The Ninjaken, also known as the Ninjato or Shinobigatana, are blades characterized by being straight, chiseled edged swords. They appear to be largely a media creation, based on the design of the Wakizashi, or perhaps the Chokutō.
What sets a Ninjaken apart from more commonly recognized blades such as Katana are the shorter blade, straight edge, and square guard. This was said to be a more efficient and sensible weapon for the ninja, who relied heavily on stealth tactics.
Leonardo is commonly shown using Katana throughout most continuities. In the 2007 film, however, his blades were clearly rendered as being straight with square guards. Confusion between the two different swords may be since the Ninjaken and Katana have minute differences that do not affect their combat effectiveness, and the fact that both weapons share the same cutting power and penetration.