Basic science of human body Chapter one: Bone- brief about its structure, growth, modeling and remodeling

1. Basic Science
Part-I: Bone
Radhika Chintamani

2. Content
Definition
Seven categories of bone
Stress strain curves
Important points
Theory of muscle fatigue leading to fracture
Bone growth, Modeling and remodeling

3. Definition
A substance that forms the skeleton of the body composed
of calcium phosphate and calcium carbonate.
Total 206 bones exist in human body.

4. Seven categories of Bone
1. Long bones: are tubular, longer
than they are wider and having a
shaft or body with convex or
concave articular ends. The shaft
usually contains a hollow center,
called the medullary cavity. Eg: of
long bones are the femur and
humerus.
2. Short bones: generally small and
cube shaped. Usually have 6
surfaces of which no more than 4
are articular, and two or more
serve as attachment site for
tendons and ligaments or as entry
sites for blood vessels. Eg: carpal
and tarsal bones.

5. 3. Flat bones: consists of two plates
of compact bone, with trabecular
bone and marrow located
between the two plates. Eg: The
skull, sternum and scapula.
4. Irregular bones: such as facial
and vertebrae, usually composed
of various non-uniform shapes
and do not fit in the other
examples.
5. Pneumatic bones: contain air
cells or sinuses with cortical
shell of bone surrounding the air
spaces. Eg: The mastoid portion
of temporal bone.

6. 5. Sesamoid bone: also called as
round or oval bone. Are located
within tendons. They not only
protect tendons from friction and
compression, but also increase
the mechanical advantage for
producing strength, by increasing
the angle of application of force
at muscle attachment sit (i.e.
angle of pull). Eg: patella.
6. Accessory or supplementary
bones: develop when an
ossification center appears in the
main part of the bone or normally
occurring ossification center fails
to fuse with main part of bone.

7. Stress strain curves
An isotropic material has a similar curve as follows
independent of direction of application of load.

8. Flexural strength of material is defined as its ability to
resist deformation under load.
Flexural strength is nothing but bending strength of
bone.
The resistance provided by the bone against bending is
called as bending stiffness.
Bending strength of human compact bone=1.26 power

9. Important Points
Muscle contraction can decrease tensile loading on a bone.
Bony protuberances develop as a result of tensile stress.
Tensile stresses at the location of a crack progresses the
crack, whereas compressive stresses do not.
Fracture stress at the bone can be given by the formula:
fracture stress = 4 x Young’s modulus x surface energy
Depth of crack
Also, maximal stress before bone failure can be
approximated by:
Stress= -My
I
Where, M= Bending moment, y= distance from the
neutral axis to the extreme fibers and I= moment of
inertia

10. Mechanical strengths of different bones depending on
Young’s Modulus:
Fibula and Radius> Tibia> Ulna> Femur and Tibia::
18.6> 18.1> 18.0> 17.2

11. Theory of muscle fatigue leading
to fracture:

12. Bone undergoes both longitudinal as well as radial
growth, modeling and remodeling during life.
Longitudinal growth: occurs at growth plate by cartialge
proliferation in the epiphyseal areas of long bones before
subsequent mineralization to form new bone.
Radial growth: occurs by deposition of new bone on the
periosteum.
Bone GrowthBone Growth

14. Modeling is the process by which bones change its shape
in response to physiological stress induces leading to
gradual adjustment of the skeleton to the surrounding
forces it is being encountered.
Wolff’s Law
Describes that “ the bones accommodate to stresses placed
on them”.
ModelingModeling

15. Bone remodeling: the process by which bone is renewed
to maintain its strength and mineral homeostasis. It is the
process of resorption of the bone and continues with bone
growth: i.e. new bone formation.
Resorption occurs by osteoclasts whose activity is
regulated by the ratio of receptor activator: RANKL to
Osteoprotegerin,, IL-1 and IL-6, Colony-stimulating
factor, parathyroid hormone, 1,25-dihydroxyvitamin D
and Calcitonin.
RemodelingRemodeling

16. Stages of Bone Remodeling
 Quiescence: site with resting lining cells
and remains dormant until next cycle.
 Formation: mature into osteblasts at the
surface and release osteoids, further
mineralization is carried out.
 Reversal: osteoclasts disappear and
mesenchymal stem cells (the pre-cursor
of osteoblasts) appear along the pit/
burrow tunnel
 Resorption: osteoclasts dig out a cavity
called a resorption pit, in spongy bone
or burrow a tunnel in compact bone.
 Activation: Pre-osteoclasts are attracted
to the remodeling site and they fuse to
form multinucleated osteoclasts