Hi Andy, sorry to hear you're suffering, I'm surprised they've taken your cast off after just 2 weeks as they normally leave it around 4-6, but it tends to depend on the type of procedure or technique used. Often when you have the pins or plates holding the joint together while it fuses, you need less casting time, as the cast is mainly just to protect the area rather than holding the bones together, like it would be for a fracture for example. As you have internal fixation, the molded brace should be enough to protect your skin and surgery site as it continues to heal, so long as you're careful and don't use your hand too much of course. If you're concerned though contact your doctor/surgeon for advice.
Try to keep your arm raised to reduce the swelling, pressure and pain, keep on top of the discomfort with painkillers and follow any instructions your doctor gave you regarding movement. It will get easier, but unfortunately fusion surgery is painful and does take time to heal. I've had several fusion surgeries done, including thumbs and wrist, and face several more in coming years too, so I can fully empathise with how painful the process is.
Take care and hang in there, it is worth it in the end honest!
