Humerus

 The word "humerus" is derived from Latin: humerus, umerus meaning upper arm, shoulder. The humerus is a long bone in the arm that runs from the shoulder to the elbow also known as adele. It is the longest bone of upper limb. The proximal aspect of the humerus articulates with the glenoid fossa of the scapula, forming the glenohumeral joint. Distally, at the elbow joint, the humerus articulates with the head of the radius and trochlear notch of the ulna.

It has an upper end, shaft and a lower end.

Upper End

• The head of humerus is directed medially, backwards and upwards. It articulates with the glenoid cavity of the scapula to form the shoulder joint. The head forms about 1/3rd of a sphere and is much larger than the glenoid cavity. 
• The anatomical neck is the line separating head from the rest of the upper end.
• The lesser tubercle (Latin lump) 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 3 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 represents downward prolongations of the lesser and greater tubercles.
• The narrow line separating the upper end of the humerus from the shaft is known as surgical neck of humerus.
  

Shaft 

The shaft is rounded in the upper half and triangular in the lower half. It has 3 borders and 3 surfaces.

Borders

1. The upper 1/3rd of the anterior border forms the lateral lip of the bicipital groove. Its medial part forms the anterior margin of the deltoid tuberosity. Its lower half of the anterior border is smooth and rounded.
2. 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 upto the posterior surface of the greater tubercle. In the middle part, it is interrupted by the radial or spiral groove.
3. The medial border's upper part 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.
   

SURFACE 

1. The 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 the radial groove runs downwards and forwards across the surface.
2. The anteromedial surface lies between the anterior and medial borders. Its upper 1/3rd is narrow and forms the floor of the bicipital groove. A nutrient foramen is seen on this surface near its middle, near the medial border.
3. The posterior surface lies between the medial and lateral borders. Its upper part is marked by an oblique ridge. The middle 1/3rd is crossed by the radial groove.

Lower End 

The lower end of the humerus forms the condyle which is expanded from side to side and has articular and non articular surfaces.

Articular Surfaces 

• Capitulum is a rounded projection which articulates with the head of the radius.
• Trochlea is a pulley shaped surface. It articulates with the trochlear notch of the ulna. The medial edge of the trochlea projects down 6mm more than the lateral edge. This results in the formation of the carrying angle.

Non Articular Surface

• 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.
• Lateral epicondyle is smaller than the medial epicondyle. Its anterolateral part has a muscular impression.
• Lateral supracondylar ridge is a sharp lateral margin which is just above the lower end.
• Medial supracondylar ridge is similar ridge on the medial side.
• Coronoid fossa is a depression just above the anterior aspect of the trochlea. It accommodates the coronoid process of the elbow when flexed.
• Radial fossa is a depression present just above the anterior aspect of the capitulum. It accommodates the head of the radius when elbow is flexed.
• 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.

MUSCULAR ATTACHMENTS

The humerus serves as the origin and insertion site of many upper limb muscles that divide into the following distinctions: scapulohumeral muscles, anterior compartment muscles, posterior compartment muscles.

Scapulohumeral muscles: The deltoid muscle, which defines the shoulder contour of the upper limb, originates at three locations: the acromion of the scapula, the spine of the scapula and clavicle, with each part inserting into the deltoid tuberosity of the humerus. The deltoid muscle allows internal/external rotation and abduction/adduction of the humerus. The pectoralis major muscle originates at the clavicle, manubrium, sternum body, and true ribs and inserts into the intertubercular sulcus of the humerus. It allows adduction, flexion, extension, and medial rotation of the humerus. Four muscles form the rotator cuff; the subscapularis, supraspinatus, infraspinatus, and teres minor. The subscapularis muscle originates from the subscapular fossa of the scapula and inserts onto the lesser tubercle of the humerus, facilitating internal rotation of the humerus. The supraspinatus muscle originates in the supraspinous fossa of the scapula and inserts into the greater tubercle of the humerus, facilitating the abduction of the humerus. The infraspinatus muscle originates in the infraspinous fossa and scapula spine and inserts into the greater tubercle of the humerus as well and allows external rotation of the humerus. The teres major originates the inferior angle of the scapula and inserts into the lesser tuberosity of the humerus, allowing internal rotation and adduction. The teres minor originates on the lateral border of the scapula and inserts into the greater tubercle and allows external rotation.

