General structure and interosseous membrane
Throughout the forearm, its fascia (fascia antebrachii) functions as a strong, dense "cuff". Deep connective tissue septa extend from its inner surface and attach directly to the crests of the radius and ulna.
The interosseous membrane of the forearm (membrana interossea antebrachii) is stretched between these bones. Its structure includes the following important elements:
- Oblique cord (chorda obliqua) — an additional fibrous band that strengthens the bony union.
- Proximal aperture — allows the passage of vessels and nerves in the upper third of the forearm.
- Distal aperture — ensures the passage of neurovascular bundles in the lower third of the forearm.
Distal fascia and retinacula
Distally, as it transitions to the hand, the forearm fascia thickens significantly and forms specialized ligaments — tendon retinacula. These are essential for preventing tendon displacement during muscle contraction.
- On the dorsal side: the extensor retinaculum (retinaculum extensorum) is formed.
- On the palmar side: the flexor retinaculum (retinaculum flexorum) is formed. This structure is also known as the transverse carpal ligament. It spans across the carpal bones to form a dense roof over the carpal tunnel, through which tendons and nerves pass.
Boundaries of adjacent spaces
When studying fascial compartments and the topography of the upper limb, it is important to clearly understand the boundaries of related anatomical regions (specifically, the axillary fossa, which transitions into the fascia of the arm and forearm).
According to the classification, the following walls and boundaries are distinguished:
- Apex (aperture): located between the clavicle, the first rib, and the superior border of the scapula. This is where the cervicoaxillary canal (canalis cervicoaxillaris) is formed.
- Base: formed by the skin, subcutaneous fat layer, and axillary fascia.
- Anterior wall: formed by the pectoralis major and minor muscles, as well as the clavipectoral fascia.
- Posterior wall: includes the latissimus dorsi, teres major, and subscapularis muscles.
- Medial wall: formed by the serratus anterior muscle and the thoracic wall (ribs).
- Lateral wall: bounded by the intertubercular sulcus of the humerus (sulcus intertubercularis).
Briefly: answers to topical questions
Which specific tendons and nerves pass through the carpal tunnel?
Inside the carpal tunnel pass the flexor tendons and a single nerve.
- Tendons of the flexor digitorum superficialis (m. flexor digitorum superficialis) — occupy the medial part of the canal.
- Tendons of the flexor digitorum profundus (m. flexor digitorum profundus) — lie deeper and more medially.
- Tendon of the flexor pollicis longus (m. flexor pollicis longus) — lies lateral to the tendons of digits II–V.
- Median nerve (n. medianus) — lies superficially in the adipose tissue between the tendons.
How many osseo-fibrous canals are located beneath the extensor retinaculum (retinaculum extensorum), and which tendons pass through them?
There are 6 osseo-fibrous canals located beneath the extensor retinaculum. The following tendons pass through them:
- In the 1st canal — APL and EPB.
- In the 2nd canal — ECRL and ECRB.
- In the 3rd canal — EPL.
- In the 4th canal — tendons of the extensor digitorum (m. extensor digitorum) and extensor indicis (m. extensor indicis).
- In the 5th canal — tendon of the extensor digiti minimi (m. extensor digiti minimi).
- In the 6th canal — ECU.
What fascial compartments are distinguished in the forearm thanks to deep fascial septa?
Three fascial compartments are distinguished in the forearm, formed by the deep fascia, bones, and intermuscular septa.
- Anterior compartment (compartimentum antebrachii anterius) — bounded anteriorly by the fascia, and posteriorly by the bones and interosseous membrane.
- Posterior compartment (compartimentum antebrachii posterius) — bounded anteriorly by the bones and membrane, and posteriorly by the deep fascia.
- Lateral compartment (compartimentum antebrachii lateralis) — bounded anteriorly and laterally by the fascia, and medially by the anterior radial septum and the radius.
To which carpal bones does the flexor retinaculum (retinaculum flexorum) attach?
The flexor retinaculum attaches to four carpal bones.
- Scaphoid (os scaphoideum) — on the radial (lateral) side.
- Trapezium (os trapezium) — on the radial (lateral) side.
- Pisiform (os pisiforme) — on the ulnar (medial) side.
- Hamate (os hamatum) — on the ulnar (medial) side.
Class and exam questions
Which structure forms the roof of the carpal tunnel?
The roof of the carpal tunnel is formed by the flexor retinaculum (retinaculum flexorum), which is also called the transverse carpal ligament.
What elements make up the interosseous membrane of the forearm?
The interosseous membrane (membrana interossea antebrachii) comprises the oblique cord (chorda obliqua) as well as proximal and distal apertures through which vessels and nerves pass.
What bounds the apex (aperture) of the space leading to the cervicoaxillary canal?
The apex is bounded by the clavicle, the 1st rib, and the superior border of the scapula, forming the canalis cervicoaxillaris.
What else to study on this topic
- Passage of vessels and nerves through the proximal and distal apertures of the interosseous membrane
- Anatomy and contents of the carpal tunnel
- Connection of the clavipectoral and axillary fasciae with the fasciae of the free upper limb
- Topography of the cervicoaxillary canal (canalis cervicoaxillaris)
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