Your knee is a complex joint that has many moving parts. This makes it more prone to injury.

As we age, the stress of everyday movements and activities can be enough to trigger symptoms of pain and fatigue in our knees.

If you’re going about your everyday activities and feel sudden knee pain, it can be hard to know what to do next. Some causes of sudden knee pain are health emergencies requiring attention from a medical professional. Other knee conditions you can treat at home.

In this article, we’ll walk you through conditions that cause sudden knee pain so you can spot the differences and plan your next steps.

Causes of sudden knee pain

Knee pain that appears out of nowhere may seem like it couldn’t be related to an injury. But the knee is a tricky body part. It consists of many parts that can become:

• stretched out
• worn
• aggravated
• partially torn
• fully ruptured

Fracture

A fracture can cause sudden knee pain. A tibial plateau fracture involves the shinbone and kneecap. This kind of fracture causes:

• swelling
• sharp pain
• inability to move your joint

Distal femoral fractures involve the lower thigh and kneecap and cause similar symptoms. A broken kneecap can also occur, causing intense pain and swelling.

Fractures that involve these bones can occur from traumatic injuries or simple falls.

Tendinitis

Tendons connect your joints to your bones. Repetitive actions (such as walking or running) can cause your tendons to become inflamed and swollen. This condition is known as tendinitis.

Tendinitis of the knee is fairly common. Patellar tendinitis (jumper’s knee) and quadriceps tendinitis are specific subtypes of this condition.

Tightness, swelling, and a dull ache are the signature symptoms of tendinitis in your knee. You may also be unable to move the affected joint until after you rest it.

Runner’s knee

Runner’s knee refers to knee pain that starts behind or around your kneecap. This condition is common in active adults.

Symptoms include a dull throbbing behind your kneecap, especially where your knee meets your femur, or thigh bone. Runner’s knee can also cause your knee to pop and grind.

Torn ligament

Commonly injured ligaments in your knee are the anterior cruciate ligament (ACL) and the medial collateral ligament (MCL).

The PCL, LCL, and MPFL ligaments in your knee can also become torn. These ligaments connect the bones above and below your kneecap.

It’s not unusual for either one of those ligaments to become torn, especially in athletes. Sometimes you can pinpoint the moment the tear happened to a tackle on the football field or an overextension playing tennis.

Other times, the cause of the injury is less traumatic. A hit to the knee at a bad angle can tear the ACL, for example.

If you tear either one of these ligaments, you’ll typically hear a popping sound, followed by swelling. Severe knee pain usually follows. You may be unable to move the joint without assistance from a brace.

Osteoarthritis

Sudden knee pain could indicate the onset of osteoarthritis (OA). OA is the most common type of arthritis.

Older people, especially athletes and people in trades like construction who often performed repetitive movements, are most at risk for this condition.

Pain, tenderness, and inflammation of the knee are signs that OA is beginning to develop. In most cases, the pain in your knee won’t present suddenly. More likely, it will cause gradually increasing levels of pain.

While OA can affect only one knee, it’s more likely it would impair both knees.

Bursitis 

The bursae are fluid-filled sacs between your joints. Bursae can become inflamed around your knees, causing bursitis.

Repeatedly bending your knees or bleeding in your bursae can cause a sudden onset of bursitis symptoms. Bursitis of the knee isn’t one of the most common places for this condition to occur, but it isn’t rare.

Acute pain and swelling in one or both knees are the most common symptoms of bursitis.

Injured meniscus

Menisci are pieces of cartilage in your knee. An injured or torn meniscus is a common condition that results from forcibly twisting your knee.

If you injure your meniscus, you may hear a popping sound followed by an immediate sharp pain as well as swelling. The affected knee might feel locked into place. This condition tends to affect only one knee at a time.

Gout

A buildup of uric acid in the body causes gout. The acid tends to collect in your feet, but it can also affect both knees.

Gout is common, especially for middle-aged men and postmenopausal women.

The condition causes intense pain and a lot of swelling. Gout comes on in spurts that last for a few days. If you’ve never had knee pain before and it comes on suddenly, it could be the beginning of gout.

Infectious arthritis

Infectious arthritis is an acute form of arthritis that develops from infected fluid surrounding your joint. If left untreated, the fluid can become septic.

Septic arthritis is considered a medical emergency that requires emergency surgery.

This condition causes sudden pain in only one knee. Having a history of arthritis, gout, or a weakened immune system can increase your risk for infectious arthritis.

Treatment for sudden knee pain

Treatment for knee pain depends on the cause.

For fractures and broken bones

Broken bones in your knee will need to be assessed by a healthcare provider. You may need a cast or a splint to stabilize the knee while the bones heal.

In the case of more severe fractures, you may need surgery, followed by a splint and physical therapy.

For tendinitis, runner’s knee, gout, and bursitis

The treatment for conditions that cause swelling, redness, and dull, burning pain usually starts with resting the joint. Ice your knee to control swelling. Elevate and stay off your joint to promote healing.

Your doctor may recommend or prescribe NSAIDs like ibuprofen. Lifestyle changes, such as wearing protective kneepads and going to physical therapy, can help you manage pain and experience fewer symptoms.

You may need to make changes to your diet, especially if you’re treating gout.

For ligament, cartilage, and joint tears

Ligament, cartilage and joint tears in your knee will need to be addressed by your doctor.

After imaging diagnostics and a clinical assessment, your doctor will let you know if your treatment will include physical therapy and anti-inflammatory medication, or if you’ll need to undergo surgery to repair the injury.

Recovery from knee surgery can take some time. It may take anywhere from 6 months to a year to resume your normal activities.