Background The incidence of lateral patellar dislocation becomes higher with increased participation in sports activities in adolescence. A screening test to diagnose asymptomatic lateral patellar instability could be useful for prevention of the sports-related patellar dislocation, however, has not been established.

Objective To quantitatively analyze patellar tracking in the adolescent population using a video system.

Design Cross-sectional study.

Setting Healthy students participating sports activities in 2nd year junior high school.

Participants 38 female and 49 male students with no past traumatic history or complaints of the knees.

Assessment of risk factors The subject was seated on a testing table with reflective markers placed on the medial and lateral femoral condyles (knee markers). To identify the medial and lateral edge of patella, the examiner pinched the patella with the thumb and index finger with reflective markers attached to the finger tips (patellar markers). The subject actively extended the knee from 90° to full extension, and the examiner followed the patella. A frontal view of knee joint was recorded with a digital video camera.

Main outcome measurements The patella position was defined as patellar lateral index (PLI), which was a percentage of the distance from the medial knee marker to the midpoint of the medial and lateral patellar markers divide by the distance between the medial and lateral knee markers. The PLI at 0% and 80% of extension phase (approximately 90° and 20° of flexion) was determined.

Results The average PLI was 57.0 ± 2.8% and 55.7 ± 4.1% at 0% and 80% of extension phase, respectively. Four students showed patellar lateral shift with more than 5% increase in PLI according to the extension.

Conclusion The excessive increase in PLI at the knee extension might be a risk factor for the patellar dislocation. Follow-up study is required to determine the relationship between PLI and the clinical presence of patellar instability.