Tort principles in Motor Vehicle Accident Benefit cases in Ontario
In the Ontario Court of Appeal decision of Monks v. ING, the Court of Appeal affirmed the conclusion of the trial judge.  The case dealt with Statutory Accident Benefits (SABS) and the application of the tort principles of crumbling skull and thin skull. The court concluded that these tort principles have no place in the assessment of accident benefits.  The entitlement to Accident Benefits flows not from tort principles but from insurance contract principles, specifically, motor vehicle insurance policies.  The entitlement to SABS is a matter of contract, and the Court recognized, that there is nothing in the SABS that allows for a reduction or apportionment of a SABS payment based on the degree of contribution of the accident to the condition of the insured person.  The Court applied a simple test.  If the accident contributed to the over-all medical/health condition of the insured, then the full amount of the SABS benefit is payable and the insured is entitled to full benefits without deduction.  This is the case even though the injured person may have had health issues prior the accident.  
The tort principles of “thin skull” and “crumbling skull” are interesting principles.  Both principles arise from concepts of negligence and indemnification of personal injury victims.  SABS entitlement is not based on negligence at all. SABS benefits is entirely contract based and therefore the only real issue is whether or not the accident caused injury and if so, then the full amount of SABS is available to treat the whole person regardless of their “condition” at the time of the accident.
The thin skull rule, sometimes called the eggshell skull rule means that the tortfeasor (person who was negligent/responsible for the accident) must take the accident victim as they are at the time of the accident, even if they are particularly susceptible to injury (such as having fragile bones or pre-existing depression). It is a well established legal principle in Ontario.  The crumbling skull rule is a similar legal principle. It is like the thin skull principle, that limits a tort defendant’s exposure to a plaintiff’s injuries to the plaintiff’s condition at the time of the tort. In the case of the crumbling skull principle, a person who is in a stable condition before the accident and – but for – the accident would have remained so, but suffers injuries that continues to deteriorate because the accident has accelerated the injuries / medical conditions. When applying the crumbling skull principle, the defendant may not be held responsible for the whole of the post-accident medical conditions of the injured person. Also, the defendant’s actions would not be considered as having been the sole cause of the entire post-accident medical conditions of the plaintiff. The defendant’s actions are considered as merely an aggravating cause of the injuries. 
Marc Quinn
Ottawa car accident and injury lawyer