Lateral Rectus Muscle and Anterior Segment Optical Coherence Tomography (AS-OCT) in Two Recession Surgeries

* Background and study aims : The investigators conducted this study to compare the movement of extraocular muscle after two types of recession surgery with non-invasive tool called AS-OCT. * Who can participate? patients who will undergo two types (conventional method and hang-back method) of typical bilateral lateral rectus recession surgery for correcting intermittent exotropia * What does the study involve? Volunteers will attend a clinic for four visits over three months. They will undergo AS-OCT at every visits during follow up period. AS-OCT is non-invasive test to evaluate operation site. The test only requires their cooperation when they undergo AS-OCT. * What are the possible benefits and risks of participating? This is non-invasive test for your postoperative follow up. Therefore, there will be no risks about this test. * Where is the study run from? Samsung Medical Center * When is the study starting and how long is it expected to run for? From April 2015 to Dec 2015