I have left the crani flap loose or hinged before and not had problems.  It generally seals down when the ICP goes down.  As long as the flap edge doesn't put upward pressure on the skin, which will break down the wound, it's fine.  I don't leave the dura open though.

Thanks holly. Do you have any reference justifying this practice. The American Guidelines do suggest this as an option but dosent quote any study on the same.

Craniotomy bone flap can be put loosely...if underlying brain bulge or increased ICP is there.. proper duroplasty is ideal before keeping the flap..

As CSF leakage induce meningeal infection, meninx should be closed tightly for CSF leakage sometimes with an aid of artificial meninx. Bone flap could be placed subcutaneously at the abdomen for later use. 

As I have described, most important complication after closing brain surgery is to avoid intra cranial high pressure and leakage of CSF. I have suggested the way to avoid the two complications after surgery.  

Dear Dr Ito

Many thanks for your participation.

Would it be surprising if I said that there has been no CSF leak inspite of keeping the the dura open in my practice. And that is precisely the question - why should we need to a tight dural closure.  I would appreciate if you could give relevant reference.