The term dorsal digital expansion refers to the small triangular aponeurosis covering the dorsum of the proximal phalanx with its base at the metacarpophalangeal joint. The main tendon of the extensor digitorum occupies the central part of the expansion and is separated from the metacarpophalangeal joint by a bursa.
[Aponeurosis is a flat sheet or ribbon of tendon-like material that anchors a muscle or connects it with the part that the muscle moves. Aponeuroses are often found at the insertion sites of the muscles and increase the strength of insertion and spread it over a wide area.]
The posterolateral corners of the extensor expansion are joined by tendons of the interossei and of a lumbrical muscle.
The deep transverse metacarpal ligament is attached to the corners.
Other names of dorsal digital expansion are
- Extensor expansion
- Extensor hood
Structure of Dorsal Digital Expansion
All the finger extensors (extensor digitorum, extensor indicis, extensor digiti minimi) have a characteristic configuration. Extensor digitorum is main extensor of all the digits except the thumb. Extensor indicis is an additional extensor muscle to the index finger. Extensor digiti minimi is another additional extensor muscle that is involved in the movement of the little finger.
Thumb has two extensor muscles- extensor pollicis brevis and extensor pollicis brevis.
[Know more about extensor muscles]
As these tendons cross the metacarpophalangeal joint, the deepest fibers of the tendons pass to adhere to the joint capsule with the bulk crosses the joint. After crossing the metacarpophalangeal joint, the fibers latten and fan out as they pass the dorsal proximal phalanx.
These fibers form three slips and separate into 3 slips.
- One central slip inserts on to the base of the middle phalanx.
- The two lateral slips move further with a slight diversion from the central slip to insert on the base of the distal phalanx.
Also, a band known as the retinacular ligament runs obliquely along with the middle phalanx and connects the fibrous digital sheath on the anterior side of the phalanges to the extensor expansion.
Palmar/dorsal interossei and lumbricals tendons reinforce the lateral slips.
This whole arrangement is called dorsal digital expansion.
The extensor expansion extends from the distal metacarpal head to the proximal phalanx and serves to hold the extensor tendons in place.
It also serves to allow the extensors, lumbricals, and interossei to effect extension at the proximal and distal interphalangeal joints.
The dorsal digital expansion of the thumb is different. There is no proper extensor expansion proper.
As we know from anatomy, the tendons of the extensor pollicis brevis and longus are inserted in the proximal and distal phalanx respectively.
The tendon of the extensor pollicis longus receives a fibrous expansion from the abductor pollicis brevis and adductor pollicis. This arrangement serves a similar purpose as the dorsal digital expansion – to hold the extensor tendon in place on the dorsal aspect of the surface.
Functions of Dorsal Digital Expansion
Dorsal digital expansion is a mobile structure during flexion and extension of the fingers, the extensor hood moves distally to the metacarpophalangeal joint during flexion.
When the fingers are extended, it moves proximally.
These movements during flexion and extension of the fingers allow a greater variation in extension and is more efficient than the situation when a single extensor tendon was used for each joint.
This makes complex movements of finger flexion and extension quite coordinated. For example, the presence of dorsal digital expansion allows the extension of the interphalangeal joints when metacarpophalangeal joints are flexed by the flexor muscles.
Moreover, the dorsal expansion often adheres to the collateral ligaments. This creates an important anchorage that unites the extensor apparatus to the immobile flexor tunnel.