This document provides an overview of biomechanics and biomaterials. It begins with an introduction to biomechanics as the application of mechanics to biological systems. It describes the three branches of mechanics - rigid body, deformable body, and fluid mechanics. It then discusses various biomechanical concepts like kinetics, kinematics, osteokinematics, arthrokinematics, and the three classes of levers found in the body. The document concludes with a section on biomaterials, discussing their properties, criteria for success, material properties like stress/strain, and examples of brittle versus ductile materials.

1. BIOMECHANICS AND
BIOMATERIAL
Birhanu Ayinetaw
OSR R1
3/9/2021 1

2. Outline
Introducion
Osteokinematics
arthrokinematics
Biomechanical levers systems
biomaterials
3/9/2021 2

3. Introduction
Mechanics is a branch of physics studying
motion and the forces causing it
Three branches
Rigid body mechanics
Deformable body mechanics
Fluid mechanics
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4. Rigid body mechanics
Assumes an object is rigid and deformation in
its shape is so small that it can be ignored
Never happen in real object but resonable for
most biomechanical studies of major segment
of the body
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5. Rigid body…
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6. Major branches
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7. Statics - study of systems that are in a constant state of
motion, whether at rest with no motion or moving at a
constant velocity without acceleration
– Statics involves all forces acting on the body being in balance
resulting in the body being in equilibrium
Dynamics - study of systems in motion with acceleration
– A system in acceleration is unbalanced due to unequal forces
acting on the body
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8. Biomechanics
Biomechanics: is The Science that examines the internal
and external force acting on the body and the effects
produced by these forces
It applies newtons law of mechanics on
models of biological objects to describe their
behaviour and function
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9. Newton’s Laws of Motion
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10. conti
 Kinetics
• examines the forces acting on the body during movement
 Kinematics
• A branch of biomechanics that study the relationship b/n
postions ,velocities and aceleration of rigid bodys without
concern for how motions are caused .it simply descirbes
geometry of motion
Kinesiology is the the study of human movement and
motion
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11. Areas of biomechanics
Orthopedic biomechanics:focuses on the
effects of motion and deformation forces and
moments acting on tissues such as bone
,cartillage ,growth plate ,ligaments
,meniscus,synovial fluid and tendons
Clinical biomechanics:evaluates specifics
pathologic condtions through study of joint
instability,gait pathology and fracture healing
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12. Conti…..
• Sport biomechanics
• Occupational biomechanics
• Forensic biomechanics…….etc
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13. ANATOMY PHYSIOLOGY MECHANICS
BIOMECHANICS
Other disciplines
SPORTS
MEDICINE
Integration of Disciplines
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14. continued
Why Study Biomechanics and Biomaterials?
 The basis of all implants and devices we use
The basis for most trauma we see
 The basis for most of our interventions
Orthopedics surgery is as much a
biomechanical science as it is a surgical
craft!!!!!!!!!!!!!
3/9/2021 14

15. Principle Quantities
Basic Quantities
• Length
• Time
• Mass
Derived Quantities
• Velocity (length/time)
• Acceleration (length/time2)
• Force (mass length/time2)
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16. Scalars and Vectors
Scalar quantities have magnitude but no
direction.
• Time, speed (not velocity), mass, volume
 Vector quantities have magnitude and
direction.
• E.g Velocity, Force, Acceleration,WT
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17. A vector is portrayed by anarrow; its direction
is indicated by the direction of the arrow, and
its magnitude is represented by the length of
the arrow. F
the weight of a limb or body segment must
always be portrayed by a vector in the
direction of gravity (down), with its length
defined relative to the weight of the limb
applied at its center of gravity
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18. Vector representation
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19. Importance of direction
3/9/2021 19

20. Vectors
 A vector can be resolved into its individual
components
F = Fx + Fy + Fz,
Vectors can be added to form a new vector by
adding their components or graphically by the
parallelogram method
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21. 3/9/2021 21

