Structure
The humerus has an expanded upper end for the shoulder, a shaft that is cylindrical above and triangular below, and a flattened lower end for the elbow.
Upper end
- Head: a smooth hemisphere facing medially, upwards and backwards towards the glenoid cavity.
- Anatomical neck: the narrow groove at the edge of the articular surface, where the capsule attaches.
- Greater tubercle: the lateral projection, with three facets on its upper and posterior surfaces.
- Lesser tubercle: the smaller projection on the front.
- Intertubercular sulcus (bicipital groove): between the tubercles, carrying the tendon of the long head of biceps, bridged by the transverse humeral ligament.
- Surgical neck: the narrowing of the shaft just below the tubercles, the usual fracture site.
Shaft
The deltoid tuberosity is a roughened V halfway down the lateral surface. The radial groove runs obliquely down and laterally across the back of the shaft. The medial and lateral supracondylar ridges rise from the epicondyles on the lower third.
Lower end
The capitulum (lateral) and trochlea (medial) form the articular surface. The coronoid and radial fossae lie above them in front and the olecranon fossa behind. The medial epicondyle is large and prominent; the lateral epicondyle is smaller.
Attachments
The humerus gives insertion to the shoulder muscles above and origin to the elbow and forearm muscles below.
| Site | Muscle |
|---|---|
| Greater tubercle: superior, middle and inferior facets | Supraspinatus, infraspinatus, teres minor |
| Lesser tubercle | Subscapularis |
| Lateral lip of intertubercular sulcus | Pectoralis major |
| Floor of intertubercular sulcus | Latissimus dorsi |
| Medial lip of intertubercular sulcus | Teres major |
| Deltoid tuberosity | Deltoid |
| Middle of the medial border | Coracobrachialis |
| Lower half of the anterior surface | Brachialis |
| Posterior surface above the radial groove | Lateral head of triceps |
| Posterior surface below the radial groove | Medial head of triceps |
| Lateral supracondylar ridge | Brachioradialis (upper two-thirds), extensor carpi radialis longus (lower third) |
| Medial epicondyle | Common flexor origin |
| Lateral epicondyle | Common extensor origin, supinator, anconeus |
The long head of triceps does not reach the humerus; it arises from the infraglenoid tubercle of the scapula.
Relations
Four nerves are in contact with or close to the humerus, one at each common fracture site.
- Surgical neck: the axillary nerve and posterior circumflex humeral artery wind round the back of it after passing through the quadrangular space.
- Radial groove: the radial nerve and profunda brachii artery run between the lateral and medial heads of triceps.
- Supracondylar region: the brachial artery and median nerve cross the front of the lower humerus.
- Medial epicondyle: the ulnar nerve lies in the groove behind it.
The nutrient artery, from the brachial artery, enters the shaft near its middle on the medial side, pointing towards the elbow. The anterior and posterior circumflex humeral arteries encircle the surgical neck and supply the head.
Clinical relevance
Humeral fractures are described by site, and each site has a nerve to examine.
- Surgical neck and proximal humerus fractures: common in older people after a fall. The axillary nerve is at risk, so deltoid function and regimental badge sensation are checked. Fractures that separate the head from its blood supply at the anatomical neck risk avascular necrosis of the head.
- Greater tuberosity fracture: often with anterior dislocation; supraspinatus pulls the fragment upwards and backwards.
- Shaft fracture: the radial nerve in the radial groove is at risk, giving wrist drop and numbness over the first dorsal web space, usually with triceps preserved.
- Supracondylar fracture: in children, a threat to the brachial artery and median nerve.
- Medial epicondyle fracture: ulnar nerve injury.
Deforming forces follow the attachments: after a surgical neck fracture pectoralis major, latissimus dorsi and teres major draw the shaft medially, while the rotator cuff holds the head.
On the specimen
The humerus stations use a dry bone with painted attachment sites and pins on the bony landmarks.
- Anatomical or surgical neck: the anatomical neck hugs the articular edge of the head; the surgical neck is lower, below the tubercles, where the shaft narrows.
- Greater or lesser tubercle: the greater is lateral and carries three facets; the lesser points forwards, with the sulcus lateral to it.
- Intertubercular sulcus: identify its lateral lip, floor and medial lip, then name pectoralis major, latissimus dorsi and teres major in that order.
- Posterior surface: the lateral head of triceps arises above and lateral to the radial groove; the medial head below and medial to it.
- Lower end: the capitulum, trochlea and coronoid fossa are in front; the olecranon fossa is behind. The medial epicondyle is the larger.
To side a humerus, hold it with the head facing medially and the lesser tubercle facing forwards; the head then points to the side the bone comes from.