What is the test for determining damages for a plaintiff's injuries in a motor vehicle accident?

The defense has argued and I accept as correct the proposition that where the plaintiff suffers from a pre-existing condition, a court must be careful not to put the plaintiff in a better position than he or she was in prior to the accident. In other words, the defendant ought not to pay for something which was not the defendant's fault. The defense also argues that the plaintiff's non-pecuniary damages should be reduced from the norm to account for the fact that the physical restrictions of the plaintiff complains of, are and will be in large measure, attributable to factors unrelated to the accident, in particular, the plaintiff's pre-existing left hip problem. In advancing this argument, the plaintiff relies on the words of Sopinka J. in Athey v. Leonati, (1996) 1996 CanLII 183 (SCC), 3 S.C.R. 458: " Separation of distinct and divisible injuries is not truly apportionment; it is simply making each defendant liable only for the injury he or she has caused, according to the usual rule. Separation is also permitted where some of the injuries have tortious causes and some have non-tortious causes. Again, such cases merely recognize that the defendant is not liable for injuries which were not caused by his or her negligence...." Again, I agree that in assessing the plaintiff's non-pecuniary damages, I must take into account the fact that her hip problem is not related to the fall.