Layered structure of the posterior forearm region
The anatomy of the posterior forearm region includes six main layers arranged sequentially from the surface inward:
- Skin (cutis) — on the posterior surface, it is thicker compared to the anterior region. In the proximal parts (closer to the elbow joint), the skin has limited mobility.
- Subcutaneous adipose tissue (tela subcutanea) — a loose layer containing superficial veins and cutaneous nerves.
- Superficial fascia (fascia superficialis) — represented by a thin connective tissue plate.
- Deep fascia of the forearm (fascia antebrachii) — a dense structure that forms fascial compartments and gives rise to intermuscular septa.
- Muscles — in the posterior compartment, they are arranged in two layers: superficial and deep.
- Forearm bones and interosseous membrane (membrana interossea antebrachii) — form the deep osteofibrous framework. They also serve as the posterior wall for the anterior compartment and the anterior wall for the posterior compartment.
Features of the deep fascia
The deep fascia of the forearm (fascia antebrachii) has uneven density throughout its extent. In the proximal part, it is thick and shiny, but distally (toward the hand) it gradually thins out.
On the medial (ulnar) side, this fascia fuses tightly with the posterior border of the ulna along its entire length. In the lateral part, it gives off two radial intermuscular septa deep into the tissue, which attach to the radius:
- Anterior radial intermuscular septum — runs strictly along the medial margin of the brachioradialis muscle (m. brachioradialis).
- Posterior radial intermuscular septum — descends along the lateral margin of the same muscle.
In addition, the deep lamina of the deep fascia divides the muscle mass of the posterior compartment into superficial and deep layers.
Fascial compartments of the forearm
The forearm bones, interosseous membrane, and intermuscular septa divide the forearm space into three fascial compartments: anterior, posterior, and lateral. Each of them has distinct walls.
| Compartment | Latin name | Boundaries and walls |
|---|---|---|
| Anterior | compartimentum antebrachii anterius | Anterior: deep fascia (fascia antebrachii). Posterior: forearm bones and interosseous membrane. Lateral: anterior radial intermuscular septum. Medial: deep fascia fused with the posterior border of the ulna. |
| Posterior | compartimentum antebrachii posterius | Anterior: forearm bones and interosseous membrane. Posterior: deep fascia. Lateral: posterior radial intermuscular septum. Medial: fusion of the fascia with the posterior border of the ulna. |
| Lateral | compartimentum antebrachii lateralis | Anterior and lateral: deep fascia. Medial: anterior radial intermuscular septum and radius. Posterior: posterior radial intermuscular septum. |
Examination of tendon reflexes
Knowledge of the anatomical landmarks of the forearm and arm is necessary for the proper examination of tendon reflexes using a neurological hammer.
1. Biceps brachii tendon reflex
- Anatomical substrate: biceps brachii muscle (m. biceps brachii).
- Technique: the patient's arm should be relaxed and slightly flexed at the elbow joint. The physician places the thumb of their hand on the muscle tendon in the cubital fossa. The strike is applied indirectly — on the phalanx of the examiner's finger rather than directly on the tendon.
- Response: contraction of the muscle leading to flexion of the forearm.
2. Triceps brachii tendon reflex
- Anatomical substrate: triceps brachii muscle (m. triceps brachii).
- Technique: the patient's arm is flexed at the elbow joint (hanging freely or supported by the physician). The strike is applied directly to the muscle tendon, just above the olecranon of the ulna (olecranon).
- Response: contraction of the muscle leading to extension of the forearm.
3. Brachioradialis reflex
- Anatomical substrate: brachioradialis muscle (m. brachioradialis).
- Technique: the patient's arm is semi-flexed at the elbow joint, with the hand in slight pronation. The strike is applied to the radial styloid process (processus styloideus radii) or the lower third of the radius.
- Response: contraction of the muscle leading to flexion of the forearm.
The structure of the posterior forearm compartment is easy to remember by analogy with the leg: the deep lamina of the deep fascia similarly divides the muscles into superficial and deep layers.
Class and exam questions
How to properly elicit the biceps brachii tendon reflex?
The neurological hammer strike is applied not to the tendon itself, but indirectly — to the phalanx of the physician's finger placed on the tendon in the cubital fossa.
Where does the anterior radial intermuscular septum run?
It originates from the deep fascia of the forearm, attaches to the radius, and runs along the medial margin of the brachioradialis muscle.
What structures form the anterior wall of the posterior fascial compartment?
The anterior boundary of the posterior compartment is formed by the forearm bones and the interosseous membrane stretched between them.
What else to study on this topic
- Fascial spaces of the anterior forearm region
- Topography of the neurovascular bundles of the forearm
- Muscles of the anterior and posterior fascial compartments (superficial and deep layers)
- Innervation and blood supply of the skin and subcutaneous adipose tissue
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