SKELETAL SYSTEM

INTRODUCTION

A bone may appear to be inert because of nonliving material in the extracellular matrix of bone tissue.

However, bone also includes active, living tissues: bone tissue, cartilage, dense connective tissue, blood, and

nervous tissue. Bones are not only alive, but also multifunctional. Bones, the organs of the skeletal system,

support and protect softer tissues, provide points of attachment for muscles, house blood-producing cells,

and store inorganic salts.

Skeletal system performs several basic functions:

1. Support: The skeleton serves as the structural framework for the body by supporting soft tissues and

providing attachment points for the tendons of most skeletal muscles.

2. Protection: The skeleton protects the most important internal organs from injury. For example, cranial

bones protect the brain, vertebrae (backbones) protect the spinal cord, and the rib cage protects the heart and

lungs.

3. Assistance in movement: Most skeletal muscles attach to bones; when they contract, they pull on bones

to produce movement.

4. Mineral homeostasis (storage and release): Bone tissue stores several minerals, especially calcium and

phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body’s calcium.

On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to

distribute the minerals to other parts of the body.

5. Blood cell production: Within certain bones, a connective tissue called red bone marrow produces red

blood cells, white blood cells, and platelets, a process called hemopoiesis. Red bone marrow consists of

developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. It is

present in developing bones of the fetus and in some adult bones, such as the hip bones, ribs, breastbone,

vertebrae (backbones), skull, and ends of the bones of the arm and thigh.

6. Triglyceride storage: Yellow bone marrow consists mainly of adipose cells, which store triglycerides.

The stored triglycerides are a potential chemical energy reserve. In a newborn, all bone marrow is red and is

involved in hemopoiesis. With increasing age, much of the bone marrow changes from red to yellow.

Bone Structure:

The bones of the skeletal system vary greatly in size and shape. However, bones are similar in structure,

development, and function.

Bone Classification

Bones are classified according to their shapes long, short, flat, or irregular.

Long bones have long longitudinal axes and expanded ends. Examples of long bones are the

forearm and thigh bones.

Short bones are cubelike, with roughly equal

lengths and widths. The bones of the wrists

and ankles are this type.

Flat bones are platelike structures with broad

surfaces, such as the ribs, scapulae, and some

bones of the skull.

Irregular bones have a variety of shapes and

are usually connected to several other bones.

Irregular bones include the vertebrae that

comprise the backbone and many facial bones.

A typical long bone consists of the following parts:

1. The diaphysis is the bone’s shaft or body—the long,

cylindrical, main portion of the bone.

2. The epiphyses (singular is epiphysis) are the

proximal and distal ends of the bone.

3. The metaphyses (singular is metaphysis) are the

regions between the diaphysis and the epiphyses.

In a growing bone, each metaphysis contains an

epiphyseal (growth) plate, a layer of hyaline cartilage

that allows the diaphysis of the bone to grow in length.

When a bone ceases to grow in length at about ages

18–21, the cartilage in the epiphyseal plate is replaced

by bone; the resulting bony structure is known as the

epiphyseal line.

4. The articular cartilage is a thin layer of hyaline

cartilage covering the part of the epiphysis where the

bone forms an articulation (joint) with another bone.

Articular cartilage reduces friction and absorbs shock at

freely movable joints.

5. The periosteum surrounds the external bone surface

wherever it is not covered by articular cartilage. It is

composed of an outer fibrous layer of dense irregular

connective tissue and an inner osteogenic layer that

consists of cells. Some of the cells of the periosteum

enable bone to grow in thickness, but not in length.

6. The medullary cavity or marrow cavity is a hollow, cylindrical space within the diaphysis that contains

fatty yellow bone marrow in adults.

7. The endosteum is a thin membrane that lines the internal bone surface facing the medullary cavity. It

contains a single layer of cells and a small amount of connective tissue.

Composition:

Four types of cells are present in bone tissue: osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

1. Osteogenic cells are unspecialized stem cells derived from mesenchyme, the tissue from which almost all

connective tissues are formed. They are the only bone cells to undergo cell division; the resulting cells

develop into osteoblasts. Osteogenic cells are found along the inner portion of the periosteum, in the

endosteum, and in the canals within bone that contain blood vessels.

