Patellofemoral pain syndrome, also known as patellofemoral syndrome, is characterized by pain with weight bearing on a flexed knee, pain in the front of the knee or anterior knee pain, or pain around or behind the patella. Related terms that are often used interchangeably include retropatellar pain syndrome, patellofemoral syndrome, anterior knee pain, runner's knee, or patellofemoral chondromalacia. It usually affects adolescents and young adults; it is more common in girls and women than in boys and men. It is the most common cause of knee pain. Because it is so common and patients present to many different specialties (such as primary care, orthopedics, sports medicine, family medicine), it is difficult to estimate the true prevalence.

There are many biomechanical and biological factors that may predispose to patellofemoral syndrome:

• Increased Q angle (eg, the angle formed by the line from anterior superior iliac spine to mid-patella and the line from mid-patella to tibial tuberosity that represents one measurement of coronal plane patellar alignment)
• Short / tight hamstrings (patients with patellofemoral pain syndrome can have tightness or lack of flexibility in the hamstrings, which can contribute to anterior knee pain; stretching exercises for hamstrings is a crucial part of management)
• Decreased quadriceps and hamstring strength are associated
• Elevated contact pressures between the patella and femoral groove (trochlea)
• Increased mileage in runners will predispose
• Chondromalacia or softening of the cartilage of the patellofemoral joint