Each long bone has an elongated shaft or diaphysis and two expanded ends (epiphyses) which are smooth and articular. The shaft typically has 3 surfaces separated by 3 borders, a central medullary cavity, and a nutrient foramen directed away from the growing end. Limb bones are typical long bones. Examples are humerus, radius, ulna, femur, tibia and fibula, metacarpals, metatarsals, and phalanges.
[Know about other types of bones in the body]
Development of Long Bone
In early fetal life, the bone is just a framework formed by the cartilage. The body needs to convert this into the bony material. This is achieved by ossification centers, the nidus from where bone tissue is constructed and laid over the cartilage framework.
Typically, three ossification centers are responsible for ossification of long bone cartilage framework.
One is in the shaft which appears early and two are at the ends of the bone which appear late. Special cells called osteoblasts lay down mineralized channels into the cartilage.
Ultimately, all the cartilage is resorbed and replaced by bone tissue except for two transverse thin plates of cartilage that remain for future growth of the bone.
This cartilage tissue is called physis and responsible for the growth of the bone. One of the physis is responsible for the greater growth of length than other.
Just behind the physis [towards the center of the bone] are widened areas of bone called metaphysis. Metaphyses has cancellous bone as compared to the cortical bone of the shaft [which is the length of the bone between two metaphyses]. As the bone grows and adds more bone, the part of the metaphysis farther from physis [towards the center of the bone] is remodeled and incorporated into the shaft as cortical bone.
The shaft is called diaphysis.
The epiphysis is the segment of the bone between physis and end of the bone.
Thus, two epiphyses, two physes, two metaphyses, and one shaft constitutes a long bone.
Thus a typical long bone ossifies into three parts, the two ends from secondary centers, and the intervening shaft from a primary center.
Parts of Long Bone
Before ossification is complete the following parts of the bone can be defined. [After ossification, physis fuses with rest of bone and bone has a shaft, metaphysis, and articular ends.
The end and tips of a bone which ossify from secondary centers are called epiphyses. These are of the following types:
This is articular and takes part in the transmission of the weight. Examples: head of the femur, the lower end of radius, etc
It is nonarticular and does not take part in the transmission of the weight. It always provides attachments to one or more tendons which exert a traction on the epiphysis. The traction epiphyses ossify later than the pressure epiphyses. Examples: trochanters of femur and tubercles of the humerus.
It is phylogenetically an independent bone which in man becomes fused to another bone. Examples: coracoid process of scapula and os trigonum.
It is the elongated shaft of the bone which ossifies from a primary center.
The ends of the shaft towards physis are called metaphyses. Each metaphysis is the zone of active growth. Before epiphyseal fusion, the metaphysis is richly supplied with blood through end arteries forming ‘hair-pin’ bends.
This is the common site of osteomyelitis in children because, the bacteria or emboli are easily trapped in the hair-pin bends, causing infarction. After the epiphyseal fusion, vascular communications are established between the metaphyseal and epiphyseal arteries.
Epiphyseal Plate of Cartilage
It separates epiphysis from metaphysis. The proliferation of cells in this cartilaginous plate is responsible for the lengthwise growth of a long bone. After the epiphyseal fusion, the bone can no longer grow in length.
The growth cartilage is nourished by both the epiphyseal and metaphyseal arteries.
Sections of Bone
Naked eye examination of the longitudinal and transverse sections of a bone shows the following features:
From without inwards, it is composed of periosteum, cortex and medullary cavity.
Periosteum is a thick fibrous membrane covering the surface of the bone. It is made up of an outer fibrous layer, and an inner cellular layer which is osteogenic in nature. Periosteum is united to the underlying bone by Sharpey’s fibers, and the union is particularly strong over the attachments of tendons and ligaments.
At the articular margin, the periosteum is continuous with the capsule of the joint. The abundant periosteal arteries nourish the outer part of the underlying cortex also. Periosteum has a rich nerve supply which makes it the most sensitive part of the bone.
Cortex is made up of a compact bone which gives it the desired strength to withstand all possible mechanical stains.
Medullary cavity is filled with red or yellow bone marrow. At birth, the marrow is red everywhere with widespread active hemopoiesis.
As the age advances the red marrow at many places atrophies and is replaced by yellow, fatty marrow, with no power of hemopoiesis.
Red marrow persists in the cancellous ends of long bones. In the sternum ribs, vertebrae and skull bones the red marrow is found throughout life.
Ends of Bone
In adults, the cut section would show cancellous bone with articular margins.
Blood Supply of Long Bones
The blood of a long bone is derived from the following sources –
Nutrient artery enters the shaft through the nutrient foramen, runs obliquely through the cortex, and divides into ascending and descending branches in the medullary cavity.
Each branch divides into a number of small parallel channels which terminate in the adult metaphysis by anastomosing with the epiphyseal, metaphyseal and periosteal arteries.
The nutrient artery supplies the medullary cavity, inner two-thirds of cortex and metaphysis.
The nutrient foramen is directed away from the growing end of the bone.
Periosteal arteries are especially numerous beneath the muscular and ligamentous attachments. They ramify beneath the periosteum and enter the Volkmann’s canals to supply the outer one-third of the cortex.
Epiphyseal vessels are derived from periarticular vascular anastomosis and enter through vascular foramina.
The number and size of these foramina may give an idea of the relative vascularity of the two ends of a bone.
These are derived from the systemic vessels in the vicinity of the bone. They pass directly into the metaphysis and reinforce the metaphyseal branches from the primary nutrient artery.
Lymphatics have not been demonstrated within the bone, although some of them do accompany the periosteal blood vessels, which drain to the regional lymph nodes.
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