Knee injuries: Injuries such as ligament sprains (such as an anterior cruciate ligament or ACL tear), meniscus tears, or fractures can cause knee pain. These injuries often occur during sports activities, accidents, or falls.

Osteoarthritis: Osteoarthritis is a degenerative joint disease that commonly affects the knees. It occurs when the protective cartilage on the ends of the bones wears down over time, leading to pain, stiffness, and reduced range of motion.

Tendinitis: Tendinitis is inflammation or irritation of the tendons in the knee, often caused by overuse, repetitive movements, or sudden excessive force. This can result in pain and swelling in the affected area.

Bursitis: Bursitis is inflammation of the fluid-filled sacs (bursae) that cushion and lubricate the joints. Inflammation of the bursae in the knee can cause pain and swelling.

Patellofemoral pain syndrome: This condition, also known as runner's knee, involves pain around the front of the knee, especially during activities that involve bending the knee, such as running, squatting, or climbing stairs.

IT band syndrome: The iliotibial (IT) band is a thick band of tissue that runs along the outside of the thigh, from the hip to the shin. Overuse or tightness of the IT band can lead to pain on the outside of the knee.

Knee cap dislocation: The kneecap (patella) can dislocate from its normal position, causing pain, swelling, and instability in the knee.

Osgood-Schlatter disease: This condition primarily affects growing adolescents and causes pain and swelling just below the kneecap, where the patellar tendon attaches to the shinbone.

Rheumatoid arthritis: Rheumatoid arthritis is an autoimmune disease that can affect multiple joints, including the knees. It causes chronic inflammation, pain, swelling, and stiffness in the affected joints.

Other causes: Other factors that can contribute to knee pain include gout, infections, tendon or ligament strains, cartilage damage, certain systemic conditions, or referred pain from the hip or lower back.