Chapter 6

Skeleton

Learning objectives

After studying this chapter, the student should be able to:

• enumerate the types of skeleton and list the bones in each type

• list the functions of the bone

• enumerate the different type of cells of bone

• classify the bones according to their shape and give an example of each

• describe the sesamoid bones and list their characteristic features

• outline the parts of a growing long bone and discuss its arterial supply

• describe intramembranous ossification

• describe intracartilaginous ossification

• compare and contrast the structure of compact and spongy bones

• classify different types of cartilages and give an example of each

• describe the growth of the bone

• discuss remodeling of bone and its clinical significance

• correctly solve the review questions given at the end of the chapter

Introduction

The skeleton of the body is composed of bones and cartilages. Both bones and cartilages are made up of specialized connective tissue called skeletal (sclerous) tissue, which can bear weight without bending and has considerable tensile strength. The skeletal tissue consists of same components (e.g. cells, matrix) as that of general connective tissue, but physically differs from it as its matrix is solidified.

The functions of skeletal system are as follows:

1. Forms a rigid framework of the body.

2. Provides protection to the viscera (viz. brain, heart, lungs, etc.) of the body from external injury.

3. Provides leverage for the body movements.

Types of the skeleton

For the convenience of study, the skeleton is divided into two types (Fig. 6.1):

1. Axial skeleton

2. Appendicular skeleton

Axial skeleton

The axial skeleton consists of bones and cartilages that lie close to the central axis of the body. It includes skull, auditory ossicles, hyoid bone, vertebral column, and rib cage:

Appendicular skeleton

The appendicular skeleton is bilaterally symmetrical and comprises the bones of the upper and lower extremities (or limbs) and bony girdles which anchor these extremities with the axial skeleton. Thus, it includes bones of pectoral girdle, upper limb, pelvic girdle and lower limb (Fig. 6.1).

1. Pectoral girdle: It consists of paired scapulae and clavicles. It is not a complete girdle as it is attached only anteriorly.

2. Bones of upper limb: Each upper limb contains the humerus within the arm (brachium); the radius and ulna within the forearm (antebrachium); the carpal bones, the metacarpal bones and the phalanges within the hand.

3. Pelvic girdle: It consists of two hip bones. It is a complete girdle as it is united anteriorly with symphysis pubis and posteriorly with the sacrum of vertebral column.

4. Bones of lower limb: Each lower limb contains the femur within the thigh; the tibia and fibula within the leg; the tarsal bones, the metatarsal bones and the phalanges within the foot.

The bones forming axial and appendicular skeleton are listed in Table 6.1 and shown in Figure 6.1.

Table 6.1

Number of bones present in axial and appendicular skeleton

	Number of bones
	Axial skeleton
Skull
Cranial bones	8
Facial bones	14
Hyoid	1
Ear ossicles (3 in each ear)	6
Vertebral bones	26
Rib cage
Sternum	1
Ribs	24
Total	80
	Appendicular skeleton
Pectoral (shoulder) girdles
Clavicle	2
Scapula	2
Upper limbs
Humerus	2
Ulna	2
Radius	2
Carpals	16
Metacarpals	10
Phalanges	28
Pelvic (hip) girdle
Hip bone	2
Lower limbs
Femur	2
Fibula	2
Tibia	2
Patella	2
Tarsals	14
Metatarsals	10
Phalanges	28
Total	126
Total bones of axial skeleton	80
Total bones of appendicular skeleton	126
Total bones of the body	206

Bones

The bones are the hard structures, which form the rigid framework of the body.

Structurally, bone is a highly vascular mineralized connective tissue consisting of cells and dense intercellular organic matrix impregnated with inorganic salts. The organic material mainly consists of collagen fibers and forms one-third of the bone. It provides resilience to the bone. The inorganic material mainly consists of calcium phosphate and traces of other salts. It provides hardness and rigidity to the bone and makes it radiopaque in x-ray films.

The bone is not an inert material as often thought of by some students but it is a living structure. It is supplied by blood vessels, lymph vessels and nerves. It has the power of regeneration and repair. In fact, bones have greater regenerative power than any other tissues of the body, except blood.

The bones are subject to diseases like any other tissue of the body.

Clinical correlation

Bone health: The bones undergo atrophy, i.e. become thinner and weaker if not used, e.g. bones of a paralyzed limb atrophy; on the other hand, they hypertrophy, i.e. become thicker and stronger if subjected to support an increased weight. Therefore, regular exercise is essential to maintain the health of the bones.

N.B.

• The skeletal system of adult human is composed of approximately 206 bones.

• At birth, the skeleton consists of approximately 270 bones (in adult the number of bones decreases due to gradual fusion of separate bones).

Functions of bones

The following are the functions of bones:

Microscopic structure of bones

The bone is a specialized connective tissue and consists of the following three components:

1. Cells

2. Ground substance

3. Fibers

The ground substance and fibers together form the intercellular substance or matrix which gets mineralized.

Cells:

The bone cells are of three types: osteoblasts, osteocytes and osteoclasts.

Osteoblasts

N.B.

The blood alkaline phosphatase level can be monitored clinically to assess osteogenesis during bone repair.

Osteocytes

1. The osteocytes are derived from osteoblasts. Once an osteoblast becomes surrounded by matrix, it becomes an osteocyte.

2. The osteocytes are flattened cells with numerous cytoplasmic processes. They are located in spaces within matrix called lacunae and canaliculi, respectively. The cytoplasmic processes of neighboring cells communicate by gap junctions.

3. The osteocytes unlike osteoblasts do not undergo mitosis.

4. The canaliculi radiate from each lacuna and permit diffusion of nutritive material (Fig. 6.2). Thus, cells remain alive in calcified matrix and maintain the bone by their balanced osteogenic and osteoclastic activity. They secrete alkaline phosphatase to maintain calcification. When bone cells die, the matrix is decalcified.

Fig. 6.2Osteocytes, lacunae and canaliculi.

