6 Common Causes of Knee Pain and How to Identify Them

Whether you’re a weekend warrior who pushed it too hard, you feel a new ache on your daily walk, or you simply notice stiffness when you stand up, knee pain is a near-universal experience. The knee is a complex joint, and that "twinge" or "ache" can be a sign of anything from a simple strain to a more complex long-term condition.

Understanding the type of pain you have is the first critical step toward finding the right solution. This guide will walk you through six of the most common causes of knee pain, helping you identify the symptoms and understand what might be happening inside your joint.

Table of Contents

1. Osteoarthritis (OA)

2. Patellofemoral Pain Syndrome (Runner's Knee)

3. Meniscus Tear

4. Knee Bursitis

5. Ligament Injuries (e.g., ACL Tear)

6. Post-Traumatic Arthritis (PTA)

7. When to See a Medical Professional

We can generally group knee pain into two categories: acute injuries (from a sudden event) and chronic conditions (which develop over time). Here are some of the most common culprits.

1. Osteoarthritis (OA)

What it is: Often called "wear-and-tear" arthritis, Osteoarthritis is a degenerative condition where the protective cartilage in the joint wears down over time. As the bones lose their cushion, they can rub together, causing pain and stiffness.

How to Identify It (Common Symptoms):

• Pain that gets worse with activity, but may feel stiff after rest (like in the morning).

• A "grinding" or "grating" sensation (called crepitus) when you move the knee.

• Stiffness or a feeling of "locking" after sitting for long periods.

• Swelling and tenderness around the joint.

2. Patellofemoral Pain Syndrome (Runner's Knee)

What it is: This is a common overuse injury, not just for runners. Patellofemoral pain syndrome (PFPS) refers to pain around or under your kneecap (patella). It’s often caused by muscular imbalances or alignment issues that put extra stress on the kneecap.

How to Identify It (Common Symptoms):

• A dull, aching pain at the front of the knee.

• Pain that gets worse when walking up or down stairs.

• Pain after sitting for a long time with your knees bent (like in a car or at a movie theater).

• A "popping" or "cracking" sound when you bend or straighten your knee.

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3. Meniscus Tear

What it is: The meniscus is a C-shaped piece of tough cartilage that acts as a "shock absorber" between your thighbone (femur) and shinbone (tibia). A sudden, forceful twist or pivot can cause this cartilage to tear.

How to Identify It (Common Symptoms):

• A distinct "pop" at the time of the injury.

• Pain, swelling, and stiffness, which often get worse over the 2-3 days after the injury.

• A feeling of your knee "catching" or "locking" as you move.

• Feeling like your knee is "giving way" or is too weak to support you.

• According to the American Academy of Orthopedic Surgeons, these tears can be traumatic (from sports) or degenerative (in older adults).

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4. Knee Bursitis

What it is: Your knee has small, fluid-filled sacs called bursae that cushion the joint and reduce friction. Knee Bursitis occurs when one or more of these sacs becomes inflamed, often from repetitive kneeling (like gardening or plumbing) or a direct blow.

How to Identify It (Common Symptoms):

• A "squishy," swollen lump on the front of your kneecap.

• The area feels warm to the touch.

• Tenderness and pain, especially when you kneel or put pressure on the knee.

• Stiffness when moving the joint.

5. Ligament Injuries (e.g., ACL Tear)

What it is: Ligaments are tough bands of tissue that connect bones to each other. The four main ligaments in the knee (ACL, PCL, MCL, LCL) provide stability. An ACL injury is a common sports injury caused by a sudden stop, pivot, or jump.

How to Identify It (Common Symptoms):

• A loud pop or tearing sensation at the moment of injury.

• Severe pain and the inability to continue activity.

• Rapid and significant swelling within the first few hours.

• A strong feeling of instability, as if the knee will "give out" if you try to put weight on it.

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6. Post-Traumatic Arthritis (PTA)

What it is: This is a form of arthritis that develops after a physical injury to the knee, such as a ligament tear, meniscus tear, or fracture. Even after the initial injury heals, the joint damage can accelerate the wear-and-tear process, leading to osteoarthritis years later.

How to Identify It (Common Symptoms):

• The symptoms are nearly identical to standard osteoarthritis (pain, stiffness, swelling).

• The key identifier is the patient's history—the knee pain began months or years after a significant knee injury.

• According to experts, PTA is one of the leading causes of knee joint disability.

When to See a Medical Professional

While mild aches and pains can often be managed at home with rest, ice, and over-the-counter medication, you should not ignore persistent or severe pain.

See a doctor if you experience:

• You cannot put your full weight on the knee.

• You have severe swelling or a visible deformity.

• You heard a "pop" and your knee immediately gave out.

• You have a fever, redness, and warmth around the joint (a sign of infection).

• Your pain is chronic and interferes with your daily life.

A medical professional can provide an accurate diagnosis through a physical exam and imaging (like an X-ray or MRI), setting you on the path to the correct treatment.

Once you’ve identified the potential cause of your pain, the next step is finding relief. Explore the full roadmap to recovery in our Ultimate Guide to Understanding and Treating Knee Pain.

(Disclaimer: This article is for informational purposes only. This does not substitute medical treatment. Always consult a medical professional for a proper diagnosis and treatment plan.)