The document defines and describes the anatomy of the extensor mechanism of the hand. It notes that the extensor mechanism is composed of the extensor expansion, extensor assembly, and extensor apparatus. It describes the structures that make up the extensor mechanism including the extensor tendons, extensor hood, lumbrical and interosseous muscles. It explains how tension in the extensor mechanism allows for independent extension of the PIP and DIP joints through the oblique retinacular ligaments. Ruptures of the central slip can cause Boutonniere deformity from the unopposed pull of the lateral bands.

2.  Extensor expansion
 Extensor assembly
 Extensor apparatus
 Dorsal digital expansion
 Dorsal aponeurosis
 Aponeurotic sleeve

3.  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

4.  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

5.  Stabilized by numerous
transverse fibres & by links
that extend to the deep
transverse metacarpal
ligaments – separates the
phalangeal attachment of dorsal
interosseus from the rest of the
muscle & also palmar
interosseus from the lumbricals.

6.  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.

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

8.  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 ?

9.  We can understand these finding by learning the
structure of the
“EXTENSOR MECHANISM”
&
“BIOMECHANICS FOR FINGER
EXTENSION”

10.  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.

11.  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.

12.  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.

13.  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.

14.  Lumbricals
 Palmar interossei
 Dorsal interossei

15.  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
flexion. Also PIP and DIP
extension when they introduce
tension into the extensor
mechanism.

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

17.  Specifically, lumbrical activity:
 increases passive tension in the extensor
mechanism.
 decreases passive tension in FDP tendon's distal
portion.

18.  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.

19.  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.

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

21.  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.

22.  The extensor mechanism's fibers have lines
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.

23.  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.

24. 1. Rupture of the central slip
2. Volar dislocation of the lateral band
3. More efficient pull of the lateral band on the DIP
J

25.  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

26.  Lateral bands – act as flexors on volar side
 Contracted oblique retinacular ligaments & lateral
bands force DIP into hyperextension – increases
by passively extending the PIP.

27.  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

28.  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.