Anterior compartment muscles: The biceps brachii muscle has a long head and short head but does not have an actual origin or insertion sites on the humerus. However, there is a transverse humeral ligament which projects from the lesser tubercle to the greater tubercle of the humerus and converts the humeral intertubercular groove into a canal which the tendon of the biceps brachii long head travels through on its way from its origination on the supraglenoid tubercle of the scapula to the radius.  The coracobrachialis originates on the coracoid process of the scapula and inserts onto the humerus and its medial surface, allowing flexion and internal rotation. The brachialis muscle originates on the anterior surface of the distal humerus and inserts onto the ulna for flexion of the forearm.

Posterior compartment muscles: The triceps brachii muscle contains three heads, with the medial head originating on the posterior aspect of the humerus, inferior to the spiral groove, and the lateral head originating on the posterior surface, both insert into the olecranon process of the ulna and allow extension of the forearm at the elbow joint. Of note, the humerus along with the long head of the triceps, teres major and teres minor, form a quadrangular space through which the posterior circumflex artery and vein and radian nerve travel.

BLOOD SUPPLY AND LYMPHATICS

The primary blood supply of the proximal humerus comes from anastomoses between the anterior and posterior circumflex humeral arteries. These are branches of the distal third of the axillary artery.  Recent studies suggest the posterior humeral circumflex artery to be the primary source of blood supply to the humeral head. The terminal division of the anterior humeral circumflex artery is the arcuate artery, which supplies the majority of the greater tuberosity.  

The axillary artery continues to become the brachial artery, which, along with one of its branches, the profunda brachii artery, will give off peri-osteal arteries to provide the blood supply for the rest humerus and its attached muscles. The inner portions of the humerus are vascularized by nutrient arteries that also branch off from the brachial artery in the vicinity of the middle of the humerus.

NERVE SUPPLY

The axillary nerve, formed from the posterior cord of the brachial plexus, wraps around the surgical neck of the humerus, and provides innervation to the deltoid and rotator cuff muscles, specifically the teres minor. Innervation of the anterior portion of the brachium is supplied by the musculocutaneous nerve, which forms as a division of the lateral cord of the brachial plexus. This nerve pierces through the coracobrachialis muscle and travels between the biceps brachii and coracobrachialis, ultimately finding its termination as the lateral cutaneous nerve of the forearm. The radial nerve from the posterior cord of the brachial plexus courses through the spiral groove of the humerus, posterior to the brachial artery, and anterior to the long head of the triceps. It innervates the posterior muscles of the arm, forearm, and overlying skin. The radial nerve is also responsible for the innervation of the lateral and medial epicondyle of the humerus.

EMBRYOLOGY & OSSIFICATION

As one of the many long bones within the appendicular skeleton, the humerus develops via endochondral ossification. This process is characterized by the replacement of a cartilage template by bone. Initially, a relatively tiny cartilage model is laid down by mesenchymal cells that form cartilage secreting chondrocytes. Second, in the center of the cartilage template (ossification center), there is chondrocyte hypertrophy and secretion factors such as alkaline phosphatase to promote calcification of that cartilage. This action forms a blockade of nutrients and causes chondrocyte death. However, before dying, these cells also secrete vascular endothelial growth factor (VEGF) to support angiogenesis towards the interior of this calcifying cartilage. Meanwhile, under the influence of Indian hedgehog homolog (IHH) protein, cells outside in the perichondrium differentiate into osteoblasts and form a layer of bone surrounding the center of cartilage known as the bony collar.

As time progresses, a central area of dead chondrocytes develops, with little pieces of calcified cartilage left behind, a shell of bone around the center, and the vascular supply going around the interior. Vascular supply brings into the interior mesenchymal cells that differentiate into more osteoblasts and monocytes that form osteoclasts. In the center of the initial cartilage template, bone is resorbed on the inside and deposited on the outside, creating the growth in width of a hollow entity (forming a marrow). Simultaneously, the proximal and distal ends of the cartilage template have chondrocytes that continue to proliferate as secondary ossification centers. This activity allows cartilage growth at the end of the bones, providing vertically expansive capabilities. Ultimately, the areas where bone meets cartilage forms the epiphyseal plate (a linear zone of cartilage), where bone replaces cartilage continuously until bone growth completely fuses and stops around puberty. In the humerus specifically, there are eight ossification centers, at the head of the humerus, humeral shaft, greater and lesser tubercles, medial and lateral epicondyles, trochlea, and olecranon. 

The ossification of the shaft occurs at eight weeks gestation, with the humeral head ossification occurring at birth/after birth. The greater tubercle's ossification takes place during the first year of life and the lesser tubercle ossification within the first six years of life. By the time of adolescence, all proximal ossification centers of the humerus have fully fused with the shaft. The distal ossifications at the condyle and trochlea and olecranon take place between early and later adolescence and fuse with the shaft of the humerus by late adolescence.

Complied & Written by Dr. Palak Shah