Senior Science
9.3 Medical Technology – Bionics
Section 3
Bionics
And the
Skeleton
© P Wilkinson 2002-04
Section 3
:::
Bionics and the skeleton
9.3.3
The wide range of movements, continual absorption of shocks
and diseases make the skeletal system vulnerable to damage but new
technologies are allowing the replacement of some damaged structures
9.3.3 a
Identify the role of the skeletal system particularly in relation to maintaining an
upright stance and protecting vital organs
9.3.3 b
Describe the different types of synovial joints as
– ball and socket
– hinge
– double hinge
– sliding
– pivot
and identify their location
9.3.3 c
Describe the role of cartilage and synovial fluid in the operation of joints
9.3.3 d
Identify the properties of silicone that make it suitable for use in bionics
9.3.3 e
Explain why silicone joints would be suitable substitutes for small joints in the
fingers and toes that bear little force
9.3.3 f
Describe the properties that make ultrahigh molecular weight polyethylene
(UHMWPE) a suitable alternative to cartilage surrounding a ball and socket joint in
terms of its
– biocompatability with surrounding tissue
– low friction
– durability
9.3.3 g
Explain why artificial joints have the articulating ends covered in polyethylene
9.3.3 h
Describe the properties of the materials, including superalloy that make a ball
and stem for the bone components of a large joint including:
– high strength
– low weight
– good compatibility with body tissue
– inertness
9.3.3 i
Identify that artificial implants can be either cemented or uncemented into place
9.3.3 j.
Describe the properties of the cement that is used in implants and discuss how
an uncemented implant forms a bond with bone
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9.3.3 i.
Perform a first-hand investigation to remove calcium compounds from chicken
bones to examine the flexible nature of bones
9.3.3 ii
Perform an investigation to examine the relationship between cartilage, muscle,
tendon and bone in an animal limb
9.3.3 iii
Perform an investigation to demonstrate the different types of joints and the
range of movements they allow
9.3.3 iv
Process secondary information to compare the shock absorbing abilities of
different parts of bones
9.3.3 v
Plan, choose equipment or resources for and perform a first-hand investigation
to demonstrate properties of silicone such as acid resistance, flexibility and
imperviousness to water that make it suitable for use in bionics
9.3.3 vi
Analyse secondary information to compare the strength of UHWPE and
‘superalloy’ metal
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9.3.3 a
Identify the role of the skeletal system particularly in relation to maintaining an
upright stance and protecting vital organs
The skeletal system
There are 206 bones in the human body.
These bones are part of the living skeleton
containing bone tissue, cartilage, fibrous
connective tissue, blood vessels and
nerves. Bones are hard and rigid because
of the presence of calcium salts.
The skeleton has five main functions
1. PROTECTS vital organs such as the
brain, heart and the lungs.
2. SUPPORTS the body. It gives us the
body shape we have.
3. ALLOWS MOVEMENT of the body
because muscles are attached to
bones.
4. MAKES blood cells
5. STORES important minerals such as
calcium.
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9.3.3 i.
Perform a first-hand investigation to remove calcium compounds from chicken
bones to examine the flexible nature of bones
Calcium in bones
Bone consists of two materials – minerals and organic substances. Calcium is the main mineral
and collagen (a fibrous protein) is the main organic material in bones. The mineral matter can be
removed by soaking a long bone in acid (1 molar) leaving the organic matter. The organic matter
can be removed by soaking the bone in boiling water, leaving the mineral matter. Changes in
the composition of bone changes the properties of the bone, particularly in relation to flexibility.
Osteoporosis is a common bone disorder in old age. It is caused by a lack of calcium being
absorbed by the bone. Bone mass decreases and the compact bone becomes more porous.
The bone becomes brittle and can fracture more easily.
Investigation
Aim
To OBSERVE how the flexibility of bone changes as the calcium content is changed.
Method
1. Cut the meat from the bones of a chicken wing.
2. Collect three similar bones. This can be done by using three wings, or by swapping with
other groups.
