As I haven't really been on track with this device for some time I thought I better update the .org on where I'm at.
From previous videos and posts the LEGO hand is approximately the size of an average male, wrist turning motion is via similar means to a biological limb and though I haven't counted, articulation is similar to that of a real hand.
One of the hardest things for me to replicate has been the thumb. LEGO doesn't really make the parts to do that well, correctly. I am on thumb #5 or 6 and still not happy with it.
Another difficulty was wrist motion; how to facilitate natural motion without twisting operational mechanisms or wires? More recently I have thought "Why does the limb NEED to rotate at the 'wrist'?" This is for a prosthetic, not a real limb. Moving rotation(still restricted to ~human motion) to just above the elbow would not be a bad idea in some cases. Moving power transfer and control contacts have been around for years. My previous idea of biological style turning created space problems within the device. When the "bones" twist either direction you must account for that with any mechanism you place inside it. If wrist turning is needed though, as in Jon K's situation, a modular device self-contained within a sleeve type attachment would be viable.
The modular version illustrated below could be plugged into a sleeve mounted attachment and the entire mechanism would rotate inside the sleeve. A full forearm unit could simply rotate at the base of the elbow(also illustrated).