I've been Googling for the effects of long-term usage of trientine as a treatment for Wilson's disease and could find none. My mom's had Wilson's disease for over 40 years now (she was diagnosed way too late, she had a clinical death before they could figure out what was wrong), and she's been using trientine dihydrochloride since she's severely allergic to penicillamine. Over the course of the last 5 years she has had a rather rapid onset with severe joint pain (especially in knees and hips), and her cartilage in those areas is determined to be nearly non-existent.
I've spent a lot of time researching the various effects of Wilson's disease so I doubt it's directly related to it, but rather the side-effects of medications she's been using.
If you could provide links to studies of side-effects of Trientine it'd really be helpful.