What is the test for determining whether a drill slipped and caused damage to the radial nerve?

10 The plaintiff's argument is that the damage to the radial nerve could not have occurred without negligence on the part of the defendant. The admission of the defendant that the drill slipped leads to an inference of negligent conduct on his part. In support of this position the plaintiff relies on Snell v. Farrell (1990), 1990 CanLII 70 (SCC), 4 C.C.L.T. (2d) 229, in which Sopinka J. per curiam, said at p. 244: In many malpractice cases, the facts lie particularly within the knowledge of the defendant. In these circumstances, very little affirmative evidence on the part of the plaintiff will justify the drawing of an inference of causation in the absence of evidence to the contrary. and, at p. 245: The legal or ultimate burden remains with the plaintiff, but in the absence of evidence to the contrary adduced by the defendant, an inference of causation may be drawn although positive or scientific proof of causation has not been adduced. These extracts do not support the plaintiff's argument: they deal only with the issue of causation, not whether negligence has been proven.