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- (^) Bone is a highly specialized tissue designed to perform multiple vital
functions of the body. Some of them are as follows:
◼ It gives shape to the body in the form of the human skeleton
◼ It gives protection to the body’s vital organ systems by forming
compact cavities (e.g., chest cavity protecting the heart and the lungs)
◼ It forms a major organ of our life, i.e., movement through systematic
arrangement of the joints and muscular attachments
◼ It serves as a storehouse for important nutrients and minerals
(e.g.,99% of the body’s calcium is stored in the bones) required to
maintain the perfectly balanced metabolism of our body
THE BONES AND JOINTS
◼ The body’s defence in the form of WBCs and platelets are also
produced in the bone marrow.
◼ Although it appears to be an inactive solid mass, bone factually is a
very active tissue, e.g., immediately following a fracture, the activity of
bone formation begins at the fractured bony ends. Therefore, fracture
reduction and achieving a correct anatomical alignment of the
fractured bony ends must be done as early as possible.
The skeleton is composed of 206 bones and is categorized into two
basic types:
- Axial skeleton – It is composed of 80 bones and forms the upright
axis of the whole body.
- Appendicular skeleton – It is composed of 126 bones which mainly
include both the shoulder and the hip girdles and the long bones of
both the upper and lower extremities
- (^) The 206 bones of the human skeleton are shaped and sized according
to their functions.
- (^) 1. The long bones (e.g., femur, tibia): These serve as levers to facilitate
muscular action.
- (^) 2. Short bones: These are generally found where only limited
movements are required. Mainly, these bones provide strength and
stability to the strong muscular action by providing a stable base (e.g.,
carpal bones).
- (^) 3. Flat bones: These are composed of a thin layer of cancellous bone
covered on both the sides by a parallel layer of compact bones, (e.g.,
skull, scapula, pelvic bones). The scapula provides free but stable
mobility to the shoulder and arm, whereas the skull and the pelvis
protect vital soft organ systems.
Types of bones
4. Irregular bones: These are irregular in
shape with small bony appendages, e.g.,
vertebrae. These bones provide stable
flexibility to the trunk while protecting the
spinal cord.
5. Sesamoid bones: These are small
rounded or triangular bones which develop
in the substance of a tendon or fascia to
protect and facilitate muscular action, e.g.,
patella – a typical resemblance to the
‘sesame seed’.
Microscopic composition of bone
- (^) The structural unit of a lamellar bone is Osteon, which consists of a
series of concentric laminations surrounding the central longitudinal
Haversian canal which contains a nutrient artery supplying blood to
the bone
- (^) Bone cells There are three types of bone cells:
- (^) 1. Osteoblasts – bone forming cells
- (^) 2. Osteoclasts – bone resorbing cells
- (^) 3. Osteocytes – resting cells which can act as both osteoblasts and
osteoclasts as per the need
Microscopically, bone is classified as follows:
◼ Woven – It is an immature bone where the cells and the collagen are
arranged in a random pattern. It is found during the initial stage of bone
formation after fracture – when the bone is in the process of formation.
◼ Lamellar – The bone is in a fully matured stage where the cellular
distribution is set in an orderly fashion and the collagen fibres are also
properly oriented. The dense arrangement of lamellae is present in the
cortical bone, whereas the lamellae are arranged loosely in a cancellous
bone.
- (^) Development of a cartilaginous model: The bone begins to develop
during the embryonic life from cartilaginous primordia (enchondral
ossification) by the process of condensation of the mesenchyme – as
hyaline cartilage surrounded by pericondrium.
Ossification of a cartilaginous model to bone: Around the fifth week
of the intrauterine period, the ossification begins by the appearance
of the primary centre of ossification (PCO) at the middle of the
bone, when it is invaded by capillaries. The pericondrium is
converted to periosteum and the osteoid is produced around the
shaft. The shaft begins to grow longitudinally by the resorption of
the inner surface.
- (^) During the late fetal stage or early childhood, two secondary centres of
ossification (SCO) appear (the epiphyses). The first SCO appears at the upper
epiphysis followed by the second SCO at the lower epiphysis.
Both the SCOs are responsible for the radial growth of a bone.
The consolidation and thickening of the periosteum occurs by the process of
subperiosteal new bone deposition. The SCOs are also called apophysis (e.g.,
apophysis of the greater trochanter).
As the bone approaches adult size, the diaphysis and epiphyses are gradually
ossified fully, but are still separated by epiphyseal plates (cartilage). As the
epiphyseal plates cease to grow, the longitudinal growth of a bone stops.
At the end of the growth period of a bone, the epiphysis fuses with the
diaphysis leaving a definite area of hyaline layer at the articular surface of a
bone.
- (^) Epiphysis: In growing children, the long bone consists of two ends known
as epiphysis. It forms a support for the joint surface. It is susceptible to
developmental problems (epiphyseal dysplasia), degenerative changes,
injury and avascular necrosis due to ischaemia.
- (^) Diaphysis: A large part of the long bone or shaft constitutes diaphysis. It
is made up of strong cortical bone but due to mechanical disadvantage,
it always remains susceptible to fracture with angulation.
The process of healing is slow as compared to metaphysis. It may
develop dysplasias or infection. Bone has the ability for remodelling or
changing its shape in response to stress.
General structure of a long bone
- (^) Metaphysis: The portion of the bone adjacent to each epiphysis is
known as metaphysis. It is made up of cancellous bone where the
process of healing is fast. It is susceptible to bone infection, dysplasia
and tumours.
Growth plate :There is a thin plate of growth cartilage, one at each
end called ‘growth plate’. At the time of maturity, this growth plate
fuses with metaphysis.
The articular surfaces of the epiphyses are covered with an articular
cartilage. The rest of the bone is covered with periosteum which
provides attachment to tendons, ligaments and muscles. Although
mechanically weak, it helps in longitudinal growth of a bone. It is
susceptible to injury or slipping (slipped femoral epiphysis), tumour
and osteomyelitis. It is also susceptible to growth arrest as well as
deformed growth. Note:
Blood supply to a long bone
- (^) The blood supply to the long bone is derived from four vessels:
- (^) Metaphyseal vessels: Metaphyseal vessels are numerous small vessels
derived from the anastomosis at the joint. They enter the metaphysis
along the line of attachment of the joint capsule.
- (^) Nutrient artery: The major vessel providing nourishment to the long
bone enters the bone around the middle of the shaft and immediately
bifurcates running in opposite directions towards the proximal and
distal end of the bone as medullary vessels. Then each one further
divides into a number of parallel vessels towards the respective
metaphysis
- (^) Epiphyseal vessels: Epiphyseal vessels enter the epiphysis providing
independent nutrition to the epiphysis or epiphyseal site.
Periosteal vessels: The periosteum is richly supplied with blood
which it receives from a number of small vessels. These directly
enter the bone and supply mainly the cortical area of a bone. These
vessels play an important role in the process of the healing of the
bone following fracture.
