Finger extension and flexion

Mechanism for finger flexion

• FDP: flexor digitorum profundus (the deeper of the two)
• FDS: flexor digitorum superficialis (the more superficial muscle)

Although the FDP is deep to the FDS over most of its course, it attaches to the skeleton more distally, because it passes through a 'split' in the FDS tendon.

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Mechanism for finger extension

We can extend the PIP and DIP joints without also extending the MP joints.

But we can't extend the PIP joint without extending the DIP joint at the same time.

Flexing only the DIP joint without also flexing the PIP joint is difficult.

Full (active or passive) flexion of the PIP joint prevents active extension of the DIP joint.

We can understand these finding by learning the structure of the EXTENSOR MECHANISM, also known as the:

extensor expansion extensor assembly extensor apparatus

dorsal aponeurosis aponeurotic sleeve

The extensor mechanism is an elaboration of the extensor digitorum comunis (EDC) tendon on the dorsum of each phalanx. The extensor indicis (EI) and the extensor digiti minimi (EDM) insert into the extensor mechanisms of the second and fifth digits, respectively.

Several tendinous structures comprise the extensor mechanism:

The EDC tendon attaches by a tendinous slip to the proximal phalanx, through which it extends the MP joint. The central tendon (or "slip") proceeds dorsally to attach to base of middle phalanx, where tension can extend the PIP joint.

3. the lateral bands proceed on either side of dorsal midline and rejoin before attaching to the distal phalanx. Tension in the lateral bands extends the DIP joint.	4. the extensor hood surrounds the MP joint laterally, medially, and dorsally, and receives tendinous fibers from the lumbricales and interossei.
5. Fibers of the oblique retinacular ligament (ORL) attach at the sides of the proximal phalanx and digital tendon sheaths, and proceed to distal portion of lateral bands. Thus, the ORL's line of application is volar to the PIP joint's lateral axis and dorsal to the DIP joint's lateral axis.
PIP extension (produced by other tissues in the extensor mechanism) elongates the ORL, creating passive tension that extends the DIP. The DIP extension helps open the hand.	DIP flexion (produced by the FDP) elongates the ORL, creating passive tension that flexes the PIP. The PIP flexion assists in finger closure.