What (unknown) implant system is this?

Don't know the system but I would try to flap it and clean it out, maybe graft. Twenty years is great!

You can go online and google "what type of dental implant". There is a web site to help identify implant systems from radiographs. Good Luck, Geoff

It seems a Nobel Biocare. External hexagon. Best regards from Spain

I would say standard branemark clone ( not absolutely clear due to the angle of the x ray) and by the shape of the apex I'd say restore. Cheers

I think branemark polished surface

Because of the apical holes, looks like an IMTEC, Branemark compatible platform. Machines surface

Go to: http://whatimplantisthat.com/ they have a library of hundreds of different implants including radiographs and specs. You should be able to easily find it that way. If not, the site will do a custom search for you for a small fee.