Board Thread:Questions and Answers/@comment-27127950-20151027213016/@comment-1210805-20160109031250

It mentioned them as examples when asking how Aizen would have made them without Urahara's Hōgyoku. The answer is simple, if he made modified Arrancars before gaining Urahara's Hōgyoku, he would have done so using his own Hōgyoku, which he had long before Urahara created his. While an Arrancar can be made without a Hōgyoku, I would think that Aizen's Espada were mostly made with the Hōgyoku, making them more powerful.