Loose abutment screw will not tighten or back out: recommendations?

Authors

I have a patient with a cement retained implant bridge #6-11. The implant used was not appropriate for this application, and the abutment screws in #6 and 11 have apparently been loosening and causing problems for years. On both 6 and 11, the cement is holding, but the abutment screws are now loose again. I have accessed the abutment screws through the labial and can securely engage the hex. But, I cannot back out the screw or screw it down to tighten. I was trying to avoid cutting off the bridge. Do you have any recommendations? Thank you.