The approach is to inlet/bury structural pieces within the wrist by drilling out matching cavaties in the existing wood. There is sufficient wood left around the outside to form a register that maintains alignment and hides the new structure. Wood is grand stuff, but not very strong in the greater scheme of strength. The new members can be stronger wood (with optimal grain orientation), metal, or composite. The cavaties are sized with enough clearance to allow alignment from the perimeter wood. Epoxy is usually used as the cement to join the structure members. Channels are usually drilled to avoid blind cavaties in the interest of filling, without overfilling (injection or escape routes for the cement). Properly done, you have a stronger area than original. Skillfully done, it is virtually undetectable.
Wrist repair images.
Round mortise with large round rod tenon
Rectangular mortise with rectangular bar tenon
Round mortises with small rod tenons
These methods can use a varity of tenon materials and may be used in conjunction with each other. Epoxy is the usual cement and fill/drain channels can be dirlled to assure proper filling between the mortise and tenon.
Does that help clarify the idea any?
