Outcomes encompass multiple factors, not just success and failure. They include other considerations such as being workable and profitable to the dentist and safe and acceptable to the patient. The more biological outcomes are most frequently measured. These measurements are on a scale from least to most predictable outcomes of treatment by the dentist, as shown on the evidence pyramid. Impact trauma is an example of a general scarcity of high-level evidence with treatment because of the nature and individuality of the injuries and the difficulty (or impossibility) of designing controlled clinical trials in humans. Much of the evidence is cohort (retrospective or observational studies), case reports, animal studies, and "expert" opinion. Thus, guidelines are largely based on lower levels of evidence. Nonetheless, the current guidelines provide the clinician with the best evidence that is available. The best opportunity for randomized controlled trials is in prevention. Specifically, mouthguard design and usage can be subjected to higher levels of controlled research. Revitalization and regeneration are relatively new procedures. To date, there is a lack of high levels of evidence to be able to substantiate whether these are clinically feasible to become a routine in practice.