2. Osteoblasts (blasts =buds or sprouts) are bone-building cells. They synthesize and secrete collagen

fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate

calcification. As osteoblasts surround themselves with extracellular matrix, they become trapped in their

secretions and become osteocytes. (Note: The ending -blast in the name of a bone cell or any other

connective tissue cell means that the cell secretes extracellular matrix.)

3. Osteocytes (cytes = cells), mature bone cells, are the main cells in bone tissue and maintain its daily

metabolism, such as the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do

not undergo cell division.

4. Osteoclasts (clast = break) are huge cells derived from the fusion of as many as 50 monocytes and are

concentrated in the endosteum. The osteoclast’s plasma membrane is deeply folded into a ruffled border.

Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components

of the underlying bone matrix. This breakdown of bone extracellular matrix, termed resorption, is part of

the normal development, maintenance, and repair of bone. In response to certain hormones, osteoclasts help

regulate blood calcium level. They are also target cells for drug therapy used to treat osteoporosis.

The extracellular matrix is about 25% water, 25% collagen fibers, and 50% crystallized mineral salts. The

most abundant mineral salt is calcium phosphate. It combines with another mineral salt, calcium hydroxide,

to form crystals of hydroxyapatite. As the crystals form, they combine with still other mineral salts, such as

calcium carbonate, and ions such as magnesium, fluoride, potassium, and sulfate. As these mineral salts are

deposited in the framework formed by the collagen fibers of the extracellular matrix, they crystallize and the

tissue hardens. This process, called calcification, is initiated by bone-building cells called osteoblasts.

Bone is not completely solid but has many small spaces between its cells and extracellular matrix

components. Some spaces serve as channels for blood vessels that supply bone cells with nutrients. Other

spaces act as storage areas for red bone marrow. Depending on the size and distribution of the spaces, the

regions of a bone may be categorized as compact or spongy. Overall, about 80% of the skeleton is compact

bone and 20% is spongy bone.

Factors Affecting Bone Growth:

Normal bone metabolism; growth in the young depends on several factors. These include adequate dietary

intake of minerals and vitamins, as well as sufficient levels of several hormones.

1. Minerals: Large amounts of calcium and phosphorus are needed while bones are growing, as are smaller

amounts of magnesium, fluoride, and manganese. These minerals are also necessary during bone

remodeling.

2. Vitamins: Vitamin A stimulates activity of osteoblasts. Vitamin C is needed for synthesis of collagen,

Vitamin D helps build bone by increasing the absorption of calcium from foods in the gastrointestinal tract

into the blood. Vitamins K and B12 are also needed for synthesis of bone proteins.

3. Hormones: During childhood, the hormones most important to bone growth are the insulin like growth

factors (IGFs), which are produced by the liver and bone tissue. IGFs stimulate osteoblasts, promote cell

division at the epiphyseal plate and in the periosteum, and enhance synthesis of the proteins needed to build

new bone. IGFs are produced in response to the secretion of human growth hormone (hGH) from the

anterior lobe of the pituitary gland. Thyroid hormones (T3 and T4) from the thyroid gland also promote

bone growth by stimulating osteoblasts. In addition, the hormone insulin from the pancreas promotes bone

growth by increasing the synthesis of bone proteins.

At puberty, the secretion of hormones known as sex hormones causes a dramatic effect on bone growth. The

sex hormones include estrogens (produced by the ovaries) and androgens such as testosterone (produced by

the testes). Although females have much higher levels of estrogens and males have higher levels of

androgens, females also have low levels of androgens, and males have low levels of estrogens. These

hormones are responsible for increased osteoblast activity and synthesis of bone extracellular matrix.

Divisions of the Skeletal System

The adult human skeleton consists of 206 named bones, most of which are paired, with one member of each

pair on the right and left sides of the body. Bones of the adult skeleton are grouped into two principal

divisions: the axial skeleton (80 bones) and the appendicular skeleton (126 bones).

The axial skeleton consists of the bones that lie around the longitudinal axis of the human body, skull

bones, auditory ossicles (ear bones), hyoid bone, ribs, sternum, and bones of the vertebral column.

