Most people that sit behind the desk end up with their knees rotating outward.  This outward rotation for long periods of time could cause back issues.

Easy fix is to use a strap to hold your legs hip distance apart.  But make sure you introduce this to your body slowly.  May be 1 hour a day for a week, then 1.5-2 hours, the second week, and so on...  Otherwise, your body might scream at you.  

Let me know if you have any questions.  And if this has been helpful.