ULNA

The ulna is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. It runs parallel to the radius, the other long bone in the forearm. The ulna is usually slightly longer than the radius, but the radius is thicker.

The ulna has a bony process, the olecranon process, a hook-like structure that fits into the olecranon fossa of the humerus. This prevents hyperextension and forms a hinge joint with the trochlea of the humerus. There is also a radial notch for the head of the radius, and the ulnar tuberosity to which muscles attach.

The long, narrow medullary cavity of the ulna is enclosed in a strong wall of cortical tissue which is thickest along the interosseous border and dorsal surface. At the extremities the compact layer thins. The compact layer is continued onto the back of the olecranon as a plate of close spongy bone with lamella parallel. From the inner surface of this plate and the compact layer below its trabecula arch forward toward the olecranon and coronoid and cross other trabecula, passing backward over the medullary cavity from the upper part of the shaft below the coronoid. Below the coronoid process there is a small area of compact bone from which trabecula curve upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of compact bone. The trabecula at the lower end has a more longitudinal direction.

The ulna has an upper end, shaft and a lower end.

GENERAL FEATURES

Upper End

It presents the olecranon and coronoid processes and the trochlear and radial notches.

1. Olecranon process projects upwards from the shaft. It has 5 surfaces. Anterior surface is articular which forms the upper part of the trochlear notch. Posterior surface forms a triangular subcutaneous area which is separated from the skin by bursa. Inferiorly it is continuous with the posterior border of the shaft of the ulna and upper part forms the point of the elbow. Medial surface is continuous inferiorly with the medial surface of the shaft. Lateral surface is smooth and Superior surface in its posterior part shows a roughened area.

2. Coronoid process projects forwards from the shaft just below the olecranon and has 4 surfaces. Superior surface forms the lower part of the trochlear notch. Anterior surface is triangular and rough & its lower corner forms the ulnar tuberosity. Lateral surface upper part is marked by radial notch for the head the radius. Annular ligament is attached to the anterior and posterior margins of the notch. Lower part forms a depressed area to accommodate the radial tuberosity. It is limited behind by a ridge called supinator crest. Medial surface is continuous with the medial surface of the shaft.

3. Trochlear notch forms an articular surface which articulates with trochlea of the humerus to form the elbow joint.

4. Radial notch articulates with the head of the radius to form the superior radioulnar joint.

Shaft

The shaft of the Ulna at its upper part is prismatic in form and curved so as to be convex behind and lateralward; its central part is straight; its lower part is rounded, smooth, and bent a little lateralward. It tapers gradually from above downward and has three borders and three surfaces.

Borders

1.The interosseous or lateral border is sharpest in its middle two-fourths. Inferiorly, it can be traced to the lateral side of the head. Superiorly, it is continuous with the supinator crest.

2.The anterior border is thick and rounded. It begins above on the medial side of the ulnar tuberosity, passes backwards in its lower one-third, and terminates at the medial side of the styloid process.

3.The posterior border is subcutaneous. It begins, above, at the apex of the triangular subcutaneous area at the back of the olecranon and terminates at the base of the styloid process.

Surfaces

1.The anterior surface lies between the anterior and interosseous borders. A nutrient foramen is seen on the upper part of this surface. It is directed upwards. The nutrient artery is derived from the anterior interosseous artery.

2.The medial surface lies between the anterior and posterior borders.

3.The posterior surface lies between the posterior and interosseous borders. It is subdivided into three areas by two lines. An oblique line divides it into upper and lower parts. The lower part is further divided by a vertical line into a medial and a lateral area.

Lower End 

The lower end of the ulna presents an articular surface, part of which, of an oval or semilunar form, is directed downward, and articulates with the upper surface of the triangular articular disc which separates it from the wrist-joint; the remaining portion, directed lateralward, is narrow, convex, and received into the ulnar notch of the radius.

Near the wrist, the ulnar, with two eminences; the lateral and larger is a rounded, articular eminence, termed the head of the ulna; the medial, narrower and more projecting, is a non-articular eminence, the styloid process.

The head presents an articular surface, part of which, of an oval or semilunar form, is directed downward, and articulates with the upper surface of the triangular articular disk which separates it from the wrist-joint; the remaining portion, directed lateralward, is narrow, convex, and received into the ulnar notch of the radius. 

