Definition of Skeletal System .
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| Human skeleton: (a) Anterior view, (b) Posterior view . |
1. Skeletal System includes bones and cartilages. It forms the main supporting framework of the body and is primarily designed for a more effective production of movements by the attached muscles.
2. Bone is one-third connective tissue. It is impregnated with calcium salts which constitute two-thirds part. The inorganic calcium salts (mainly calcium phosphate, partly calcium carbonate and traces of other salts) make it hard and rigid, which can afford resistance to compressive forces of weight-bearing and impact forces of jumping.
3. The organic connective tissue (collagen fibres) makes it tough and resilient (flexible), which can afford resistance to tensile forces. In strength, bone is comparable to iron and steel.
4. Despite its hardness and high calcium content the bone is very much a living tissue. It is highly vascular, with a constant turn-over of its calcium content. It shows a characteristic pattern of growth.
5. It is subjected to disease and heals after a fracture. It has greater regenerative power than any other tissue of the body, except blood. It can mould itself according to changes in stress and strain it bears. It shows disuse atrophy and overuse hypertrophy.
Functions of bone in Skeletal System .
.jpg "Divisions of the Skeletal System")
1. Bones give shape and support to the body, and resist any forms of stress .
2. These provide surface for the attachment of muscles, tendons, ligaments, etc.
3. These serve as levers for muscular actions.
4. The skull, vertebral column and thoracic cage protect brain, spinal cord and thoracic viscera, respectively.
5. Bone marrow manufactures blood cells.
6. Bones store 97% of the body calcium and phosphorus.
7. Bone marrow contains reticuloendothelial cells which are phagocytic in nature and take part in immune responses of the body.
8. The larger paranasal air sinuses affect the timber of the voice .
CLASSIFICATION OF BONES .
A. According to Shape of bones.
1. Long bones:
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| Long bones: (a) Humerus, (b) Metacarpals, (c) Clavicle |
[1]. Each long bone has an elongated shaft (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.
[2]. Examples: (a) typical long bones like humerus, radius, ulna, femur, tibia and fibula;
(b) miniature long bones have only one epiphysis like metacarpals, metatarsals and phalanges; and
(c) modified long bones have no medullary cavity like clavicle .
2. Short bones:
Their shape is usually cuboid, cuneiform, trapezoid, or scaphoid. Examples: tarsal and carpal bones .
3. Flat bones
It resemble shallow plates and form boundaries of certain body cavities. Examples: bones in the vault of the skull, ribs, sternum and scapula .
4. Irregular bones:
Examples: vertebra, hip bone, and bones in the base of the skull .
5. Pneumatic bones:
Certain irregular bones contain large air spaces lined by epithelium Examples: maxilla, sphenoid, ethmoid, etc. They make the skull light in weight, help in resonance of voice, and act as air conditioning chambers for the inspired air .
6. Sesamoid bones:
These are bony nodules found embedded in the tendons or joint capsules. They have no periosteum and ossify after birth. They are related to an articular or nonarticular bony surface, and the surfaces of contact are covered with hyaline cartilage and lubricated by a bursa or synovial membrane. Examples: patella, pisiform, fabella, etc. .
Functions of the sesamoid bones are:
(a) to resist pressure;
(b) to minimise friction;
(c) to alter the direction of pull of the muscle; and
(d) to maintain the local circulation.
7. Accessory (supernumerary) bones .
Accessory (supernumerary) bones are not always present. These may occur as ununited epiphyses developed from extra centres of ossification. Examples: sutural bones, os trigonum (lateral tubercle of talus), os vesalianum (tuberosity of 5th metatarsal), etc. In medicolegal practice, accessory bones may be mistaken for fractures. However, these are often bilateral, and have smooth surfaces without any callus.
8. Heterotopic bones:
Bones sometimes develop in soft tissues. Horse riders develop bones in adductor muscles (rider’s bones).
B. Developmental Classification of bones .
1. Membrane (dermal) bones .
Membrane (dermal) bones ossify in membrane (intramembranous or mesenchymal ossification), and are thus derived from mesenchymal condensations. Examples: bones of the vault of skull and facial bones.
2. Cartilaginous bones .
Cartilaginous bones ossify in cartilage (intracartilaginous or endochondral ossification), and are thus derived from preformed cartilaginous models. Examples: bones of limbs, vertebral column and thoracic cage.
3. Membrano-cartilaginous bones .
Membrano-cartilaginous bones ossify partly in membrane and partly in cartilage. Examples: clavicle, mandible, occipital, temporal, sphenoid.
