Humerus

Anatomical Position in the Upper Limb

The free part of the upper limb (pars libera membri superioris) is anatomically divided into three key regions:

1. Arm (brachium) — its skeletal framework consists exclusively of the humerus (humerus).
2. Forearm (antebrachium) — here the skeleton is formed by two bones: the ulna (ulna) and the radius (radius).
3. Hand (manus) — formed by multiple carpal, metacarpal, and phalangeal bones (ossa manus).

As a typical long bone, the humerus features a central shaft (diaphysis) and two expanded extremities: the proximal epiphysis (closest to the trunk) and the distal epiphysis (furthest from the trunk).

Proximal Epiphysis

The upper end of the bone participates in the formation of the glenohumeral joint. It features a smooth, hemispherical head of the humerus (caput humeri), which serves as the articular surface for articulation with the glenoid cavity of the scapula.

Immediately bordering the articular margin of the head lies a narrow groove called the anatomic neck (collum anatomicum), corresponding to the metaphyseal zone. Lateral to this neck are two bony elevations for muscle attachment:

• Lesser tubercle (tuberculum minus) — directed anteriorly/medially.
• Greater tubercle (tuberculum majus) — occupies a lateral position.

Extending downward from each tubercle are corresponding crests: the crest of the greater tubercle (crista tuberculi majoris) and the crest of the lesser tubercle (crista tuberculi minoris). The depression between these elevations forms the intertubercular sulcus (sulcus intertubercularis).

Just distal to the tubercles, the bone narrows significantly. This site is called the surgical neck (collum chirurgicum) — fractures occur here most frequently.

Shaft of the Humerus (Diaphysis)

The diaphysis, or shaft of the humerus (corpus humeri), is cylindrical in its upper portion and becomes triangular distally. It clearly presents three surfaces:

• Posterior surface (facies posterior)
• Anteromedial surface (facies anteromedialis)
• Anterolateral surface (facies anterolateralis)

In the middle third of the anterolateral surface lies the roughened deltoid tuberosity (tuberositas deltoidea). Inferior to this tuberosity, spiraling obliquely across the bone, runs the radial groove (sulcus nervi radialis), which lodges the radial nerve and profunda brachii artery.

Distal Epiphysis

The lower end of the bone is widened and forms the condyle of the humerus (condylus humeri), covered with articular cartilage for articulation with the forearm bones. The condyle consists of two main articular components:

• Laterally, the spherical capitulum (capitulum humeri) — articulates with the head of the radius.
• Medially, the pulley-shaped trochlea (trochlea humeri) — articulates with the trochlear notch of the ulna.

Flanking the condyle are the epicondyles: the medial epicondyle (epicondylus medialis) and lateral epicondyle (epicondylus lateralis). Corresponding supraepicondylar ridges (crista epicondyli medialis et lateralis) extend proximally from them along the shaft. On the posterior aspect of the medial epicondyle, the sulcus for the ulnar nerve (sulcus nervi ulnaris) is readily palpable.

To accommodate full range of motion at the elbow joint, the distal epiphysis features several fossae:

1. Anteriorly, above the capitulum — the radial fossa (fossa radialis).
2. Anteriorly, above the trochlea — the coronoid fossa (fossa coronoidea).
3. Posteriorly, above the trochlea — a large and deep olecranon fossa (fossa olecrani).

Side Determination (Right vs. Left)

In practical anatomy, determining whether a humerus belongs to the right or left side of the body relies on a structured orientation algorithm:

1. Superior/Inferior: Orient the bone so that the proximal epiphysis with the rounded head is pointing upward, and the distal epiphysis with the condyle points downward.
2. Anterior/Posterior: Locate the deep olecranon fossa on the distal epiphysis — it is always situated on the posterior surface. The smaller radial and coronoid fossae must face anteriorly.
3. Lateral/Medial: The humeral head (caput humeri), lesser tubercle, trochlea, and larger medial epicondyle are always directed medially (toward the trunk). The greater tubercle, deltoid tuberosity, and capitulum face laterally.