3. Introduction
3
Bone tissue, or osseous tissue, - a type of
connective tissue used in forming bones.
Bones protect the vital organs and help
support the body.
Bone is composed mainly of collagen, or
ossein, fibers, and bone cells called osteocytes.
There are two types of bone tissue, referred to
as cortical bone and cancellous bone.
20. Basic Biomechanics
Load to Failure
Continuous
application of force
until the material
breaks (failure point
at the ultimate load).
Common mode of
failure of bone and
reported in the
implant literature.
Fatigue Failure
Cyclical
subthreshold loading
may result in failure
due to fatigue.
Common mode of
failure of
orthopaedic implants
and fracture fixation
constructs.
21. Basic Biomechanics
Material properties of bones:
Anisotropic
Mechanical
properties
dependent upon
direction of loading
Viscoelastic
Stress-Strain
character dependent
upon rate of applied
strain (time
dependent).
22. Bone Biomechanics
Bone is anisotropic - its modulus is dependent
upon the direction of loading.
Bone is weakest in shear, then tension, then
compression.
Ultimate Stress at Failure Cortical Bone
Compression
Tension
Shear
< 212 N/m2
< 146 N/m2
< 82 N/m2
23. Bone Biomechanics
Bone is viscoelastic: its forcedeformation characteristics are
dependent upon the rate of
loading.
Trabecular bone becomes stiffer
in compression the faster it is
loaded.
24. Bone Mechanics
Bone Density
Subtle density
changes greatly
changes strength
and elastic
modulus
Density changes
Normal aging
Disease
Use
Disuse
Cortical Bone
Trabecular Bone
Figure from: Browner et al: Skeletal Trauma
2nd Ed. Saunders, 1998.
28. Fracture Mechanics
Torsion
The diagonal in the direction of the applied force is in
tension – cracks perpendicular to this tension
diagonal
Spiral fracture 45º to the long axis
Figures from: Tencer. Biomechanics in Orthopaedic
Trauma, Lippincott, 1994.
The human skeletal system is primarily composed of two types of skeletal tissue, bones and cartilage. Bones provide the framework for the human body and help protect internal organs from trauma.
They also aid in red blood cell production, store minerals, and support movement. Cartilage keeps the bones connected, helps them move, and prevents friction damage at the ends of joined bones. It provides flexible support and shaping in external structures like the nose and ears as well as internal structures, such as the wind pipe.The skeletal system includes all the bones of the body plus joints where they attach to each other. Skeleton system protects internal organs and provides a framework, allows standing upright and moving. Skeleton system stores minerals that a body needs to function properly and produce blood cells.The skeletal system is made up 206 different bones which come in 4 basic shapes.
Long bones are characterized by a shaft, the diaphysis, that is much longer than it is wide. They are made up mostly of compact bone, with lesser amounts of marrow, located within the medullary cavity, and spongy bone. Most bones of the limbs, including those of the fingers and toes, are long bones.
Short bones are roughly cube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. The bones of the wrist and ankle are short bones.
Flat bones are thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones.
Irregular bones do not fit into the above categories. They consist of thin layers of compact bone surrounding a spongy interior. By the name, their shapes are irregular and complicated. Often this irregular shape is due to their many centers of ossification or because they contain bony sinuses. The bones of the spine, Pelvis, and some bones of the skull are irregular bones.
The majority of bone is made of the bone matrix. It is composed primarily of inorganic hydroxyapatite and organic collagen. Bone is formed by the hardening of this matrix around entrapped cells. When these cells become entrapped from osteoblasts they become osteocytes.It is much less dense and comparatively weaker than cortical bone tissue, cancellous tissue serves an important purpose. There are two kinds of bone marrow. Red bone marrow is almost completely made up of specialized cells that make the majority of the body's red and white blood cells, and platelets.
This image shows trabecular bone structure in the lower spine of a young adult compared to an osteoporotic elderly adult. When a child is born, all of the bone marrow in his body is typically red bone marrow.As a person grows older, more and more red bone marrow is replaced by yellow bone marrow, which is made mostly of fat. During times of extreme starvation, the body will use these fat stores, and, in some cases, the yellow marrow can change back into red marrow if needed. Roughly half of all marrow in an adult's body is yellow.
Let’s talk about Mechanical properties.The primary tissue of boneis a relatively hard and lightweight composite material. It is mostly made up of a composite material incorporating the mineral calcium phosphate in the chemical arrangement termed calcium hydroxyl apatite (this is the osseous tissue that gives bones their rigidity) and collagen, an elastic protein which improves fracture resistance. It has relatively high compressive strength of about 170 MPabut poor tensile strength of up to 121 MPa and very low shear stress strength (51.6 MPa), meaning it resists pushing forces well, but not pulling or torsional forces. While bone is essentially brittle, it does have a significant degree of elasticity, contributed chiefly by collagen. All bones consist of living and dead cells embedded in the mineralized organic matrix that makes up the osseous tissue.
