What is the case law on expert testimony in medical malpractice cases?

First, the fact that this argument was not raised at trial is fatal to its success in this appeal. Generally, this court will not entertain entirely new issues on appeal for fairness reasons: Kaiman v. Graham, 2009 ONCA 77, 245 O.A.C. 130, at para. 18; Whitby (Town) v. G&G, 2020 ONCA 654, at para. 9. Moreover, having presented and argued the case at trial on the basis that the standard of care depended on expert evidence (which is the typical approach in medical malpractice cases), it is not open to the appellant, after the trial judge rejected her expert evidence, to say for the first time on appeal, that the trial judge did not need expert evidence to decide the standard of care issues in this case.