4. Somatic bones.
Somatic bones: Most of the bones are somatic.
5. Visceral bones .
Visceral bones: These develop from pharyngeal arches. Examples are hyoid bones, part of mandible and ear ossicles.
C. Regional Classification of bones.
1. Axial skeleton includes skull, vertebral column, and thoracic cage.
2. Appendicular skeleton includes bones of the limbs.
D. Structural Classification of bones.
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| Structural components of a bone |
I. Macroscopically, the architecture of bone may be compact or cancellous .
1. Compact bone .
Compact bone is dense in texture like ivory, but is extremely porous. It is best developed in the cortex of the long bones. This is an adaptation to bending and twisting forces (a combination of compression, tension and shear).
2. Cancellous or spongy, or trabecular bone .
[1]. Cancellous or spongy, or trabecular bone is open in texture, and is made up of a meshwork of trabeculae (rods and plates) between which are marrow containing spaces. The trabecular meshworks are of three primary types, namely: (a) meshwork of rods, (b) meshwork of rods and plates, and (c) meshwork of plates (Singh, 1978).
[2]. Cancellous bone is an adaptation to compressive forces. Bones are marvellously constructed to combine strength, elasticity and lightness in weight. Though the architecture of bone may be modified by mechanical forces, the form of the bone is primarily determined by heredity.
Wolff’s law .
[1]. According to Wolff’s law (Trajectory Theory of Wolff, 1892), the bone formation is directly proportional to stress and strain. There are two forces, tensile force and compressive force. Both the tensile and compressive forces can stimulate bone formation in proper conditions.
[2]. The architecture of cancellous bone is often interpreted in terms of the trajectorial theory. Thus the arrangement of bony trabeculae (lamellae) is governed by the lines of maximal internal stress in the bone.
[3]. Pressure lamellae are arranged parallel to the line of weight transmission, whereas tension lamellae are arranged at right angles to pressure lamellae. The compact arrangement of pressure lamellae forms bony buttress, for additional support, like calcar femorale .
II. Microscopically, the bone is of five types, namely lamellar (including both compact and cancellous), woven, fibrous, dentine and cement.
1. Lamellar bone .
Most of the mature human bones, whether compact or cancellous, are composed of thin plates of bony tissue called lamellae. These are arranged in piles in a cancellous bone, but in concentric cylinders (Haversian system or secondary osteon) in a compact bone.
2. Woven Bone .
Woven bone seen in fetal bone, fracture repair and in cancer of bone
3. Fibrous bone .
Fibrous bone is found in young foetal bones, but are common in reptiles and amphibia.
4. Dentine occur in teeth .
5. Cement occur in teeth.
Difference between the Compact and cancellous bone .
Gross Structure of an Adult Long Bone .
Naked eye examination of the longitudinal and transverse sections of a long bone shows the following features.
1. Shaft of bones .
From without inwards, it is composed of periosteum, cortex and medullary cavity .
(a) Periosteum .
[1]. 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 fibres, and the union is particularly strong over the attachments of tendons, and ligaments.
[2]. 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.
(b) Cortex .
Cortex is made up of a compact bone which gives it the desired strength to withstand all possible mechanical strains.
(c) Medullary cavity .
Medullary cavity is filled with red or yellow bone marrow. At birth the marrow is red everywhere with widespread active haemopoiesis. As the age advances, the red marrow at many places atrophies and is replaced by yellow, fatty marrow, with no power of haemopoiesis.
Red marrow persists in the cancellous ends of long bones. In the sternum ribs, iliac crest, vertebrae and skull bones the red marrow is found throughout life.
2. The two ends of a long bone are made up of cancellous bone covered with hyaline (articular) cartilage .
Parts of a young Bone .
A typical long bone ossifies in three parts, the two ends from secondary centres, and the intervening shaft from a primary centre . Before ossification is complete the following parts of the bone can be defined.
1. Epiphysis .
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| Types of epiphyses |
The ends and tips of a bone which ossify from secondary centres are called epiphyses. These are of the following types.
(a) Pressure epiphysis is articular and takes part in transmission of the weight. Examples: head of femur; lower end of radius, etc.
(b) Traction epiphysis is nonarticular and does not take part in the transmission of the weight. It always provides attachment 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 humerus .
(c) Atavistic epiphysis is phylogenetically an independent bone which in man becomes fused to another bone. Examples: coracoid process of scapula and os trigonum or lateral tubercle of talus,
(d) Aberrant epiphysis is not always present. Examples: epiphysis at the head of the first metacarpal and at the base of other metacarpal bones.