Functionally, the auditory ossicles in the middle ear, which vibrate in response to sound waves that strike

the eardrum, are not part of either the axial or appendicular skeleton, but they are grouped with the axial

skeleton for convenience.

Function of axial skeleton:

1. Axial skeleton serves as a support system and protects various organs of the body like spinal cord,

heart, lungs and brain.

2. It also plays a vital role in maintaining blood calcium levels by taking part in storage and release of

calcium.

3. Axial skeleton mainly supports the head, neck and trunk portions of the body.

Skull: Skull is made up of 22 bones and is situated on the superior end of the back bone (vertebral column).

Cranial bones (8) and facial bones (14) together constitute the skull.

Axial Skeleton

Skull 22 bones

Vertebral column 26 bones

Thorax 25 bones

Auditory ossicles 06 bones

Hyoid 01bone

Total 80 bone

Cranium is considered as a protective cover for the brain and is made up of eight cranial bones. Face of an

individual is made up of 14 bones known as facial bones. The 14 bones constitute the face.

Associated structures of the skull:

The skull is not only made of bones, it also includes certain structures such as sutures, paranasal sinuses and

fontanels which are considered as the unique features of skull.

1.Sutures: the skull bones are held together by means of an immovable joint known as suture. The different

sutures that appear in the skull are tabulated below.

Skull (22)

Cranial bones (08) Facial bones (14)

Frontal bone 01 Nasal bones 02

Parietal bones 02 Maxillae 02

Temporal bones 02 Mandible 01

Occipital bone 01 Zygomatic bones 02

Sphenoid bone 01 Lacrimal bones 02

Ethmoid bone 01 Palatine bones 02

Inferior nasal conchae 02

Vomer 01

Total 08 bones 14 bones

Suture Bones which the suture unites

Coronal sutures Fuses frontal bone and parietal bones

Lambdoid suture Fuses the parietal bones with occipital bone

Squamous suture Fuses parietal bones and temporal bones

Sagital suture Fuses the two parietal bones

Frontozygomatic suture Fuses frontal bone with the zygomatic bone

Sphenoparietal suture Fuses sphenoid and parietal bones

2. Paranasal sinuses: On the lateral sides of nasal cavity, both cranial and facial bones contain air-filled spaces

known as paranasal sinuses. These sinuses are composed of ciliated mucous membranes that open into the

nasal cavity. These sinuses are present in the sphenoid, ethmoid, maxillary and frontal bones. These sinuses

apart from purifying the inhaled air also play a role in resonating the voice in order to improve the quality of

voice.

3. Fontanels or soft spots: The skull of a fetus or

newborn is different in many ways from an adult

skull. The skeleton of adults is entirely composed

of bones, but this is not the same with the skeleton

of fetus or infants. The skeleton of foetus is not yet

completely formed, it contains certain areas of

hyaline cartilage or mesenchyme that are yet to

undergo ossification. By birth almost all the

cartilage is replaced by bone through the process of

ossification. But at birth, certain areas between

cranial bones do not undergo ossification and

remain as mesenchyme-filled spaces known as

fontanels or soft spots. These fontanels are of

significance as they provide flexibility to the skull

in order to facilitate its expulsion through the birth

canal during parturition. Apart from that they also

assist in rapid growth of the brain. These fontanels

remain until 20-40 months after birth.

Vertebral column: The vertebral column is made up of bones called vertebrae. These vertebrae are

interspersed with discs known as intervertebral discs and because of these discs that the vertebral column is

flexible. The sternum, ribs and vertebral column together constitute the trunk of the body. The vertebral

column encloses a very delicate structure within it known as spinal cord and serves to protect it. It also

attaches the muscles of ribs, girdle, pelvis, upper limbs etc.

It is formed by 26 irregular bones. The number of bones that make up the vertebral column during early

development is 33. With the progression of child growth vertebrae present in sacral and coccygeal regions

articulate to form a vertebral column made of 26 bones. The vertebral column is divided into five different

portions. The names to the vertebrae are given based on their location. Of the 26 vertebrae, 24 vertebrae are

single bones, while the last two are sacrum and coccyx these are formed by the fusion of 5 and 4 bones

respectively.

