- Sudden knee pain after an injury, especially with swelling and instability, may require prompt medical evaluation.
- Persistent knee pain, redness, deformity, fever, or painful locking or catching can be signs of a more serious condition.
- Early evaluation and appropriate treatment can help prevent further joint damage and improve recovery.
Knee pain when bending can be caused by patellar tendinitis, osteoarthritis, a Baker’s cyst, or an injury from a fall or direct trauma. These injuries may affect the knee joint or its ligaments.
Depending on the cause, the pain may be severe enough to interfere with everyday activities. It can occur behind the knee, above or below the kneecap, or in another part of the joint.
It is important to have persistent, severe, or unexplained knee pain when bending evaluated by a healthcare provider. They can identify the cause and recommend the most appropriate treatment.
Main causes
The main causes of knee pain when bending include:
1. Traumatic injury
A traumatic knee injury may result from a fall, bruise, direct blow, twisting movement, dislocation, or fracture.
In these cases, knee pain when bending usually begins suddenly and may be severe. The pain may occur behind the knee, above or below the kneecap, or in another area, depending on where the injury occurred. Swelling and difficulty moving the knee may also develop.
What to do: For a mild injury without signs of a fracture, rest the knee and apply an ice pack for 15 minutes, two to three times a day. If a fracture or dislocation is suspected, seek emergency care immediately.
Physical therapy may also be recommended to relieve pain and support recovery, even after a mild injury.
2. Torn ligament
A tear in a knee ligament, such as the medial collateral ligament, lateral collateral ligament, anterior cruciate ligament, or posterior cruciate ligament, can cause pain when bending the knee or fully straightening the leg. It may also lead to swelling and a limited range of motion.
A person may hear or feel a sudden pop when the injury occurs. The knee may then feel unstable or become difficult to move.
What to do: Treatment for a torn knee ligament should be guided by an orthopedic specialist. Depending on the type and severity of the injury, treatment may include a knee brace, rest, physical therapy, or surgery. A knee brace may be recommended initially to help stabilize and protect the joint.
3. Patellar tendinitis
Patellar tendinitis is inflammation of the tendon that connects the kneecap to the shinbone. It can cause pain when bending the knee, sitting for a long time, squatting, or climbing stairs. Swelling or stiffness after waking up may also occur.
Patellar tendinitis, also known as jumper’s knee, is usually associated with repetitive strain in athletes and people who participate in intense physical activities, such as running, soccer, or cycling.
What to do: Rest the knee, avoid activities that worsen the pain, and apply an ice pack for up to 20 minutes at a time, two to three times a day. An orthopedic specialist may also recommend physical therapy, anti-inflammatory medication, or, in some cases, surgery.
4. Knee bursitis
Knee bursitis is inflammation of one of the 11 bursae around the knee. These small, fluid-filled sacs cushion the joint and reduce friction between the bones, tendons, and muscles.
When a bursa becomes inflamed, it can cause pain when bending the knee, swelling, warmth, and pain above or below the kneecap.
Knee bursitis is often caused by repeated kneeling or knee bending, falls, direct blows, or sports such as Brazilian jiu-jitsu, soccer, and volleyball.
What to do: Rest and apply an ice pack for 15 minutes, two to three times a day. An orthopedic specialist may also recommend physical therapy, anti-inflammatory medication, a corticosteroid injection into the bursa, or surgery.
5. Meniscus injury
Pain when bending the knee, pain on the inner side of the knee, or pain when climbing stairs or squatting may indicate a meniscus injury. The meniscus is a cartilage disc that cushions the knee and absorbs shock inside the joint.
Meniscus injuries are commonly caused by twisting or pivoting movements during sports and other physical activities. They may also develop because of age-related degeneration of the cartilage.
What to do: Rest and avoid activities that place excessive stress on the knee. Physical therapy may be recommended to strengthen the muscles that support the knee and improve joint function.
In some cases, an orthopedic specialist may recommend surgery, particularly if the tear is severe or symptoms persist. If a meniscus injury is accompanied by inflammation or knee arthritis, corticosteroid or hyaluronic acid injections may also be considered to help relieve symptoms.
6. Quadriceps tendinitis
Quadriceps tendinitis is inflammation of the quadriceps tendon, which connects the quadriceps muscle at the front of the thigh to the kneecap.
It can cause pain above the kneecap when bending the knee, along with stiffness, swelling, or warmth around the joint. It most often develops because of repetitive stress from activities such as running and jumping or after a knee injury.
What to do: Tendinitis treatment should be supervised by an orthopedic specialist and may include rest, avoiding activities that worsen symptoms, applying ice packs, taking anti-inflammatory medication, and participating in physical therapy.
Surgery may be recommended in severe cases or when symptoms do not improve with conservative treatment.
