In 2013 Google launched its Hummingbird update focusing on the actual meaning of keywords. This changed the SEO world on multiple levels none of which more important than having proper keyword research that also focuses on the meaning of those keywords.
To rank probably after the Hummingbird update you don't only need to use certain keywords that have searches but also contextual words, like for example if you are talking about a certain medical condition you also need to integrate words like "treatment"; symptoms and so on.
Therefore here are a couple of tips and tricks for a better keyword research:
1. Answer questions in your content - there are a lot of keyword phrases that have a low number of monthly searches that are question-based. Example: " how to treat tooth abscess" - If your content focuses on having healthy teeth you may also want to include answers related to common questions regarding teeth. So also focus on searching those type of keywords and include them in your content;
2. Revitalize old content by re-doing a keyword research. Google loves when you update old articles with new content, it sees your struggles on creating relevant content and will rank you higher. Just because 2-3 years ago for a certain piece of content you didn't find that many keywords it doesn't mean that at the moment new keywords haven't evolved and increased in popularity;
3. Use common words and phrases, it's important to have exact technical keywords, but those words may not exist in the user vocabulary, do a separate keyword research for common words that people use.