The styloid process projects from the medial and back part of the bone; it descends a little lower than the head, and its rounded end affords attachment to the ulnar collateral ligament of the wrist joint.

The head is separated from the styloid process by a depression for the attachment of the apex of the triangular articular disk, and behind, by a shallow groove for the tendon of the extensor carpi ulnaris.

PARTICULAR FEATURES:

MUSCLES

The ulna serves as the attachment site for numerous muscles with a myriad of actions. The following are organized in terms of the direction and where on the ulna is the attachment of the muscle’s fibers.

The following muscles insert into the ulna:

Triceps brachii – posterior section of the superior surface of the olecranon Anconeus – olecranon Brachialis – the volar surface of the coronoid process

The following muscles originate from the ulna:

• Pronator teres – the medial surface of the coronoid process
• Flexor carpi ulnaris – olecranon process
• Flexor digitorum superficialis – coronoid process
• Flexor digitorum profundus – anteromedial surface
• Pronator quadratus – distal anterior shaft
• Extensor carpi ulnaris – posterior border
• Supinator – proximal ulna
• Abductor pollicis longus – posterior surface
• Extensor pollicis longus – dorsal shaft
• Extensor indicis – posterior distal shaft
  

Blood vessels and Lymphatics

The main blood supply of the ulna originates from the ulnar artery or the ulnar recurrent artery. The ulnar artery then branches into a common interosseous artery that then further subdivides into the posterior and anterior interosseous vessels. These vessels are vital for the nutrients of the ulnar diaphysis. The ulnar metaphysis is supplied by the end branches of the anterior interosseous artery, while the head of the ulna receives its supply from small divisions off of the ulnar artery proper.

The lymphatics of the ulna drain either to the supratrochlear lymph node or directly travel to the adjacent cluster of axillary lymph nodes.

Nerves

Three main nerves run in the forearm in the proximity of the ulna: the median, ulnar, and radial nerves. The median nerve runs anterolaterally and innervates the muscles responsible for flexion of the wrist and the fingers (digits 1 to 3 and radial aspect of the fourth). The ulnar nerve is positioned more anteromedially, running in between the flexor digitorum superficialis and flexor digitorum profundus. This nerve is also involved in flexion of the wrist and fingers (the ulnar aspect of 4th and all of the 5th) but also is responsible for spanning the fingers. A major branch of the median nerve is the anterior interosseous nerve. Impingement or entrapment of the anterior interosseous nerve results in the characteristic clinical finding of weakness related to the "pincer" movement. The radial nerve extends posteriorly to the ulna and the radius and is the sole nerve involved in the muscles responsible for extension of the forearm, wrist, and fingers.  The posterior interosseous nerve is a branch of the radial nerve with both motor and sensory functions. Posterior interosseous nerve impingement or entrapment occurs in the region of the radial tunnel resulting in weakness related to the extension of the digits.

EMBRYOLOGY AND OSSIFICATION

The ulna develops from the induction of the lateral plate mesoderm. From this, it differentiates into the somatic mesoderm, which, in turn, gives rise to all of the bones, ligaments, connective tissue, and blood vessels of the extremities. Endochondral ossification allows for the induction of bone from previously laid hyaline cartilage. The ulna itself is ossified in three sections. First, it commences at the body of the ulna. Around week eight of gestation, the chondrocytes in the middle of the ulna lay down collagen and fibronectin to appropriately calcify the bone template. At the time of birth, the ends of the bone are still cartilaginous and not ossified. Around age four, an ossification center arises in the middle of the ulnar head and then encompasses into the styloid process. At age ten, a subsequent center appears in the olecranon, which is then met by the growth of the epiphysis at approximately age sixteen. The ossification centers about the elbow occur in a predictable order and understanding/knowledge of these stages of development is essential for identifying traumatic injuries about the elbow

SCAPULA

The scapula also known as the shoulder bone, shoulder blade, wing bone or blade bone, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side of the body being roughly a mirror image of the other. The name derives from the Classical Latin word for trowel or small shovel, which it was thought to resemble.

The scapula forms the back of the shoulder girdle. In humans, it is a flat and thin bone, roughly triangular in shape, placed on a posterolateral aspect of the thoracic cage.

