The humerus is the only bone of the upper arm. It is both the longest and largest bone of the upper limb. It consists of expanded ends and a shaft. It articulates with the scapula at the shoulder joint and the radius and ulna at the elbow joint.
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When the shoulder is abducted, it may be possible to feel the head of the humerus deep inside the axilla, particularly with internal and external rotation of the shoulder. The lesser tuberosity may be felt through the bulk of the deltoid about 1 inch (3cm) below the acromion, particularly on rotation, whilst the greater tuberosity is entirely covered by muscle. Likewise, the shaft is not directly palpable due to its muscular surrounds.
The medial and lateral epicondyles are both subcutaneous and can be both seen and felt on their respective sides of the elbow. The posterior surface of the medial epicondyle is crossed by the ulnar nerve, which can be felt by rolling against the bone. The cubital fossa lies anteriorly to the elbow joint, bounded superiorly by a line joining the two epicondyles.
The upper end of the humerus is dominated by the smooth hemispheric head, which points posteriorly and superomedially to act as an articular surface with the scapula. In life it is covered with hyaline cartilage which is thicker centrally.
The anatomical neck directly adjoins the margin of the head and is only a slight constriction in the diameter of the humerus. It marks an area between the head and the attachment of the shoulder joint capsule, which is only deficient at the intertubercular sulcus, where the tendon of the long head of biceps passes. Medially, the capsule continues an additional 1cm onto the shaft.
The greater tubercle (greater tuberosity) is the most lateral part of the proximal humerus. In the anatomical position it lies inferolateral to the acromion. Posterosuperiorly there are three notches for the tendons of supraspinatus, infraspinatus and teres minor. The lesser tubercle (lesser tuberosity) lies anteriorly and just beyond the anatomical neck. Between the two is the intertubercular sulcus, which continues distally as the shallow bicipital groove for the tendon of biceps brachii (the two are sometimes used interchangeably). Beyond the tubercles, the proximal end tapers to the rather ill-defined surgical neck, which lies lateral to the axillary nerve and posterior humeral circumflex artery.
The shaft is cyclindrical proximally, but becomes triangular in cross section distally and anteroposteriorly compressed. It has three borders (anterior, lateral and medial) and three surfaces (anterolateral, anteromedial and posterior). The radial groove (spiral groove, radial sulcus) is an oblique depression that runs posteriorly from medial to lateral sides between the attachments for the lateral and medial heads of triceps, ending about half way down the shaft beneath the deltoid tuberosity. An oblique ridge runs above this groove. The deltoid tuberosity sits just proximal to the midpoint of the humerus on the lateral border. The nutrient foramen of the humerus is near the midpoint of the anteromedial surface, its canal directed distally.
The distal humerus is widened transversely into a 'condyle'. The medial epicondyle is a blunt projection marking the end of the medial border, which widens first into the medial supracondylar ridge. The lateral epicondyle similarly marks the end of the lateral border after formation of the lateral supracondylar ridge. The medial epicondyle projects beyond the joint surface of the elbow, however the lateral epicondyle does not. The joint surface itself is formed from the lateral capitulum, a rounded surface that articulates with the radial head, and the medial trochlea, which is shaped somewhat like a pulley and articulates with the trochlear notch of the ulna. The olecranon fossa is a deep hollow posterior to the condyle, into which the olecranon slides in full extension. A medial coronoid fossa and lateral radial fossa on the anterior surface accommodate the coronoid process of the ulna and radial head in full flexion respectively.
- Triceps has three heads, two of which arise from the humerus:
- The lateral head arises from a line on the upper half of the posterior surface
- The medial head arises from a large area inferomedial to the radial groove, extending over almost all the distal half of the posterior surface
- Brachialis arises from a large area covering most of the anterior surface of the distal half of the humerus
- Brachioradialis arises from the proximal two thirds of the lateral supracondylar ridge
- Extensor carpi radialis longus arises from the distal third of the lateral supracondylar ridge
- The humeral head of pronator teres arises from the medial epicondyle and medial supracondylar ridge
- Anconeus arises from the distal part of the posterior aspect of the lateral epicondyle
- The common flexor tendon arises from the medial epicondyle
- The common extensor tendon arises from the lateral epicondyle.
- The four muscles of the rotator cuff:
- Supraspinatus inserts into the superior facet of the greater tubercle
- Subscapularis inserts into the lesser tuberosity and upper medial lip of the bicipital groove
- Infraspinatus inserts into the middle facet of the greater tubercle (posteriorly)
- Teres minor inserts into the inferior facet of the greater tubercle
- Teres major inserts into the medial lip of the bicipital groove
- Latissimus dorsi inserts into the floor of the bicipital groove
- Pectoralis major inserts into the lateral lip of the bicipital groove, as well as the anterior surface of the deltoid tuberosity
- The deltoid inserts into the middle of the lateral surface including the deltoid tuberosity
- Coracobrachialis inserts into a portion of the middle third of the medial border.
- The glenohumeral (shoulder) joint
- The elbow joint, consisting of humeroulnar and humeroradial joints.
- The brachial artery runs along the medial surface of the humerus for most of its course, before tracking laterally to enter the cubital fossa
- The anterior humeral circumflex artery and posterior humeral circumflex artery run around the surgical neck of humerus, anastamosing laterally and giving branches to the shoulder joint and muscles
- The profunda brachii artery passes in the radial groove adjacent to the radial nerve
- The median nerve runs directly lateral to the brachial artery in the upper arm, crossing medially at midshaft to lie medially in the cubital fossa
- The radial nerve runs in the radial groove on the posterior aspect of the humerus, piercing the intermuscular septum at midshaft to enter the lateral forearm, where it continues distally to divide into terminal branches over the lateral epicondyle
- The ulnar nerve lies medial to the brachial artery, piercing the medial intermuscular septum at midshaft to lie along this septum, and passes posterior to the medial epicondyle.
- The musculocutaneous nerve runs laterally between biceps and brachialis, usually adherent to the deep surface of biceps
- The axillary nerve runs medially around the surgical neck of the humerus
- The intertubercular sulcus contains the long tendon of biceps brachii and an ascending branch of the anterior humeral circumflex artery
- A subacromial bursa may separate the greater tubercle from the deltoid muscle.
There are eight centres of ossification, namely the head, shaft, greater and lesser tubercles, both epicondyles, and two within the trochlea. The shaft ossifies centrally beginning in the 8th week. By the sixth year the head and tubercles unite. The distal centres may not unite until puberty, with the medial epiphysis often remaining unattached until the twentieth year.
A supracondylar process up to 2cm in length may project from the anteromedial surface of the shaft roughly 5cm proximal to the medial epicondyle. This may hook over and connect to the medial border by a fibrous band, forming a tunnel that protects the median nerve and/or brachial artery.