PFS is an abbreviation of 'patellofemoral syndrome'.
A group of symptoms
PFS is a group of symptoms relating to pain being felt in the front of the knee. It is a non-specific diagnosis going by several different names, but patients tend to have a pattern of dynamic valgus at the knees (Knock knee) with flat feet. It may be more frequent in adolescents who are rapidly growing.
Patellofemoral pain syndrome. Petersen W, Ellermann A, Gösele-Koppenburg A, Best R, Rembitzki IV, Brüggemann G-P and Liebau C. Knee Surg Sports Traumatol Arthrosc. 2014; 22(10): 2264–2274.