Boundaries and External Landmarks
Determining this region is quite simple. The upper boundary runs along a horizontal line 4 cm below the level of the elbow crease, and the lower boundary runs 2 cm above the tip of the radial styloid process. On the sides, the region is separated from the posterior surface of the forearm by vertical lines connecting the epicondyles of the humerus to the corresponding styloid processes.
On the surface of the arm, one can easily palpate the main landmarks: the radial and ulnar sulci, the dense muscle belly of the brachioradialis muscle (m. brachioradialis), and in the lower third of the forearm, the tendons of the flexor carpi radialis and palmaris longus contouring under the skin.
Superficial layers and fasciae
The skin here is thin, with loose and layered subcutaneous adipose tissue lying beneath it. The major superficial veins of the arm pass through it: v. cephalica on the lateral side, and v. basilica on the medial side. They are accompanied by cutaneous nerves providing sensitivity.
The superficial fascia is very weakly developed. Important clinical nuance: the skin flap along with the adipose tissue is extremely easy to detach from deeper tissues over a large extent. By the mechanism of injury, this resembles scalp avulsion injuries.
The deep fascia (fascia antebrachii) is unevenly structured: in the upper part, it is dense and shiny, while closer to the wrist joint, it becomes significantly thinner. Medially, it fuses rigidly with the ulna, and gives off anterior and posterior intermuscular septa toward the radius. As a result, three isolated fascial compartments are formed: anterior, posterior, and lateral.
Muscle layers of the anterior compartment
Beneath the deep fascia lies a massive complex of flexors and pronators, grouped into four layers. Almost all muscles of the first (superficial) layer originate from the medial epicondyle of the humerus and appear at the top as a single muscle belly.
| Layer | Muscle composition |
|---|---|
| First | Flexor carpi ulnaris, palmaris longus, flexor carpi radialis, pronator teres |
| Second | Flexor digitorum superficialis (broad muscle beneath the first layer) |
| Third | Flexor digitorum profundus, flexor pollicis longus |
| Fourth | Pronator quadratus (located only in the lower third) |
In the lateral fascial compartment, there are only two muscles—the brachioradialis and the supinator. The deep branch of the radial nerve runs within the thickness of the supinator muscle, which is extremely important to consider during surgeries.
Pirogov-Parona fascial space
This is a critically important deep space of the anterior compartment, located between the third and fourth muscle layers. Posteriorly, it is bounded by the interosseous membrane and the pronator quadratus; anteriorly, by the muscles of the third layer; and laterally, by the intermuscular septum and the fusion of the fascia with the ulna.
The space is notable for its impressive volume—from 100 to 300 ml of pus or blood can accumulate in it. The main feature is that it lacks a lower wall and directly transitions into the carpal tunnel (canalis carpi). Because of this, purulent processes from the hand easily spread upward to the forearm. Upward or backward infection rarely occurs, but a breakthrough into the posterior compartment is possible through a natural opening in the interosseous membrane where the anterior interosseous artery passes.
To memorize the order of the muscles of the first layer from the ulnar to the radial edge: "Ulnar Palm Radial Pronator" — m. flexor carpi ulnaris, m. palmaris longus, m. flexor carpi radialis, m. pronator teres.
Class and exam questions
Where can pus spread from the Pirogov-Parona space?
First of all, purulent streaks descend distally through the carpal tunnel onto the palmar surface of the hand. Transition to the posterior compartment of the forearm along the course of the anterior interosseous artery is also possible.
What is the feature of the blood supply to the anterior region of the forearm?
Three large arteries (radial, ulnar, and anterior interosseous) pass here, forming a dense network of muscular anastomoses. Thanks to this, the radial artery can be safely used as a shunt in heart surgeries.
Which muscle of the anterior compartment of the forearm may be normally absent?
The palmaris longus muscle (m. palmaris longus), belonging to the first muscle layer, is often absent in humans as a variant of normal anatomy.
What else to study on this topic
- Projection lines of major vessels and nerves of the forearm
- Syntopy of deep formations and bifurcation of the brachial artery
- Topography and contents of the lateral fascial compartment
- Features of the passage of the radial nerve and its deep branch
- Anatomy and contents of the carpal tunnel
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