22. 3/9/2021 22

23. Force
• Definition:a push or pull on an object resulting
from the object's interaction with another
object
• equation
– force = mass x acceleration, F=ma
– 1 Newton = force required to give 1 kg mass an
acceleration of 1 m/s2
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24. Moment (torque)the tendency of a force to
rotate a body around an axis
• When a force is applied to an object that is
not on line with the center of the object,
theforce will create a torque that tends to
rotate the object
– equation
• moment (torque) = force(perpendicular) X distance
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25. 3/9/2021 25

26. KINEMATICS
 Translation-When all parts of a “body” move in the
same direction as every other part
• Rectilinear motion = straight line motions (sliding
surfaces)
• Curvilinear motion = curved line of motion (the motion
of a ball when tossed)
 Rotation-the arc of motion around a fixed axis of
rotation or a “pivot point”
• Joints have “pivot points” which are used as reference
points from which to measure the range of motion
(ROM) of that joint
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27. ‘Pivot point’
3/9/2021 27

28. Kinematics of walking
 The hips are moving
forward and marked to
indicate the curvilinear
path that they take in
the translatory motion
of walking.
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29. Kinematics of motion
• Movement of the body =
translation of the
translation of the body’s
center of mass
• Center of Mass/Center of
Gravity
 Center of gravity- COG-
balance point of an object
at which torque on all
sides are equal
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30. Active VS passive motion
Active
• Generated by muscle
contraction
Passive
 Occur due to stresses
placed on the tissue
other than muscle
contraction
 Gravity
 Resistance
 An applied stretch from
someone or something
else
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31. 3/9/2021 31

32. Osteokinematics
Motion of bones through a range of motion
relative to the 3 cardinal planes of the body and
around the axis in that joint
Planes:
Saggital or Median
• Flexion & extension
Frontal or Coronal
• ABD & ADD
Horizontal or Transverse
• Rotational motions
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33. 3/9/2021 33

34. Conti….
Anatomic postion
• Standard Reference Point
• Axis of rotation
• Planes of motion
• Actions of muscles are referenced from
anatomic position
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35. Body described in relative to anatomic
postion
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36. Contin…
Axis of Rotation = “pivot point” It’s ALWAYS
perpendicular to the plane of motion!
Degrees of Freedom The number of planes of
motion allowed to a joint
• The shoulder and hip have 3
• the elbow and knee have just 1
• The wrist has 2
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37. Arthrokinematics
• Manner in which adjoining joint surfaces
move in relation to each other or how they fit
together
• helps to improve the movement of the joint
• Parts may move in
the same direction
the opposite direction
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38. Fundamental joint surfaces
movements
 Roll
• Multiple points maintain
contact throughout the
motion
 Slide
• A single point on one
surface contacts multiple
points throughout the
motion
 Spin
• A single point on one
surface rotates on a single
point on the other surface
3/9/2021 38

39. Types of machines found in the body
Musculoskeletal system may be thought of as a
series of simple machines
Machines function in four ways
– balance multiple forces
– enhance force in an attempt to reduce total force
needed to overcome a resistance
– enhance range of motion & speed of movement
so that resistance may be moved further or faster
than applied force
– alter resulting direction of the applied force
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40. Cont…
Musculoskeletel system arrangement provides
for 3 types of machines in producing
movement
– Levers (most common)
– Wheel-axles
– Pulleys
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41. Mechanical advantage:ratio between the
force arm (distance between the force and the axis)and
the resistance arm( distance between the resistance
and the axis )
when the FA is greater than the RA The MA is
greater than 1
• The force arm has more force than the RA It
takes less force on your part if you apply
resistance distally rather than proximaly
Mechanical disadvantage
– Levers designed for speed/ROM
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42. F R
Mechanical advantage or disadvantage?
How does mechanical advantage affect
movement of the lever?
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43. Advantage: Small effort moves
big resistance
Disadvantage: Big movement
required to move resistance a
small distance
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44. F R
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45. Knee Joint and MA
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46. Biomechanical levers
• Levers rotate about an axis as a result of force
(effort, E) being applied to cause its
movement against a resistance or weight
• In the body
– bones represent the bars
– joints are the axes
– muscles contract to apply force
3/9/2021 46

