Musculoskeletal

SKELETAL SYSTEM

BONES AND CARTILAGES

A skeletal cartilage is made of some variety of cartilage tissue, which consists primarily of water.

The high water content of cartilage accounts for its resilience, that is, its ability to spring back to its original shape after being compressed.

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The cartilage, which contains no nerves or blood vessels, is surrounded by a layer of dense irregular connective tissue, the perichondrium.

The perichondrium acts like a girdle to resist outward expansion when the cartilage is compressed.

HYALINE CARTILAGES

Which look like frosted glass when freshly exposed, provide support with flexibility and resilience.

• They are the most abundant skeletal cartilages.
• When viewed under the microscope, their chondrocytes appear spherical.

The only fiber type in their matrix is fine collagen.

Skeletal hyaline cartilages include:

• Articular cartilages, which cover the ends of most bones at movable joints.
• Costal cartilages, which connect the ribs to the sternum (breastbone)
• Respiratory cartilages, which form the skeleton of the larynx (voicebox), and reinforce other respiratory passageways.
• Nasal cartilages, which support the external nose.

ELASTIC CARTILAGES

Look very much like hyaline cartilages, but they contain more stretchy elastic fibers and so are better able to stand up to repeated bending.

They are found in only two skeletal locations the external ear and the epiglottis (the flap that bends to cover the opening of the larynx each time we swallow).

FIBROCARTILAGES

Are highly compressible and have great tensile strength.

The perfect intermediate between hyaline and elastic cartilages, fibrocartilages consist of roughly parallel rows of chondrocytes alternating with thick collagen fibers.

Fibrocartilages occur in sites that are subjected to both heavy pressure and stretch, such as the padlike cartilages (menisci) of the knee and the discs between vertebrae

CLASSIFICATION OF BONES

The 206 named bones of the human skeleton are divided into two groups: axial and appendicular.

The axial skeleton forms the long axis of the body and includes the bones of the skull, vertebral column, and rib cage.

Generally speaking these bones are most involved in protecting, supporting, or carrying other body parts.

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The appendicular skeleton consists of the bones of the upper and lower limbs and the girdles (shoulder bones and hip bones) that attach the limbs to the axial skeleton.

Bones of the limbs help to get from place to place (locomotion) and to manipulate our environment.

Bones come in many sizes and shapes.

For example, the pisiform bone of the wrist is the size and shape of a pea, whereas the femur (thigh bone) is nearly 2 feet long in some people and has a large ball-shaped head.

The unique shape of each bone fulfills a particular need.

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For the most part, bones are classified by their shape as long, short, flat, and irregular . 1. Long bones, as their name suggests, are considerably longer than they are wide.

• A long bone has a shaft plus two ends.
• All limb bones except the patella (kneecap) and the wrist and ankle bones are long bones.

Notice that these bones are named for their elongated shape, not their overall size.

2. Short bones are roughly cube shaped.

The bones of the wrist and ankle are examples.

Sesamoid bones are a special type of short bone that form in a tendon (for example, the patella).

They vary in size and number in different individuals.

3. Flat bones are thin, flattened, and usually a bit curved.

The sternum (breastbone), scapulae (shoulder blades), ribs, and most skull bones are flat bones.

• Irregular bones have complicated shapes that fit none of the preceding classes.

Examples include the vertebrae and the hip bones.

FUNCTIONS OF BONES

Besides contributing to body shape and form, our bones perform several important functions:

• SUPPORT. Bones provide a framework that supports the body and cradles its soft organs.

For example, bones of lower limbs act as pillars to support the body trunk when we stand, and the rib cage supports the thoracic wall.

2. PROTECTION. The fused bones of the skull protect the brain.

The vertebrae surround the spinal cord, and the rib cage helps protect the vital organs of the thorax.

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• MOVEMENT. Skeletal muscles, which attach to bones by tendons, use bones as levers to move the body and its parts.
• As a result, we can walk, grasp objects, and breathe.

The design of joints determine the types of movement possible.

4. MINERAL AND GROWTH FACTOR STORAGE. Bone is a reservoir for minerals, most importantly calcium and phosphate.

The stored minerals are released into the bloodstream as needed for distribution to all parts of the body.

• Blood cell formation. Most blood cell formation, or hematopoiesis, occurs in the marrow cavities of certain bones.

BONE STRUCTURE

Because they contain various types of tissue, bones are organs.

Although bone (osseous) tissue dominates bones, they also contain nervous tissue in their nerves, cartilage in their articular cartilages, fibrous connective tissue lining their cavities, and muscle and epithelial tissues in their blood vessels.