Osteoclasts

Ground substance:

It consists of the following components:

1. Proteoglycans containing a side chain of glycosaminoglycans (GAGs), specifically chondroitin sulfate and keratan sulfate.

2. Glycoproteins, such as osteonectin and osteocalcin (a calcium binding protein).

3. A mineral component that includes hydroxyapatite (calcium phosphate crystals), citrate ions and bicarbonate ions. Mineral component mainly consists of hydroxyapatite crystals of calcium phosphate (Ca₁₀[PO₄]₆[OH]₂), and contributes to the hardness/rigidity of bone.

4. Water (tissue fluid), which contributes to a low degree of bone hydration (7%).

Fibers:

They are type I collagen fibers and provide tensile strength to the bone.

Classification of bones

The bones are usually classified in three ways:

(a) According to the shape

(b) According to the structure (structural classification)

A. Classification according to the shape

Depending on the size, shape, etc., the bones are classified into the following types (Fig. 6.3):

Long bones are those in which length exceeds the breadth and thickness. The long bones are of two types: typical and miniature/short.

The examples of miniature long bones are metacarpals, metatarsals and phalanges. All the metacarpals and metatarsals have epiphysis at their distal end except first metacarpal and first metatarsal which have epiphysis at their proximal end.

Short bones are small in size and usually cuboidal in shape, presenting six surfaces. These bones are found in wrist (carpal bones) and foot (tarsal bones).

N.B.

All the short bones ossify in cartilage after birth except talus, calcaneus and cuboid which start ossifying before birth.

Flat bones are flat and shallow plate-like bones. They form boundaries of certain body cavities, e.g. cranium, thoracic cavity. The examples of flat bones include: (a) bones forming cranial vault (frontal, parietal, occipital, etc.), and (b) bones forming thoracic cage (scapulae, ribs, sternum, etc.).

Irregular bones are highly irregular in shape, viz. hip bone, vertebrae, bones forming base of the skull.

Pneumatic bones are a variety of irregular bones which contain air-filled cavity or cavities within them. These bones are mainly located around the nasal cavity, viz. maxilla, frontal, sphenoid and ethmoid bones. The air-filled cavities in these bones are called paranasal air sinuses.

The paranasal air sinuses not only make the skull light but also add resonance to the voice. They also act as air conditioning chambers for the inspired air.

Sesamoid bones (Arab. sesame = seed) are seed-like bony nodules which develop in certain muscle tendons where they rub against convex bony surfaces during the movements of the joint. The rubbing surface of the sesamoid bone is covered with an articular cartilage.

Sites of sesamoid bones

Functions of sesamoid bones

1. Act as pulleys for muscle contraction.

2. Alter the direction of the pull of the muscle.

3. Minimize the friction of tendon against the bone to prevent its attrition.

Characteristics of sesamoid bones

1. Develop in the tendons of the muscles.

2. Are devoid of periosteum.

3. Ossify after birth usually by multiple centers.

4. Lack haversian system.

N.B.

• Patella is the largest sesamoid bone in the body and the cartilage covering, its articular surface is the thickest articular cartilage in the body.

• Sesamoid bones are not classified as true bones as they are not covered by periosteum.

Accessory bones

These are bones that are not generally present in the body. If present they do not cause any harm, sometimes clinicians confuse it with a fractured bone. Accessory bones may be formed due to:

N.B.

Heterotopic bones sometimes formed in soft tissues where they are not normally found, e.g., rider’s bone in the tendon of adductor longus muscle of the thigh.

B. Structural classification

Macroscopically (i.e. as seen on naked eye examination of a section of a long/flat bone), the architecture of bone may be compact (dense) or cancellous (spongy).

Most bones of the body have a basic structure of an outer region of compact bone and inner region of spongy (cancellous) bone. For example:

1. In long bones, the shaft consists of compact bone forming a cylinder that surrounds a central cavity called medullary cavity. The ends of long bones consist of cancellous bone surrounded by a thin layer of compact bone.

2. In flat bones of the skull, the cancellous bone is sandwiched between the plates of compact bone. The spongy part is called diploe and outer and inner plates of compact bone are called inner and outer tables.

N.B.

All the bones of body consist of both compact and spongy bones except inferior nasal concha which consists of only spongy bone.

Compact (dense) bone

The texture of compact bone is dense, ivory-like with no visible spaces on naked eye examination. The compact bone consists of (a) lamellae of collagenous sheets, and (b) haversian systems or osteons (Fig. 6.4).

Lamellae

The compact bone consists of three types of bony lamellae:

1. Concentric lamellae which surround the haversian canal.

2. Interstitial lamellae—lie between the osteons. The interstitial lamellae are remnants of old outer circumferential lamellae or old haversian systems.

3. Circumferential lamellae which are flat plates that extend around the bone.

Haversian system (osteon)

Each haversian system consists of a haversian canal (or central canal), surrounded by concentric lamellae of bone.

Haversian canal runs parallel to the long axis of the bone and contains blood vessels, nerves and loose connective tissue.

The concentric lamellae, 4–20 in number, form rings around haversian canal. The osteocytes are located between the lamellar rings in lacunae.

The lacunae communicate with one another in between and across the lamellae, and with the central canal by numerous radiating canaliculi.

Haversian canals are lined by endosteum and communicate with each other, with the medullary cavity and with the surface of bone by numerous Volkmann’s canals. The Volkmann’s canals run perpendicular to the long axis of the bone.

The blood vessels from the periosteum or endosteum enter the bone through Volkmann’s canals.

The haversian canals receive blood vessels from Volkmann’s canals.

The osteocytes receive nutrients and eliminate waste products through the canal system.

Cancellous (spongy) bone

The cancellous bone is a meshwork of bony spicules. It consists of interconnecting rods and plates of bone called trabeculae, enclosing large spaces filled with red bone marrow. The trabeculae:

(a) consist of superimposed lamellae and do not form haversian system because they get nutrition from the blood vessels of tissues around them;

(b) have a layer of osteoblasts on their surfaces.

The differences between the compact and spongy bone are given in Table 6.2.