47. The body has Three Classes of Levers
First
• Similar to a “see saw”
Second
• The axis is located at one end to provide “good
leverage”
Third
• The axis is also at one end but gravity has more
“leverage” than muscle meaning that more
muscle force is needed to lift a small load
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48.  First Class Lever
• F - A – R
• Force, Axis, Resistance
 Designed for balance
• The head sitting on the
cervical vertebrae
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49.  Second Class Lever
• A – R – F
• Designed for power
 Ankle plantar flexors are the
perfect example of a second
class lever.
 There is excellent leverage so
that the body is easily elevated
with relatively little force
generated by the plantar
flexors of the calf.
 Rare In the body
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50. Third Class lever
• A – F – R
• Designed for motion
• The most common lever in
the body because they favor
large ranges of motion
• There is mechanical
disadvantage
• Favor speed and distance
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51. PULLEYS
A Pulley
• A grooved wheel that turns on an axel with a
rope or cable riding in the groove
advantage
• To change the direction of a force
• To increase or decrease the magnitude of a
force
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52. 3/9/2021 52

53. Biomaterials
Defnition:synthetic or treated natural
materials that are used to replace or augment
tissue and organ function.
Creterias of success full biomaterial
Biocompatability
Resist corrosion and degradation
Wear resistance
Adquate mechanical property
reproducibilty
3/9/2021 53

54. Material Properties: Fundamental behaviors of a
substance independent of its geometry.
Structural Properties: related to both the
material properties and the shape of an
object.
3/9/2021 54

55. • Material Properties
– Elastic-Plastic
– Yield point
– Brittle-Ductile
– Toughness
–Independent of
Shape!
• Structural Properties
– Bending Stiffness
– Torsional Stiffness
– Axial Stiffness
– Depends on
Shape of
Material!
3/9/2021 55

56. Material properties
Basic defintions
 Stress
- Is defined as the force acting on a surface
divided by the area over which it acts.
- Used to analyze the internal resistance of a
body to load.
- Normal stress vs shear stress
- Shear stress are parallel
- Normal stress are perpendicular to the surface
3/9/2021 56

57. Strain
 Is the change in height or length of the
object under load divided by its original
height or length
relative measure of the deformation of an
object
 A relative quantity with no units. Often
expressed as a percent
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58. 3/9/2021 58

59. Material strength
Stress vs Strain Curve; Derived from axially
loading an object and plotting the stress
verses strain curve
• Each material produces different stress strain
diagram
3/9/2021 59

60. Elastic zone
– the zone where a material will return to its original shape for a
given amount of stress
• Yield point
– the transition point between elastic and plastic deformation
• Yield strength
– the amount of stress necessary to produce a specific amount of
permanent deformation
Plastic zone
– the zone where a material will not return to its orginal shape for
a given amount of stress
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61. Stress vs strain curve
3/9/2021 61

62. • Breaking point
– the object fails and breaks
• Ultimate (Tensile) strength
– defined as the load to failure
• Hooke's law
– when a material is loaded in the elastic zone, the stress is
proportional to the strain
• Young's modulus of elasticity
– measure of the stiffness (ability to resist deformation) of a
material in the elastic zone
– calculated by measuring the slope of the stress/strain curve in
the elastic zone
– a higher modulus of elasticity indicates a stiffer material
3/9/2021 62

63. young’s modulus of elasticty
 Of metals and biologics
1. Relative values of Young's
modulus of elasticity (numbers
correspond to numbers on
illustration to right)Ceramic
(Al2O3)
2. Alloy (Co-Cr-Mo)
3. Stainless steel
4. Titanium
5. Cortical bone
6. Matrix polymers
7. PMMA
8. Polyethylene
9. Cancellous bone
10.Tendon / ligament
11.Cartilage
3/9/2021 63

64. Material description
Brittle material a material that exhibits linear
stress stain relationship up until the point of
failure
undergoes elastic deformation only, and little
to no plastic deformation
examples
– ceramics
3/9/2021 64