GENERAL FEATURES

SURFACES: There are two surfaces costal and dorsal.

1. Costal surface or subscapular fossa is concave and directed medially and forwards. It is marked by 3 longitudinal ridges and one more thick ridge joins the lateral border which is almost rod - like. At the upper part of the fossa is a transverse depression, where the bone appears to be bent on itself along a line at right angles to and passing through the center of the glenoid cavity, forming a considerable angle, called the subscapular angle; this gives greater strength to the body of the bone by its arched form, while the summit of the arch serves to support the spine and acromion.

COSTAL SURFACE

2. Dorsal surface gives attachment to the spine of scapula which divides the surface into smaller supraspinatous fossa and a larger infraspinatous fossa. These two are connected by spinoglenoid notch which lateral to the root of the spine.

DORSAL SURFACE

BORDERS:

1. Superior border is the shortest and thinnest; it is concave and extends from the superior angle to the base of the coracoid process. It is referred to as the cranial border in animals. At its lateral part is a deep, semicircular notch, the scapular notch, formed partly by the base of the coracoid process. This notch is converted into a foramen by the superior transverse scapular ligament and serves for the passage of the suprascapular nerve; sometimes the ligament is ossified. The adjacent part of the superior border affords attachment to the omohyoideus.

Red line is Superior Border

2. Axillary border (or "lateral border") is the thickest of the three. It begins above at the lower margin of the glenoid cavity, and inclines obliquely downward and backward to the inferior angle. At the upper end it presents the infragleniod tubercle. It is referred to as the caudal border in animals.

Lateral Border

3. Medial border (also called the vertebral border or medial margin) is the thinnest and is the longest of the three borders and extends from the superior angle to the inferior angle. In animals it is referred to as the dorsal border.

Medial Border

ANGLES

1. Superior angle of the scapula or Medial angle is covered by the trapezius muscle. This angle is formed by the junction of the superior and medial borders of the scapula. The superior angle is located at the approximate level of the second thoracic vertebra. The superior angle of the scapula is thin, smooth, rounded, and inclined somewhat lateralward, and gives attachment to a few fibers of the levator scapulae muscle.

Superior angle of Scapula

2. Inferior angle of the scapula is the lowest part of the scapula and is covered by the latissimus dorsi muscle. It moves forwards round the chest when the arm is abducted. The inferior angle is formed by the union of the medial and lateral borders of the scapula. It is thick and rough, and its posterior or back surface affords attachment to the teres major and often to a few fibers of the latissimus dorsi. The anatomical plane that passes vertically through the inferior angle is named the scapular line.

Inferior angle of Scapula

3. Lateral angle of the scapula or glenoid angle also known as the head of the scapula is the thickest part of the scapula. It is broad and bears the glenoid cavity on its articular surface which is directed forward, laterally and slightly upwards, and articulates with the head of the humerus. The inferior angle is broader below than above and its vertical diameter is the longest. The surface is covered with cartilage in the fresh state; and its margins, slightly raised, give attachment to a fibrocartilaginous structure, the glenoidal labrum, which deepens the cavity. At its apex is a slight elevation, the supraglenoid tuberosity, to which the long head of the biceps brachii is attached.

Lateral angle of Scapula

PROCESSES

1. Spine or spinous process is a triangular plate of bone with 3 borders and 2 surfaces.   It divides the dorsal surface of the scapula into the supraspinatus and infraspinatus   fossae. Its posterior border is called the crest of the spine. The crest has upper and   lower lips.