Cervical vertebrae 07

Thoracic vertebrae 12

Lumbar vertebrae 05

Sacrum 01

Coccyx 01

Thorax/Rib cage/Thoracic cage:

It is formed by 25 bones, one sternum and twelve pairs of ribs, coastal cartilage of ribs and bodies of

thoracic vertebrae together constitute a cone shaped cage like structure known as thoracic cage. This cone

shaped structure is narrow at its upper surface and broadens as one proceeds from superior to inferior end.

This bony cage serves as a protective cover for the organs in thoracic and superior abdominal cavities.

The appendicular skeleton consists of the bones of the upper and lower limbs (extremities), plus the bones

forming the girdles that connect the limbs to the axial skeleton. The appendicular skeleton is made up of 126

bones and it is necessary because it is responsible for the physical movements of the body. Apart from

assisting the movements, the appendicular skeleton also maintains homeostasis, protect organs like

reproductive organs.

Pectoral girdle 04 bones

Upper limbs 60 bones

Pelvic girdle 02 bones

Lower limbs 60 bones

Total 126 bones

1. Pectoral girdle: The bones of upper limbs articulate with the axial skeleton via the shoulder or pectoral

girdles each on either side of the body. Each pectoral girdle is made up of a clavicle and a scapula.

2. Upper limb bones: the two upper limbs are characterized by the presence of thirty separate bones in each

limb. The thirty bones in each limb are spread in three different areas as follows:

a. Humerus (arm bone)

b. Ulna and radius (bones of fore arm)

c. Eight carpal bones in the wrist, 5 metacarpals in palm region and 14 phalanges.

3. The pelvic (hip) girdle consists of the two hip bones. The hip bones unite anteriorly at a joint called the

pubic symphysis. They unite posteriorly with the sacrum at the sacroiliac joints. The complete ring

composed of the hip bones, pubic symphysis, and sacrum forms a deep, basin like structure called the

bony pelvis. Functionally, the bony pelvis provides a strong and stable support for the vertebral column

and pelvic and lower abdominal organs. The pelvic girdle of the bony pelvis also connects the bones of the

lower limbs to the axial skeleton.

4. Lower limb bones: Each lower limb (lower extremity) has 30 bones in four locations:

a) the femur in the thigh,

b) the patella (kneecap),

c) the tibia and fibula in the leg,

d) the 7 tarsals in the tarsus (ankle),

e) the 5 metatarsals in the metatarsus, and

f) the 14 phalanges (bones of the digits) in the foot.

JOINTS

The joints of the skeletal system contribute to homeostasis by holding bones together in ways that allow for

movement and flexibility. A joint, also called an articulation or arthrosis, is a point of contact between two

bones, between bone and cartilage, or between bone and teeth. When we say one bone articulates with

another bone, we mean that the bones form a joint. The scientific study of joints is termed arthrology. The

functional classification of joints relates to the degree of movement they permit. Functionally, joints are

classified as one of the following types:

Synarthrosis: An immovable joint. The plural is synarthroses.

Amphiarthrosis: A slightly movable joint. The plural is amphiarthroses.

Diarthrosis: A freely movable joint. The plural is diarthroses. All diarthroses are synovial joints.

They have a variety of shapes and permit several different types of movements.

Joints are classified structurally, based on their anatomical characteristics, and functionally, based on the

type of movement they permit. The structural classification of joints is based on two criteria:

(1) The presence of a space between the articulating bones, called a synovial cavity, and

(2) The type of connective tissue that binds the bones together. Structurally, joints are classified as one of

the following types:

• Fibrous joints: There is no synovial cavity, and the bones are held together by dense irregular

connective tissue that is rich in collagen fibers.

• Cartilaginous joints: There is no synovial cavity and the bones are held together by cartilage.

• Synovial joints: The bones forming the joint have a synovial cavity and are united by the dense

irregular connective tissue of an articular capsule, and often by accessory ligaments.

Fibrous Joints: Some bones, such as those that make up the cranium, are sutured together by a thin layer

of fibrous connective tissue and are immovable.

Cartilaginous Joints: Slightly movable joints are those in which the bones are joined by fibrocartilage. The

ribs are joined to the sternum by costal cartilages. The bodies of adjacent vertebrae are separated by

intervertebral disks that increase vertebral flexibility. The pubic symphysis, which occurs between the pubic

bones, consists largely of fibrocartilage. Due to hormonal changes, this joint becomes more flexible during

late pregnancy, which allows the pelvis to expand during childbirth.