65. Ductile Material
– undergoes large amount of plastic deformation
before failure
– Example
• Metal
Toughness:
 the amount of energy/volume a material can
absorb before failure
Area under the stres strain curve
3/9/2021 65

66. 3/9/2021 66

67. 3/9/2021 67

68. toughness
Ductile metal or cortical bone Collageneous tissues
3/9/2021 68

69. Cont…
This concept is useful because many injuries
may impart a specific energy to the body. For
example, a fall from a height may turn the
potential energy of the body weight at the
original height into the energy of deformation
of the lumbar spine or hip,causing fracture
3/9/2021 69

70. Conti….
Fatigue failure
– failure at a point below the ultimate tensile
strength secondary to repetitive loading
• depends on magnitude of stress and number of cycles
Endurance limit
– defined as the maximal stress under which an
object is immune to fatigue failure regardless of
the number of cycles
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71. 3/9/2021 71

72. Viscoelastic material
– a material that exhibits a stress-strain relationship that
is dependent on the load and the rate by which the
load is applied (syringe with narrow needle)
• a function of the internal friction of a material
• examples
– ligaments
– Bone
Creep phenomenon of progressive deformation
of in response to a constant force over an
extended period of time
Stress relaxation the force required to maintain
the deformation will diminish with time until
equilibrium is reached
3/9/2021 72

73. 3/9/2021 73

74. Time dependent
behaviour of strain
Elasticc modulus ,yield
stress and ultimate
stress incresing where
as strain rate and
ductility deacreasing
as loading rate
increased
• Cortical bone -viscoelasticty
3/9/2021 74

75. hysteresis
• The loading and unloading
curves do not overlap,
forming a closed loop called
a hysteresis loop
• Represents the energy
dissipated within the
material, primarily by
internal friction within the
material.
3/9/2021 75

76. Conti…..
Anisotropic materials
– possess different mechanical properties depending on
the direction of the applied load
– examples
• ligaments
• Bone
• Composites (plaster cast,fiber glass)
Isotropic materials
– possess the same mechanical properties in all
directions
• example
– golf ball
3/9/2021 76

77. Anisotropy
 bone is an anisotropic
material hence failure
depends on load direction
and loading type
Cortical bone
3/9/2021 77

78. Anisotropic behavior can often be predicted
by considering the material’s structure.
• Bone tissue at an ultrastructural structure
scale has both collagen fibrils and mineral
crystals both aligned in a generally
longitudinal direction in the cortices of the
long bones of the skeleton
why the tissue has its greatest strength in the
longitudinal direction
3/9/2021 78

79. METALS
• Metal alloys have found wide spread use in
orthopedics
• Alloys are metals composed of mixtures or
solutions of metallic and non metalic elements
• Combination imparts high
strength,ductility,corrosion resitance and
biocompatibility
The 3 commons alloys:stainless steel ,cobalt
chromium alloys and titanium alloys
3/9/2021 79

80. Conti…
Stainless Steel (316L)
– Components
• primarily iron-carbon alloy with lesser elements of
– Chromium form chromium oxide layer
– molybdenum
– Manganese
– Silicon
– nickel
– advantages
• very stiff
• fracture resistant
– disadvantages
• susceptible to corrosion
• stress shielding of bone due to superior stiffness
– Used
• Fracture fixation and spinal stabilization until healing occurs
3/9/2021 80

81. Conti…
Cobalt alloy
– components
• cobalt
• chromium
• molybdenum
– advantages
• very strong
• better resistance to corrosion than stainless steel
3/9/2021 81

82. Titanium and titanium alloys
Titanium –aluminium-vanadium
widely used
– very biocompatable
– forms adherent oxide coating through self
passivation(osteointergrated)
» corrosion resistant
– low modulus of elasticity makes it more similar to biologic
materials as cortical bone
– Better than both stinless steel an d cobalt interms of corrosion
resistant and biocmpatiblity
3/9/2021 82

83. uses
– fracture plates
– screws
– intramedullary nails
– some femoral stems
disadvantages
– poor resistance to wear (notch sensitivity) (do not use as a
femoral head prosthesis)
– generates more metal debris than cobalt chromium
3/9/2021 83

