Osgood Schlatter's disease is an 'overuse' condition, where the attachment of the patellar tendon to the tibia bone becomes stressed.
The attachment area of the patellar tendon is called the tibial tuberosity (or tibial tubercle). The bony bump is actually an apophysis - ie a small distinct growth plate - so repetitive abnormal traction here can trigger bony enlargement of the bump. This is thus called a traction apophysitis but is more commonly called Osgood Schlatter's disorder (or 'disease').
How does Osgood Schlatter's disorder start?
Usually the patient developing a traction apophysis is a young person who engages in jumping sports, and who may over time become very distressed by pain at the tibial tuberosity due to traction on the growth plate. There is usually no single actual traumatic incident that starts it off, but rather just repetitive traction.
What triggers a flareup in Osgood Schlatter's?
The bony area is likely to be active in any growing youth, but any undue repetitive traction on the area can trigger inflammation. The condition is very common and it is usually the highly competitive youth that suffers from these flare-ups.