hello and welcome to HCM,
You seem to be having a unique problem but let us see what we can do about it.
You are right in assuming that a wire prosthesis
may have been used and this makes MRI
not a possibility also if any metal prosthesis like the titanium was used, even that makes MRI difficult to perfom.
Many a times if the drum is clear, using just a microscope you can examine the ear properly (you can even use otoendoscope) and find out the type of piston used
A CT may not diagnose a metal per say but it will definitely give you a better picture and it can diagnose a teflon piston which can exclude other pistons so go for CT
go for the above two and hope it helps.
Meanwhile remember that a deterioration of hearing after so many years of stapes surgery is normal so do not worry but get it evaluated
Dr Sriram Nathan