Arthritis – rheumatoid arthritis, osteoarthritis, gout, and other connective tissue disorders like lupus.

Baker's cyst – when the posterior region of the knee is inundated with fluid, swelling and arthritic inflammation occurs. This engorged cyst can burst, causing pain to travel down the back of the calf.

Bursitis – symptoms include inflammation from constant knee stress (e.g., prolonged kneeling, excessive joint use, or injury).

Disorders of the hip – pain causing ailments which affect the knee. The Iliotibial Band Syndrome is one such example. This injury occurs in the thick band of tissue that extends from the hip to the knee exterior.

Joint infection.

Kneecap dislocation.

Knee injuries – these injuries can result in internal knee bleeding, thereby aggravating the pain.

Strains and sprains – these are minor ligament injuries that are caused by abrupt or irregular joint wrenching motions that may cause ligament trauma.

Tendinitis (tendonitis) – characterized by anterior knee pain that is exacerbated by activities like going up or down stairs and other inclines. Athletes like runners, skiers and cyclists are more prone to this disease than most.

Meniscus tears (cartilage tears) - these type of injuries result in pain in the interior or exterior knee joint.

ACL tear (ligament tear) – the resulting pain and damage also causes the knee joint to become unstable.

Knee pain can also result from other less common illnesses which include:

Bone tumours
Osgood-Schlatter disease