Knox and her former boyfriend Sollecito are on trial in Perugia, central Italy, on charges of murder and sexual violence in the death of Knox's roommate Meredith Kercher. Both deny wrongdoing.
Introna, who was called to the stand by Sollecito's defense lawyers, testified that the cut on Kercher's neck was made with a knife with a 3-3 1/2-inch-long (8-9 centimeters long) blade, ANSA said.
Prosecutors say that a 6 1/2-inch (16.5 centimeters) knife found at Sollecito's house matched the victim's wounds and could be the murder weapon. They say the knife had Kercher's DNA on the blade and Knox's on the handle.
Prosecutors allege that Kercher was killed during what had begun as a sex game. Her body was found in the apartment she shared with Knox on Nov. 2, 2007.
A third person, Rudy Hermann Guede of the Ivory Coast, was convicted in a separate trial last year and sentenced to 30 years in prison. He denies wrongdoing and has appealed his conviction.
Introna said that the bedroom where Kercher was killed was too small and that it would be "physically impossible" that three people could have attacked her, ANSA said.