BONE MARKINGS

The external surfaces of bones are rarely smooth and featureless.

Instead, they display projections, depressions, and openings that serve as sites of muscle, ligament, and tendon attachment, as joint surfaces, or as conduits for blood vessels and nerves

These bone markings are named in different ways.

Projections (bulges) that grow outward from the bone surface include heads, trochanters, spines, and others, each having distinguishing features and functions.

In most cases, bone projections are indications of the stresses created by muscles attached to and pulling on them or are modified surfaces where bones meet and form joints

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Depressions and openings include fossae, sinuses, foramina, and grooves; these usually serve to allow passage of nerves and blood vessels.

BONE TEXTURES: COMPACT AND SPONGY BONE

• Every bone has a dense outer layer that looks smooth and solid to the naked eye.

This external layer is compact bone.

Internal to this is spongy bone, a honeycomb of small needle-like or flat pieces called trabeculae.

In living bones the open spaces between trabeculae are filled with red or yellow bone marrow.

DIAPHYSIS

A tubular diaphysis, or shaft, forms the long axis of the bone.

It is constructed of a relatively thick collar of compact bone that surrounds a central medullary cavity or marrow cavity.

In adults, the medullary cavity contains fat (yellow marrow) and is called the yellow bone marrow cavity.

EPIPHYSES

• The epiphyses are the bone ends.
• In many cases, they are more expanded than the diaphysis.

Compact bone forms the exterior of epiphyses; their interior contains spongy bone.

The joint surface of each epiphysis is covered with a thin layer of articular (hyaline) cartilage, which cushions the opposing bone ends during joint movement and absorbs stress

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Between the diaphysis and each epiphysis of an adult long bone is an epiphyseal line, a remnant of the epiphyseal plate, a disc of hyaline cartilage that grows during childhood to lengthen the bone.

The region where the diaphysis and epiphysis meet, whether it is the epiphyseal plate or line, is sometimes called the metaphysis.

MEMBRANES

A third structural feature of long bones is membranes.

The external surface of the entire bone except the joint surfaces is covered by a glistening white, double-layered membrane called the periosteum .

The outer fibrous layer is dense irregular connective tissue.

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The inner osteogenic layer, abutting the bone surface, consists primarily of bone-forming cells, osteoblasts , and bone-destroying cells, osteoclasts .

The periosteum is richly supplied with nerve fibers, lymphatic vessels, and blood vessels, which enter the diaphysis via a nutrient foramen.

Internal bone surfaces are covered with a delicate connective tissue membrane called the endosteum .

The endosteum covers the trabeculae of spongy bone and lines the canals that pass through the compact bone.

Like the periosteum, the endosteum contains both osteoblasts and osteoclasts.

STRUCTURE OF SHORT, IRREGULAR, AND FLAT BONES

They all consist of thin plates of periosteum-covered compact bone on the outside and endosteum-covered spongy bone within.

However, these bones are not cylindrical and so they have no shaft or epiphyses.

They contain bone marrow (between their trabeculae), but no significant marrow cavity is present.

LOCATION OF HEMATOPOIETIC TISSUE IN BONES

Hematopoietic tissue, red marrow, is typically found within the trabecular cavities of spongy bone of long bones and in the diploë of flat bones.

For this reason, both these cavities are often referred to as red marrow cavities.

In newborn infants, the medullary cavity of the diaphysis and all areas of spongy bone contain red bone marrow

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In most adult long bones, the fat-containing medullary cavity extends well into the epiphysis, and little red marrow is present in the spongy bone cavities.

Hence, blood cell production in adult long bones routinely occurs only in the heads of the femur and humerus.

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The red marrow found in the diploë of flat bones (such as the sternum) and in some irregular bones (such as the hip bone) is much more active in hematopoiesis, and these are the sites routinely used for obtaining red marrow samples when problems with the blood-forming tissue are suspected

BONE DEVELOPMENT

• Ossification and osteogenesis are synonyms meaning the process of bone formation.

In embryos this process leads to the formation of the bony skeleton

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Later another form of ossification known as bone growth goes on until early adulthood as the body continues to increase in size.

• Bones are capable of growing in thickness throughout life.

However, ossification in adults serves mainly for bone remodeling and repair.

FORMATION OF THE BONY SKELETON

Before week 8, the skeleton of a human embryo is constructed entirely from fibrous membranes and hyaline cartilage.

Bone tissue begins to develop at about this time and eventually replaces most of the existing fibrous or cartilage structures.

When a bone develops from a fibrous membrane, the process is intramembranous ossification, and the bone is called a membrane bone

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Bone development by replacing hyaline cartilage is called endochondral ossification, and the resulting bone is called a cartilage, or endochondral, bone.

