Are adhesions possible after subacromial decompression arthroscopic surgery?

Yes - according to my surgeon.

I am not a doctor, but my source is my surgeon who uses a preparation called Hyaluronan which replaces lost synovial fluid after surgery. One of the benifits he cites is a lower instance of adhesions following surgery. He has discussed this in the context of Subacromial Decompression.

More info here: http://www.trbchemedica.co.uk/joint-pain/viscoseal