes relates to juvenile osteochondritis dissecans. This condition affects the bone and cartilage beneath the joint surface and may cause swelling, catching, or locking, especially in active young athletes.

When should you worry about sudden knee pain?

You should worry about sudden knee pain when you cannot take four steps, the swelling appears quickly, the knee looks deformed, the joint locks, or you feel marked instability. These features increase concern for fracture, major ligament injury, loose body, or a significant meniscal tear.

Urgent medical review is also sensible if the knee is hot and red, you have fever, numbness, severe calf swelling, or symptoms are worsening quickly. Healthdirect advises prompt care when you cannot walk because of a knee injury or when the knee is badly swollen or changed shape.

How is sudden knee pain assessed?

Assessment starts with the injury story. Your physiotherapist or doctor will ask what you were doing, whether you heard a pop, how quickly the swelling developed, and whether the knee now locks or gives way. That history often points strongly toward the likely structure involved.

The physical examination usually checks swelling, joint line tenderness, ligament stability, kneecap position, range of motion, and weight-bearing ability. Imaging is not always needed straight away, but X-ray may be important if fracture is possible, while MRI is more useful for ligament, meniscus, cartilage, or osteochondral injuries.

If your pain is very recent, an acute soft tissue injury approach often helps settle the knee until a clearer diagnosis is made. In the first phase, this commonly means protecting the knee, managing swelling, and restoring safe movement before loading harder.

What should you do if your knee suddenly starts hurting?

If your knee suddenly starts hurting, stop the aggravating activity, reduce load, use ice if helpful, consider compression, and avoid pushing through unstable or locking symptoms. Early assessment is usually the safest next step because the right plan depends on what structure has been injured.

Physiotherapy may help you identify the injured tissue, decide whether imaging is needed, reduce swelling, restore movement, and begin a staged rehab plan. As the knee settles, treatment may progress to knee exercises, strength work, balance retraining, and return-to-sport guidance. If you are unsure who to see first, this FAQ on doctor or physio for a knee injury may help.

FAQs about sudden knee pain

Can sudden knee pain happen without a major accident?

Yes. Sudden knee pain can start after a smaller twist, awkward squat, stumble, or change in training load. The movement may seem minor, but if the force is poorly timed or the knee is vulnerable, it can still irritate the meniscus, ligaments, tendon, kneecap, or joint lining.

Does a pop always mean I tore my ACL?

No. A pop raises suspicion for an ACL injury, but it is not specific to the ACL. Meniscus tears, kneecap dislocations, and other joint injuries can also produce a pop or shift sensation. Rapid swelling and instability make ACL injury more likely, but proper assessment is still needed.

Why did my knee swell so quickly?

Rapid swelling within a few hours can suggest bleeding inside the joint, which is more common with ACL tears, patellar dislocation, fracture, or other significant internal injuries. Delayed swelling is more often seen with some meniscus or overload problems, although patterns can overlap.

Should I keep walking on a suddenly painful knee?

Light walking may be reasonable if your symptoms are mild and the knee feels stable. However, you should avoid pushing through if you are limping badly, the knee buckles, swelling is increasing, or you cannot take four normal steps. Those signs suggest the injury needs earlier review.

Do I need an MRI straight away?

Not always. Many acute knee injuries can be assessed well from the history and physical examination first. X-ray is often more important early if fracture is suspected. MRI is more useful when ligament, meniscus, cartilage, or osteochondral injury is suspected, or when recovery is not progressing as expected.

How can physiotherapy help sudden knee pain?

Physiotherapy may help by identifying the likely injured structure, reducing pain and swelling, improving range of motion, guiding safe loading, and progressing strength and control. It also helps you decide when you can return to work, sport, stairs, squatting, and other everyday tasks with more confidence.

What to do next

If your knee has suddenly started hurting, do not guess the diagnosis based on pain location alone. The same area of pain can come from very different structures, and the correct treatment depends on what has actually been injured.

