Humerus Bone Anatomy, Attachments and Side Determination

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, writing and others which require movements of the shoulder joint.

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 bone acts as an attachment site for many important muscles and ligaments.

Humerus Bone anatomy

For describing the anatomy of the humerus bone, it is divided in three parts

• Proximal end
• Shaft
• Distal end

Proximal End of Humerus

Head is the 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.

The 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 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 the attachment 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.

Borders

The anterior border is a continuation of the lateral lip of the intertubercular sulcus or bicipital groove.  In its middle part, it forms the anterior margin of the deltoid tuberosity. The lower half of the anterior border is smooth and rounded.

The lateral border is prominent only at the lower end where it forms the lateral supracondylar ridge. In the middle part, it is interrupted by the radial (spiral) groove which holds the radial nerve.

Medial lip of the intertubercular sulcus or biciptal groove continues as the medial border.  About its middle, it presents a rough strip. It is continuous below with the medial supracondylar ridge.

Surfaces

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.

Between the medial and lateral borders lies posterior surface. It is marked by an oblique ridge in upper part and is crossed by the radial groove in the middle third.

Lower End of Humerus

Below the deltoid tuberosity, the humerus gradually widens, doubling its width as it approaches the elbow.

The lower end of the humerus forms the condyle which is expanded from side to side and shows lateral supracondylar ridge and medial supracondylar ridge is a similar ridge on the medial side.

Articular part

• 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 a carrying angle or the angle by which forearm projects from humeral axis.

Nonarticular part

On medial and lateral side, there are prominent bony projections called epicondyle.  Medial is called the medial epicondyle and lateral one is termed as lateral epicondyle.

The lateral epicondyle is smaller than the medial epicondyle.

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.

Attachments of Humerus

Upper Humerus

The subscapularis is inserted into the lesser tuberosity.

The greater tuberosity gives attachement to insertion of supraspinatus, infraspinatus andteres minor.

• Supraspinatus -uppermost part
• Infraspinatus – Middle
• Teres minor-  lower impression

Intertubercular sulcus also gives attachement to insertion of three muscles. The pectoralis major inserts into lateral lip of the intertubercular sulcus in two layers. The teres major is inserts into the medial lip of the intertubercular sulcus. The latissimus dorsi is inserted into the floor of the intertubercular sulcus.

[The contents of the intertubercular sulcus are  the tendon of the long head of the biceps, and its synovial sheath along with 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. On the medial side, 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 for the tendon of the long head of the biceps leaves the joint cavity

Shaft of Humerus

The deltoid tuberosity provides an insertion to deltoid muscle.

A rough area on the middle of medial border is for insertion of coracobrachialis.

The brachialis arises from the lower halves of the anteromedial and anterolateral surfaces of the shaft extending in part on to the posterior aspect.

Lower End

Upper upper two thirds of the lateral supracondylar ridge give origin to brachioradialis where as lower one third gives rise to extensor carpi radialis longus.

The anteromedial surface, near the lower end of the medial supracondylar ridge gives origin to humeal head of pronator teres. {The other end arises from ulna on the medial side of the coronoid}

The common flexor origin on anterior aspect of medial epicondyle gives rise to superficial flexors of wrist and hand, commonly called flexor forearm muscels.

The common extensor origin on lateral epicondyle provides origin to extensor muscles of forearm.

Posterior surface of lateral epicondyle gives rise to anconeus muscle.

The capsular of the elbow joint is attached to along a line that reaches proximal to the radial and coronoid fossae, anteriorly, and  on the posterior side, proximal to the olecranon fossa posteriorly.

On medial side, the line of attachment of the capsule 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 capitellum, the trochlea, and one for each epicondyle.

There are one primary and seven secondary centres .

Primary Center

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 6 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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