Osteoarthritis: This is the most common cause of knee pain, especially in older adults. It is characterized by wear and tear of the joint cartilage, leading to pain, stiffness, and limited mobility.

Ligament Sprains: Injuries to the ligaments surrounding the knee, such as the anterior cruciate ligament (ACL) or medial collateral ligament (MCL), can cause knee pain, especially in athletes or individuals who engage in physical activities.
Meniscal Tears: Tears in the menisci, which are cartilage structures in the knee joint, can cause pain, swelling, and locking of the knee.
Tendinitis: Inflammation of the tendons, such as the patellar tendon, can cause knee pain, especially in individuals who engage in repetitive activities like jumping or running.
Bursitis: Inflammation of the bursae, which are fluid-filled sacs that cushion the joints, can cause knee pain and swelling.

Septic Arthritis: A bacterial infection of the joint, which can cause severe pain, swelling, and fever. If left untreated, it can lead to joint destruction and long-term disability.
Osteonecrosis: A condition where the bone tissue dies due to lack of blood supply, which can cause severe knee pain and collapse of the joint.
Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the leg, which can cause knee pain, swelling, and warmth. If left untreated, it can lead to pulmonary embolism, which can be life-threatening.

Rheumatoid Arthritis: An autoimmune disease that causes inflammation and pain in the joints, including the knee.
Gout: A condition characterized by sudden, severe attacks of pain, swelling, and redness in the joints, often caused by elevated uric acid levels.
Pseudogout: A condition similar to gout, but caused by calcium pyrophosphate dihydrate crystals.
Osgood-Schlatter Disease: A condition that affects the growth plate in the knee, causing pain and swelling, typically in adolescents.