Table 6.2

Difference between compact and spongy bone

	Compact bone	Spongy bone
Density	Dense like an ivory	Porous like a sponge
Haversian systems	Present	Absent
Arrangement of bony lamellae	Regular	Irregular
Location in bone	Outer region	Inner region
Bone marrow	Absent	Present
Amount in the body by weight	75%	25%

Wolff’s Law (Trajectory theory of Wolff, 1892): According to this law, the bone formation (osteogenesis) is directly proportional to stress and strain. The tensile force favors bone formation, whereas compressive force favors bone resorption. This theory, however, no longer holds good because now it is noticed that both tensile and compressive forces can stimulate bone formation in appropriate conditions.

The architecture of cancellous bone is often interpreted in terms of Wolff’s law. Thus, the arrangement of bony lamellae is governed by the lines of maximal stress (Fig. 6.5):

1. Pressure lamellae are arranged parallel to the line of weight transmission.

2. Tension lamellae are arranged at right angles to the pressure lamellae.

N.B.

The compact arrangement of pressure lamellae form bony buttresses to provide additional support, viz. calcar femorale (Fig. 6.5).

Clinical correlation

The trabeculae are oriented along the line of stress within a bone. If the direction of weight-bearing stress is changed slightly, for example, because of a fracture that heals improperly, the trabecular pattern realigns with new lines of stress. For this reason, patient often complains of pain at the fracture site in later life.

Microscopically, bones are of two types:

1. Woven

2. Lamellar

This classification is based on the manner in which the different components of the matrix are arranged.

Woven bone:

The collagen fascicles and bone crystals within a woven bone are arranged randomly forming a network resembling the warp and weft of a woven fabric and hence the name ‘woven bone’. The examples of woven bone are young fetal bones (most immature bone), callus at fracture repair sites.

Lamellar bone:

In lamellar bone, the mineralized matrix is arranged in thin layers (lamellae). The lamellae are arranged in two different patterns:

1. In number of concentric cylindrical units (haversian system), e.g. compact bone.

2. In number of branching and anastomosing curved plates, e.g. spongy bone.

N.B.

All the adult bones are lamellar bones.

C. Developmental classification

According to the process of bone development (ossification), the bones are of the following three types:

1. Membranous bones (membrane bones), developed by membranous ossification.

2. Cartilaginous bones (cartilage bones), developed by endochondral ossification.

3. Membrano-cartilaginous bones, developed by both membranous and endochondral ossification.

For details see development of bones on page 77.

The examples of bones according to their development are enumerated in Table 6.3.

Table 6.3

Examples of bones according to their development

Types of bones	Examples
Membranous	Bones of cranial vault Facial bones
Cartilaginous	Bones of limbs Bones of base of skull Bones of vertebral column (vertebrae) Bones of thoracic cage (ribs, sternum)
Membrano-cartilaginous	Mandible Clavicle Occipital bone Temporal bone Sphenoid bone

Gross structure of a typical long bone

A typical long bone consists of three parts: a shaft (or body) and two ends (Fig. 6.6).

1. The shaft is an elongated part between the two expanded ends. It is made of an outer thick shell of compact bone (cortex) enclosing a cavity called medullary cavity, thus providing maximum strength with minimal material and weight.

The medullary cavity is filled with yellow bone marrow (remember that at birth it is filled with red bone marrow and is involved in active hemopoiesis).

2. The ends are knobby (expanded) and largely made up of cancellous bone covered by a thin shell of a compact bone. The ends of long bone are covered by an articular hyaline cartilage and take part in the formation of joints.

Periosteum and endosteum

The outer surfaces of all the bones are covered by a thick fibrous membrane called periosteum, whereas their inner surfaces are lined by a thin fibrous membrane called endosteum. Both periosteum and endosteum contain osteoprogenitor cells, osteoblasts and osteoclasts which are essential for formation, remodeling and repair of bone.

The periosteum (Figs 6.4 and 6.6) is a thick fibrous membrane covering the entire surface of bone except at the areas covered by an articular cartilage. At the articular margin, the periosteum is continuous with the joint capsule.

It is made up of two layers:

1. Outer dense fibrous layer.

2. Inner cellular layer which is osteogenic in nature.

The periosteum is united to the underlying bone by extrinsic collagen fibers, the Sharpey’s fibers. These are coarse collagenous fibers from inner layer which extends inward to penetrate the bone matrix (outer cortical bone tissue) like spikes of a shoe, thus bolting the periosteum to the bone.

Functions of periosteum

1. Protects and maintains the shape of the bone.

2. Nourishes the outer part of the underlying cortex by abundant periosteal arteries.

3. Provides attachment to ligaments, tendons, muscles and intermuscular septa.

4. Provides regenerative capability to the bone.

The endosteum is cellular membrane that lines the medullary cavity of the shaft. The osteoblasts and osteoprogenitor cells within the endosteum play an important role in bone remodeling and repair.

Nutrient foramen is usually present near the middle of shaft for the entry of nutrient artery.

Clinical correlation

The periosteum is important in the repair of fractures.

It has unique capability of forming new bone, hence bone stripped of its periosteum will not survive.

The periosteum is very sensitive to pain, particularly to tearing and tension. Therefore, drilling into the bone without anesthesia is very painful. For the same reason, during intramuscular injection, if needle penetrates periosteum, severe pain is felt by the patient.

Bone marrow

The bone marrow is a soft, loose, vascular tissue consisting of delicate network of reticular fibers and various types of cells. It is the main site of hemopoiesis.

It is present in the medullary cavity of the long bones and in the cavities of the spongy bone.

Types of bone marrow

Two types of bone marrow are described according to their appearance on gross examination.

