Next we’ll move on to the bones and joints of the fingers themselves. The proximal and middle phalanges are flattened on their flexor aspects. The flexor tendons run along here. The sheath that surrounds the flexor tendons is attached to these ridges. The tip of the distal phalanx is flattened. The fibrous pulp of the fingertip is attached here. The bed of the fingernail is attached here.
Now let’s look at the metacarpophalangeal joint, the MP joint. It’s the joint at which the finger becomes separate from the hand. We’ll take the other fingers away, so that we can see it from all sides.
The articular surface of the metacarpal head is curved in two planes, from side to side, and from front to back. The base of the proximal phalanx has a concave articular surface that’s also curved in two planes.
The shape of the bones allows a wide range of flexion and extension at the MP joints. It also allows a range of side to side movement that’s greater when the joints are extended, less when they’re flexed. We’ll see why that is in a minute. Let’s see how the joint looks in the living body.
The MP joint has a capsule that’s loose on the back to allow the joint to flex. On the front, the capsule thickens remarkably, into a tough piece of fibrocartilage, the palmar plate, also called the palmar ligament. The palmar plate moves along with the proximal phalanx when the joint flexes.
Here’s the palmar plate incised, to show how thick it is. As we’ll see, some important structures are attached to the palmar plate, or merge with it. One of them we’ve seen already, the deep transverse metacarpal ligament. It goes here.
Here we’ve removed most of the joint capsule, so that we can see the two massive collateral ligaments which hold the MP joint together. The collateral ligaments run obliquely from the back of the metacarpal head, to the front of the base of the proximal phalanx. The collateral ligaments are loose when the joint is extended, but when it’s flexed, they become tight. So when the joint is extended, side to side movement can occur readily, but when the joint is flexed, the tightness of the ligaments prevents side to side movement.
We need to understand the names that are given to those side to side movements at the MP joints. Spreading all the fingers apart is called abduction. Bringing them all together is adduction. Those are useful terms for describing those collective movements of the fingers, but when we’re speaking of an individual finger, it’s often simpler to speak instead of ulnar deviation and radial deviation.
Now let’s move on to the interphalangeal joints. The proximal and distal IP joints are very much alike. They’re different from the MP joints in that they only allow flexion, and extension.
The head of the phalanx is curved mainly from front to back, with a slight depression in the middle. The base of the adjoining phalanx has a corresponding curve to it.
The capsule of an IP joint is much like that of an MP joint, but the collateral ligaments are different, in that they’re equally tight in flexion and in extension.