2. Acromion has 2 borders, 2 surfaces and a facet.

3. Coracoid process is directed forwards and slightly laterally.

ATTACHMENTS

MUSCLES

• Subscapularis arises from the medial 2/3rds of the subscapular fossa.
• Supraspinatus arises from medial 2/3rds of supraspinous fossa including upper surface of the spine
• Infraspinatus arises from medial 2/3rds of infraspinous fossa, including lower surface of spine.
• Deltoid arises from lower border of the crest of spine and from lateral border acromion.
• Latissimus Dorsi lower fibres originate from inferior angle of scapula.
• Trapezius is inserted into the upper border of the crest of the spine and into medial border of the acromion.
• Serratus anterior is inserted along the medial border of costal surface; 1 digitation from the superior angle to the root of the spine, 2 digitations to the medial border, 5 digitations to the inferior angle.
• The long head of biceps brachii arises from supraglenoid tubercle and the short head from the lateral part of the tip of the coracoid process.
• Coracobrachialis arises from medial part of tip of the coracoids process
• Pectoralis minor is inserted into the medial border and superior surface of coracoid process.
• The long head of triceps arises from infraglenoid tubercle
• Teres minor arises from upper 2/3rds of rough strip on the dorsal surface along the lateral border.
• Teres major arises from lower 1/3rd of rough strip on the dorsal aspect of lateral border
• Levator scapulae is inserted along the dorsal aspect of the medial border, from superior angle up to root of spine
• Rhomboideus minor is inserted into medial border (dorsal aspect) opposite to root of spine
• Rhomboideus major is inserted into the medial border (dorsal aspect) between the root of spine and inferior angle
• Inferior belly of omohyoid arises from upper border near suprascapular notch.
  

LIGAMENTS

• The margin of glenoid cavity gives attachment to the capsule of shoulder joint and to the glenoid labrum
• The margin of the facet on the medial aspect of the acromion gives attachment to the capsule of the acromioclavicular joint
• The coracoacromial ligament is attached to the lateral border of the coracoids process and to the medial side of the tip of the acromion process
• The coracohumeral ligament is attached to the root of the coracoids process.
• The coracoclavicular ligament is attached to the coracoid process; the trapezoid part on the superior aspect, and the conoid part near the root. The coracoclavicular ligament is made up of 2 bands: the conoid and the trapezoid, both of which provide vertical stability. The coracoacromial ligament connects the coracoid process to the acromion.
• The suprascapular ligament bridges across the suprascapular notch and converts it into a foramen which transmits the suprascapular nerve. The suprascapular ligament lies above the ligament.
• The spinoglenoid ligament bridges the spinoglenoid notch. The suprascapular vessels and nerve pass to it.
• The acromioclavicular ligament connects the distal end of the clavicle to the acromion and provides horizontal stability
  

BURSAE

1. Scapulothoracic Bursa, between the serratus and the thorax, and
2. Subscapularis Bursa, between the subscapularis and the serratus.

OSSIFICATION

The scapula is ossified from 7 or more centers: one for the body, two for the coracoid process, two for the acromion, one for the vertebral border, and one for the inferior angle. Ossification of the body begins about the second month of fetal life, by an irregular quadrilateral plate of bone forming, immediately behind the glenoid cavity. This plate extends to form the chief part of the bone, the scapular spine growing up from its dorsal surface about the third month. Ossification starts as membranous ossification before birth. After birth, the cartilaginous components would undergo endochondral ossification. The larger part of the scapula undergoes membranous ossification. Some of the outer parts of the scapula are cartilaginous at birth and would therefore undergo endochondral ossification.

At birth, a large part of the scapula is osseous, but the glenoid cavity, the coracoid process, the acromion, the vertebral border and the inferior angle are cartilaginous. From the 15th to the 18th month after birth, ossification takes place in the middle of the coracoid process, which as a rule becomes joined with the rest of the bone about the 15th year.

Between the 14th and 20th years, the remaining parts ossify in quick succession, and usually in the following order: first, in the root of the coracoid process, in the form of a broad scale; secondly, near the base of the acromion; thirdly, in the inferior angle and contiguous part of the vertebral border; fourthly, near the outer end of the acromion; fifthly, in the vertebral border. The base of the acromion is formed by an extension from the spine; the two nuclei of the acromion unite and then join with the extension from the spine. The upper third of the glenoid cavity is ossified from a separate center (sub coracoid), which appears between the 10th and 11th years and joins between the 16th and the 18th years. Further, an epiphysial plate appears for the lower part of the glenoid cavity, and the tip of the coracoid process frequently has a separate nucleus. These various epiphyses are joined to the bone by the 25th year.

Failure of bony union between the acromion and spine sometimes occurs (see os acromiale), the junction being affected by fibrous tissue, or by an imperfect articulation; in some cases of supposed fracture of the acromion with ligamentous union, it is probable that the detached segment was never united to the rest of the bone.

Complied & Written by Dr. Palak Shah

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