1. Red bone marrow

2. Yellow bone marrow

Red bone marrow:

It is vascular and appears red in color due to the presence of red blood cells. It consists of network of fine reticular fibers containing blood forming cells, showing all stages of development: immature (nucleated) and mature (non-nucleated) red blood cells, myeloblasts, myelocytes, granulocytes, megakaryocytes, etc. At birth it is present in all the bones at all sites, and is an important site of hemopoiesis, but as the age advances the marrow in the medullary cavity of long bones is gradually replaced by yellow marrow. Hence in the adults, the red marrow is found in the cancellous bone. The sites (Fig. 6.7) are as follows:

(a) Ends of long bones

(b) Sternum

(d) Skull bones

(e) Iliac crests of hip bones

(f) Vertebrae

Clinical correlation

• Bone marrow aspiration: It is often performed by the clinicians for diagnostic purpose. The sites commonly used for bone marrow aspiration are: sternum, anterior and posterior iliac spines of hip bones, ribs, lumbar spinous processes and upper end of tibia.

• In the adult, the common sites of bone marrow aspiration are manubrium of sternum and spinous processes of lumbar vertebrae.

• In the children, the common sites of bone marrow aspiration are iliac crests of hip bones.

Yellow bone marrow:

It mainly consists of adipose tissue. However, few hemopoietic elements may be found in it. Under certain conditions, such as severe bleeding or hypoxia, yellow bone marrow converts back into red bone marrow.

N.B.

In the old age, sometimes the red marrow in the skull bones degenerates to produce jelly-like material (gelatinous mass). It is known as gelatinous marrow.

Parts of growing (young) long bone

A typical long bone develops from a preformed model of hyaline cartilage in three parts: two ends and an intervening shaft. The two ends are formed by secondary centers of ossification and the shaft is formed by primary center of ossification. Before the ossification is complete, the following parts can be defined in the young long bone (Fig. 6.8):

Epiphyses:

These are ends of long bones which ossify from secondary centers.

Types of epiphyses

The epiphyses are of the following four types (Fig. 6.9):

1. Pressure epiphysis: It is covered by an articular cartilage and takes part in the transmission of body weight, e.g. head of femur, humerus, lower end of radius, etc.

2. Traction epiphysis:

(a) It is produced by a pull of the muscle or muscles, e.g. greater and lesser trochanters of femur, greater and lesser tubercles of humerus, etc.

(b) It is always nonarticular and provides attachment to the muscle or muscles.

3. Atavistic epiphysis: It is an independent bone in lower mammals, which in man gets fused to the nearest bone to receive nutrition from the host bone. It grows like a parasite, e.g. coracoid process of scapula, posterior tubercle of talus which is also known as os trigonum.

4. Aberrant epiphysis: It is an epiphysis which appears at unusual end of a short long bone, e.g. epiphysis at the head of first metacarpal. Remember, normally epiphysis appears at the base of the first metacarpal bone.

Epiphyseal (growth) plate:

It is a plate of hyaline cartilage which intervenes between the epiphysis and diaphysis of a growing long bone. The proliferation of cells in the epiphyseal cartilage is responsible for growth in length of a long bone (for details see page 85). After the fusion of epiphysis with diaphysis, the bone can no longer grow in length. The epiphyseal plate/cartilage is nourished by both epiphyseal and metaphyseal arteries.

N.B.

The fusion between epiphysis and diaphysis occurs 2 or 3 years earlier in females, hence they are shorter in length than males.

Metaphysis:

The end of diaphysis toward the epiphyseal cartilage is called metaphysis. It is the most actively growing area of a long bone. Before fusion of epiphysis, the metaphysis is profusely supplied by blood from nutrient, periosteal and juxta-epiphyseal arteries. These are end arteries and form hair-pin-like bends. Therefore, metaphysis is the common site of osteomyelitis in children for bacteria and emboli are easily trapped in the hair-pin bends leading to infarction. After the epiphyseal fusion the communications are established between the epiphyseal and metaphyseal arteries, as a result metaphysis contains no more end arteries. For this reason, osteomyelitis in this region is rare in adults.

Diaphysis:

It is the elongated part of bone between the metaphyses. It develops from primary ossification center.

Blood supply of bones

1. Blood Supply of a Long Bone: The long bone is supplied by the following four sets of arteries: nutrient artery, periosteal arteries, metaphyseal arteries and epiphyseal arteries (Fig. 6.10).

(a) Nutrient artery: It enters the middle of the shaft through a nutrient foramen, runs obliquely through the cortex and then divides into ascending and descending branches in the medullary cavity. Each branch then subdivides into a number of smaller parallel vessels which enter the metaphysis and form hair-pin loops. These loops anastomose with epiphyseal, metaphyseal (juxta epiphyseal) and periosteal arteries. Therefore, the metaphysis is the most vascular zone of the long bone.

The nutrient artery supplies the medullary cavity containing bone marrow and inner two-third of the outer shell of compact bone of diaphysis and metaphysis.

N.B.

• Oblique direction of nutrient foramen in shaft is opposite to the growing end of the long bone.

• Before entering the nutrient foramen the nutrient artery is tortuous, so that it is not affected during the movements of the bone.

• The nutrient artery of tibia is the largest nutrient artery of the body.

(b) Periosteal arteries: They are numerous and ramify beneath the periosteum. They enter the bone through Volkmann’s canals to supply the outer one-third of the cortex. The periosteal vessels are especially numerous beneath the muscular and ligamentous attachments.

(c) Metaphyseal (juxta-epiphyseal) arteries: They are derived from neighboring arteries and enter the metaphysis directly along the attachment of joint capsule.

(d) Epiphyseal arteries: They are derived from arterial anastomosis around the joint (circulus vasculosus). They enter the epiphysis either directly or after piercing the epiphyseal cartilage.

2. Blood Supply to Short Long Bones: To an extent it is similar to that of a long bone except that:

(a) The nutrient artery enters the middle of the shaft and immediately divides to form a plexus.

(b) The periosteal vessels supply the major part of the bone in adult and replace the nutrient vessels.

