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Humerus

The humerus is a long bone forming the skeleton of the upper arm. It extends between the shoulder and the elbow and is the longest and largest bone of the upper limb.

The humerus is connected with the scapula at one end, and with both forearm bones (radius and ulna) on the other end. The proximal end of the humerus articulates with the glenoid cavity of the scapula at the glenohumeral joint. At the distal end, the humerus articulates with the head of the radius and the trochlear notch of the ulna, forming the elbow joint.

Like other long bones, the humerus has three main parts - a proximal end, a shaft, and a distal end. All of these parts have important anatomical landmarks.

Proximal end of humerus

The proximal end or extremity of the humerus consists of the head, an anatomical neck, and the greater and lesser tubercles.

The head of the humerus has a hemispherical shape and has a smooth articular surface, which is covered by hyaline cartilage. In the anatomical position, the head faces in a medial, superior, and posterior direction, where it articulates with the glenoid cavity of the scapula.

The anatomical neck of the humerus is a narrowing below the articular surface of the head. The joint capsule of the shoulder joint attaches here.

The greater tubercle is the most lateral part of the proximal end of the humerus. Three impressions mark it at the upper posterior aspect, all of which serve for the attachment of muscles. From superior to inferior, the muscles that attach at these impressions are three of the rotator cuff muscles:

The lateral aspect of the greater tubercle is covered by the deltoid muscle, which gives the shoulder its usual rounded shape. Multiple vascular foramina also mark the lateral aspect of the greater tubercle.

The lesser tubercle is found anterior to the anatomical neck and has a smooth, palpable muscular impression. The lateral aspect of this tubercle forms the medial margin of the intertubercular sulcus. The lesser tubercle of the humerus gives attachment to the subscapularis muscle. Also, the transverse ligament of the shoulder attaches here.

There is a deep groove separating the two tubercles, which is called the intertubercular sulcus. It is also known as the bicipital groove. The long tendon of the biceps brachii and a branch of the ascending circumflex humeral artery lie within the intertubercular sulcus. A lateral lip and a medial lip form the intertubercular sulcus. The lateral lip is also known as the crest of the greater tubercle, and it gives attachment to the tendon of the pectoralis major muscle. The medial lip serves as the attachment site for the teres major muscle. Also, the tendon of the latissimus dorsi muscle attaches to its posterior aspect.

There is also a slight narrowing below the tubercles, which is known as the surgical neck of the humerus. It is a common fracture site. The axillary nerve and the posterior circumflex humeral artery lie close to the bone here. Here, the proximal extremity ends and continues as the shaft of the humerus.

Shaft of humerus

The shaft or body is the middle part of the humerus, and it gives attachment to several muscles. Cross-section views reveal that the proximal half of the shaft is circular, while its distal half is triangular and flattened. The shaft of the humerus has three borders and three surfaces.

The borders of the shaft:

  • the anterior border extends from the greater tubercle downward almost to the end of the humerus, and its proximal end is continuous with the lateral lip of the intertubercular sulcus;
  • the lateral border begins distal to the greater tubercle, and it gets ticker distally to form the lateral supracondylar ridge, its middle portion is adjacent to the rough “V” shaped area called the deltoid tuberosity;
  • the medial border forms the medial supracondylar ridge distally, and it is interrupted in its medial third by the radial groove, where the radial nerve and the deep brachial artery are situated.

The surfaces of the shaft:

  • the anterolateral surface is the area between the anterior and lateral borders of the shaft, its proximal surface is smooth and is mostly covered by the deltoid muscle; the deltoid inserts into the deltoid tuberosity, which is located around the middle of this surface; the lateral part of the brachialis originates from the distal portion of the anterolateral surface, as well as from the proximal two-thirds of the lateral supracondylar ridge;
  • the anteromedial surface is the area between the anterior and medial borders; the coracobrachialis muscle attaches approximately in the mid-portion of this surface, while the distal half of the anteromedial surface is mostly covered by the medial portion of the brachialis muscle;
  • the posterior surface is located between the medial and lateral borders of the shaft. Most of it is covered by the medial head of the triceps brachii muscle, while a ridge on its proximal-third also gives attachment to the lateral head of the triceps brachii.

Distal end of humerus

The distal end or extremity of the humerus has both articular and non-articular parts. The articular part is a modified condyle, which consists of a medial trochlea and a lateral capitulum, separated by a faint groove. This part of the humerus articulates with both ulna and radius. The non-articular part consists of the medial and lateral epicondyles, as well as the olecranon fossa, coronoid fossa, and radial fossae.

The trochlea is a projection that is shaped like a pulley and located medially, extending onto the posterior aspect of the humerus. It articulates with the ulna at the trochlear notch.

Lateral to the trochlea is the capitulum, which is a rounded and convex projection on the distal end of the humerus. It articulates with the head of the radius.

The medial epicondyle is a blunt projection that forms the end of the medial border of the humerus. It is located superomedially to the medial condyle. The ulnar nerve passes in a groove on the posterior aspect of the medial epicondyle. The anterior surface of the medial epicondyle provides origin sites for the following superficial muscles of the anterior compartment of the forearm:

The lateral epicondyle is smaller than the medial, and it is where the lateral border of the humerus ends. It is marked by an impression where seven superficial muscles of the lateral and posterior compartments of the upper limb, which include the following muscles:

The olecranon fossa is a deep depression on the posterior surface of the distal end, superior to the trochlea. During the extension of the elbow, the tip of the ulnar olecranon process lodges into this fossa.

The coronoid fossa is a smaller depression that is also located superior to the trochlea but on the anterior surface of the humerus. Upon elbow flexion, the coronoid process of the ulna lodges into this fossa.

Lateral to the coronoid fossa and superior to the capitulum of the humerus is another hallow area called the radial fossa—the margin of the head of the radius lodges into this fossa upon full elbow flexion.