Step 2
Shift-click to make one copy of each bone at the same time.

Step 3
Hide everything but the copies you just made. Go into the Character menu and bring up the Bone Tools window. Click on Refine. What this does is let you break one bone into two without breaking the chain.

Step 4
Break the copy of the large forearm bone you just made into four equal sections as shown below. Do this by clicking as close as you can to the middle of the large bone, as then as close to middle as you can of the two new bones.

Step 5
Now we've got four "mini bones" that together are exactly the same length as the original forearm bone. And while we've got the Bone Tools window open, add some fins to the new bones so you can see them rotating easier.

Step 6
Create a spline with two points, one at either end. It doesn't matter where you create it, but the order of the points is very important. Create the points as shown below.

Step 7
We need to position the line exactly inside the chain of bones we just created. So with the spline still selected, go into the Modify panel, down into the Vertex sub-object mode, and select the first point on the line. Then click on the Align tool and then on the first bone in the chain.

Step 8
Use the settings below to align the first point of the line correctly.

Step 9
Do the same thing to the second point, but align it with the last bone in the chain.

Step 10
Use the settings below.

Step 11
Now we're going to apply the SplineIK. The way it works is you select one end of the bone chain, then the other, then the line you're using as the IK spline. So select the mini bone at the end of the chain and then go up to the Animation menu, down into the IK Solvers group and select SplineIK Solver.

Step 12
Select the last bone in the chain.

Step 13
And now select the line. Don't worry about not being able to see the line, because Max is smart enough at this point to only select line objects. So you wouldn't be able to select one of the bones even if you wanted to.

Step 14
You'll notice that the entire bone chain flips 90 degrees as soon as you complete the SplineIK.

Step 15
We'll fix that by going into the Motion panel, IK Solver Properties, and changing the Twist Start Angle to 90.

Step 16
Here's comes the wiring. The cool thing about this setup is that we only have to do this once. Unhide the other bones and helpers and select the main IK Goal we're using to animate with. Right-click on it and select Wire Parameters. Now navigate to the X Rotation controller and select it.

Step 17
Select the smaller IK Goal that got added when we created the SplineIK chain. Choose Twist End Angle from the submenu.

Step 18
The wiring window pops up. First click on the bottom arrow to tell Max that the control for these two objects is going from the main IK Goal to the secondary IK Goal and not the other way around. Then type in the expression below:

(X_Rotation)-(degtorad(90))

Basically, we're telling the main IK Goal to twist the wrist around as it's rotated. Technically you could do without the "-(degtorad(90))" part and it will still work. This is merely a housekeeping thing to keep the forearm bone chain looking nice. Try it without that last part and you'll see that the bone rotates 90 degrees as soon as you hit the Update button. So that last part of the equation just tells Max to leave the bone where it is.

Step 19
The only thing left to do is the link the new bones to the existing arm chain so that everything moves together. Select the dummy box at the wrist and link it to the main IK Goal.

Step 20
Then select the dummy box at the elbow and link it to the upper arm bone.

Step 21
That's it. Now you can rotate the IK Goal and watch both the hand and the forearm twist together.