Clinical correlation

• Osteomyelitis: In typical long bones, infection occurs in the metaphyseal regions firstly because they are richly supplied with blood and secondly because the blood supply in the metaphyses is through end arteries forming ‘hair-pin bends’. Therefore, the bacteria and emboli are easily trapped in these hair-pin bends, leading to infarction (necrosis) of bone, resulting in a clinical condition called osteomyelitis. In short long bones, the infection begins in the middle of the shaft because nutrient artery enters the middle of the shaft and immediately divides to form the plexus.

• The blood supply of bone is so rich that the passing of a metal pin cannot interrupt blood supply sufficiently to kill the bone.

3. Blood Supply of a Vertebra: It is supplied by three sets of vessels:

(a) One or more large but short vessels enter the body from its posterior aspect through basivertebral foramen.

(b) A set of small vessels pierce the anterolateral surface of the body.

(c) A set of long vessels pierce the root of the transverse processes and supply the vertebral arch, and transverse and spinous processes.

4. Blood Supply of a Rib: It is supplied by two sets of vessels:

(a) The nutrient artery which enters it just behind the tubercle.

(b) The periosteal arteries.

N.B.

• The veins are numerous in cancellous bones; for example, in the vertebrae they join to form large basivertebral vein. In the compact bone they accompany the arteries in the Volkmann’s canals.

• There are no lymphatics in the bone, however some lymph vessels do accompany the periosteal blood vessels and drain in the regional lymph nodes.

Fracture of bone

The fracture is a break in the continuity of the bone. It is the most common type of bone injury. Fracture mostly occurs due to trauma (traumatic fractures), however, they may occur spontaneously due to disease that weakens the bone (spontaneous or pathological fractures).

A fracture may be simple or compound.

1. Simple or closed fracture: the fractured bone is not exposed to the exterior through skin.

2. Compound fracture: the fractured bone is exposed to the exterior through an opening in the skin.

When a bone fractures, the treatment involves realigning the broken ends and then immobilizing them until the fracture is healed (repaired).

Repair of a fractured bone

It involves the following steps:

1. When bone is fractured, the blood collects and coagulates to form fracture hematoma in and around the fracture site.

2. Two or three days later, new blood capillaries and uncommitted cells from the surrounding tissue invade the hematoma.

3. After 1 week, some of the uncommitted cells differentiate into fibroblasts, which produce a fibrous network between fractured ends. The other cells differentiate into chondroblasts, which produce islets of fibrocartilage in the fibrous network forming a fibrocartilage callus (temporary callus).

4. The osteoprogenitor cells in the periosteum and endosteum around the fracture site proliferate and differentiate into the bone-forming cells, the osteoblasts, which invade the temporary callus and lay down the woven-bone (primary bone tissue or osteoid) which unites the extremities of the fractured bone. It is called bony callus.

5. The primary bone tissue of the callus is gradually replaced by a secondary bone (mature bone).

6. Remodeling takes place and restores the original bone structure.

Figure 6.11 depicts repair of a fractured bone.

Fig. 6.11Repair of a fractured bone: A, bone fracture with formation of hematoma; B, formation of fibrocartilage callus; C, formation of bony callus; D, healed fracture.

Development and growth of bones

Development

All the bones develop from a mesenchyme by a process called ossification.

Ossification

As stated above it is the process of bone formation.

Types of ossification

There are two types of ossifications:

1. Intramembranous ossification.

2. Intracartilaginous (endochondral) ossification.

In intramembrane ossification mesenchymal models of bones undergo ossification, while in intracartilaginous ossification, cartilaginous models of bones undergo ossification. The details are as under:

Intramembranous ossification:

It involves the following steps:

Step 1: Mesenchymal tissue condenses to form a membranous sheet model. Osteoprogenitor cells located in this sheet differentiate into osteoblasts. The sites where the osteoblasts first appear are called ossification centers.

Step 2: Osteoblasts secrete organic substance (ground substance and collagen fibers) in the intercellular spaces to form osteoid tissue or bone matrix.

Step 3: Under the influence of alkaline phosphatase secreted by osteoblasts, the osteoid tissue is mineralized with calcium salts to become bone. The osteoblast now becomes trapped in mineralized matrix and are called osteocytes.

The intramembranous ossification is shown in Figure 6.12. Bones formed by the intramembranous ossification include flat bones of skull, viz. frontal, parietal, occipital, clavicle, etc.

Fig. 6.12Steps of intramembranous ossification. Source: (Fig. 8.1, Page 85, *Textbook of Clinical Embryology*, Vishram Singh. Copyright Elsevier 2012, All rights reserved.)

Clinical correlation

Cleidocranial dysostosis: It is a congenital condition presenting the following features:

(a) Complete or partial absence of clavicles.

(b) Defective development of cranial vault with large fontanelles and delayed closing of the sutures. This condition occurs due to defective intramembranous ossification.

Intracartilaginous (endochondral) ossification:

The intracartilaginous ossification is more complicated, but clinically more important because all the long bones of limbs (and many others) are ossified by this process.

It involves the following steps (Fig. 6.13):

Fig. 6.13Steps of intracartilaginous ossification (A to D). Source: (Fig. 8.2, Page 86, *Textbook of Clinical Embryology*, Vishram Singh. Copyright Elsevier 2012, All rights reserved.)

Step 1: The cartilage cells (chondroblasts) of cartilaginous model enlarge and matrix surrounding them is calcified under the influence of alkaline phosphatase secreted by cartilage cells.

Step 2: The cartilage cells die and disappear leaving behind empty spaces called primary areolae.

Step 3: The cells on the surface of periosteum differentiates into osteoblasts which enter at the site of ossification along with blood vessels (periosteal bud).

Step 4: The most of calcified matrix is absorbed forming large empty spaces called secondary areolae, leaving behind only thin bars of calcified matrix.

Step 5: The new bone (osteoid) is laid down on the surface of calcified bars of matrix.

Step 6: The mineralization of osteoid.

Ossification of a long bone

The following is a simplified account of intracartilaginous ossification of a long bone (Fig. 6.14).

Step 1: First the bone is laid down as a hyaline cartilaginous model surrounded by perichondrium.

Step 2: A primary centre of ossification then appears in the center of the shaft and this spreads toward the ends to form the diaphysis.

