I had this a few years ago. It was very painful. I used to open my hand in the morning after first giving it a soak in quite hot water. I think what sorted it for me was massage, I used to have a monthly massage and the lady used to massage my hand, wrist and forearm with soothing oils. I used to massage the hand myself through the day and I used those Chinese balls that you often see in charity shops to exercise the hand - you basically circle them around each other very gently. I think it's an inflammatory condition and I suspect I used a bit of ibuprofen gel too. I imagine a splint would help.
Mine has totally vanished now so take heart, it can be improved!