Skeletal System



Skeletal System



Learning Objectives



1. List and describe five functions of the skeletal system.


2. Explain the difference between compact and spongy bone.


3. Classify bones according to size and shape.


4. Identify the general features of a long bone.


5. Explain the process by which long bones grow in length.


6. Explain the difference between the axial and appendicular skeletons.


7. Identify the bones of the skull.


8. Identify the structural features of vertebrae.


9. List and describe the divisions of the vertebral column.


10. Describe the structural features of the sternum and ribs.


11. Identify the parts of the pectoral girdle.


12. Identify the bones of the upper extremities.


13. Identify the parts of the pelvic girdle.


14. Identify the bones of the lower extremities.


15. List and describe the different types of joints.


16. Describe ways in which the aging of an individual affects the skeletal system.


17. Identify pathology related to the skeletal system.



Key Terms













Introduction to the Skeletal System


The skeletal system consists of the bones and the cartilage, ligaments, and tendons associated with the bones. It accounts for about 20% of the body weight. Bones are rigid structures that form the framework for the body. People often think of bones as dead, dry, inert pipes and plates because that is how they are seen in the laboratory. In reality, the living bones in our bodies contain active tissues that consume nutrients, require a blood supply, use oxygen and discharge waste products in metabolism, and change shape or remodel in response to variations in mechanical stress. The skeletal system is strong but lightweight. It is well adapted for the functions it must perform. It is a masterpiece of design.



Overview of the Skeletal System


Functions of the Skeletal System


The skeletal system gives form and shape to the body. Without the skeletal components, we would appear as big “blobs” inefficiently “oozing” around on the ground. Besides contributing to shape and form, our bones perform several other functions and play an important role in homeostasis.








Structure of Bone Tissue


There are two types of bone tissue: compact and spongy. As the names imply, the two types differ in density, or how tightly the tissue is packed together. Three types of cells contribute to bone homeostasis: osteoblasts, osteoclasts, and osteocytes. Osteoblasts are bone-forming cells, osteoclasts resorb or break down bone, and osteocytes are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains bone tissue.



Compact Bone

The microscopic unit of compact bone is known as the osteon (haversian system). The osteon consists of a central canal called the osteonic (haversian) canal, which is surrounded by concentric rings (lamellae) of hard, calcified matrix. Between the rings of matrix, the bone cells (osteocytes) are located in spaces called lacunae. Small channels (canaliculi) radiate from the lacunae to the osteonic (haversian) canal to provide passageways through the hard matrix. In compact bone the haversian systems are packed tightly together to form what appears to be a solid mass. The osteonic canals contain blood vessels that are parallel to the long axis of the bone. These blood vessels interconnect, by way of perforating (Volkmann) canals, with vessels on the surface of the bone. The microscopic structure of compact bone is illustrated in Figure 7-1.


image
Figure 7-1 Structure of compact and spongy bone. Note the osteons packed together for compact bone and trabeculae of spongy bone.


Highlight on the Skeletal System




Osteoporosis: Osteoporosis is a bone disorder caused by decreased osteoblast activity. It is characterized by loss of the organic matrix, collagenous fibers, and minerals in the bone tissue. People with osteoporosis are susceptible to deformities of the vertebral column and fractures because the bones are too weak to support the weight of the body. Osteoporosis occurs most frequently in postmenopausal Caucasian women. Factors that influence its occurrence are aging, malnutrition, lack of exercise, and hormone imbalance. Supplemental estrogen after menopause may be of benefit, and exercise is always important in maintaining bone strength.


Epiphyseal plate: The epiphyseal plates of specific long bones ossify at predictable times. Radiologists frequently can determine a young person’s age by examining the epiphyseal plates to see whether they have ossified. A difference between bone age and chronologic age may indicate some type of metabolic dysfunction.


Mastoiditis: The mastoid air cells are separated from the cranial cavity by only a thin partition of bone. A middle ear infection that spreads to the mastoid air cells (mastoiditis) is serious because there is danger that the infection will spread from the air cells to the membranes around the brain.


