It May be Osgood-Schlatters Disease? Osgood-Schlatter’s disease causes pain in the front of the knee. It is commonly seen in boys and girls between the ages of 9-16. It features a painful lump just below the knee, this is because of inflammation of the patellar ligament at the tibial tuberosity. The pain usually occurs during physical activity such as running, jumping, squatting and going up and down stairs. Who Is Affected? Around 75% of cases affect boys and occurs in up to 20% of sporty children compared to 4% of a group of all activity