Here's my suggestion:
Don't think about "questions that need to be answered." Think about "what kinds of issues can I reasonably investigate?"
First step is to look at what your resources are where you are doing your research. Is there a particular type of surgery that they perform a whole lot of? Do they keep a database of patients with validated outcome scores assessed at set time points after surgery? Do they have a cadaver lab with access to bodies and equipment that could be used for research? Do they have biomechanical testing equipment? What are your mentor's areas of interest?
Once you figure that out, then you can start formulating a research question. The question should be narrowly focused. "Is there neurological re-growth into an ACL graft?" is a very good research question (though likely difficult to answer). "Efficacy of hip scopes for femoro-acetabular impingement syndrome" is a very very bad question. The usual error people make is to come up with a research question that is extremely broad. Narrow. You want narrow.
Once you think you have a research question, you need to assess how much time you need versus how much time you have. The best studies are prospective, but these take a long time to complete. I am wrapping up a year of research. Two of my prospective studies have taken almost 6 months just to get approved by the IRB. They will likely still be going when I graduate (or they will have died). Retrospective studies are quicker, but you need to know if the charts/databases you are reviewing have the data you need to answer your question (attending surgeons frequently over-estimate the quality of their documentation; be weary of anyone who tells you that ROM, strength, and neurologic exams are all documented in the chart). Cadaver studies and biomechanical studies are very fast, especially if you have a technician who can help you with the specimens and machinery. However, they are very resource intensive, and many places do not have the infrastructure necessary to perform these studies.
Finally, make sure you do a very thorough literature review to see what others have published on your topic. There is nothing wrong with repeating someone else's study (especially in surgery, where the "surgeon" is an important but often under-appreciated variable). But you need to know what's been done, because it may prompt additional questions that you could answer in the same study.
Good luck.