Sinus problems: The bones with paranasal sinuses are the frontal, the sphenoid, the ethmoid, and the two maxillae. The sinuses are lined with mucous membranes that are continuous with the nasal cavity. Allergies and infections cause inflammation of the membranes, which results in sinusitis. The swollen membranes may reduce drainage from the sinuses so that pressure within the cavities increases, resulting in sinus headaches.


Soft spots: The bones in the skull of a newborn are not completely joined together but are separated by fibrous membranes. The six large areas of membranes are called fontanels, or soft spots. The anterior fontanel is on the top of the head, at the junction of the frontal and parietal bones. The posterior fontanel is at the junction of the occipital and parietal bones. On each side of the head there is a mastoid (posterolateral) fontanel near the mastoid region of the temporal bone and a sphenoid (anterolateral) fontanel just superior to the sphenoid bone.


Abnormal spinal curvatures: An abnormally exaggerated lumbar curvature is called lordosis, or swayback. This is often seen in pregnant women as they adjust to their changing center of gravity. An increased roundness of the thoracic curvature is kyphosis, or hunchback. This is frequently seen in elderly people. Abnormal side-to-side curvature is scoliosis. Abnormal curvatures may interfere with breathing and other vital functions.


Yes and no: The atlas holds up the skull and permits you to nod “yes.” The axis allows you to rotate your head from side to side to indicate “no.”


Marrow biopsy: The sternum is frequently used for a red marrow biopsy because it is accessible. The sample for biopsy is obtained by performing a sternal puncture, in which a large needle is inserted into the sternum to remove a sample of red bone marrow.


Fractured clavicle: The clavicle is the most frequently fractured bone in the body because it transmits forces from the arm to the trunk. The force from falling on the shoulder or outstretched arm is often sufficient to fracture the clavicle.


Tennis elbow: Tennis elbow is an inflammation of the tissues surrounding the lateral epicondyle of the humerus. Six muscles that control movement of the hand attach in this region, and repeated contraction of these muscles irritates the attachments. The medical term for tennis elbow is lateral epicondylitis.


Pelvic outlet and childbirth: The female pelvis is shaped to accommodate childbearing. Because the fetus must pass through the pelvic outlet, the physician carefully measures this opening to make sure there is enough room. The distance between the two ischial spines is a good indication of the size of the pelvic outlet. If the opening is too small, a cesarean delivery is indicated.


Broken hip: Elderly people, particularly those with osteoporosis, are susceptible to “breaking a hip.” The femur is a weight-bearing bone, and when it is weakened, it cannot support the weight of the body and the neck of the femur fractures under the stress. Instead of saying, “Grandma fell and broke her hip,” often it is more appropriate to say, “Grandma broke her hip, then fell.”


Bunion: Poorly fitted shoes may compress the toes so that there is a lateral deviation of the big toe toward the second toe. When this occurs, a bursa and callus form at the joint between the first metatarsal and proximal phalanx. This creates a bunion.


Gout: Gout was commonly known as the disease of the kings because it was believed to be caused by a rich diet and fine wines. Gout is an equal-opportunity disease, however, and occurs across the entire population. A rich diet and fine wines may contribute to the disease, but they are not the definitive cause. Gout is caused by the excessive accumulation of uric acid that forms needle-like crystals within the joint, producing pain and inflammation. The great toe is the most commonly affected joint. The disorder is diagnosed by aspirating joint fluid and observing the crystals under the microscope. Although there is no cure for gout, it can be effectively controlled with antiinflammatory drugs and dietary measures.


Knee problems: The term torn cartilage refers to a damaged meniscus, usually the medial, in the knee. Frequently this can be repaired with relatively minor arthroscopic surgery. A torn ligament in the knee usually involves one of the cruciate ligaments. The surgical procedure to repair this damage is quite involved, and recovery of function may require months of rehabilitative therapy. image



Spongy (Cancellous) Bone

Spongy (cancellous) bone is lighter and less dense than compact bone (see Figure 7-1). Spongy bone consists of plates and bars of bone adjacent to small, irregular cavities that contain red bone marrow. The plates of bone are called trabeculae (trah-BEK-yoo-lee). The canaliculi, instead of connecting to a central haversian canal, connect to the adjacent cavities to receive their blood supply. It may appear that the trabeculae are arranged in a haphazard manner, but they are organized to provide maximum strength in the same way that braces are used to support a building. The trabeculae of spongy bone follow the lines of stress and can realign if the direction of stress changes.



