Background The methodological quality of studies on treatment of anterior cruciate ligament (ACL) injuries in skeletally immature children after ACL injury is low, and no prospective studies have evaluated the functional outcomes following a non-operative treatment algorithm.

Purpose To report changes in knee function and activity level in skeletally immature children following a non-operative treatment algorithm for a minimum of 2 years after ACL injury.

Study design Prospective cohort.

Methods 46 skeletally immature children aged 12 years and younger were evaluated at baseline and subsequent yearly follow-ups using patient-reported outcome measurements, isokinetic muscle strength measurements, single-legged hop tests and clinical examinations over a minimum period of 2 years. Participation in physical activities was monitored using a monthly online activity survey, and the main leisure-time sport activity was registered at the yearly follow-ups.

Results 36 (78%) of the children did not undergo an ACL reconstruction during the follow-up. Statistically significant changes with questionable clinical relevance were discovered with the patient-reported outcome measurements or hop tests. Leg symmetry indexes were consistently above 90% for muscle strength and single legged hop tests throughout the study, and the isokinetic muscle strength improved significantly in the injured limb. Ninety-one per cent maintained participation in pivoting sports and/or physical education in school, although 38% of the ACL deficient children changed their main activity from a level 1 to a level 2 activity.

Conclusions A non-operative treatment algorithm may be appropriate for ACL injured skeletally immature children, although a reduced participation in level 1 activities may be necessary for some children.