Hand anatomy

Segments of the upper extremity and anatomy of the hand

The free upper extremity is anatomically divided into three key segments. The proximal segment is the arm (based on the humerus). The middle segment is the forearm, which includes the radius and ulna. The distal segment is the hand itself, consisting of the carpal bones, metacarpals, and phalanges.

In clinical practice, it is important to strictly adhere to anatomical terminology: the arm refers only to the segment from the shoulder to the elbow joint, the region above it is the shoulder girdle, and the area from the elbow to the wrist is called the forearm.

Radiocarpal joint and biomechanics

The radiocarpal joint is a complex synovial articulation. It is formed by the radius and ulna, the eight carpal bones, and the metacarpal bones.

• The distal radioulnar joint (DRUJ) plays a crucial role in providing rotational movements of the forearm — supination and pronation.
• The biomechanics of the hand are structured such that the main movements are executed through the proximal row of carpal bones (scaphoid, lunate, and triquetrum). This row functions as an elastic bridge (intercalated segment) between the forearm bones and the stiffer distal carpal row.
• The role of the "keystone" (keystone) in this structure is played by the lunate bone (lunate).

To assess the proper anatomical alignment of the carpal bones on posteroanterior radiographs, Gilula's arcs (arcs of Gilula) are analyzed — lines reflecting the continuity of the articular surfaces.

Ligaments and stabilizing complexes

Wrist stability depends on a robust ligamentous apparatus:

1. Intrinsic ligaments (scapholunate — scapholunate and lunotriquetral — lunotriquetral) maintain connections between individual bone segments within the proximal row.
2. Extrinsic ligaments (numerous dorsal — dorsal and palmar — volar) provide general stability between the forearm and both rows of carpal bones.

Of particular importance is the triangular fibrocartilage complex (TFC complex). It stabilizes the distal radioulnar and ulnocarpal joints. Anatomically, it is a cartilaginous disc that extends from the ulnar margin of the distal radius to the fovea (fovea) and the apex of the ulnar styloid process. The edges of the disc blend tightly with the radioulnar ligaments and the tendon sheath of the extensor carpi ulnaris (ECU).

The first carpometacarpal joint (CMC), located between the trapezium (trapezium) and the first metacarpal bone, deserves special mention. It has an isolated synovial cavity and features maximum mobility to ensure thumb function.

Carpal tunnel and retinacula

Located on the palmar aspect of the wrist is the carpal tunnel (Carpal tunnel). This is a narrow anatomical space through which the finger flexor tendons and the median nerve (median nerve) pass into the hand.

• Superficially, the canal is closed by the flexor retinaculum (Flexor retinaculum). This dense structure is stretched between the hook of the hamate (hook of hamate) and the pisiform (pisiform) on one side, and the scaphoid (scaphoid) and trapezium on the other.
• On the dorsal side of the wrist is the extensor retinaculum (Extensor retinaculum), which fixes and stabilizes the extensor tendons at the level of the first row of carpal bones.

Clinical anatomy: radius fractures

Distal radius fractures at a typical site are closely linked to the biomechanics of hand trauma. There are two main types:

• Colles fracture (extension type). Occurs when falling on an extended hand. The distal fragment is displaced dorsally and radially. On the lateral radiograph, the angle of the articular surface exceeds 90° (normal palmar tilt is lost).
• Smith's fracture (flexion type). Occurs when falling on a flexed hand. The fragment is displaced palmarward, and the radiographic angle is less than 80°.

Normally, on the lateral view, the angle between the diaphyseal axis and the articular surface line is 80°–90°. On the anteroposterior view, the radioulnar angle (radial inclination angle) is assessed, which is normally 30°.

Neurological examination: brachioradialis reflex

Knowledge of wrist anatomy is essential for performing a neurological examination. When testing the brachioradialis reflex, the function of the brachioradialis muscle (m. brachioradialis) is evaluated.

The patient's arm should be semi-flexed at the elbow joint, and the hand in a slight pronation position. The neurological hammer strikes the styloid process of the radius (processus styloideus radii) or its lower third. The normal response is muscle contraction, resulting in forearm flexion.

Class and exam questions

What else to study on this topic

• Diagnostic imaging and injuries of the ulnar collateral ligament (UCL)
• Osteochondritis dissecans of the capitellum on MRI
• Early signs of elbow osteoarthritis on CT
• Technique for examining biceps and triceps tendon reflexes

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.