TENDONS, ZONES & PULLEYS OF THE HAND

    TENDONS

Extensor Tendons
The six extensor compartments of the wrist serve as tunnels for tendons to pass from the forearm to the wrist.
• 1st compartment is comprised of the Abductor pollicis longus (APL) and Extensor pollicis brevis (EPB) tendons.
• 2nd compartment is comprised of the Extensor carpi radialis brevis (ECRB) and Extensor carpi radialis longus (ECRL) tendons.
• 3rd compartment is comprised of the Extensor pollicis longus (EPL) tendon.
• 4th compartment is comprised of the Extensor indicis proprius (EIP) and Extensor digitorum communis (EDC) tendons.
• 5th compartment is comprised of the Extensor digiti minimi (EDM) tendon.
• 6th compartment is comprised of the Extensor carpi ulnaris (ECU) tendon.

Flexor Tendons

There are six long flexor muscles of the hand which contribute to flexion of the hand, wrist, and fingers.
• Flexor carpal radialis muscle
• Flexor carpal ulnaris muscle
• Flexor digitorum profundus muscle
• Flexor digitorum superficialis muscle
• Flexor pollicis longus muscle
• Palmaris longus muscle

Flexor Digitorum Profundus
• It is the main mass flexion (gripping) muscle. It acts best when the wrist is extended.
• It is the only flexor of the DIP joints of digit 2nd to 5th.
• It also acts as a flexor of the wrist, metacarpophalangeal, and PIP joints of the index, middle, ring, and little fingers along with Flexor digitorum superficialis.
• As the lumbricals arise from the flexor digitorum profundus tendons, flexor digitorum profundus muscle aids the lumbricals in fully extending the fingers at the interphalangeal joints and flexing the fingers at the metacarpophalangeal joints.

      ZONES (Flexor and Extensor)

There are 5 Flexor and 9 Extensor Zones of the Hand. They are used to describe the location of a tendon injury, guide post-operative therapy and predict prognosis.

Extensor Zones
The dorsum of the hand, wrist, and forearm are divided into nine anatomic zones to facilitate classification and treatment of extensor tendon injuries. Odd numbers are used for regions overlying articular structures, with even numbers being assigned to the regions between joints, as follows:

Zone 1 (distal interphalangeal [DIP] joint)
Zone 2 (middle phalanx)
Zone 3 (proximal interphalangeal [PIP] joint)
Zone 4 (proximal phalanx)
Zone 5 (metacarpophalangeal [MCP] joint)
Zone 6 (dorsum of hand)
Zone 7 (wrist)
Zone 8 (distal forearm)
Zone 9 (proximal forearm)

Flexor Zones
Zone 1 – Flexor Digitorum Profundus only
Zone 2 – Flexor Digitorum Superficialis and Flexor Digitorum Profundus
Zone 3 – Central palm (from the carpal tunnel to distal palmar crease)
Zone 4 – Carpal tunnel
Zone 5 – Proximal to carpal tunnel
Thumb – Specific to the thumb

Zone 2- Referred to as Bunnell’s no-man’s land because of the risk of adhesions around lacerations.

Bunnell, in 1918, coined the term “no man’s land” to describe zone 2 in the hand because at that time it was felt that no man should attempt repair within this zone.

    PULLEYS

The pulley system is made of thickened areas of the flexor tendon sheath. The flexor pulley system of the hand is a complex structure that co-ordinates flexion of the digits. The main role of the flexor pulley system is to hold the flexor tendons against the phalanges; preventing them from pulling away and bowstringing. This allows efficient flexion of the individual digits by the long flexor muscles.

It consists of:
a. Long flexor tendons – and their associated synovial sheaths.
b. Annular pulleys – 5 associated with each finger, 2 associated with the thumb.
c. Cruciate pulleys – 3 associated with each finger.
d. Oblique pulley – 1 associated with the thumb.

The annular pulleys (or ligaments) represent five areas where the fibrous flexor sheaths are reinforced by circular fibres. It is thought that A2 and A4 are most important in preventing bowstringing of the flexor tendons.
• A1 – overlies the metacarpophalangeal joint
• A2 – overlies the proximal aspect of the proximal phalanx
• A3 – overlies the proximal interphalangeal joint
• A4 – overlies the mid-portion of the middle phalanx
• A5 – overlies the distal interphalangeal joint

The cruciate pulleys refer to areas where the fibrous flexor sheaths are reinforced by cruciform fibres:
• C1 – located between A2 and A3
• C2 – located between A3 and A4
• C3 – located between A4 and A5

The fibrous flexor sheath of the thumb is reinforced by three pulleys:
•  A1 – overlies the metacarpophalangeal joint
• Oblique – overlies the proximal half of proximal phalanx
• A2 – overlies the distal half of proximal phalanx

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Resources:
https://www.theplasticsfella.com/zones-of-the-hand/
https://teachmeanatomy.info/upper-limb/misc/flexor-system-hand/