Evidence based ortho research is based on the following studies:
Evidence-based medicine pyramid. 
The levels of evidence are appropriately represented by a pyramid as each level, from bottom to top, reflects the quality of research designs (increasing) and quantity (decreasing) of each study design in the body of published literature. For example, systematic reviews are higher quality and more labor intensive to conduct, so there is a lower quantity published.
Cohort
Cross Sectional
Systematic studies
RCT