Step 3: At the same time, periosteum lays down a collar of bone (periosteal collar) around the circumference of the shaft.

Step 4: Later, one or more secondary centres of ossification develop at each end of the cartilaginous model and form the epiphysis.

Step 5: The epiphyseal cartilage separating epiphysis from diaphysis and articular cartilage at each end of bone remain cartilaginous.

Step 6: The epiphyseal plate of cartilage continues to produce new cartilage and thus enables the bone to grow in length. For this reason, epiphyseal plate of cartilage is also called growth plate. Once the growth in length is completed, the remaining epiphyseal cartilage ceases to proliferate and becomes ossified leaving only an epiphyseal scar called epiphyseal line.

Ossification centres

These are sites, where bone formation begins. The ossification centres are generally classified into two types: primary and secondary.

1. Primary centres of ossification, appear before birth (usually between the 7th to 12th week) with some exceptions (viz. primary centers of tarsal and carpal bones appear after birth except those of talus, calcaneum and cuboid).

2. Secondary centres of ossification, appear as a rule after birth (usually from the time of birth to 5 years of age).

A primary centre forms centres diaphysis, and the secondary centre form epiphyses. The fusion of epiphysis with the diaphysis starts at puberty and is usually complete by the age of 25 years, therefore after the age of 25 years, no more bone growth can take place.

In general, the appearance of secondary centres and fusion of epiphyses occur about 1 year earlier in females than in males.

N.B.

All the secondary centers appear after birth except the one at the lower end of femur which appears in the 9th month of intrauterine life and sometimes at the upper end of tibia and humerus.

Clinical correlation

• The visibility of centre of ossification in the distal end of femur in radiograph of newborn child found dead is of medicolegal importance because it indicates that the child was full term (Fig. 6.15). In x-ray of newborns, the shaft of long bone presents a white (radiopaque) shadow, whereas ends and joints cast black (radiolucent) shadow. This is because, at birth the shaft of cartilaginous model is ossified, i.e. bony, but ends are still unossified, i.e. cartilaginous.

• On the other hand in x-rays of growing children, the shaft and epiphyses cast white (radiopaque) shadow but the epiphyseal plate between the epiphysis and diaphysis cast a black (radiolucent) shadow.

• Achondroplasia (dwarfism): It is a congenital condition with the following clinical features:

1. Individual is abnormally short (dwarf).

2. Disproportionately shorter limbs than trunk.

3. Dorsal kyphosis and lumbar lordosis.

This condition occurs due to defective endochondral ossification.

Growing end of the long bone

In most of the long bones the two epiphyses do not fuse simultaneously. The epiphysis at one end always fuses few years before than that at the other end.

The end at which fusion occurs later is called the growing end and this is also the end at which secondary center of ossification appears first.

The growing end is always located opposite to the direction of nutrient foramen.

The direction of nutrient foramen is easily remembered by a ‘dictum’ that says, ‘To the elbow I go and from knee I flee’.

In the milking cow position, the direction of nutrient foramina is always downward (Fig. 6.16).

Fig. 6.16Milking cow position showing direction of nutrient foramina.

Thus in upper limb, shoulder end of humerus and wrist ends of ulna and radius are growing ends, whereas in the lower limb, the knee ends of femur, tibia and fibula are growing ends.

N.B.

The nutrient foramen in the shaft of all long bones is always directed opposite to the growing end except that in the shaft of fibula which is directed toward the growing end.

Law of union of epiphyses (law of ossification)

According to this law, the epiphyseal centre (secondary centre of ossification) which appears first unites last and the epiphyseal centre which appears later unites first except in case of fibula where the epiphyseal centre for lower end appears first and also unites first. On the other hand, the epiphyseal centre for upper end appears late and also unites late (Fig. 6.17). Thus, fibula is the only long bone which violates the law of union of epiphyses.

Fig. 6.17Violation of the law of union of epiphyses by the fibula.

Growth of a long bone

The growth of long bone after birth occurs by two methods:

1. Appositional growth

2. Endochondral growth

N.B.

The bones cannot grow by an interstitial growth like tendons, ligaments and cartilage.

Appositional growth:

The appositional growth refers to the addition, i.e. growth, at the periphery of the bones resulting in an increase in the diameter of the long bones.

In appositional growth, the osteoblasts on the surface of bones proliferate. The superficial osteoblasts produced from these divisions remain as such or divide again. The deep osteoblasts resulting from these divisions produce bone matrix, and when they are surrounded by matrix, they become osteocytes. Consequently, a new layer of bone is deposited on the surface of the bone.

In cancellous bone, appositional growth adds bone matrix to the outer surface of trabeculae.

Endochondral growth:

The endochondral growth is responsible for the increase in the length of long bones.

The endochondral growth occurs due to multiplication of the cells of the epiphyseal plate of cartilage.

The surface of epiphyseal plate facing toward epiphysis gives rise to new cartilage which pushes the epiphysis away from diaphysis.

The amount of new cartilage produced is equal to the amount of cartilage replaced by spongy bone toward the metaphyseal surface of the epiphyseal plate leading to increase in the length of diaphysis. The thickness of epiphyseal plate remains almost the same. The details are as under:

The epiphyseal plate is organized into four zones (Fig. 6.18); from superficial to deep these are:

1. Zone of resting cartilage

2. Zone of proliferation

3. Zone of hypertrophy

4. Zone of calcification

The zone of resting cartilage is nearest to the epiphysis and contains randomly arranged chondrocytes which do not divide rapidly.

In the zone of proliferation, the chondrocytes proliferate and form longitudinal columns of young cartilage cells resembling stacks of plates or coins.

In the zone of hypertrophy, the chondrocytes mature and hypertrophy. They secrete alkaline phosphatase.

Thus, maturation gradient exists in each column of chondrocytes. The cells near the zone of resting cartilage are younger and actively proliferating, whereas cells progressively near the diaphysis are older and are undergoing hypertrophy.

