Most of the 3D-printed prosthetic hands have adjustable tensioners that tune the tendons to each finger so that the all work together as the wrist flexes and pulls on the tendon cords. This grip is not optimal for grasping odd-shaped objects.

The design was updated with the Whippletree Tensioner stl which has a larger slot that is cut out for a 3rd peg at the pivot.
The Whippletree idea came from Steve Wood (Gyrobot) created triangle-shaped connectors that allow fingers to flex individually until they all meet resistance.http://www.thingiverse.com/thing:392970

This allows the grip to conform to the shape of the object being grasped. 

With pegs made from 3mm filament serving as pulleys, this device simplifies Wood’s mechanism by replacing his small triangles with posts (pegs made from 3mm filament) that serve as pulleys. It works with the Raptor Adaptor https://www.youmagine.com/designs/the-raptor-adaptor-v1-0.