Objective: To demonstrate factors that can affect interpretation of ACL tear using anterior tibial translation sign and to compare the cut-off value of anterior tibial translation sign in the present study with previous studies.
Material and Method: This was a retrospective descriptive research study. The authors included all patients who underwent both MRI and arthroscopy of the knee in King Chulalongkorn Memorial Hospital from January 2002 to March 2010. Anterior tibial translation distance was measured. For patients with intact ACL, tests for correlation between anterior tibial translation distance and demographic data were performed. For patients with ACL tear, a receiver operating characteristic (ROC) analysis was performed in order to determine the best cut-off value for an anterior tibial translation sign.
Results: One hundred seventeen patients were enrolled in this study and classified as follows: intact ACL (n = 58), partial ACL tear (n = 19), and complete ACL tear (n = 40). Anterior tibial translation distances for each subgroup were 1.5, 5.0, and 7.6 mm, respectively. Significant mean distance differences for each pair of subgroups were found. No significant correlations between anterior tibial translation distance and sex, height, and weight were found. There was, however, a significant correlation between anterior tibial translation distance and age. For diagnosis of partial and complete ACL tear, cut-off distances of 3.5 mm and 5.5 mm provided the best accuracy, respectively.
Conclusion: There is correlation between anterior tibial translation distance and age. The authors may possibly imply that, using anterior tibial translation distance in young age group patients for diagnosing ACL tear may increase the false-positive rate. The authors introduce a cut-off distance of 3.5 mm to classify patients as having intact ACL or ACL tear.

Keywords: Anterior cruciate ligament (ACL), Anterior tibial translation, Arthroscopy, Magnetic resonance imaging (MRI)