The arm is the region between the shoulder and the elbow and consists of one bone, the humerus. The humerus, the longest and largest bone of the upper limb, articulates with the scapula proximally at the glenohumeral joint and has distal articulations with the radius and ulna at the elbow joint.
The distal end of the humerus consists of two rounded prominences referred to as the medial and lateral epicondyles. The medial epicondyle lies on a more posterior plane than the lateral epicondyle, resulting in the humerus appearing medially rotated when the arm is by the side of the trunk. This is important when considering movements of the arm and forearm. The humerus consists of a proximal end, a shaft and a distal end, all which contain important anatomical landmarks. This article will talk about these aspects in detail, including muscular attachments and anatomical landmarks, followed by an overview of clinical pathology related to the humerus.
The proximal end of the humerus consists of a head, an anatomical neck and the greater and lesser tubercles.
The head is a hemispheroidal shape, with hyaline cartilage covering its smooth articular surface. In the anatomical position, the head faces in a medial, superior and posterior direction where it articulates with the glenoid fossa of the scapula.
The anatomical neck is a slight narrowing below the articular surface of the head. Here, the joint capsule of the shoulder joint is attached.
The greater tubercle is the most lateral portion of the proximal end of the humerus. It consists of three smooth and flat impressions at the posterosuperior aspect for the attachment of muscles. From superior to inferior, the muscles that attach at these impressions are the:
The deltoid muscle covers the lateral aspect of the greater tubercle, resulting in the normal rounded shape of the shoulder. The lateral aspect also contains multiple vascular foramina.
The lesser tubercle is located anterior to the anatomical neck and has a smooth, palpable muscular impression. The lateral part forms the medial margin of the intertubercular sulcus. The subscapularis muscle attaches at this tubercle and the transverse ligament of the shoulder also attaches on its lateral part.
The intertubercular sulcus is an indentation located between the two tubercles. It is sometimes referred to as the bicipital groove. The long tendon of the biceps brachii and a branch of the ascending circumflex humeral artery are located within the sulcus. The sulcus consists of a lateral lip and a medial lip. The tendon of the pectoralis major muscle attaches on to the lateral lip, while the teres major tendon attaches on to the medial lip. In addition, the tendon of lattisimus dorsi attaches to the posterior aspect.
There is also a narrowing below the tubercles referred to as the surgical neck, which is a common fracture site. It is in close proximity to the axillary nerve and the posterior circumflex humeral artery. This is where the proximal end of the humerus joins with the long shaft.
The proximal half of the shaft is of a cylindrical shape, whereas the distal half is triangular. It consists of three borders known as the anterior, lateral and medial borders. The shaft also contains three surfaces referred to as the anterolateral, anteromedial and posterior surfaces.
The anterior border begins at the greater tubercle and runs downward almost to the end of the bone. The proximal third of the border forms the lateral lip of the intertubercular sulcus.
The lateral border thickens distally to form the lateral supracondylar ridge. The middle part of the border has a rough V shaped area referred to as the deltoid tubercle.
The medial border is similar to the lateral border in that it forms the medial supracondylar ridge distally. The radial groove is a shallow groove that interrupts the medial border in its medial third. The radial nerve and profunda brachii artery are located in this groove.
The anterolateral surface has a smooth upper surface and is covered by the deltoid muscle. The deltoid inserts into the deltoid tubercle around the middle of the surface. The distal fibres of the brachialis muscle originate from the distal part of this surface. The lateral supracondylar ridge is a site for two muscular attachments: the brachioradialis on the proximal two thirds and the extensor carpi radialis longus on the distal third.
The upper third of the anteromedial surface forms the floor of the intertubercular sulcus and there is a small area distal to this sulcus where there is an absence of muscular attachment. However, the lower half of the surface is covered by the medial portion of the brachialis muscle. Coracobrachialis is also attached to this surface in the middle part as well as pronator teres, which is attached distally to the medial supracondylar ridge.