Book an assessment if you have swelling, instability, locking, trouble walking, or symptoms that are not settling quickly. Early guidance often helps you avoid setbacks and gives you a safer path back to work, exercise, and sport.

Not Sure What You’ve Injured?

If your knee pain started suddenly, getting the right diagnosis early can make a big difference. Many injuries look similar at first but need very different management.

A physiotherapy assessment can help identify the structure involved and guide your next steps with more confidence.

References

1. Jadidi S, Lee AD, Pierko EJ, Choi H, Jones NS. Non-operative Management of Acute Knee Injuries. Curr Rev Musculoskelet Med. 2024;17(1):1-13. doi:10.1007/s12178-023-09875-7
2. Sims JI, Chau MT, Davies JR. Diagnostic accuracy of the Ottawa Knee Rule in adult acute knee injuries: a systematic review and meta-analysis. Eur Radiol. 2020;30(8):4438-4446. doi:10.1007/s00330-020-06804-x
3. Akkawi I, Zmerly H, Draghetti M, Felli L. Juvenile Osteochondritis Dissecans: Current Concepts. Cureus. 2024;16(7):e65496. doi:10.7759/cureus.65496
4. Howell M, Khalid A, Nelson C, Doonan J, Jones B, Blyth M. Long term outcomes following tibial plateau fracture fixation and risk factors for progression to total knee arthroplasty. Knee. 2024;51:303-311. doi:10.1016/j.knee.2024.10.003

Functional knee assessment can guide your next step.

Choosing between a doctor or physio for knee injury care depends on your symptoms. Some knee injuries need urgent medical review. However, many movement-related knee injuries can start with a physiotherapy assessment, especially when pain follows sport, work, exercise, or gradual overload.

If you have severe trauma, suspected fracture, broken skin, infection signs, or extreme pain, see a doctor or attend an emergency department first. If your knee injury is less severe, a physiotherapist can assess your knee, guide early care, and advise whether imaging or medical review is needed.

For a broader overview of common causes and treatment options, visit our knee pain and injury guide.

When Should You See a Doctor for a Knee Injury?

See your doctor or attend an emergency department urgently if your symptoms suggest a more serious injury or medical problem.

• Your knee looks deformed or your leg alignment has changed after trauma.
• You have broken skin, a deep cut, or you may need wound care or stitches.
• You have severe pain that does not settle with rest or simple pain relief.
• You cannot put weight through the leg after a fall, twist, tackle, or impact.
• Your knee is hot, red, swollen, and painful, especially with fever or feeling unwell.
• You heard a pop with rapid swelling and your knee feels unstable.
• You cannot straighten or bend the knee normally after injury.

In a non-urgent setting, your doctor may examine your knee, discuss pain relief, request imaging, or refer you to a physiotherapist or orthopaedic specialist.

When Should You See a Physiotherapist for a Knee Injury?

A physiotherapist is often a practical first contact when your knee pain follows a twist, awkward landing, knock, training change, or gradual overload.

Physiotherapists assess how your knee moves, how it responds to load, and which structures may be irritated or injured. They can also screen for signs that need medical review.

For treatment options, read more about knee treatment and physiotherapy.

How Do You Decide Who to See First?

Use the warning signs first. If the knee injury feels severe, unsafe, or unusual, choose medical review first. If the injury is movement-related and you can still walk with manageable symptoms, physiotherapy is often a suitable starting point.

Your situation	Best first step
Major trauma, deformity, deep wound, fever, or red hot swelling	Doctor or emergency department
Cannot weight-bear after injury	Doctor or emergency department first
Twist, awkward landing, swelling, or sport injury without emergency signs	Physiotherapist, with referral if needed
Gradual pain with stairs, running, squats, or kneeling	Physiotherapist
Unclear injury type or concern about MRI	Physiotherapist or GP, depending on severity

Weight-bearing ability helps guide care decisions.

Do You Need a Referral?

You do not need a GP referral to see a physiotherapist for a private knee injury appointment. You can book directly with PhysioWorks.

