Extensor Hood Mechanism Hand

Original Editor - Lucinda Hampton 

Top Contributors - Lucinda hampton and Uchechukwu Chukwuemeka

Introduction

The extensor hoods are triangular aponeuroses by which the extensor tendons insert onto the phalanges.

Extensor Hood.jpg
  • The tendons of the extensor digitorum flatten as they reach the metacarpals and become extensor hoods which fan out and wrap around the metacarpal and proximal phalanx joining onto the palmar plate (structures present on the palmar side of each Metacarpophalangeal and interphalangeal joint which limit hyperextension of the digit).
  • The extensor hood spreads out further distally into a median band which attaches to the middle phalanx and two lateral bands which attach to the distal phalanx. 
  • Contraction of the extensor digitorum muscle tightens this tendon which acts on these attachments and extends the fingers.[1]

Anatomy

Extensor digitorum muscles.jpg

Extensor Hoods are 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 hoods of the second and fifth digits, respectively.

Several tendinous structures comprise the extensor hoods:

  1. The EDC tendon attaches by a tendinous slip to the proximal phalanx, through which it extends the Metacarpophalangeal (MCP) joint.

2. The central tendon (or "slip") proceeds dorsally to attach to the base of the middle phalanx, where tension can extend the proximal interphalangeal (PIP) joint.

3. The lateral bands proceed on either side of the dorsal midline and rejoin before attaching to the distal phalanx. Tension in the lateral bands extends the distal interphalangeal (DIP) joint.

4. The extensor hood surrounds the MCP joint laterally, medially, and dorsally, and receives tendinous fibers from the lumbricals 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 the 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.

6. Oblique retinacular ligament

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 Flexor digitorum profundus) elongates the ORL, creating passive tension that flexes the PIP. The PIP flexion assists in finger closure.[2]

Injuries

see Traumatic Extensorhood Injuries
Extensor hood anatomy.jpg
Extensor hood rupture
  • Is a rare injury associated with boxing and other professional sports.
  • Extensor tendon rupture occurs secondary to trauma, inflammatory arthropathies, and steroid injection.
  • In boxing, blunt trauma is usually the cause[3]

References

  1. Teach me anatomy Structures of the hand Available from:https://teachmeanatomy.info/upper-limb/misc/structures-hand/ (accessed 18 March 2020)
  2. Biomechanics of the hand Gwenda Sharp OTR and Dave Thompson PT. Available from:https://ouhsc.edu/bserdac/dthompso/web/namics/hand.htm (accessed 17 March 2020)
  3. Hashem M. Traumatic Extensor Hood Rupture. Available from: https://www.physio-pedia.com/Traumatic_Extensor_Hood_Rupture (accessed 18 March 2020)