Humerus : Structure, Attachments, Side Determination, Ossification, Functions & Clinical Anatomy
Overview
- The humerus is the bone of the arm that runs from the shoulder to the elbow.
- It is the longest bone of the upper limb.
- It connects the scapula and the two bones of the lower arm, the radius and ulna.
Structure
It has two ends – an upper end, & a lower end and a shaft.
Shaft
The shaft is rounded in the upper half and triangular in the lower half. It has three borders and three surfaces.
Borders
- Anterior border : 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.
- Lateral border : The lateral border is prominent only at the lower end where it forms the lateral supracondylar ridge. In the upper part, it is barely traceable up to the posterior surface of the greater tubercle. In the middle part, it is interrupted by the radial or spiral groove.
- Medial border : 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
- Anterolateral surface lies between the anterior and lateral borders. The upper half 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.
- 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. - 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.
Ends
Upper End
- 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.
- 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.
- 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.
- The narrow line separating the upper end of the humerus from the shaft is called the surgical neck.
Lower End
The lower end of the humerus forms the condyle which is expanded from side to side, and has articular
and non-articular parts.
The articular part includes the following.
- The capitulum is a rounded projection which articulates with the head of the radius.
- 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 non-articular part includes the following.
- 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.
- The lateral epicondyle is smaller than the medial epicondyle. Its anterolateral part has a muscular impression.
- The sharp lateral margin just above the lower end is called the lateral supracondylar ridge.
- The medial supracondylar ridge is a similar ridge on the medial side.
- 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.
Side Determination
- 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 intertubercular sulcus or bicipital groove.
Attachments
Ossification
The humerus ossifies from one primary centre and 7 secondary centres.
- The primary centre appears in the middle of the diaphysis during the 8th week of development
- The upper end ossifies from 3 secondary centres: one for the head (first year), one for the greater tubercle (second year), and one for the lesser tubercle (fifth year).
- The 3 centres fuse together during the sixth year to form one epiphysis, which fuses with the shaft during the 20th year. The epiphyseal line encircles the bone at the level of the lowest margin of the head. This is the growing end of the bone (remember that the nutrient foramen is always directed away from the growing end).
- The lower end ossifies from 4 centres which form 2 epiphyses. The centres include: one for the capitulum and the lateral flange of the trochlea (first year), one for the medial flange of the trochlea (9th year), and one for the lateral epicondyle (12th year): all three fuse during the 14th year to form one epiphysis, which fuses with the shaft at about 16 years.
- The centre for the medial epicondyle appears during 4-6 years, forms a separate epiphysis, and fuses with the shaft during the 20th years.
Functions
- It connects the scapula and the two bones of the lower arm, the radius and ulna.
Clinical Anatomy
- Nerve Injury : Three nerves are directly related to the humerus and are, therefore, liable to injury: the axillary at the surgical neck, the radial at the radial groove, and the ulnar behind the medial epicondyle.
- Humerus fracture : The common sites of fracture are the surgical neck, the shaft, and the supracondylar region. The humerus has a poor blood supply at the junction of its upper and middle thirds. Fractures at this site show delayed union or non-union.
- Supracondylar fracture is common in young age. It is produced by a fall on the outstretched hand. The lower fragment is mostly displaced backwards, so that the elbow is unduly prominent, as in dislocation of the elbow joint. However, in fracture, the three bony points of the elbow form the usual equilateral triangle. This fracture may cause injury to the median nerve. It may also lead to Volkmarm’s isehaemic contracture, and myositis ossiflcans.
- Humerus dislocation : The head of the humerus commonly dislocates inferiorly.
