Consider: I may have a medical record that doesn't indicate knee issues because I have correctly been diagnosed with no knee issues. Or I may have such a record because I have been incorrectly so diagnosed, despite the presence of knee issues. Or I may have such a record because I haven't been to a doctor about my knee issues in the years since they've developed, and so no opportunity has arisen for my knee issues to be documented. Simply from a medical record reflecting no evidence of knee issues, you cannot know which of these, if any, is true.
As I mentioned in a reply to your neighbor comment, clinical studies are designed to exclude such confounders as these - and that's a considerable part of why such studies are so expensive to design and carry out. It's very difficult to see how such exclusion could be achieved with data of the quality which seems to be involved here.
Do you mean that it's not sufficient to have a medical record that doesn't indicate knee issues? That you would need a medical record that confirms there are no knee issues?