My advice is - you don't. You most probably have another unidentified selection effect
at work in your sampling frame.
An effect so large in a common complaint would have been so obvious that you would
not have to ask the question, so there must be something peculiar with your data.
Now if only a fraction of 0.1% of the population develop arthritis then difference
in rates might have gone unnoticed, but even then it is difficult to believe that half
of the patients at an arthritis clinic being LHed would have not been noticed decades