The zone of calcification is very thin. It consists of matrix mineralized with calcium carbonate. When the hypertrophied chondrocytes die, the blood vessels from diaphysis grow into this area. The connective tissue surrounding the blood vessels contains osteoblasts from the endosteum. The osteoblasts line up on the surface of the calcified cartilage and deposit bone.

Clinical correlation

Growth in height: The epiphyseal (growth) plates of long bones of upper and lower limbs are responsible for growth in height of an individual. Once the epiphyses unite with diaphyses, the growth in height stops.

The sex hormones stimulate the ossification of epiphyseal plates leading to union of epiphysis with diaphysis. In females, estrogen causes early closure of epiphyses with diaphysis as compared to that of testosterone. Consequently, females usually do not reach the same height as males.

The damage to the epiphyseal plate at the growing end of long bones of limbs of growing children will lead to shortening of the limb because damage of epiphyseal plate interferes with elongation of that bone.

Remodeling of bone

The bone does not grow by multiplication of its cells or by increase in its intercellular material (i.e. interstitial growth), hence its shape cannot be maintained.

The bone grows by deposition of new bone by osteoblasts on its ends and on its surface in a random fashion. Therefore to maintain its shape, the unwanted bone is removed by osteoclasts. This process of bone removal is called remodeling.

Surface remodeling

As the bone grows in length, there occurs subperiosteal bone deposition in the shaft while subperiosteal bone resorption occurs in conical region toward the end. Thus diverging regions are straightened to become the part of the shaft (Fig. 6.19).

Fig. 6.19Remodeling of bone: A, as the long bone grows, the sites once occupied by expanded ends become part of slender shaft; B, sites of new bone deposition (black areas).

Internal remodeling

As the bone grows in diameter by deposition of new layers of bone deep to periosteum, the periosteal bone becomes thicker and thicker which is neither necessary nor desirable. Hence as the bone is laid down outside the shaft, it is removed from inside by osteoclasts. Consequently the shaft grows in diameter, and at the same time its wall does not become too thick.

The osteoclasts also remove trabeculae in the center of bone that were formed by endochondral ossification. This leads to the formation of marrow cavity.

N.B.

Remodeling of developing bone is done by: (a) deposition of bone by osteoblasts and (b) selective absorption of bone by osteoclasts.

Clinical correlation

Paget’s disease: Normally the bone is continuously removed and repaired (replaced) by a new bone throughout life. In Paget’s disease (osteitis deformans) which occurs in later life, there is uncontrolled osteoclast activity causing wide spread bone resorption which is followed by hectic, osteoblastic activity producing woven bone (osteoid) that fills the erosion but the repair stops at the osteoid stage. As a result, the healthy mature bone is gradually replaced by thick, bulky, weak osteoid bone. Consequently in individuals suffering from Paget’s disease, the bones are thick and bent, especially the femora, tibiae and spine. The skull is enlarged and thick, the spine is bent forward (kyphosis).

Factors affecting growth of bones

1. Nutritional factors:

(a) Vitamin A co-ordinates the activity of osteoblasts and osteoclasts. The deficiency of vitamin A slows down the activity of osteoclasts, consequently the size of spinal and cranial foramina is reduced, with eventual compression of nerve roots. On the other hand, the high concentration of Vitamin A causes rarefaction and resorption of bone.

(b) Vitamin C is essential for the synthesis of organic intercellular matrix (i.e. collagenous fibers and ground substance) by osteoblasts. The deficiency of vitamin A leads to defective development of growing ends of the bone. It also leads to rupture of capillaries producing painful subperiosteal hematoma.

(c) Vitamin D is essential for the absorption of calcium and phosphate from intestine. Deficiency of vitamin D leads to defective mineralization (calcification) of osteoid tissue. As a result children suffer from rickets (characterized by bowing of legs, knobby metaphyseal regions); and adults suffer from osteomalacia (i.e. softening of the bones).

2. Hormonal factors: The hormones play very important role in bone growth. The hormones which are essential for proper development and growth of bone include pituitary hormones, thyroid hormones, parathyroid hormones and sex hormones. Their defective secretion can cause number of skeletal defects.

3. Genetic factors: A clinical condition ‘chondrodystrophia foetalis’ occurs due to defective autosomal dominant inheritance. In this condition, endochondral ossification fails to occur properly.

4. Mechanical factors: Tensile force helps in bone formation whereas compressive forces favor bone resorption. In practice, orthopedic surgeons provide traction for quick healing of fractures.

Cartilage

The cartilage is a specialized connective tissue which provides rigidity along with elasticity. Hence it is found in those areas of the body, where both rigidity and elasticity are required. It consists of chondrocytes embedded in a gel-like matrix.

Phylogenetically, the cartilage tissue is older than bone tissue. Most of the bones in the intrauterine life are preformed in cartilage. The cartilages which are replaced by bones are called temporary cartilages and those that persist throughout life are called permanent cartilages.

Function of cartilage

The following are the functions of the cartilage:

1. Provides rigidity and support to soft tissues.

2. Provides smooth gliding surface for articulation.

3. Enables development and growth of long bones.

N.B.

Both bone and cartilage are specialized connective tissue, but the basic difference between the two is that in bone matrix it is mineralized while in cartilage it is not mineralized.

Structure

The cartilage has all common features of connective tissue, which include the following:

1. Cells

2. Fibers

3. Ground substance

Cells:

Cells of the cartilage are of the following three types:

1. Chondrogenic cells: found in the perichondrium, where they undergo mitosis and differentiate into chondroblasts.

2. Chondroblasts: young cartilage cells occupying small spaces (lacunae) and may undergo mitosis.

3. Chondrocytes: mature cartilage cells which reside in lacunae. They form isogenous cell clusters surrounded by territorial matrix.

Fibers:

Cartilage has the following two types of fibers:

1. Type I collagen fibers in fibrocartilage.

2. Type II collagen fibers in hyaline and elastic cartilages.

Ground substance:

It consists of:

1. Proteoglycans, specifically chondroitin sulphate and keratan sulphate.

2. Glycoproteins, viz. chondronectin and chondrocalcin (a calcium-binding protein).

3. Water (tissue fluid) contributes to 75% hydration of the ground substance (high degree of hydration).

N.B.

