The prevalence of adolescent knee pain is 33%, and patellofemoral pain (PFP) is the most common diagnosis with a nontraumatic onset. The 5-year prognosis of adolescent PFP compared with other types of knee pain is unknown.

In 2011, a cohort of 2200 adolescents aged 15 to 19 years answered an online questionnaire on musculoskeletal pain. Of these, 504 reported knee pain, and 153 of these were clinically diagnosed with PFP. Now, after 5 years, the 504 adolescents, as well as 252 randomly selected adolescents who did not report knee pain in 2011, will be contacted again to gain insights into their prognosis.

From the 504, 153 were diagnosed with PFP, the criteria for which included:

- insidious onset of anterior knee or retropatellar pain for longer than 6 weeks that was provoked by at least 2 of the following: prolonged sitting or kneeling, squatting, running, hopping, or stair climbing; tenderness on palpation of the patella; pain when stepping down; or double-leg squatting.

Exclusion criteria were:

• concomitant injury or pain from the hip, lumbar spine, or other knee structures
• self-reported patellofemoral instability
• knee joint effusion.

In September 2011, students at 4 upper secondary schools in Aalborg were invited to answer an online questionnaire and to be part of the APA2011 cohort. From 2846 potential responders, 2200 adolescents responded to the questionnaire, corresponding to a response rate of 77%. A total of 504 adolescents indicating knee pain at least monthly were successfully contacted (a response rate of 83% of those who reported their telephone numbers) and were asked standardized questions on the telephone. This forms the cohort of 504 adolescents with knee pain. In addition a random selected group of adolescents without knee pain in 2011 will be contacted and asked the same questions as those with knee pain.