Perhaps you should try some spelling variations:
decrease periodontal » increased periodontal (Expand search), disease periodontal (Expand search), diseases periodontal (Expand search)
periodontal effect » periodontal effects (Expand search), periodontais effect (Expand search), periodontal defect (Expand search)
decrease fear » decreased fear (Expand search), decrease fetal (Expand search), decreased heart (Expand search)
fear decrease » break decrease (Expand search), merr decreases (Expand search), can decrease (Expand search)
decrease that » decrease the (Expand search), decrease at (Expand search), decrees that (Expand search)
that decrease » that increase (Expand search), that increases (Expand search), not decrease (Expand search)
in decrease » in decreased (Expand search), on decrease (Expand search), pain decrease (Expand search)

You may be able to get more results by adjusting your search query.

• The words AND, OR and NOT may confuse the search; try adding quotes: "(( decrease fear decrease in decrease ) OR ( decrease that decrease periodontal effect ))~".
• Adding a wildcard symbol may retrieve word variants: (( decrease fear decrease in decrease ) OR ( decrease that decrease periodontal effect ))*.