Page 95 - Human Inspired Dexterity in Robotic Manipulation

Approaching Human Hand Dexterity Through Highly Biomimetic Design 93

metacarpophalangeal (MCP) joints are formed by the connection of the
phalanges to the metacarpals. The MCP joint works like a ball joint when
the finger bends, straightens, and wiggles from side to side. Depending
on the distance to the MCP joint, there exist two more types of joints,
namely, the proximal interphalangeal (PIP) joint and distal interphalangeal
(DIP) joint. Based on this definition, the thumb only has one DIP joint
between the two thumb phalanges.

6.3.2 Joint Ligaments

The range of motion (ROM) at each finger joint is restricted by the length of
ligaments. As shown in Fig. 6.3, ligaments are tough bands of fibrous tissues
inserted on both sides of the two adjacent bones. Two important branches
are called collateral ligaments. Similar structures can be found in all the finger
joints with variations in length. Their function is to stabilize the joint, shape
the ROM, and prevent the abnormal sideways bending of each joint. For
example, at the MCP joint, the collateral ligaments originate from the dorsal
side of the metacarpal bone and end near the palmar side of the adjacent fin-
ger phalanx. In this way, the collateral ligaments get taut when the finger
bends, and become relaxed once the finger straightens. This is why our
index finger can easily move from side to side when it extends, but has very
limited side motions once it fully bends.

Fig. 6.3 Schematic drawing showing the function of collateral ligaments at the MCP
joint. Their function is to stabilize the joint, shape the ROM, and prevent abnormal
sideways bending of each joint.