Referrals are usually required if you need an orthopaedic surgeon review, some types of diagnostic imaging, or care under a specific funding pathway. Your physiotherapist can guide this if your assessment suggests it is needed.

Do You Need an X-ray or MRI?

Not every knee injury needs imaging straight away. The first step is usually a clinical assessment to check the injury pattern, swelling, weight-bearing ability, movement, strength, and warning signs.

An X-ray may be more appropriate if fracture is suspected. An MRI may be considered when the diagnosis remains unclear, a significant ligament or cartilage injury is suspected, or symptoms are not improving as expected.

For more detail, read How Do I Know If I Need an MRI on My Knee?

Common Knee Injuries a Physio May Assess

Knee injuries can involve ligaments, cartilage, tendons, kneecap joint loading, or surrounding muscles. Your symptom pattern often gives useful clues.

• ACL injury: often linked with pivoting, a pop, rapid swelling, and instability.
• MCL injury: commonly causes inner knee pain after a side impact or twist.
• Meniscus tear: may cause joint-line pain, swelling, catching, or locking.
• Patellofemoral pain syndrome: often causes pain around or behind the kneecap with stairs, squats, running, or sitting.

If you are unsure what type of injury you have, read How Do I Know What Type of Knee Injury I Have?

Still Unsure Who to See First?

If you are still unsure whether to see a doctor or physio for knee injury care, start with the safety check. Severe trauma, infection signs, broken skin, deformity, or inability to weight-bear should be medically reviewed first.

If those warning signs are not present, a physiotherapist can ask screening questions, assess your knee, and guide the safest next step. This may include physiotherapy care, GP review, imaging advice, or specialist referral if needed.

Related Knee Injury Information

• Knee Pain & Injury: Causes, Treatment & Prevention
• Knee Pain FAQs
• How Do I Know What Type of Knee Injury I Have?
• How Do I Know If I Need an MRI on My Knee?
• Knee Treatment and Physiotherapy
• Knee Pain Products

Doctor or Physio for Knee Injury FAQs

Should I see a doctor or physio for a knee injury?

See a doctor first if you have severe trauma, deformity, broken skin, signs of infection, extreme pain, or you cannot put weight through the leg. For many other knee injuries linked to movement, sport, or overload, a physiotherapist is often an appropriate first contact.

When should I see a doctor urgently for knee pain?

Seek urgent medical care if your knee is badly swollen, has changed shape, is red and hot with fever, or the injury followed major trauma. You should also seek medical care if pain is severe or you cannot walk because of the injury.

Can a physiotherapist diagnose a knee injury?

A physiotherapist can assess and diagnose many common knee injury patterns. They check swelling, movement, strength, walking, joint loading, and symptom behaviour. If your signs suggest fracture, infection, major joint injury, or another concern, they will guide medical review.

Do I need an MRI before seeing a physiotherapist?

Most knee injuries do not need an MRI before physiotherapy. A clinical assessment is usually the first step. MRI may be considered if a significant structural injury is suspected, the diagnosis remains unclear, or your symptoms are not improving as expected.

Can I book a physio without a GP referral?

Yes. You can book a private physiotherapy appointment without a GP referral. Some funded pathways, specialist reviews, and imaging requests may still require a medical referral.

Guided rehab can support safer movement.

What to Do Next

If your knee injury has urgent warning signs, seek medical review first. If your knee pain is movement-related and does not have red flags, a physiotherapy assessment can help clarify the likely injury and guide safe early recovery.

PhysioWorks physiotherapists can assess your knee, explain your options, and help you decide whether physiotherapy, imaging, GP review, or specialist referral is the right next step.

References

1. healthdirect Australia. Knee pain. Healthdirect Australia. Accessed June 18, 2026.
2. Wall C, Satalich J, Yung A, et al. Acute sport-related knee injuries. Aust J Gen Pract. 2023;52(11).
3. BMJ Best Practice. Assessment of knee injury. BMJ Best Practice. Updated June 30, 2022. Accessed June 18, 2026.