The posterior surface is bounded by the medial and lateral borders and is covered mostly by the medial head of the triceps brachii muscle. A ridge on the proximal third also gives attachment to the lateral head of the triceps brachii.
The distal end consists of both articular and non-articular parts. The articular part of the humerus is a modified condyle and is wider transversely. It articulates with both the ulna and radius and consists of a medial trochlea and a lateral capitulum, which are separated by a faint groove. The non-articular part consists of the medial and lateral epicondyles as well as the olecranon process, coronoid fossa and radial fossae.
The trochlea has a surface shaped like a pulley and covers the anterior, posterior and inferior surfaces of the medial condyle of the humerus. It articulates with the ulna at the trochlear notch. When the elbow is in the extended position, the posterior and inferior aspects of the trochlea are in contact with the ulna. However, when the elbow is flexed the posterior part is no longer in contact, as the trochlear notch slides towards the anterior aspect of the humerus.
The capitulum is a convex and rounded projection that covers the anterior and inferior surfaces of the lateral condyle of the humerus. Unlike the trochlea, it doesn’t cover the posterior surface. It articulates with the head of the radius. In extension, the inferior surface is in contact with the ulna but in the flexed position the radial head slides towards the anterior aspect of the humerus.
The medial epicondyle is a blunt projection superomedial to the medial condyle, which forms at the end of the medial border of the humerus. The ulnar nerve crosses its smooth posterior surface and is palpable in this location. The superficial muscles of the anterior compartment of the forearm originate from the anterior surface of the medial epicondyle. These muscles are the:
- flexor carpi ulnaris
- palmaris longus
- flexor carpi radialis
- pronator teres
The lateral border of the humerus ends at the lateral epicondyle. There is an impression on the lateral and anterior surfaces where the seven muscles of the superficial group of the posterior compartment of the forearm originate. These include the:
- extensor carpi radialis longus
- extensor carpi radialis brevis
- extensor digitorium
- extensor digiti minimi
- extensor carpi ulnaris
The olecranon fossa is a deep hollowed area on the posterior surface, superior to the trochlea. In elbow extension, the tip of the ulnar olecranon process lodges into this fossa.
The coronoid fossa is a smaller hollow that is also located superior to the trochlea, but on the anterior surface. During flexion of the elbow, the coronoid process of the ulna lodges into the coronoid fossa. Lateral to the coronoid fossa and superior to the capitulum is another depression referred to as the radial fossa. It is so named as the margin of the head of the radius lodges there in full flexion.
Fractures of the humerus are relatively common and can occur at any location on the humerus. At the proximal end, most fractures are located at the surgical neck and are most common in the elderly, especially those with osteoporosis.
An impact fracture, often the result of a humeral fracture, is where one bone fragment is driven into the spongy bone of another bone fragment. This is usually due to the force of a fall on the hand.
When the greater tubercle is pulled away from the head of the humerus the result in an avulsion fracture. It is most commonly seen in the middle-aged and in the elderly. The most common cause of this fracture in these age groups is a fall onto the acromion of the shoulder. In young adults, it can result from falling on the hand when the arm is in abduction.
A direct blow to the arm can result in a transverse fracture of the humeral shaft. The pull of the deltoid muscle causes the proximal fragment to displace laterally.
A fall onto the outstretched hand can also cause a spiral fracture of the shaft of the humerus. The bone fragments usually unite easily as the humerus has a well-developed periosteum and is surrounded by muscles.
An intercondylar fracture can occur due to a fall on the elbow whilst it is in flexion. This results in separation of one or both of the condyles from the shaft of the humerus.
The following nerves are located on the following aspects of the humerus:
- The axillary nerve: surgical neck
- The radial nerve: radial groove
- The median nerve: distal humerus
- The ulnar nerve: medial epicondyle
If any of these aspects of the humerus are fractured, there may be damage to these nerves.