Extensor Compartments

There are a whole lot of wrist/finger extensors trying to fit in the wrist and anatomically these are divided into 6 compartments.

1. First compartment – it’s this that is affected in de Quervain tenosynovitis
   • APL (abductor pollicis longus): attaches to 1st MC
   • EPB (extensor pollicis brevis): attaches to base of proximal phalanx
2. Second compartment
   • ECRB (extensor carpi radialis brevis): attaches to 3rd MC
   • ECRL (extensor carpis radialis longus): attaches to 2nd MC
3. Third compartment
   • EPL (extensor pollicis longus): passes around Lister’s tubercle of radius and inserts on distal phalanx of thumb (extends thumb IPJ)
4. Fourth compartment – the posterior interosseus nerve lies on the floor of this compartment
   • EDC (extensor digitorum communis): no direct attachment to phalanx, attaches to the extensor expansions
   • EIP (extensor indicis proprius): lies ulnar to 1st EDC tendon)
5. Fifth compartment
   • EDM (extensor digiti minimi): attaches to extensor expansion of little finger
6. Sixth compartment
   • ECU (extensor carpi ulnaris): attaches to base of 5th MC

Extensor Zones

• Zone I: over the DIP (this is where mallet finger injuries occur)
• Zone II: middle phalanx
• Zone III: over the PIP
• Zone IV: proximal phalanx
• Zone V: over the MCP
• Zone VI: dorsum of hand/metacarpals
• Zone VII: over the extensor retinaculum/carpals
• Zone VIII: proximal wrist

Juncturae Tendinum

• This is the connections of fascia between the EDC tendons and why you can’t stick your ring finger up alone, as it prevents independent movement.
• It can also lead to confusion about whether an extensor tendon has been cut as the juncture tendinum transmits MCP joint extension even if a tendon is cut (as long as it’s cut distal to the JT)
• But it’s also helpful as it prevents the cut tendon from retracting up into the forearm