Synovial Joints: All synovial joints are freely movable because, unlike the joints discussed so far, the two

bones are separated by a joint cavity. The cavity is lined by a synovial membrane, which produces synovial

fluid, a lubricant for the joint. The absence of tissue between the articulating bones allows them to be freely

movable but means that the joint has to be stabilized in some way.

Types of Synovial Joints

Although all synovial joints are similar in structure, the shapes of the articulating surfaces vary; thus, many

types of movements are possible. Synovial joints are divided into six categories based on type of movement:

planar, hinge, pivot, condyloid, saddle, and ball-and-socket.

Planar Joints: The articulating surfaces of bones in a planar joint are flat or slightly curved. Planar joints

primarily permit back-and-forth and side-to-side movements between the flat surfaces of bones. Many planar

joints are biaxial because they permit movement around two axes. An axis is a straight line around which a

rotating (revolving) bone moves. Examples of planar joints are the intercarpal joints (between carpal bones at

the wrist), intertarsal joints (between tarsal bones at the ankle), sternoclavicular joints (between the

manubrium of the sternum and the clavicle), acromioclavicular joints (between the acromion of the scapula

and the clavicle), sternocostal joints (between the sternum and ends of the costal cartilages at the tips of the

second through seventh pairs of ribs), and vertebrocostal joints (between the heads and tubercles of ribs and

transverse processes of thoracic vertebrae).

Hinge Joints: In a hinge joint, the convex surface of one bone fits into the concave surface of another

bone. As the name implies, hinge joints produce an angular, opening-and-closing motion like that of a

hinged door. In most joint movements, one bone remains in a fixed position while the other moves around

an axis. Hinge joints are monaxial (uniaxial) because they typically allow motion around a single axis.

Hinge joints permit only flexion and extension. Examples of hinge joints are the knee (actually a modified

hinge joint), elbow, ankle, and interphalangeal joints.

Pivot Joints: In a pivot joint, the rounded or pointed surface of one bone articulates with a ring formed

partly by another bone and partly by a ligament. A pivot joint is monaxial because it allows rotation only

around its own longitudinal axis. Examples of pivot joints are the atlanto-axial joint, in which the atlas

rotates around the axis and permits the head to turn from side to side as when you shake your head “no”, and

the radioulnar joints that enable the palms to turn anteriorly and posteriorly.

Condyloid Joints: In a condyloid joint or ellipsoidal joint, the convex oval-shaped projection of one bone

fits into the oval-shaped depression of another bone. A condyloid joint is biaxial because the movement it

permits is around two axes (flexion–extension and abduction–adduction). Examples of condyloid joints are

the wrist and metacarpophalangeal joints for the second through fifth digits.

Saddle Joints: In a saddle joint, the articular surface of one bone is saddleshaped, and the articular surface

of the other bone fits into the “saddle” as a sitting rider would sit. A saddle joint is a modified condyloid

joint in which the movement is somewhat freer. Saddle joints are triaxial, permitting movements around

three axes (flexion–extension, abduction–adduction, and rotation). An example of a saddle joint is the

carpometacarpal joint between the trapezium of the carpus and metacarpal of the thumb.

Ball-and-Socket Joints: A ball-and-socket joint consists of the ball-like surface of one bone fitting into a

cuplike depression of another bone. Such joints are triaxial, permitting movements around three axes

(flexion–extension, abduction–adduction, and rotation). Examples of ball-and-socket joints are the shoulder

and hip joints. At the shoulder joint, the head of the humerus fits into the glenoid cavity of the scapula. At

the hip joint, the head of the femur fits into the acetabulum of the hip bone.

Skeletal Disorders

Rheumatism is any painful disorder of the supporting structures of the body—bones, ligaments, tendons, or

muscles, that is not caused by infection or injury.

Arthritis is a form of rheumatism in which the joints are swollen, stiff, and painful. It is the leading cause

of physical disability among adults over age 65.

Osteoarthritis is a degenerative joint disease in which joint cartilage is gradually lost. It results from a

combination of aging, obesity, irritation of the joints, muscle weakness, and wear and abrasion.

