How does PFS happen?

The vastus medialis (VM) muscle is usually underdeveloped in the majority of the population.  The VM serves to prevent the knee cap from sliding too far to the outside of the leg.  This happens when the vastus lateralis muscle becomes too strong and pulls on the knee cap.  When the knee cap rubs against the femur, the cartilage that lines the bottom of the knee cap begins to wear away causing bone to rub on bone and creates pain in the front of the knee.  This can also cause minimal swelling which can decrease  the function of the entire quadriceps group and create more pain.