3. Observe and describe the flexibility of the bones
4. Put one bone in 1M HCl. Place in a refrigerator overnight.
5. Gently heat one bone using a Bunsen burner.
 Hold the bone over a Bunsen flame using a set of metal tongs.
 CAUTION - Be careful as the oil in the bone could catch alight.
 Store the bone in a refrigerator overnight.
6. Put the third bone in a container. Store in a refrigerator overnight.
7. Next day, compare the three bones for flexibility.
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Conclusion
Write an appropriate conclusion
Discussion
Discuss various features of the investigation
[7 marks]
The discussion could include
 A description of some good features of the method.
 An explanation of why the procedure used is appropriate for the aim.
 An outline of modifications to the method or wording of the method; and suggestions of
how these modifications might effect the investigation.
Marking Criteria
Factors
H12
Mark Allocation
Perform first-hand investigations
a. Carrying out the planned procedure
b. Efficiently undertaking the planned procedure to minimise
hazards and wastage of resources
c. Disposing carefully and safely of waste materials
d. Complete a risk assessment
2-0
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2-0
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1-0
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2-0
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3–0
2-0
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2-0
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7–0
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H13 Present information by
a. Using the report scaffold
 Writing a heading
 Observe and record results
H11.3 Choose equipment
a Identify equipment needed
Discussion questions
Total
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9.3.3 ii
Perform an investigation to examine the relationship between cartilage, muscle,
tendon and bone in an animal limb
Cow and Chicken joints
Aim
To OBSERVE cartilage, muscle, tendons and bones in a cow joint.
To OBSERVE cartilage, muscle, tendons and bones in a chicken wing.
Risk assessment
Identify possible risks associated with this investigation.
Method
1. Observe a knee joint of a cow.
2. Move the bones of the knee joint and observe any structures involved.
3. Cut the meat from the bones of the knee joint. Take care not damage any structures around
the joint.
4. Expose any important structures in the joint.
5. Observe and describe the relationship between the various structures in the joint.
Results - description
Movement of the human body involves a large number of structures.
The following sentences describe some of the structures involved in movement, their function
and their effect on other structures.
1. Read these sentences.
2. Organize the sentences into a logical order.
3. Use the sentences to write a paragraph that shows an understanding of the relationship
between cartilage, muscle, tendons and bone.
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
When bones meet a joint is formed.
Tendons connect bones and muscles.
Muscles move bones by pulling on tendons.
One end of the tendon arises in the muscle.
The other end of the tendon is woven into the substance of a bone.
Cartilage is a bluish white rubbery tissue found in humans.
The cartilage is found at the ends of long bone and cushions the bone against shock
It also prevents them from rubbing against each other.
Ligaments tie the bones together at the joints.
A tendon is a strong white cord.
Muscles also hold the bones of the skeleton together.
Muscles make the body move.
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9.3.3 b
Describe the different types of synovial joints as
– ball and socket
– hinge
– double hinge
– sliding
– pivot
and identify their location
9.3.3 iii
Perform an investigation to demonstrate the different types of joints and the
range of movements they allow
Bone Joints
Bones don’t work on their own. The bones join together to form joints.
There are three basic joints
 fibrous (immovable)
 cartilaginous (slightly movable)
 synovial (freely movable)
Synovial Joints
Freely movable joints (synovial joints) provide stability as well as allowing movement. A large
range of twisting, turning and rotating movements are possible because of movement at joints.
A synovial joint has the following features:
1.
2.
3.
4.
5.
6.
A joint capsule surrounds the joint completely.
A joint cavity
surrounded by the joint capsule.
A synovial membrane lines the inside of the joint capsule.
Synovial fluid
is secreted by the synovial membrane
Bones
come together to form the joint.
Cartilage
covers the bone ends
There may be other structures present in or
near the joint such as disks, cartilage,
tendons and ligaments.