Osteoarthritis is the most common type of arthritis. Osteoarthritis is a progressive disorder of synovial

joints, particularly weight-bearing joints. Articular cartilage deteriorates and new bone forms at the margins

of the joint. The cartilage slowly degenerates, and as the disease progresses, the exposed bone tissue

thickens and forms bony spurs (osteophytes) that enlarge the bone ends and may restrict joint movement.

Patients complain of stiffness on arising that lessens somewhat with activity. The affected joints may make

a crunching noise, called crepitus (krep′ĭ-tus), as they move and the roughened articular surfaces rub

together. The joints most often affected are those of the cervical and lumbar spine and the fingers, knuckles,

knees, and hips. Osteoarthritis is the most common reason for hip- and knee-replacement surgery.

Rheumatoid arthritis (RA) is a chronic inflammatory and autoimmune disease in which the immune

system of the body attacks its own tissues RA begins with inflammation of the synovial membrane

(synovitis) of the affected joints. Inflammatory cells (lymphocytes, neutrophils, and others) migrate into the

joint cavity from the blood and release a flood of inflammatory chemicals that destroy body tissues when

released inappropriately in large amounts as in RA. Synovial fluid accumulates, causing joint swelling and

in time, the inflamed synovial membrane thickens into a pannus, an abnormal tissue that clings to the

articular cartilages. The pannus erodes the cartilage (and sometimes the underlying bone) and eventually

scar tissue forms and connects the bone ends. Later this scar tissue ossifies and the bone ends fuse together,

immobilizing the joint. This end condition, called ankylosis (“stiff condition”), often produces bent,

deformed fingers. Not all cases of RA progress to the severely crippling ankylosis stage, but all cases do

involve restriction of joint movement and extreme pain. Though it usually arises between the ages of 40 and

50, it may occur at any age. It affects three times as many women as men. In the early stages of RA, joint

tenderness and stiffness are common. If untreated, the membrane thickens, and synovial fluid accumulates.

The resulting pressure causes pain and tenderness. Many joints, particularly the small joints of the fingers,

wrists, ankles, and feet, are afflicted at the same time and bilaterally. For example, if the right elbow is

affected, most likely the left elbow is also affected, although often not to the same degree. Other

manifestations may include anemia, osteoporosis, muscle atrophy, and cardiovascular problems.

A wonder drug for RA sufferers is still undiscovered. RA therapy utilizing aspirin, long-term antibiotic

therapy, and physical therapy to a more progressive treatment course using immunosuppressants such as

methotrexate, or anti-inflammatory drugs.

Gouty Arthritis: Uric acid is a waste product produced during the metabolism of nucleic acid (DNA and

RNA) subunits. A person who suffers from gout either produces excessive amounts of uric acid or is not

able to excrete as much as normal. The result is a buildup of uric acid in the blood. This excess acid then

reacts with sodium to form a salt called sodium urate. Crystals of this salt accumulate in soft tissues such as

the kidneys and in the cartilage of the ears and joints. In gouty arthritis, sodium urate crystals are deposited

in the soft tissues of the joints. Gout most often affects the joints of the feet, especially at the base of the big

toe. The crystals irritate and erode the cartilage, causing inflammation, swelling, and acute pain. Eventually,

the crystals destroy all joint tissues. If the disorder is untreated, the ends of the articulating bones fuse, and

the joint becomes immovable. Treatment consists of pain relief (ibuprofen, naproxen, colchicine, and

cortisone) followed by administration of allopurinol to keep uric acid levels low so that crystals do not form.

Ankylosing spondylitis is an inflammatory disease of unknown origin that affects joints between vertebrae

(intervertebral) and between the sacrum and hip bone (sacroiliac joint). The disease, which is more common

in males, sets in between ages 20 and 40. It is characterized by pain and stiffness in the hips and lower back

that progress upward along the backbone. Inflammation can lead to ankylosis (severe or complete loss of

movement at a joint) and kyphosis (hunchback). Treatment consists of anti-inflammatory drugs, heat,

massage, and supervised exercise.