2. Diaphysis .
It is the elongated shaft of a long bone which ossifies from a primary centre .
3. Metaphysis .
[1]. The epiphysial ends of a diaphysis are called metaphyses. Each metaphysis is the zone of active growth. Before epiphysial fusion, the metaphysis is richly supplied with blood through end arteries forming ‘hair-pin’ bends.
[2]. This is the common site of osteomyelitis in children because the bacteria or emboli are easily trapped in the hair-pin bends, causing infarction.
[3]. After the epiphysial fusion, vascular communications are established between the metaphysial and epiphysial arteries. Now the metaphysis contains no more end-arteries and is no longer subjected to osteomyelitis.
4. Epiphysial Plate of Cartilage .
It separates epiphysis from metaphysis. Proliferation of cells in this cartilaginous plate is responsible for lengthwise growth of a long bone. After the epiphysial fusion, the bone can no longer grow in length. The growth cartilage is nourished by both the epiphysial and metaphysial arteries.
Blood Supply of Bones .
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| Blood supply of a long bone |
1. Long Bones .
The blood supply of a long bone is derived from the following sources .
(a) Nutrient artery .
[1]. It enters the shaft through the nutrient foramen, runs obliquely through the cortex, and divides into ascending and descending branches in the medullary cavity.
[2]. Each branch divides into a number of small parallel channels which terminate in the adult metaphysis by anastomosing with the epiphysial, metaphysial and periosteal arteries.
[3]. The nutrient artery supplies medullary cavity, inner 2/3 of cortex and metaphysis.
[4]. The nutrient foramen is directed away from the growing end of the bone; their directions are indicated by a jingle, ‘To the elbow I go, from the knee I flee’ .
[5]. The growing ends of bones in upper limb are upper end of humerus and lower ends of radius and ulna. In lower limb, the lower end of femur and upper end of tibia are the growing ends.
(b) Periosteal arteries .
[1].These are especially numerous beneath the muscular and ligamentous attachments.
[2]. They ramify beneath the periosteum and enter the Volkmann’s canals to supply the outer 1/3 of the cortex.
(c) Epiphysial arteries .
[1]. These are derived from periarticular vascular arcades (circulus vasculosus) found on the nonarticular bony surface.
[2]. Out of the numerous vascular foramina in this region, only a few admit the arteries (epiphysial and metaphysial), and the rest are venous exits.
[3]. The number and size of these foramina may give an idea of the relative vascularity of the two ends of a long bone (Tandon, 1964).
(d) Metaphysial arteries .
[1]. These are derived from the neighbouring systemic vessels.
[2]. They pass directly into the metaphysis and reinforce the metaphysial branches from the primary nutrient artery.
[3]. In miniature long bones, the infection begins in the middle of the shaft rather than at the metaphysis because, the nutrient artery breaks up into a plexus immediately upon reaching the medullary cavity. In the adults, however, the chances of infection are minimized because the nutrient artery is mostly replaced by the periosteal vessels.
2. Other Bones blood supply.
[1]. Short bones are supplied by numerous periosteal vessels which enter their nonarticular surfaces. In a vertebra, the body is supplied by anterior and posterior vessels and the vertebral arch by large vessels entering the bases of transverse processes. Its marrow is drained by two large basivertebral veins.
[2]. A rib is supplied by : (a) the nutrient artery which enters it just beyond the tubercle; and
(b) the periosteal arteries. Veins are numerous and large in the cancellous, red marrow containing bones (e.g., basivertebral veins).
[3]. In the compact bone, they accompany arteries in the Volkmann’s canals.
[4]. 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.
Nerve Supply of Bones .
Nerves accompany the blood vessels. Most of them are sympathetic and vasomotor in function. A few of them are sensory which are distributed to the articular ends and periosteum of the long bones, to the vertebra, and to large flat bones.
Development and Ossification of Bones .
[1]. Bones are first laid down as mesodermal (connective tissue) condensations. Conversion of mesodermal models into bone is called intramembranous or mesenchymal ossification and the bones are called membrane (dermal) bones.
[2]. However, mesodermal stage may pass through cartilaginous stage by chondrification during 2nd month of intrauterine life. Conversion of cartilaginous model into bone is called intracartilaginous or endochondral ossification and such bones are called cartilaginous bones .
[3]. Ossification takes place by centres of ossification, each one of which is a point where laying down of lamellae (bone formation) starts by the osteoblasts situated on the newly formed capillary loops. The centres of ossification may be primary or secondary.
