What is the current state of the law on expert testimony in medical malpractice cases?

In Fisher v. R. at p. 19 Mr. Justice Aylesworth referred to the rules that an expert witness must be qualified in the fields within which he testifies and that he may not testify with respect to anything that is within the common stock of knowledge. He then continued "Subject to these rules, the basic reasoning which runs through the authorities here and in England, seems to be that expert opinion evidence will be admitted where it will be helpful to the jury in their deliberations and it will be excluded only where the jury can as easily draw the necessary inferences without it."