The humerus is a bone that connects shoulder and elbow.The humerus is the largest bone in the upper limb and the only bone in the upper arm. Humerus movements participate in many day to day essential activities like throwing, lifting, and writing.
Proximally, humerus forms part of the shoulder joint whereas distally, the humerus articulates with the radius and ulna to form the elbow joint.
The humerus acts as an attachment site for many muscles and ligaments.
For describing anatomy of humerus bone, it is divided in three parts
Proximal End of Humerus
Head is most proximal region of the bone. Head is almost spherical in shape and flattens a bit where it attaches to the scapula and clavicle.
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.
The line separating the head from the rest of the upper end is called the anatomical neck.
There are two tubercles present. Lesser tubercle is an elevation on the anterior aspect of the upper end. 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.
Bicipital groove separates the 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 and serves for attachement of ligaments and muscles
The line separating the upper end of the humerus from the shaft is called the surgical neck.
Shaft of Humerus
The shaft is rounded in the upper half and triangular in the lower half. It has three borders and three surfaces.
Anterior border is continuation of the lateral lip of the intertubercular sulcus or bicipital goove. In its middle part it forms the anterior margin of the deltoid tuberosity. The lower half of the anterior border is smooth and rounded.
Anterolateral surface lies between the anterior and lateral borders. The upper half of this surface is covered by the deltoid. It is marked by a V shaped deltoid tuberosity a little above the middle.
The anteromedial surface lies between the anterior and medial borders. Its upper one third is narrow and forms the floor of the bicipital groove. Near the medial border, at almost middle, a nutrient foramen is seen.
Lower End of Humerus
Below the deltoid tuberosity, the humerus gradually widens, doubling its width as it approaches the elbow.
- The capitellum is rounded projection which articulates with the head of the radius. to form radio-capitellar joint.
- The trochlea is a pulley shaped surface. It articulates with the trochlear notch of the ulna to form elbow joint. The medial edge of the trochlea projects down 6 mm more than the lateral edge. This leads to carrying angle or the angle by which forearm projects from humeral axis.
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.
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.
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.
Attachements of Humerus
- The subscapularis is inserted into the lesser tubercle.
- The supraspinatus is inserted into the uppermost impression on the greater tubercle.
- The infraspinatus is inserted into the middle impression on the greater tubercle.
- The teres minor is inserted into the lower impression on the greater tubercle.
- The pectoralis major is inserted into the lateral lip of the intertubercular sulcus. The insertion is bilaminar.
- The latissimus dorsi is inserted into the floor of the intertubercular sulcus.
- The teres major is inserted into the medial lip of the intertubercular sulcus.
- 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.
- Anatomical neck provides attachment to the capsular ligament of the shoulder joint 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. An aperture is provided at the sulcus through which the tendon of the long head of the biceps leaves the joint cavity.
Shaft of Humerus
- The deltoid is inserted into the deltoid tuberosity.
- The coracobrachialis is inserted into the rough area on the middle of the medial border.
- 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.
- The brachioradialis arises from the upper two thirds of the lateral supracondylar ridge.
- The extensor carpi radialis longus arises from the lower one third of the lateral supracondylar ridge.
- Humeal head of pronator teresarises from the anteromedial surface, near the lower end of the medial supracondylar ridge.
- 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.
- The superficial extensor muscles of the forearm have a common origin from the lateral epicondyle. This is called the common extensor origin.
- The anconeus arises from the posterior surface of the lateral epicondyle.
- 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. 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 capitellum.
Side Determination of Humerus Bone
- The upper end is rounded to form the head. The lower end is expanded from side to side and flattened from before backwards.
- The head is directed medially and backwards.
- The lesser tubercle projects from the front of the upper end and is limited laterally by the intertuberacular sulcus (bicipital groove)
Ossification of Humerus
The humerus is ossified from eight centers, one for each of the following parts of humerus – the body, the head, the greater tubercle, the lesser tubercle, the capitulum, the trochlea, and one for each epicondyle.
There are one primary and seven secondary centres .
The center for the body appears near the middle of the diaphysis in the eighth week of fetal life, and soon extends toward the extremities. This is the primary center.
At birth the humerus is ossified in nearly its whole length, only the extremities remaining cartilaginous.
Upper End of Humerus
The ossification centers appear as follow
Head of the bone – First year
Greater tubercle – second year
Lesser tubercle – Fifth year.
Centers for the head and tubercles join by six years to form a single large epiphysis. This fuses with the body about the twentieth year.
Lower End of Humerus
The ossification centers for lower end are rembered by CRITOE. That is the order of appearance.
The timeline is as follows
- Capitellum – 1-2 year
- Radius – 3-4year
- Internal or medial epicondyle – 5-6 years
- Trochlea – 8 years
- Olecranon – 9-10 years
- External or lateral epicondyle – 11-12 years
About the sixteenth or seventeenth year, the lateral epicondyle and both portions of the articulating surface, having already joined, unite with the body, and at the eighteenth year the medial epicondyle becomes joined to it.
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