Osteoporosis is literally a condition of porous bones. The basic problem is that bone resorption outpaces

bone deposition. In large part this is due to depletion of calcium from the body—more calcium is lost in

urine, feces, and sweat than is absorbed from the diet. Bone mass becomes so depleted that bones fracture,

often spontaneously, under the mechanical stresses of everyday living.

In addition to fractures, osteoporosis causes shrinkage of vertebrae, height loss, hunched backs, and bone

pain. The disorder primarily affects middle-aged and elderly people, 80% of them women. Older women

suffer from osteoporosis more often than men for two reasons: (1) Women’s bones are less massive than

men’s bones, and (2) production of estrogens in women declines dramatically at menopause, but production

of the main androgen, testosterone, goes down gradually and only slightly in older men. Estrogens and

testosterone stimulate osteoblast activity and synthesis of bone extracellular matrix. Besides gender, risk

factors for developing osteoporosis include a family history of the disease, thin or small body build, an

inactive lifestyle, cigarette smoking, a diet low in calcium and vitamin D, more than two alcoholic drinks a

day, and the use of certain medications.

Osteoporosis is diagnosed by taking a family history and undergoing a bone mineral density (BMD) test.

BMD tests are performed like x-rays and measure bone density.

Treatment options for osteoporosis are varied. With regard to nutrition, a diet high in calcium is important to

reduce the risk of fractures. Vitamin D is necessary for the body to utilize calcium. Medications used to treat

osteoporosis are generally of two types:

(1) Antiresorptive drugs that slow down the progression of bone loss and (2) bone-building drugs that

promote increasing bone mass. Among the antiresorptive drugs are (1) bisphosphonates, which inhibit

osteoclasts (Fosamax®, Actonel®, Boniva®, and calcitonin); (2) selective estrogen receptor modulators,

which mimic the effects of estrogens without unwanted side effects and (3) estrogen replacement therapy

(ERT), which replaces estrogens lost during and after menopause, and hormone replacement therapy (HRT),

which replaces estrogens and progesterone lost during and after menopause. HRT helps maintain and

increase bone mass after menopause. Women on HRT have a slightly increased risk of stroke and blood

clots. HRT also helps maintain and increase bone mass, and women on HRT have an increased risk of heart

disease, breast cancer, stroke, blood clots, and dementia. Among the bone-forming drugs is parathyroid

hormone (PTH), which stimulates osteoblasts to produce new bone.

Rickets and Osteomalacia (malacia = softness) are two forms of the same disease that result from

inadequate calcification of the extracellular bone matrix, usually caused by a vitamin D deficiency. Rickets

is a disease of children in which the growing bones become “soft” or rubbery and are easily deformed.

Because new bone formed at the epiphyseal (growth) plates fails to ossify, bowed legs and deformities of

the skull, rib cage, and pelvis are common.

Osteomalacia is the adult counterpart of rickets, sometimes called adult rickets. New bone formed during

remodeling fails to calcify, and the person experiences varying degrees of pain and tenderness in bones,

especially the hip and legs. Bone fractures also result from minor trauma. Prevention and treatment for

rickets and osteomalacia consists of the administration of adequate vitamin D.

Osteogenic sarcoma (sarcoma =connective tissue tumor) Bone cancer that primarily affects osteoblasts and

occurs most often in teenagers during their growth spurt; the most common sites are the metaphyses of the

thigh bone (femur), shin bone (tibia), and arm bone (humerus). Metastases occur most often in lungs;

treatment consists of multidrug chemotherapy and removal of the malignant growth, or amputation of the

limb.

Osteomyelitis: An infection of bone characterized by high fever, sweating, chills, pain, nausea, pus

formation, edema, and warmth over the affected bone and rigid overlying muscles. It is often caused by

bacteria, usually Staphylococcus aureus. The bacteria may reach the bone from outside the body (through

open fractures, penetrating wounds, or orthopedic surgical procedures); from other sites of infection in the

body (abscessed teeth, burn infections, urinary tract infections, or upper respiratory infections) via the

blood; and from adjacent soft tissue infections (as occurs in diabetes mellitus).

Osteopenia (penia =poverty) Reduced bone mass due to a decrease in the rate of bone synthesis to a level

too low to compensate for normal bone resorption; any decrease in bone mass below normal. An example is

osteoporosis.

Normal Bone Osteoporotic Bone