Anatomy of Humerus

The humerus is a bone that connects shoulder and home. The humerus has an upper end, a lower end and a shaft. Following are salient features.

1.The head is directed medially, backwards and upwards. It articulates with the glenoid cavity of the scapula to form the shoulder joint. The head forms about one third of a sphere and is much larger than the glenoid cavity.
2.The line separating the head from the rest of the upper end is called the anatomical neck.
3.The lesser tubercle is an elevation on the anterior aspect of the upper end.
4.The greater tubercle is an elevation that forms the lateral part of the upper end. Its posterior aspect is marked by three impressions-upper, middle and lower.
5.The intertubercular sulcus (or bicipital groove) separates the lesser tubercle (medially) from the anterior part of the greater tubercle. The sulcus has medial and lateral lips that represent downward prolongations of the lesser and greater tubercles.
6.The line separating the upper end of the humerus from the shaft is called the surgical neck.

Humerus anterior and posterior view

The Shaft
The shaft is rounded in the upper half and triangular in the lower half. It has three borders and three surfaces.

Borders
1. The upper one third of the anterior border forms the lateral lip of the intertubercular sulcus. In its middle part it forms the anterior margin of the deltoid tuberosity. The lower half of the anterior border is smooth and rounded.
2. The lateral border is prominent only at the lower end where it forms the lateral supracondylar ridge. In the upper part is is barely traceable up to the posterior surface of the greater tubercle. In the middle part it is interrupted by the radial (spiral) groove.
3. The upper part of the medial border forms the medial lip of the intertubercular sulcus. About its middle it presents a rough strip. It is continuous below with the medial der forms the lateral lip of the intertubercular sulcus. In its middle part it forms the anterior margin of the deltoid tuberosity. The lower half of the anterior border is smooth and rounded.
4. The lateral border is prominent only at the lower end where it forms the lateral supracondylar ridge. In the upper part is is barely traceable up to the posterior surface of the greater tubercle. In the middle part it is interrupted by the radial (spiral) groove.
5. The upper part of the medial border forms the medial lip of the intertubercular sulcus. About its middle it presents a rough strip. It is continuous below with the medial supracondylar ridge.

Surfaces

1. The anterolateral surface lies between the anterior and lateral borders. The upper ½ of this surface is covered by the deltoid. A little above the middle it is marked by a V-shaped deltoid tuberosity. Behind the deltoid tuberosity the radial groove runs downwards and forwards across the surface.
2. The anteromedial surface lies between the anterior and medial borders. Its upper one third is narrow and forms the floor of the intertubercular sulcus. A nutrient foramen is seen on this surface near its middle (near the medial border).
3. The posterior surface lies between the medial and lateral borders. Its upper part is marked by an oblique ridge. The middle one third is crossed by the radial groove.

Lower End
The lower end of the humerus forms the condyle which is expanded from side to side, and has articular and nonarticular parts. The articular part includes the following.
1. The capitulum is rounded projection which articulates with the head of the radius.
2. The trochlea is a pulley shaped surface. It articulates with the trochlear notch of the ulna. The medial edge of the trochlea projects down 6 mm more than the lateral edge: this results in the formation of the carrying angle.

The nonarticular part includes the following:
1. The medial epicondyle is a prominent bony projection on the medial side of the lower end. It is subcutaneous and is easily felt on the medial side of the elbow.
2. The lateral epicondyle is smaller than the medial epicondyle. Its anterolateral part has a muscular impression.
3. The sharp lateral margin just above the lower end is called the lateral supracondylar ridge.
4. The medial supracondylar ridge is a similar ridge on the medial side.
5. The coronoid fossa is a depression just above the anterior aspect of the trochlea. It accommodates the coronoid process of the ulna when the elbow is flexed.
6. The radial fossa is a depression present just above the anterior aspect of the capitulum. It accommodates the head of the radius when the elbow is flexed.
7. The olecranon fossa lies just above the posterior aspect of the trochlea. It accommodates the olecranon process of the ulna when the elbow is extended.

How To Determine Side

1. The upper end is rounded to form the head. The lower end is expanded from side to side and flattened from before backwards.
2. The head is directed medially and backwards.
3. The lesser tubercle projects from the front of the upper end and is limited laterally by the intertuberacular sulcus (bicipital groove)

Muscles and ligments attached on humerus move shoulder and elbow. Attachments on humerus are as follows

1. The subscapularis is inserted into the lesser tubercle.
2. The supraspinatus is inserted into the uppermost impression on the greater tubercle.
3. The infraspinatus is inserted into the middle impression on the greater tubercle.
4. The teres minor is inserted into the lower impression on the greater tubercle.
5. The pectoralis major is inserted into the lateral lip of the intertubercular sulcus. The insertion is bilaminar.
6. The latissimus dorsi is inserted into the floor of the intertubercular sulcus.
7. The teres major is inserted into the medial lip of the intertubercular sulcus.
8. The contents of the intertubercular sulcus are (i) the tendon of the long head of the biceps, and its synovial sheath; and (ii) the ascending branch of the anterior circumflex humeral artery.
9. The deltoid is inserted into the deltoid tuberosity.
10. The coracobrachialis is inserted into the rough area on the middle of the medial border.
11. The brachialis arises from the lower halves of the anteromedial and anterolateral surfaces of the shaft. Part of the area extends on to the posterior aspect.
12. The brachioradialis arises from the upper two thirds of the lateral supracondylar ridge.
13. The extensor carpi radialis longus arises from the lower one third of the lateral supracondylar ridge.
14. The pronator teres (humeral head) arises from the anteromedial surface, near the lower end of the medial supracondylar ridge.
15. The superficial flexor muscles of the forearm arise by a common origin from the anterior aspect of the medial epicondyle. This is called the common flexor origin.
16. The superficial extensor muscles of the forearm have a common origin from the lateral epicondyle. This is called the common extensor origin.
17. The anconeus arises from the posterior surface of the lateral epicondyle.

18. The capsular ligament of the shoulder joint is attached to the anatomical neck except on the medial side where the line of attachment dips down by about a centimeter to include a small area of the shaft within the joint cavity. The line is interrupted at the intertubercular sulcus to provide an aperture through which the tendon of the long head of the biceps leaves the joint cavity.

19. The capsular ligament of the elbow joint is attached to the lower end along a line that reaches the upper limits of the radial and coronoid fossae, anteriorly; and of the olecranon fossa posteriorly; so that these fossae lie within the joint cavity. Medially the line of attachment passes between the medial epicondyle and the trochlea. On the lateral side it passes between the lateral epicondyle and the capitulum.

Text adapted from: Human Anatomy by BD Chaurasia

Image Credit: Wikipedia

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