The answer

It's a splint or brace for a trigger thumb. Out of nowhere, a couple of weeks ago, my thumb started acting like a spring. When I bent the middle joint it sprang back again. It was/is a most peculiar sensation. On Tuesday I saw a hand therapist for the first time. She is part of a physiotherapist practice based in a local hospital. After running several tests on my hand she made the brace out of a sheet of interesting material which becomes pliable in very hot water and then becomes solid again when it cools down. The brace is meant to stop the joint from bending to the point where it springs back. I have a series of exercises to do and I have to wear the brace for several weeks. The wonderful hand therapist made a second brace which I wear at night. It is designed to immobilse the lower joint in my thumb as well while I sleep. Such clever people.