Does a plaintiff have to prove that the accident caused or contributed to their degenerative changes?

Therefore, even where there are other potential non-tortious causes of an injury, such as degenerative changes, the defendant will still be found liable if the plaintiff can prove that the accident caused or contributed to the injury. The contribution must be material, in the sense that there is a substantial connection between the accident and the injury, beyond a de minimus range: Farrant v. Laktin, 2011 BCCA 336 at paras. 9-11.

In Blackwater v. Plint, 2005 SCC 58, McLachlin C.J.C. discussed the difference between causation as the source of the loss and the rules of damage assessment in tort, at para. 78: … The rules of causation consider generally whether “but for” the defendant’s acts, the plaintiff’s damages would have been incurred on a balance of probabilities. Even though there may be several tortious and non-tortious causes of injury, so long as the defendant’s act is a cause of the plaintiff’s damage, the defendant is fully liable for that damage. The rules of damages then consider what the original position of the plaintiff would have been. The governing principle is that the defendant need not put the plaintiff in a better position than his original position and should not compensate the plaintiff for any damages he would have suffered anyway: Athey.