The beauty of using structures (membranes and cartilages) that are flexible and resilient to fashion the embryonic skeleton is that they can accommodate mitosis

BONE HOMEOSTASIS: REMODELING AND REPAIR

Bone is a dynamic and active tissue, and small-scale changes in bone architecture occur continually.

Every week we recycle 5–7% of our bone mass, and as much as half a gram of calcium may enter or leave the adult skeleton each day!

Spongy bone is replaced every three to four years; compact bone, every ten years.

This is fortunate because when bone remains in place for long periods the calcium crystallizes and becomes more brittle—ripe conditions for fracture.

BONE REMODELING

In the adult skeleton, bone deposit and bone resorption (removal) occur both at the surface of the periosteum and the surface of the endosteum.

Together, the two processes constitute bone remodeling, and they are coupled and coordinated by packets of adjacent osteoblasts and osteoclasts called remodeling units

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In healthy young adults, total bone mass remains constant, an indication that the rates of bone deposit and resorption are essentially equal.

Remodeling does not occur uniformly, however.

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For example, the distal part of the femur, is fully replaced every five to six months, whereas its shaft is altered much more slowly.

Bone deposit occurs wherever bone is injured or added bone strength is required.

For optimal bone deposit, a healthy diet rich in proteins, vitamin C, vitamin D, vitamin A, and several minerals is essential.

BONE REPAIR

Despite their remarkable strength, bones are susceptible to fractures, or breaks. During youth, most fractures result from exceptional trauma that twists or smashes the bones.

Excessive intake of vitamin A and elevated blood levels of the protein homocysteine appear to increase fracture risk in some people.

In old age, most fractures occur as bones thin and weaken.

FRACTURES CLASSIFICATION

• Position of the bone ends after fracture.

In nondisplaced fractures the bone ends retain their normal position; in displaced fractures the bone ends are out of normal alignment.

• Completeness of the break.

If the bone is broken through, the fracture is a complete fracture; if not, it is an incomplete fracture

3. Orientation of the break relative to the long axis of the bone.

If the break parallels the long axis, the fracture is linear; if the break is perpendicular to the bone’s long axis, it is transverse

4. Whether the bone ends penetrate the skin.

If so, the fracture is an open (compound) fracture; if not it is a closed (simple) fracture.

In addition to these four either-or classifications, all fractures can be described in terms of the location of the fracture, the external appearance of the fracture, and/or the nature of the break

A fracture is treated by reduction, the realignment of the broken bone ends.

In closed or external reduction, the bone ends are coaxed into position by the physician’s hands.

In open (internal) reduction, the bone ends are secured together surgically with pins or wires.

After the broken bone is reduced, it is immobilized either by a cast or traction to allow the healing process to begin.

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For a simple fracture the healing time is six to eight weeks for small or medium-sized bones in young adults, but it is much longer for large, weight-bearing bones and for bones of elderly people (because of their poorer circulation).

Repair in a simple fracture involves four major stages

Hematoma formation.

When a bone breaks, blood vessels in the bone and periosteum, and perhaps in surrounding tissues, are torn and hemorrhage.

As a result, a hematoma , a mass of clotted blood, forms at the fracture site.

Soon, bone cells deprived of nutrition die, and the tissue at the site becomes swollen, painful, and inflamed

Fibrocartilaginous callus formation.

Within a few days, several events lead to the formation of soft granulation tissue, also called the soft callus.

Capillaries grow into the hematoma and phagocytic cells invade the area and begin cleaning up the debris.

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Meanwhile, fibroblasts and osteoblasts invade the fracture site from the nearby periosteum and endosteum and begin reconstructing the bone.

The fibroblasts produce collagen fibers that span the break and connect the broken bone ends, and some differentiate into chondroblasts that secrete cartilage matrix.

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Within this mass of repair tissue, osteoblasts begin forming spongy bone, but those farthest from the capillary supply secrete an externally bulging cartilaginous matrix that later calcifies.

This entire mass of repair tissue, now called the fibrocartilaginous callus, splints the broken bone.

Bony callus formation.

Within a week, new bone trabeculae begin to appear in the fibrocartilaginous callus and gradually convert it to a bony (hard) callus of spongy bone.

Bony callus formation continues until a firm union is formed about two months later.

Bone remodeling.

Beginning during bony callus formation and continuing for several months after, the bony callus is remodeled.

The excess material on the diaphysis exterior and within the medullary cavity is removed, and compact bone is laid down to reconstruct the shaft walls.

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The final structure of the remodeled area resembles that of the original unbroken bony region because it responds to the same set of mechanical stressors.