Chapter 6 Objectives

1. List the functions of bone tissue.
2. Describe bone classification by shape and composition.
3. Describe the gross anatomy of bone.
4. Describe the microscopic anatomy of bone.
5. Contrast spongy and compact bone.
6. Describe the chemical composition of bone.
7. Define the markings of bone with regard to structure and function (category).
8. Define the common types of bone fractures.
9. Describe the stages in healing of a fracture.

BONES AND BONE TISSUE

Functions:
1. Support: provides framework that supports and anchors all soft organs. Leg bones act as pillars to support the body trunk, and the ribs support the thorax wall.
2. Protection: skull and vertebrae surround soft tissue of the nervous system, and the rib cage protects vital thoracic organs.
3. Movement: skeletal muscles use the bones as levers to move the body.
4. Storage: fat stored in the interior of the bones. Bone matrix serves as a storehouse for various minerals.
5. Blood Cell Formation: hematopoiesis occurs within the marrow cavities of the bones.

Classification of Bones:
Bones can be classified by the two types of osseous tissue.
Bones of different shapes contain different proportions of the two basic types of osseous tissue (compact & spongy bone). Compact bone is dense and looks smooth and homogeneous. Spongy bone is composed of small needlelike or flat pieces of bone called trabeculae and has much open space. The trabeculae form an open network which is filled with bone marrow.

Bones can be classified by shape.
Bones come in varying sizes and shapes, with each unique shape fulfilling a particular function or need. The shapes are long, short, flat, and irregular.

Long bones: Longer than they are wide. Reflects the elongated shape rather than the overall size. Consist of a shaft plus two ends and are constructed primarily of compact bone, but may contain substantial amounts of spongy bone. All bones of the limbs, except the patella, wrist and ankle bones, are long bones.

Short bones: Roughly cubelike, containing mostly spongy bone. Comprise bones of the wrist and ankle. Vary in size and number in different individuals.

Flat bones: Thin, flattened, and usually a bit curved, they have two roughly parallel compact bone surfaces, with a layer of spongy bone between. Examples include the sternum, ribs, and most of the skull bones.

Irregular bones: Fit none of the previous classes. Include some skull bones, vertebrae, and hip bones. Have complicated shapes and consist mainly of spongy bone enclosed by thin layers of compact bone.

Bone Structure - Gross Anatomy:

With few exceptions, all long bones have the same general structure.

Diaphysis: "shaft" which constitutes the long axis of the bone. Constructed of a thick collar of compact bone that surrounds a cavity. The medulla cavity in adults contains fat (yellow marrow).

Epiphyses: "ends" or extremities. They are usually more expanded than the diaphyses. A thin layer of compact bone forms the exterior and the interior contains spongy bone.

Epiphyseal plate: In young bones, cartilage is present at the junction of the diaphysis and epiphysis. This is the growth area that allows bones to lengthen.

Epiphyseal line: remnant of the epiphyseal plate. After puberty the cartilage of the epiphyseal plate is converted to bone and no further growth is possible.

Periosteum: the outer surface of the diaphysis which is covered and protected by a double layered membrane. Connective tissue consisting primarily of bone-forming cells (osteoblasts). Provides an insertion or anchoring point for tendons and ligaments.

Endosteum: internal bone surfaces are covered with a delicate connective tissue membrane. Covers the trabeculae of spongy bone in the narrow cavities and lines the canals that pass through the compact bone.

Articular Cartilage: where long bones articulate at epiphyseal surfaces, the bony surfaces are covered with articular (hyaline) cartilage which cushions the bone ends and absorbs stress during joint movement.

The other bones share a simple design. They consist of thin plates of periosteum-covered compact bone on the outside and endosteum covered spongy bone within. They have no shaft or epiphyses.

Microscopic Structure of Bone:

Compact bone
Microscopically, compact bone is riddled with canals and passageways that serve as conduits for nerves, blood vessels, and lymphatic vessels. The structural unit of compact bone is called the osteon or Haversian system. Each osteon is an elongated cylinder oriented along the long axis of the bone. Osteons appear as tiny weight-bearing pillars composed of a group of hollow tubes of bone matrix, one placed inside the next. Each of the tubes is called a lamella. All the collagen fibers in a particular lamella run in a single direction, while the collagen fibers in adjacent lamella always run in different directions resulting in a very strong structure able to withstand twisting and other mechanical stresses. Running through the core of each osteon is a canal (central or Haversian canal) which contains small blood vessels and nerve fibers. Running a right angles to the long axis are canals (perforating or Volkmann's canals) which connect the vascular and nerve supplies of the periosteum to those in the central canals and medullary cavity.

Spongy Bone
Consists of trabeculae which are positioned where the bone is stressed. Only a few layers thick, the trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi. No osteons are present.

Chemical Composition of Bone:
Bone is made of both organic and inorganic components. The organic components consist of cells and the osteoid (organic part of the matrix). The osteoid makes up 1/3 of the matrix, which contributes to the bone's structure, flexibility and tensile strength. The inorganic components consist of hydroxyapatites (mineral salts), largely calcium phosphate. Calcium salts are present in the form of tiny crystals, which accounts for bones hardness.

Bone Markings:
Bones are rarely smooth. They display bulges, depressions and holes. These markings are named in different ways. Projections that grow outward from the bone surface include heads, trochanters, spines, etc. Depressions and openings are fossae, sinuses, foramina, and grooves.

Projections that are sites of muscle and ligament attachment:
Tuberosity: large rounded projection
Crest: narrow ridge of bone, usually prominent
Trochanter: very large, blunt, irregularly shaped process
Tubercle: small rounded projection or process
Epicondyle: raised area on or above a condyle
Spine: sharp, slender, often pointed projection

Projections that help form joints:
Head: bony expansion carried on a narrow neck
Condyle: rounded articular projection
Ramus: armlike bar of bone

Depressions and openings:
Meatus: canal-like passageway
Sinus: cavity within a bone, filled with air and lined with mucous membrane
Fossa: shallow basinlike depression in a bone often serving as an articular surface
Fissure: narrow, slitlike opening
Foramen: round or oval opening through a bone

Types of fractures:
simple - bone breaks cleanly, does not penetrate the skin
compound - broken ends of bone protrude through skin
comminuted - bone fragments into many pieces
compression - bone is crushed
depressed - broken bone portion is pressed inward
impacted - broken bone ends are forced into each other
spiral - ragged break occurring when excessive twisting forces are applied to bone
greenstick - bone breaks incompletely similar to a green twig

Bone Repair of a Simple Fracture:
1. Hematoma formation - A mass of clotted blood forms at fracture site. Bone cells begin to die and tissue becomes swollen, painful, and inflamed.
2. Fibrocartilaginous callus formation - Capillaries grow into the area and macrophages clean up debris. Fibroblasts and osteoblasts migrate to the area and begin reconstructing bone. Fibroblasts produce collagen fibers that begin to connect broken ends. Some fibroblasts become chondroblasts which make cartilage. Osteoblasts begin forming spongy bone. All of the repair tissue at this stage is called the fibrocartilage callus.
3. Bony callus formation - The fibrocartilage callus is converted to a hard bony callus of spongy bone by the action of the osteoblasts and osteoclasts. This begins 3-4 weeks post injury and continues until a firm union forms 2-3 months later.
4. Remodeling - During bony callus formation and the following several months remodeling occurs. Excess material outside the shaft is removed and compact bone is laid down to reconstruct the shaft. The final structure will resemble unbroken bone because it is under the same set of mechanical stimuli.