Extensor expansion Extensor assembly Extensor apparatus Dorsal digital expansion Dorsal aponeurosis Aponeurotic sleeve

Translucent, fibrous expansion on the dorsum of proximal phalanx of each digit.

An aponeurotic extension of the extensor digitorum tendons (EDC, EI, EDM).

Triangular, with base being proximally, wraps around dorsal and collateral aspects MCP joints.

Definition

A tendon of EDC blends with the expansion along it’s central axis & is separated from the MCP joint by a small bursa.

Base – connects it to the interosseous muscles on

each side

Stabilized by numerous transverse fibres & by l inks that extend to the deep transverse metacarpal l igaments – separates the phalangeal attachment of dorsal interosseus from the rest of the muscle & also palmar interosseus from the lumbricals.

Margins – are thickened – Laterally by lumbrical & interosseous ms.– Medially by interosseous tendons alone.

These attachments called

“WING TENDONS”– Proximal – interossei– Distal – lumbricals.

EDC – 3 slips insersion.

Each expansion forms a movable hood◦ Moves distally when MCP is flexed &◦ Proximally when MCP is extended.

TRY IT ? We can extend the PIP and DIP joints without

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” & “BIOMECHANICS FOR FINGER EXTENSION”

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.

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.

The extensor hood surrounds the MP joint laterally, medially, and dorsally, and

receives tendinous fibers from the lumbricales and interossei.

Fibers of the oblique retinacular l igament (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.

Lumbricals Palmar interossei Dorsal interossei

Attachments proximally tendons of the flexor digitorum profundus, and distally to the extensor mechanism on radial side at the level of the lateral bands.

Pass on the volar side of the transverse metacarpal ligament.

If they act alone, produce MP f lexion. Also PIP and DIP extension when they introduce tension into the extensor mechanism.

The lumbricals permit a dynamic interaction between flexors and extensors. Their attachments transmit their force to both the FDP tendon and the extensor mechanism.

Specifically, lumbrical activity: increases passive tension in the extensor

mechanism. decreases passive tension in FDP tendon's distal

portion.

Attach proximally to a metacarpal, and distally to the same digit's proximal phalanx and/or its extensor mechanism.

They produce MP adduction and, in certain instances, MP flexion. They also produce PIP and DIP extension when they introduce tension into the extensor mechanism.

Attach proximally between adjacent metacarpals & distally either to bone (proximal phalanx) or to soft tissue (extensor mechanism).

Produce MP abduction and, in certain instances, MP flexion. Because they attach to the extensor mechanism, they also produce PIP and DIP extension.

Extensor mechanism's fibers are tendinous, and therefore incapable of producing active force, they transmit force to their attachments.

Activity of hand's intrinsic muscles which attach to the extensor mechanism produces force that extensor mechanism communicates to its distal attachments.

The extensor mechanism develops passive tension whenever it is elongated. Hand movements that passively elongate either the extensor mechanism or a structure that attaches to the extensor mechanism produce force in the extensor mechanism itself.

The extensor mechanism's fibers have l ines of application that are always dorsal to the lateral axes of the PIP and DIP joints.

Therefore, activity in the intrinsic muscles that attach to the extensor mechanism always produces DIP and PIP extension.

Passive flexion of the MP joint (try it yourself!) elongates the extensor mechanism and extends the PIP and DIP joints.

In MP joints it differs◦ MP in flexion: Flexes the MP joint & distal shift in the

extensor hood ◦ MP in extension: Extends the MP joint, and also pulls the

extensor mechanism (including the hood) proximally.

1. Rupture of the central slip2. Volar dislocation of the lateral band3. More efficient pull of the lateral band on the DIP

J

Rupture of central slip. Loss of active extension of PIP Persistent flexion of PIP Untreated – collateral ligs. &

volar plate of PIP contracted Lateral bands subluxate

volarwards & held there by transverse retinacular ligaments also contracted

Lateral bands – act as f lexors on volar side Contracted oblique retinacular ligaments & lateral

bands force DIP into hyperextension – increases by passively extending the PIP.

Can also occur by TRAUMATIC ROTATION of

the digit at PIP while partially flexed.

Condylar herniation – rupture of triangular lig area.

Partial rupture of Extensor mechanism

Dislocation of PIP

Acute Buttonhole – diagnosed early ◦ Conservatively – SPLINTING PIP in full extension, free

DIP x 4-6 wks followed by night wear. Traumatic complete rupture◦ Surgical repair.