Perhaps you should try some spelling variations:
"donor retention" » "poor retention", "donor selection", "dauer retention"
"donor resection" » "donor selection", "donor section", "tumor resection"
"air retention" » "a retention", "water retention", "user retention"

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

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("donor retention") OR ("donor resection"))) OR ("air retention")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"donor retention\") OR (\"donor resection\"))) or (\"air retention\"))".
• Removing quotes may allow a broader search: (((( donor retention ) OR ( donor resection ))) or ( air retention )).
• Adding a wildcard symbol may retrieve word variants: (((("donor retention") OR ("donor resection"))) or ("air retention"))*.