Patellofemoral pain syndrome is a group of symptoms related to the joint between the kneecap (patella) and its underlying groove in the femur bone, as well as the soft tissues related to the patella, such as the fat pad.

Causes of patellofemoral pain syndrome
In reality, although the pain is experienced in the front of the knee, the underlying cause may be remote - such as a hip problem or a bony rotation in the hip or thigh, or even a structural problem in the foot and ankle.
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CLINICAL PUBLICATIONS
Examination of the patellofemoral joint. Manske RC and Davies GJ. Int J Sports Phys Ther. 2016 Dec; 11(6): 831–853.
Patellofemoral pain syndrome in female athletes: A review of diagnoses, etiology and treatment options. Molly Vora,1 Emily Curry,2 Amanda Chipman,3 Elizabeth Matzkin,4 and Xinning Li1 Orthop Rev (Pavia). 2017 Dec 14; 9(4): 7281.
Patellofemoral pain in athletes: clinical perspectives. Halabchi F, Abolhasani M, Mirshahi M and Alizadeh Z. Open Access J Sports Med. 2017; 8: 189–203.