Features of palmar layers
The skin (cutis) of the palm is characterized by significant thickness and a powerfully developed stratum corneum. Its main topographical feature is immobility. Numerous connective tissue septa extend from the dermis deep down to the palmar aponeurosis.
Due to these fibrous septa, the subcutaneous adipose tissue (panniculus adiposus) acquires a dense honeycomb structure. This anatomical arrangement has an important clinical consequence: purulent-inflammatory processes here do not spread along a plane (outward/wide), but rapidly spread from the surface deep into the tissues.
The superficial fascia (fascia superficialis) on the palm is absent as an isolated laminar layer, as it has completely transformed into vertical septa. The proper fascia (fascia propria) covers the muscles of the thenar and hypothenar eminences with a thin plate, and in the center of the palmar depression it firmly fuses with the aponeurosis.
Palmar aponeurosis
The central framework of the hand is formed by the palmar aponeurosis (aponeurosis palmaris). It has the shape of an isosceles triangle. Its apex is directed proximally and attaches to the lower edge of the flexor retinaculum (retinaculum mm. flexorum), where bundles of the palmaris longus muscle also blend in. The base of the triangle faces distally toward the interdigital spaces.
The aponeurosis consists of powerful longitudinal fibers that unite into 4 bundles extending to the bases of digits II–V. In the distal part, these bundles are bound by transverse fibers — fasciculi transversi. On the lateral and medial sides, the aponeurosis is clearly demarcated by the thenar and hypothenar folds, respectively.
Synovial tendon sheaths
The flexor tendon muscles glide within isolated synovial sheaths, the boundaries of which are critically important for understanding the pathways of infection spread:
- Radial sheath (vagina tendinis m. flexoris pollicis longi). Contains the tendon of the flexor pollicis longus. It begins blindly in the Pirogov-Parona space 2 cm proximal to the flexor retinaculum and extends to the base of the distal phalanx of digit I.
- Common (ulnar) sheath (vagina communis tendinum musculorum flexorum). Encloses the flexor tendons of digits II–V. Proximally, it also originates in the Pirogov-Parona space. Distally, it extends to the terminal phalanx only for the little finger (digit V). For digits II–IV, the sheath terminates at the level of the mid-metacarpals.
- Digital synovial sheaths (vaginae synoviales digitorum manus). Protect the tendons of digits II–IV from the metacarpal heads to the bases of the distal phalanges.
Segments of the tendons of digits II–IV located within the middle fascial space's subcutaneous tissue are not covered by a synovial sheath. In 10% of cases, the radial and ulnar sheaths communicate proximally, which can lead to the development of severe crossed (V-shaped) phlegmon, where pus from digit I rapidly flows to digit V (and vice versa).
Vessels, nerves, and forbidden zone
Blood supply to the superficial layers is provided by the superficial palmar arterial arch (arcus palmaris superficialis). It projects onto the proximal transverse crease of the palm. The arch is formed as an anastomosis between the main trunk of the ulnar artery (a. ulnaris) and the superficial branch of the radial artery. Common and then proper palmar digital arteries branch off from it.
Common digital nerves lie beneath the arterial arch. Three of them branch off from the median nerve (n. medianus), providing skin sensitivity for digits I, II, III, and the radial surface of digit IV.
Of exceptionally high clinical importance is Kanavel's forbidden zone (zona prohibita). It is localized in the proximal third of the thenar fold. At this point, the motor branch of the median nerve to the short muscles of digit I projects and branches off. Any incisions in this zone are strictly prohibited to avoid paralysis and loss of thumb opposition.
Briefly: answers to topical questions
Which muscles are part of the thenar eminence?
The muscles of the thenar eminence include:
- Abductor pollicis brevis muscle (m. abductor pollicis brevis).
- Opponens pollicis muscle (m. opponens pollicis).
- Flexor pollicis brevis (m. flexor pollicis brevis).
- Adductor pollicis (m. adductor pollicis).
Additionally, the tendon of the flexor pollicis longus passes through the thenar region (tendo m. flexor pollicis longus).
Which vessels form the deep palmar arterial arch?
The deep palmar arterial arch (arcus palmaris profundus) is formed by the radial artery and the deep palmar branch of the ulnar artery.
- Radial artery (a. radialis) — is the main trunk that enters the subtendinous space through the first intermetacarpal space from the anatomical snuffbox.
- Deep palmar branch of the ulnar artery (r. palmaris profundus a. ulnaris) — anastomoses with the radial artery, thereby completing the arch.
The arch is located in the cellular tissue of the subtendinous space at the level of the bases of metacarpals II–IV.
Which nerve provides sensation to the palmar surface of digit V and the ulnar half of digit IV?
Sensation to the palmar surface of digit V and the ulnar half of digit IV is provided by the ulnar nerve (n. ulnaris). Innervation is carried out via its superficial branch (r. superficialis n. ulnaris), which gives off on the palmar surface:
- Proper digital nerve (n. digitalis proprius) — runs towards the little finger.
- Common digital nerve (n. digitalis communis) — divides into two proper digital nerves for adjacent sides of digits IV and V.
Collectively, the ulnar nerve innervates 1.5 digits of the medial part of the hand on the palmar surface.
What are the walls of the Pirogov-Parona space formed by?
The walls of the Parona-Pirogov space are formed by muscles, fasciae, and bony structures of the forearm.
- Anterior wall — posterior (deep) surface of the muscles of the third layer: flexor pollicis longus (m. flexor pollicis longus) and flexor digitorum profundus (m. flexor digitorum profundus).
- Posterior wall — interosseous membrane (membrana interossea) and pronator quadratus (m. pronator quadratus) along with its fascia.
- Lateral wall — anterior radial intermuscular septum.
- Medial wall — proper fascia of the forearm fused with the ulna.
- Superior boundary — attachment site of the deep flexors to the interosseous membrane.
There is no inferior wall, as the space transitions directly into the carpal tunnel (canalis carpi).
To remember the course of synovial sheaths, the "border digits" rule is used: the sheaths of the border digits (I and V) run continuously from the forearm to the fingertips, while those of the middle digits (II, III, IV) are interrupted in the palm.
Class and exam questions
Why do purulent inflammations of the palm often spread deep?
The subcutaneous tissue of the palm has a honeycomb structure due to dense vertical fibrous septa. They prevent the spread of pus laterally across planes, directing the exudate into deep layers.
Why is a cross (V-shaped) phlegmon dangerous?
In approximately 10% of people, the synovial sheaths of the thumb and little finger communicate proximally (near the wrist). In case of infection of either digit, pus can rapidly spread to the other, affecting the entire hand.
What is Kanavel's forbidden zone?
This is an area in the proximal third of the thenar fold. Surgical incisions must not be made here because the motor branch of the median nerve to the thumb muscles runs superficially.
What else to study on this topic
- Cellular spaces of the middle compartment (subaponeurotic and subtendinous)
- Topography of the deep palmar arterial arch
- Muscles of the thenar and hypothenar eminences
- Zones of hand innervation by the ulnar nerve
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