84. polymers
 Ultra-high-molecular-weight polyethylene (UHMWPE)
 advantages
– tough
– ductile
– resilient
– resistant to wear
 disadvantages
– susceptible to abrasion
• wear usually caused by third body inclusions
– thermoplastic (may be altered by extreme temperatures)
– weaker than bone in tension
 gamma irradiation
– increases polymer chain cross-linking which improves wear characteristics
– decreases fatigue and fracture resistance
3/9/2021 84

85. UHMW polyethylene
C C C C
H H H H
H H H H
crystalline
amorphous
3/9/2021 85

86. Polymethylmethacrylate (PMMA,
bone cement)
Mechanical grout that polymerizes insitu
2 component material
– powder
» polymer
» benzoyl peroxide (initiator)
» barium sulfate (radio-opacifier)
– Liquid
» Monomer(MMA)
» toluidine (accelerant)
» hydroquinone (stabilizer)
3/9/2021 86

87. The liquid does not polymerize until it comes
into contact with the initiator, dibenzoyl
peroxide, which is mixed in with the powder
3/9/2021 87

88. Brittle material
Tensile strength = ~ 35 MPa
Compressive strength = ~ 90 MPa
Fatigue strength = ~ 6 MPa at 105 cycles
3/9/2021 88

89. functions
used for fixation and load distribution in conjunction
with orthopeadic implants
functions by interlocking with bone
may be used to fill tumor defects and minimize local
recurrence
3/9/2021 89

90. advantages
• reaches ultimate strength at 24 hours
• strongest in compression
• Young's modulus between cortical and cancellous bone
disadvantages
• poor tensile and shear strength
• failure often caused by microfracture and
fragmentation
3/9/2021 90

91. Ceramics
Solid, inorganic compounds consisting of
metallic and nonmetallic elements held
together by ionic or covalent bonding
Aluminum + Oxygen  Alumina (Al2O3)
Zirconium + Oxygen  Zirconia (ZrO2)
3/9/2021 91

92. • advantages
–best wear characteristics
–high compressive strength
– High elastic modulus (2-3x metals)
– Polished to a very smooth finish
– Excellent wettability (hydrophylic)
– Excellent scratch resistance
– Even with the presence of third bodies
– Inert/biocompatible
3/9/2021 92

93. • disadvantages
– typically brittle, low fracture toughness
– high Young's modulus
– low tensile strength
USE:
in total joint replacement components
Bone graft substitutes
3/9/2021 93

94. BONE
Bone composition
– composed of collagen and hydroxyapatite
collagen
• low Young's modulus
• good tensile strength
• poor compressive strength
hydroxyapatite
• stiff and brittle
• good compressive strength
3/9/2021 94

95. Mechanical properties
• strongest in compression
• a dynamic structure
– remodels geometry to increase inner and outer cortex to alter
the moment of inertia and minimize bending stresses
• weakest in shear
3/9/2021 95

96. • Cortical bone is stiffer than cancellous bone,
withstanding greater stress but less strain before failure.
• Cancellous bone may sustain up to 50% of strains before
yielding.
• Cortical bone yields and fractures when the strain exceeds
1.5% to 2.0%.
• Cortical bone:weak in tension and shear
• Cancellous bone is good against Compressive and shear
forces.
3/9/2021 96

97. FRACTURE
Tension
– usually leads to transverse fracture secondary to muscle
pull
compression
– due to axial loading
– leading to a crush type fracture
– bone is strongest in resisting compression
bending
– leads to butterfly fragment
torsion
– leads to spiral fracture
3/9/2021 97

98. 3/9/2021 98

99. Ligaments & Tendons
• Characteristics
– viscoelastic with nonlinear elasticity
– displays hysteresis
• Advantages
– strong in tension (can withstand 5-10% as
opposed to 1-4% in bone)
• Disadvantages
– demonstrate creep and stress relaxation
3/9/2021 99

100. references
• Orthopedic basic science
• Fundamental biomechanics
• OTA notes
3/9/2021 100

101. •Thank
u!!!!!!!!!!!!
3/9/2021 101