Keratotomy was first performed by surgeons in Europe and the United States in the late 1800s, and the basic optical and mechanical principles of the operation were defined in the 1940s and 1950s by the Japanese doctors T. Sato and K. Akiyama, who used anterior and posterior corneal incisions. The posterior incisions damaged the cornea, and the procedure was modified in the Soviet Union by doctors Fyodorov and V. Durnev to include incisions in only the anterior cornea. Since its introduction into the United States in 1978, numerous ophthalmologists have modified the procedure by introducing technical and surgical improvements such as ultrasonic methods to measure the thickness of the cornea and the use of diamond-bladed micrometer knives to make the incisions.