7. Ligaments
8. Tendons
© P Wilkinson 2002-04
connect the bones at the joint
attach muscle to bone
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Types of Synovial Joints
The main function of synovial joints is to provide both stability and mobility. Synovial joints are
classified by the shape of the bone ends and the type of movement they allow. There are a
number of different types of synovial joints.
Ball and socket joints
 One rounded head bone (ball) fits into a cup
shaped bone (socket).
 Movement in almost any direction – side to
side, back and forth, rotation.
 Examples – hip, shoulder
Hinge joints
 One bone curves out (convex) and fits a
second bone that curves in (concave).
 Movement in one plane (like a door hinge) –
backward and forward.
 Examples – elbow, knee, ankle, finger joints
Pivot joints
 A bony projection on one bone fits in a ring
shaped bony structure on the other bone.
 Movement is rotational (eg nodding no with
the head).
 Examples – elbow (ulna & radius), first and
second cervical vertebrae)
Sliding Joint
 Both bones are slightly curved. Bones slide
over each other
 Movement in all directions (side to side &
back and forth) but not rotational.
 Examples – Metacarpals in wrist, Vertebrae
Double Hinge Joint
 General term for Condyloid Joints and
Saddle Joints.
 Movements in two planes that together
allow some rotation at the joint.
 Examples: wrist, thumb
© P Wilkinson 2002-04
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Activity
Aim
Body Joints
To demonstrate the different types of joints and the range of movements they allow
Method
1. Identify parts of the body where joints occur (eg. wrist, knuckle, ankle).
2. For each part of the body
a. Identify how many bones meet at each part of the body.
b. Describe the range of movement of the body part.
Use terms like the ones listed below to describe the range of movement.
 side to side
 back and forth
 3600 rotation,
3. Record data in a table like the one below.
4. Name the type of synovial joint for each part of the body
Results
Body part
Description of Range of movement of joint
Type of synovial joint
Conclusion Write an appropriate conclusion
Discussion
[Syllabus 12.1a]
1. Identify one modification that could be made to this investigation.
2. Analyze the effect of this suggested adjustment to this investigation.
[1 mark]
[2 marks]
[Syllabus 12.1d]
3. Identify safe work practices required in this investigation.
4. Describe the location of a ball and socket joint.
5. Describe the location of a sliding joint.
[2 marks]
[1 mark]
[1 mark]
Marking Criteria
Factor
H12.1 a. Carry out the planned procedure
H13.1 a. Use appropriate text types – use report scaffold
a. Use appropriate text types – tabulate data
H14.1 a. Identify trends to write an appropriate conclusion
H12.1 a. Identify modifications & analyze the effect of this modification
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Mark range
2-0
1–0
4-0
2-0
4–0
9.3.3 c
Describe the role of cartilage and synovial fluid in the operation of joints
Cartilage
At joints, the bone ends are covered with a layer of smooth cartilage. The cartilage has two
important functions:
1. Cartilage reduces friction & wear, between the two opposing joint surfaces during movement.
The surface is very smooth and the cartilage’s coefficient of friction is low. The low coefficient
protects the bone ends by providing a smooth, gliding surface when movement occurs.
2. Cartilage absorbs and spreads the forces on the joint over a wider area. This decreases the
stresses on the contacting surfaces & on the bone shaft. The cartilage is deformable like
elastic and it recovers its shape quickly when the deforming force is removed.
Synovial Fluid
Synovial fluid is secreted into the joint cavity from the synovial membrane. Most of the fluid
comes from the blood capillaries on the membrane. Synovial fluid has two roles:
1. Nutrition
the fluid provides nutrients to the cartilage. The nutrients are delivered as
the fluid moves around inside the synovial cavity. This is necessary since the cartilage itself
has a limited blood supply. Wastes are also carried out of the joint via the synovial fluid. [The
part of the cartilage nearest the bone is impregnated with calcium. This calcified layer is a
barrier to the passage of oxygen and nutrients to the cartilage from the bone. Therefore, the
cartilage is largely dependent upon the synovial fluid for its nourishment.]
