Osteoarthritis: This is a common condition, especially in older adults, characterized by wear and tear of the joint cartilage, leading to pain, stiffness, and limited mobility. The knee is one of the most frequently affected joints.

Meniscal Tear: A tear in the cartilage of the knee, which can cause pain, swelling, and locking of the knee. It's common in individuals who participate in sports or activities that involve twisting or bending.
Ligament Sprain: Injury to the ligaments surrounding the knee, often due to sudden stops, changes in direction, or direct blows to the knee. Symptoms include pain, swelling, and instability.
Tendinitis: Inflammation of the tendons, which connect muscles to bones. Around the knee, patellar tendinitis (jumper's knee) is common, especially in athletes who frequently jump.
Bursitis: Inflammation of the fluid-filled sacs (bursae) that cushion the knee joint, often caused by repetitive motion or direct pressure on the knee.

Septic Arthritis: A bacterial infection within the joint space, which is a medical emergency requiring prompt antibiotic treatment. Symptoms include severe pain, swelling, redness, and warmth of the knee, along with systemic symptoms like fever.
Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the leg, which can cause pain, swelling, and warmth. DVT is a risk for pulmonary embolism, a potentially life-threatening condition.
Osteonecrosis (Avascular Necrosis): Death of bone tissue due to a lack of blood supply, which can cause knee pain and is more common in individuals with a history of steroid use, excessive alcohol consumption, or severe trauma.

Rheumatoid Arthritis: An autoimmune disorder that causes inflammation and pain in the joints, including the knees. It's less common than osteoarthritis but can cause significant disability if not treated.
Gout: A form of inflammatory arthritis characterized by sudden, severe attacks of pain, swelling, redness, and tenderness, often occurring at the base of the big toe but can also affect the knee.
Plica Syndrome: Irritation of the synovial folds within the knee joint, which can cause pain and snapping sensations, especially with knee movement.
Osgood-Schlatter Disease: A condition that affects the upper part of the shinbone (tibia) where the patellar tendon attaches, causing knee pain in adolescents, especially during growth spurts.