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1.3.4 Bones and joints of the thumb

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(3.06)

Now let’s move on from the fingers, to look at the bones and joints of the thumb. The first carpo-metacarpal joint is the joint which gives the thumb its special position, and a great deal of its special mobility.

Let’s take off the metacarpal heads, to see the joint surfaces. Here’s the first CMC joint. It sits in front of these other CMC joints, and at an angle to them. Because of this, the thumb and its metacarpal lie in front of the fingers and their metacarpals. Because of the angle of the carpometacarpal joint, the thumb faces not forward, as the fingers do, but sideways, across the hand.

The articular surface on the trapezium is curved in two planes, from side to side, and from back to front. The base of the first metacarpal is curved in the same way. The shape of the joint surfaces enables the first metacarpal to move in this plane, and in this plane. We’ll name those movements in a minute, but first let’s look briefly at the other two joints of the thumb.

The MP joint of the thumb is unlike the finger MP joints. It’s much more like an interphalangeal joint. It permits only flexion and extension. On its flexor aspect there are two tiny sesamoid bones, which are embedded in the joint capsule. The one interphalangeal joint of the thumb is just like the IP joints of the fingers.

Now let’s go back to the CMC joint, and see how the first metacarpal moves, and what the movements are called. Movement away from the second metacarpal is called abduction. Movement toward it is adduction. Movements at right angles to this axis are called flexion and extension.

These two sets of movements often happen in combination. As it makes these movements, ...

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(3.06)

Now let’s move on from the fingers, to look at the bones and joints of the thumb. The first carpo-metacarpal joint is the joint which gives the thumb its special position, and a great deal of its special mobility.

Let’s take off the metacarpal heads, to see the joint surfaces. Here’s the first CMC joint. It sits in front of these other CMC joints, and at an angle to them. Because of this, the thumb and its metacarpal lie in front of the fingers and their metacarpals. Because of the angle of the carpometacarpal joint, the thumb faces not forward, as the fingers do, but sideways, across the hand.

The articular surface on the trapezium is curved in two planes, from side to side, and from back to front. The base of the first metacarpal is curved in the same way. The shape of the joint surfaces enables the first metacarpal to move in this plane, and in this plane. We’ll name those movements in a minute, but first let’s look briefly at the other two joints of the thumb.

The MP joint of the thumb is unlike the finger MP joints. It’s much more like an interphalangeal joint. It permits only flexion and extension. On its flexor aspect there are two tiny sesamoid bones, which are embedded in the joint capsule. The one interphalangeal joint of the thumb is just like the IP joints of the fingers.

Now let’s go back to the CMC joint, and see how the first metacarpal moves, and what the movements are called. Movement away from the second metacarpal is called abduction. Movement toward it is adduction. Movements at right angles to this axis are called flexion and extension.

These two sets of movements often happen in combination. As it makes these movements, the first metacarpal also rotates around its own long axis, as the pen is doing. When it’s abducted and flexed, it rotates medially. When it’s adducted and extended it rotates laterally.

This rotation can’t happen in isolation, but only as part of those other, bigger movements. It happens because of the curious and complex shape of the CMC joint surfaces. This important and complex movement of the thumb is called opposition. It's a combination of abduction, flexion, and medial rotation, all occurring here at the CMC joint. Because of the rotation that occurs, the tip of the thumb ends up pointing toward the fingers. Once the thumb is in opposition, flexion at the MP and IP joints brings the tip of the thumb into contact with the fingers.

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