Square-shaped burger paddies have surpassed charbroiled slabs of meat, as The Wendys' Co. (NASDAQ: WEN) leapfrogged Burger King Holdings Inc. (NYSE: BKC) to become the second-largest U.S. hamburger chain, Technomic Inc.'s Top 500 Chain Restaurant Report said Monday.

The Dublin, Ohio-based fast food joint recorded U.S. sales of $8.5 billion, eclipsing Miami-based Burger King's $8.4 billion. Both are still far behind McDonald's Corp. (NYSE: MCD), with $34.2 billion in sales in 2011.

Wendy's has focused on promoting more expensive burgers, promoting a premium line of sandwiches under CEO Emil Brolick.

Burger King revamped its menu and rolled out new French fries in an effort to boost flagging sales.

Sales for the nation's 500 largest chain restaurants grew 3.4 percent to $242 billion last year, a boost in growth over 2010's 1.8 percent.

Wendy's closed down a penny at $5.03 a share on Monday.