BACKGROUND: The treatment of combined ANTEROLATERAL posterolateral rotary instability has been done by correcting knee alignment, anterior cruciate ligament (ACL) reconstruction plus repair or reconstruction of the posterolateral complex. Because of the technical difficulties encountered in these operations and the need for more than two stages, and considering the controversy among the role of posterolateral complex (PLC) in valgus knees, this study was designed to observe the results of treating this instability by ACL-reconstruction alone, after correction of varus, without reconstruction of the posterolateral complex or further extra-articular manipulation.METHODS: This was a clinical trial performed on 29 patients (29 knees) with combined ANTEROLATERAL posterolateral rotary instability. Subjective and objective instability signs were recorded. Arthroscopy was then performed and a valgus osteotomy was done to correct alignment. Then in a second stage, an ACL-reconstruction was carried out. Results, after a mean of 23 months follow-up, were compared to the conditions before surgery. Fisher exact test, X2 and Wilcoxon tests were used to analyze the data. P<0.05 was considered to be meaningful.RESULTS: Pain was relieved in more than half and locking was improved in all of the patients. Giving way of the knee was diminished from 79.3% to 6.9%. Special instability tests showed a significant improvement after surgery (P<0.001). Most of the patients returned to the preinjury level of work or sports.CONCLUSIONS: Based on the results of this study, ACL-reconstruction alone, after correction of varus, can be sufficient to address this combined knee instability without farther procedures on extra-articular structures and posterolateral complex, thus avoiding unnecessary complications and longer rehabilitation.