Try Subject Searching
You are probably most used to keyword searching, in which you brainstorm search words and various synonyms.
Subject searching can be used in addition to or as an alternative to keyword searching. If you use the correct subject heading, you will capture synonyms automatically (e.g. "brain injuries" will find "brain injury" and "concussion"). How do you know what the correct subject heading is? You can look them up in many databases.
- In PubMed, change the drop-down to "MeSH".
There are people whose job it is to read articles and assign subject headings to them (i.e. decide what they are "about"). It's kind of like assigning a hashtag to something in Twitter and then being able to search by that hashtag (...kind of).
Find Primary Research Articles
There's no perfect way to find primary research articles.
Most primary research articles will describe a study that has been done, so you're best bet is to use the word "study" as one of your search words.
Some databases (e.g. PubMed) have a special limit for different types of articles.