Common causes of knee pain

Injuries: Injuries to the knee can cause knee pain. Fractures, ACL injury, Torn meniscus, etc. are common causes.
Knee bursitis: When the small fluid-filled sacs that cushion the joints, called the Bursa, become inflamed, they cause pain.
Patellar tendinitis: Tendinitis is a condition that irritates of one or more tendons that attach muscles to bones.
Degeneration: Sometimes injury or degeneration of bone or cartilage can cause pain.
Arthritis: There are over 100 types of arthritis and conditions that commonly affect the knee are Osteoarthritis, Rheumatoid arthritis, Septic arthritis, Gout, and more. 

Common symptoms of knee pain

• Swelling and stiffness in the knee
• Redness and warmth 
• Weakness or instability in the knee area
• Popping or crunching noises from the knee joints
• Inability to fully straighten the knee

When should you consult a doctor for knee pain?

• Difficulty bearing weight on your knee 
• Unable to fully extend or flex your knee
• See a deformity in your leg or knee
• Fever accompanied by redness, pain, and swelling in your knee
• Severe knee pain with no visible injury