Patellofemoral pain syndrome (PFPS) aka Runners knee is the most common overuse injury of the knee. It is also known as:-

There is usually no specific history of trauma, however it may be related to biomechanical abnormalities and abnormal position and tracking of the patella (knee cap).

Patients often experience pain behind or deep in knee. The pain is often aggravated during activities that require flexion of the knee whilst under loading such as climbing stairs, walking down slopes or stairs, squatting or prolonged sitting. Crepitus (grinding) around the patella may also be present and can sometime be felt when moving the joint.