7. Hamstring tendinitis
Hamstring tendinitis affects a tendon at the back of the thigh. It may cause pain behind the knee when bending it, pain on the outer side of the knee, swelling, or pain when moving the joint.
This condition is usually associated with repetitive strain in athletes and people who participate in intense activities such as running, soccer, or cycling.
What to do: Rest the joint, avoid repetitive strain, and apply an ice pack to the knee for 20 minutes twice a day. An orthopedic specialist may also recommend physical therapy, anti-inflammatory medication, or surgery.
8. Knee osteoarthritis
Knee osteoarthritis is a gradual breakdown of the cartilage in the knee. As the quality, amount, and thickness of the cartilage decrease, chronic pain may develop.
This condition can interfere with everyday activities. It may cause pain below the knee when bending it, stiffness in the morning, or pain when walking, at the end of the day, or after standing for a long time.
What to do: A healthcare provider can recommend treatment based on the severity of the symptoms. Treatment may include physical therapy, exercise, weight management when appropriate, and pain-relieving medication.
According to the American Academy of Orthopaedic Surgeons (AAOS), nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen or diclofenac, are recommended to help improve pain and function in people with knee osteoarthritis.
Acetaminophen may be considered in some cases, although it may provide less relief than NSAIDs. A corticosteroid injection may be used for short-term pain relief.
9. Iliotibial band syndrome
Iliotibial band syndrome causes a burning pain on the outer side of the knee. The pain may spread to the thigh or hip and may worsen when bending the knee.
This condition is common among runners, cyclists, and people who participate in sports that involve repeated knee bending. It may be associated with weak or inflexible muscles, unsuitable training intensity or volume, running terrain, footwear, or poor form.
What to do: Temporarily stop the activity that is causing the symptoms. A qualified fitness professional may recommend stretching and strengthening exercises.
An orthopedic specialist may also recommend physical therapy and a topical or oral anti-inflammatory medication.
10. Rheumatoid arthritis
Rheumatoid arthritis is a chronic inflammatory autoimmune disease that causes pain, swelling, and stiffness in the joints. When it affects the knee, it can cause pain when bending the joint and pain after waking up.
Morning pain and stiffness are often most intense during the first few minutes after waking and tend to improve with movement. Knee pain and swelling that occur without an injury may also be signs of rheumatoid arthritis.
What to do: An orthopedic specialist or rheumatologist may recommend pain relievers or anti-inflammatory medications. Physical therapy may also help improve knee movement.
11. Baker’s cyst
A Baker’s cyst is a fluid-filled lump that develops behind the knee when synovial fluid builds up in the joint. It can cause pain, swelling, stiffness, and discomfort when bending the knee, especially during physical activity.
The lump may feel painful and move slightly when touched. A Baker’s cyst is also known as a popliteal cyst.
It is often associated with another knee condition, such as rheumatoid arthritis, osteoarthritis, a meniscus injury, or cartilage degeneration.
What to do: Treatment should be guided by a healthcare provider or orthopedic specialist. According to the American Physical Therapy Association Clinical Practice Guideline, exercise therapy is the primary treatment for patellofemoral pain syndrome, with hip- and knee-strengthening exercises recommended to improve pain and function.
Activity modification, ice, and pain-relieving or anti-inflammatory medications may also help relieve symptoms. Surgery is rarely needed and is generally considered only when symptoms do not improve with conservative treatment or when an underlying structural problem is present.
12. Patellofemoral pain syndrome
Knee pain when bending may also be caused by patellofemoral pain syndrome, which affects the joint between the kneecap and the thighbone. It may be associated with flat feet, knee injuries, or cartilage degeneration.
Patellofemoral pain syndrome usually causes pain at the front or inner part of the knee. The pain often worsens when going up or down stairs. Other symptoms may include swelling or popping and cracking sensations when moving the knee.
What to do: Treatment should be guided by a healthcare provider or orthopedic specialist. Physical therapy is usually the main treatment and focuses on strengthening and stretching the muscles that support the knee and improve alignment.
Activity modification, ice, and pain-relieving or anti-inflammatory medications may also help relieve symptoms. Surgery is rarely needed and is generally considered only when symptoms do not improve with conservative treatment or when an underlying structural problem is present.
When to see a doctor
See a healthcare provider if you have:
- Knee pain when bending that lasts longer than three days, even with rest and ice
- Severe or persistent pain
- Swelling, redness, or deformity of the knee
- Difficulty standing, walking, climbing stairs, or performing everyday activities
- Painful popping, locking, or catching of the knee
- An inability to bend or straighten the knee
- Fever
The healthcare provider may order imaging tests, such as an X-ray or MRI, to help determine the cause of the pain and confirm the diagnosis. Treatment will depend on the underlying condition.