Have you ever wondered why a hand is casted in the following position when it is healing from a fracture? (See photo above) This certainly is not the typical resting position of the hand, but there is indeed a rationale as to why the “safe position” works better than other positions for prolonged periods of immobilization.
From the photo we can see that the MCP joints, or first knuckles of the hand, are bent to nearly 90°. This position allows for the collateral ligaments of the MCP joint to be “taut.” When the collateral ligaments are taut, they are less likely to get stuck in a contracted position from the lengthy immobilization period. Take a look at the following illustration of the MCP joint for clarification. (Illustration #2)
We can see from this illustration that when the MCP joint is straight, the collateral ligaments (which are orange) are a bit slacked in appearance. If we were to leave the MCP joint in this position for a month, the collateral ligaments would accommodate to that position and the joint would become very, very difficult to move. This is why the preferred position of the MCP joint for casting is in flexion.
In regard to the other joints of the fingers, the collateral ligaments are actually taut when the fingers are straight. (Illustration #3)
In this illustration of the PIP joint of the finger (the next knuckle towards the end of the finger), the collateral ligament (also in orange) is in a taut position when the joint is straight. This ligament will not stiffen significantly from casting in this position. Hooray.