Following detailed preoperative examination , Group I underwent conventional cross-linking ( 36 patients , 3 mW/cm ( 2 ) for 30 min ) ; Group II ( 36 patients , 9 mW/cm ( 2 ) for 10 min ) , Group III ( 33 patients , 18 mW/cm ( 2 ) for 5 min ) , and Group IV ( 33 patients , 30 mW/cm ( 2 ) for 3 min ) underwent accelerated cross-linking .