Background Recent innovation in ACL reconstruction has provided improved knee stability and function, however, the effect to decrease the incidence of the second ACL injury has not been established.

Objective To determine the incidence of the second ACL injury after anatomical ACL reconstruction and the associated factors.

Design Retrospective study.

Setting Patients who underwent primary ACL reconstruction from 2003 to 2010 in our institute.

Patients 177 patients received single-bundle reconstruction with a bone-patellar tendon-bone graft and 257 patients received double-bundle reconstruction with hamstring tendon grafts.

Risk factor assessment The patients who suffered the second ACL injury were searched by reviewing medical records.

Main outcome measurement Gender, age at the primary ACL surgery, graft type, traumatic event which caused second ACL injury, the period from the primary ACL surgery to the second ACL injury, and the stability of ACL-reconstructed knee and muscular recovery rate of quadriceps and hamstrings immediate before the second ACL injury were investigated in the patients with the second ACL injury.

Results 34 patients (7.8%) had reconstructed ACL tear and 32 patients (7.4%) had contralateral ACL tear. The second ACL injury frequently occurred in lower age patients, while was not associated with gender, graft type. All patients except one had the second ACL injury after return to sports. Before the second ACL injury, 92.4% ACL-reconstructed knees showed 1 mm or less of side-to-side difference in KT-1000 measurements, and 89.4% knees were negative in pivot-shift test. Good muscular recovery with more than 80% of healthy side achieved in 89.4% before the second ACL injury.

Conclusions The good results in either knee stability or muscular recovery after anatomically ACL-reconstructed knee does not guarantee safe return to sports or decreased risk of the second ACL injury.