[4]. The primary centres appear before birth, usually during 8th week of intrauterine life; the secondary centres appear after birth, with a few exceptions of lower end of femur and upper end of tibia. Many secondary centres appear during puberty.
[5]. A primary centre forms diaphysis, and the secondary centres form epiphyses. Fusion of epiphyses with the diaphysis starts at puberty and is complete by the age of 25 years, after which no more bone growth can take place.
[6]. The law of ossification states that secondary centres of ossification which appear first are last to unite. The end of a long bone where epiphysial fusion is delayed is called the growing end of the bone.
GROWTH OF A LONG BONE
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| Growth of a long bone |
1. Bone grows in length by multiplication of cells in the epiphysial plate of cartilage .
2. Bone grows in thickness by multiplication of cells in the deeper layer of periosteum.
3. Bones grow by deposition of new bone on the surface and at the ends. This process of bone deposition by osteoblasts is called appositional growth or surface accretion.
4. However, in order to maintain the shape the unwanted bone must be removed. This process of bone removal by osteoblasts is called remodeling. This is how marrow cavity increases in size.
Medicolegal and anthropological aspects .
When a skeleton or isolated bones are received for medicolegal examination, one should be able to determine:
(a) whether the bones are human or not;
(b) whether they belong to one or more persons;
(c) the age of the individual;
(d) the sex;
(e) the stature; and
(f) the time and cause of death.
1. Estimation of Skeletal Age .
[1]. Up to the age of 25 years, the skeletal age can be estimated to within 1-2 years of correct age by the states of dentition and ossification, provided the whole skeleton is available.
[2]. From 25 years onwards, the skeletal age can be estimated to within ± 5 years of the correct age by the state of cranial sutures and of the bony surfaces of symphysis pubis. In general, the appearance of secondary centres and fusion of epiphyses occur about one year earlier in females than in males.
[3]. These events are also believed to occur 1-2 years (Bajaj et al, 1967) or 2-3 years (Pillai, 1936) earlier in India than in Western countries. However, Jit and Singh (1971) did not find any difference between the eastern and western races.
2. Estimation of Sex .
[1].Sex can be determined after the age of puberty. Sexual differences are best marked in the pelvis and skull, and accurate determination of sex can be done in over 90% cases with either pelvis or skull alone.[2]. However, sexual dimorphism has been worked out in a number of other bones, like sternum (Jit et al, 1980), atlas (Halim and Siddiqui, 1976), and most of the limb bones.
3. Estimation of Stature (Height) .
[1]. It is a common experience that trunk and limbs show characteristic ratios among themselves and in comparison with total height. Thus a number of regression formulae have been worked out to determine height from the length of the individual limb bones (Siddiqui and Shah, 1944; Singh and Sohal, 1952; Jit and Singh, 1956; Athawale, 1963; Kolte and Bansal, 1974; Kate and Majumdar, 1976).
[2]. Height can also be determined from parts of certain long bones (Mysorekar et al), from head length (Saxena et al, 1981), and from foot measurements (Charnalia, 1961; Qamra et al, 1980).
[3]. CR length has been correlated with diaphysial length of foetal bones (Vare and Bansal, 1977) and with the neonatal and placental parameters (Jeyasingh et al, 1980; Saxena et al, 1981).
4. Estimation of Race .
It is of interest to anthropologists. A number of metrical (like cranial and facial indices) and non metrical features of the skull, pelvis, and certain other bones are of racial significance (Krogman, 1962; Berry, 1975).
CARTILAGE
Cartilage is a connective tissue composed of cells (chondrocytes) and fibres (collagen or yellow elastic) embedded in a firm, gel-like matrix which is rich in a mucopolysaccharide. It is much more elastic than bone.
General Features of Cartilage .
1. Cartilage has no blood vessels or lymphatics. The nutrition of cells diffuses through the matrix.
2. Cartilage has no nerves. It is, therefore, insensitive.
3. Cartilage is surrounded by a fibrous membrane, called perichondrium, which is similar to periosteum in structure and function. The articular cartilage has no perichondrium, so that its regeneration after injury is inadequate.
4. When cartilage calcifies, the chondrocytes die and the cartilage is replaced by bone like tissue.
Comparison between bone and cartilage.
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| Comparison between bone and cartilage |
Types of Cartilage
There are three types of cartilages:
1. Hyaline cartilage .
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| Hyaline and elastic cartilages |
2. Fibrocartilage .
3. Elastic cartilage .
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| Comparison of three types of cartilages |
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