There is no mineral (inorganic) component in the ground substance of cartilage because it is not mineralized.

Growth of cartilage

The cartilage grows by both appositional and interstitial methods.

1. Appositional growth: In this, layers of new cartilage are deposited at the surface beneath the perichondrium. The new cartilage is formed by chondroblasts derived from perichondrium. By appositional growth the cartilage increases in width.

2. Interstitial growth: It occurs due to increase in size and the number of existing cells and by an increase in the amount of intercellular matrix, due to proliferation of chondrocytes by mitosis in the center of cartilaginous model. By interstitial growth the cartilage increases in length.

N.B.

Adult cartilage grows and repair slowly, after a severe injury.

Characteristic features of cartilage

The distinguishing features of cartilage are as follows:

N.B.

The articular cartilage and fibrocartilage are devoid of perichondrium.

Clinical correlation

• Cartilage graft: The cartilage has low antigenicity due to lack of lymphatics and isolation of chondrocytes in separate lacunae within matrix, hence homogenous transplantation of cartilage is possible without rejection.

• Cartilage repair: Damaged cartilage shows limited repair (regeneration) with defects being slowly filled with scar tissue instead of cartilage. Thus, repair of cartilage takes a long time because it is avascular.

Types of cartilage

The cartilages are classified into three types (Fig. 6.20):

1. Hyaline cartilage

2. Elastic cartilage

3. Fibrocartilage

Hyaline cartilage (Gk. hyalos = transparent stone):

It appears bluish-white and transparent because it contains very fine collagen fibers having same refractive index as that of ground substance.

It is the most widely distributed cartilage in the body (see Table 6.5). All the long bones in the body are preformed in hyaline cartilage.

N.B.

• All the hyaline cartilages are covered by perichondrium except articular cartilages.

• Nonarticular hyaline cartilage has tendency to calcify and to be replaced by bone.

Elastic cartilage:

It is made up of numerous chondrocytes embedded in matrix containing rich network of yellow elastic fibers. The sites of distribution of elastic cartilage include pinna of the ear, epiglottis.

Fibrocartilage:

It appears white and opaque due to abundance of collagen fibers in it. The collagen fibers are arranged in bundles. The chondrocytes are few, small and scattered singly or arranged in rows. It is formed at sites subjected to great pressure like intervertebral disc.

The key histological features of three types of cartilages (Fig. 6.16) are presented in Table 6.4.

Table 6.4

Salient histological features of the three types of cartilages

Feature	Hyaline cartilage	Elastic cartilage	Fibrocartilage
Color	Glossy bluish, transparent	Yellowish	White, opaque
Perichondrium	Present	Present	Absent
Chondrocytes	Present in large numbers, singly or in groups inside lacunae	Larger, more numerous and packed more closely inside lacunae as compared to hyaline cartilage	Few in number, scattered singly or arranged in rows
Matrix	Homogenous, basophilic, collagen fibers delicate and not visible	Rich in elastic fibers	Thick bundles of collagen fibers run parallel within matrix
Tendency to calcify	Common (in later life)	Less common	Absent

The distribution of the three types of cartilages in the body is enumerated in Table 6.5.

Table 6.5

Sites of hyaline, elastic and fibrocartilages

Hyaline cartilage	Elastic cartilage	Fibrocartilage
Articular cartilages of most of the joints	Pinna of external ear	Intervertebral discs
Thyroid cartilage	Epiglottis	Interpubic disc
Cricoid cartilage	Corniculate cartilage	Menisci of knee joint
Lower part of arytenoids cartilage	Cuneiform cartilage	Articular discs of temporo-mandibular, sterno-clavicular and inferior radio-ulnar joints
Tracheal rings	Apex of arytenoid cartilage	Articular cartilages of temporo-mandibular, sterno-clavicular and acromio-clavicular joints
Costal cartilages	Auditory tubes	Glenoid labrum, acetabular labrum, etc.
Bronchial cartilage	External auditory meatus
Nasal cartilages

Golden Facts to Remember

➢ Total number of bones in the body	206
➢ Largest bone in the body	Femur
➢ Smallest bone in the body	Stapes
➢ Longest bone in the body	Femur
➢ Smallest long bone in the body	Malleus
➢ Most slender long bone in the body	Fibula
➢ Largest flat bone in the body	Hip bone
➢ Largest sesamoid bone in the body	Patella
➢ Most actively growing area of long bone	Metaphysis
➢ Most vascular area of a long bone	Metaphysis
➢ Most commonly fractured bone in the body	Clavicle in children and adults, radius in people over 50 years of age
➢ Largest nutrient artery in the body	Nutrient artery of tibia
➢ Commonest site of osteomyelitis in a long bone	Metaphysis
➢ All the long bone of body lie vertically except	Clavicle which lies horizontally
➢ All the bones in the body are covered by periosteum except	Sesamoid bones and ear ossicles
➢ All the bones in the body are made up of both compact and spongy bone except	Inferior nasal concha
➢ All the bones in the body form joint/joints except	Hyoid bone
➢ Commonest site of bone marrow aspiration	Manubrium sternum in adults and iliac crest in children
➢ First bone to ossify in the body	Clavicle
➢ Commonest congenital anomaly due to defective endochondral ossification	Achondroplasia (dwarfism)
➢ Commonest congenital anomaly due to defective membranous ossification	Cleidocranial dysostosis
➢ Most common site of accessory bones	Lambdoid suture of skull
➢ All the long bones in the body follow law of ossification except	Fibula
➢ Most abundant cartilage in the body	Hyaline cartilage
➢ Most durable cartilage in the body	Fibrocartilage
➢ Largest yellow elastic cartilage in the body	Cartilage of ear auricle
➢ Largest hyaline cartilage in the body	Thyroid cartilage of larynx