Which type of knife is spear-shaped with cutting edges on both sides of the blade?

The correct answer is the interdental knife, which is specifically designed for use in periodontal procedures. It features a spear-shaped blade with cutting edges on both sides, allowing for effective access and cutting in interproximal areas between teeth, especially in the presence of gingival tissue. This design is beneficial for achieving clean and precise cuts in interdental spaces, where traditional straight blades may not be as effective.

In contrast, a surgical blade, while also used in various surgical procedures, generally does not have the same specialized shape and dual cutting edges. A periodontal knife, although used for similar purposes, may not specifically have the same spear-shaped design as the interdental knife. A scalpel, while sharp and effective for cutting tissues in a general sense, typically features a single cutting edge and is not specifically designed for interdental use. Thus, the interdental knife's unique shape and cutting capabilities make it the optimal choice for accessing and treating areas within the dental arch.