Keywords are common use terms. You've probably seen them in textbooks or used them when talking with classmates.
It is okay to use Google or Wikipedia to find keyword terms since you're looking for terms that people commonly use. If you're only using them to search (and not incorporating it into your assignment), you won't need to cite Google or Wikipedia as a source.
Brainstorm keywords and technical terms to use in your search before you start searching in databases
Anterior cruciate ligament OR ACL
(You don't need to put OR in capitals, but it makes it easier to differentiate between the two terms.)
Tip: Put one idea (and its variant terms) per line. It keeps things organized.
MeSH = Medical Subject Headings (specific medical terminology)
Many medical databases use these MeSH terms as consistent wording. Instead of using a bunch of different words (ultrasound, sonography, ultrasound scan, etc.) to describe the same thing, they use one word consistently. In this case, it's ultrasonography.
Look up the MeSH term using Medline:
Search for ultrasound in MeSH 2015, and you'll find that ultrasonography is the term they use. Check it off, hit "Search Database" and it'll pull up everything they have on the topic (63,000+ articles)
Use MeSH to structure your search by using two search features:
Explode = includes all the "smaller" terms nested under the subject heading in your search
For example, exploding "Ultrasonography" will include methods, instrumentation and other narrower aspects of the subject
Major concept = only finds articles where this subject heading is a major point of the article
How is ultrasound used to diagnose a torn ACL?
Key parts: ultrasound, ACL injury