Classification of Bones


Bones come in a variety of sizes and shapes. Bones that are longer than they are wide are called long bones. They consist of a long shaft with two bulky ends or extremities. They are primarily compact bone but may have a large amount of spongy bone at the ends. Examples of long bones are those in the thigh, leg, arm, and forearm.


Short bones are roughly cube-shaped with vertical and horizontal dimensions approximately equal. They consist primarily of spongy bone, which is covered by a thin layer of compact bone. Examples of short bones include the bones of the wrist and ankle.


Flat bones are thin, flattened, and often curved. They are usually arranged like a sandwich with a middle layer of spongy bone called the diploë (DIP-loh-ee). The diploë is covered on each side by a layer of compact bone; these layers are called the inner and outer tables. Most of the bones of the cranium are flat bones.


Bones that are not in any of the previously mentioned three categories are classified as irregular bones. They are primarily spongy bone that is covered with a thin layer of compact bone. The vertebrae and some of the bones in the skull are irregular bones.



General Features of a Long Bone


Most long bones have the same general features, which are illustrated in Figure 7-2.


image
Figure 7-2 General features of long bones.


Diaphysis: The shaft of a long bone is called the diaphysis (dye-AF-ih-sis). It is formed from relatively thick compact bone that surrounds a hollow space called the medullary (MED-yoo-lair-ee) cavity.


Medullary cavity: In adults the medullary cavity contains yellow bone marrow, so it is sometimes called the yellow marrow cavity.


Epiphysis: At each end of the diaphysis, there is an expanded portion called the epiphysis (ee-PIF-ih-sis). The epiphysis is spongy bone covered by a thin layer of compact bone. The end of the epiphysis, where it meets another bone, is covered by hyaline cartilage, called the articular cartilage. This provides smooth surfaces for movement in the joints. In growing bones, there is an epiphyseal (ep-ih-FIZ-ee-al) plate of hyaline cartilage between the diaphysis and epiphysis. Bones grow in length at the epiphyseal plate. Growth ceases when the cartilaginous epiphyseal plate is replaced by a bony epiphyseal line.


Periosteum: Except in the region of the articular cartilage, the outer surface of long bones is covered by a tough, fibrous connective tissue called the periosteum. The periosteum is richly supplied with nerve fibers, lymphatic vessels, blood vessels, and osteoblasts.


Nutrient foramina: Blood vessels enter the diaphysis of the bone through small openings called nutrient foramina.


Endosteum: The surface of the medullary cavity is lined with a thinner connective tissue membrane, the endosteum, which contains osteoclasts.


In addition to the general features that are present in most long bones, all bones have surface markings and characteristics that make a specific bone unique. Bones have holes, depressions, smooth facets, lines, projections, and other markings. These usually represent passageways for vessels and nerves, points of articulation with other bones, or points of attachment for tendons and ligaments.




Bone Growth in Length


Bones grow in length at the epiphyseal plate located between the diaphysis and epiphysis of a long bone. The hyaline cartilage in the region of the epiphyseal plate next to the epiphysis continues to grow by mitosis. The chondrocytes in the region next to the diaphysis age and degenerate. Osteoblasts move in and ossify the matrix to form bone. This process continues throughout childhood and adolescence until the cartilage growth slows and finally stops. When cartilage growth ceases, usually in the early 20s, the epiphyseal plate completely ossifies so that only a thin epiphyseal line remains and the bones can no longer grow in length. Bone growth occurs under the influence of growth hormone from the anterior pituitary gland and sex hormones from the ovaries and testes.


Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or to weight gain. The increase in diameter is called appositional (ap-poh-ZISH-un-al) growth. Osteoblasts in the periosteum form compact bone around the external bone surface. At the same time, osteoclasts in the endosteum break down bone on the internal bone surface, around the medullary cavity. These two processes together increase the diameter of the bone and at the same time keep the bone from becoming excessively heavy and bulky.

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Apr 16, 2017 | Posted by in NURSING | Comments Off on Skeletal System

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