2. Lubrication
the bone surfaces are kept well “oiled “ by the synovial fluid. Also, the fluid
acts as a cushion between the surfaces of the bones. This is because the joint surfaces do
not fit perfectly together and need to be kept apart.
The effects of Arthritis show the importance of cartilage and the synovial fluid. This disease is a
disease that effects joints, and can attack any synovial joint in the body. Victims of arthritis suffer
pain, stiffness and swelling in their joints.
Osteoarthritis occurs when a joint wears out and therefore occurs on older people. In this form of
the disease the cartilage between the bones breaks down. This causes the bone ends to rub
against each other. In the beginning this will cause a grating sensation and pain in the joint.
Later, as knobs of bone and hardened cartilage develop, deformity and swelling in the joint can
occur. Osteoarthritis cannot be cured. Drugs (aspirin for pain), surgery (to repair the joint) or
prosthetics (to replace the joint with an artificial one) can treat it.
Rheumatoid arthritis begins with swelling in the synovial membrane. A number of substances
then begin to break down the bone and cartilage in the joint. The disease results in severe pain
and deformity of the joint. Treatment includes rest, a program of exercise and pain relief, using
aspirin. In some cases joint replacement is necessary.
© P Wilkinson 2002-04
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9.3.3 iv
Process secondary information to compare the shock absorbing abilities of
different parts of bones
© P Wilkinson 2002-04
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9.3.3 v
Activity
Plan, choose equipment or resources for and perform a first-hand investigation
to demonstrate properties of silicone such as acid resistance, flexibility and
imperviousness to water that make it suitable for use in bionics
Properties of silicone
Planning
Information
Define the terms
 Acid resistance
 Flexibility
 Imperviousness
What to do
1. Write a heading for the investigation.
2. Write an aim for the investigation.
The statements below are a list of instructions 3. Write a method for the investigation.
a. Describe Test 1
that may be used in designing the
 Select appropriate instructions.
investigation. In all three tests need to be
 Add others that may be necessary.
designed.
 Make sure there is a control group
To demonstrate these properties of silicone
being compared to an experimental
will involve three tests - One involving salt
group.
water, one involving acid and one involving
 Write instructions, in point form.
flexibility of hardened gel.
b. Describe Test 2, 3
c. Draw at least one labelled diagram.
Test 1, Test 2, Test 3
Handle the hardened gel and observe its
flexibility and elasticity.
Place a piece of hardened silicone gel in acid.
Squeeze a thick line of silicone gel onto a
piece of rigid plastic.
Place a piece of gel in weak acid.
Place an uncovered nail in salt water.
Observe the differences in covered and
uncovered nails.
Cover a soluble disprin tablet in silicone
Place a piece of gel in water.
Leave for a few hours or overnight????
Completely cover a piece of iron with silicone.
Pour 150mL of acid into a bag that has been
placed inside a beaker.
© P Wilkinson 2002-04
4. Make a list of equipment needed.
5. List possible risks and other safety factors
6. Perform the investigation
method you have written.
using
the
7. Write down what you observe in the
results section.
8. Write a conclusion describing why the
silicone gel would be suitable for use in
bionics.
9. Complete the discussion questions.
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Results
1. Acid Test
Write a sentence describing the effect on the silicone by:
a. water,
b. a weak acid,
c. a strong acid.
2. Imperviousness to water test
Write a sentence describing what happened to:
a. the covered disprin and
b. the uncovered disprin.
3. Flexibility
Write a sentence describing the flexibility of the hardened gel.
Discussion and discussion questions
Answer these questions for one (test) section of your investigation (acid resistance, flexibility)
The investigation used to answer these questions is _____________________ .
1. Name the independent variable.
[1 mark]
2. Name the dependent variable.
[1 mark]
3. Name two controlled variables.
[2 marks]
4. Identify the control in this experiment?
[1 mark]
5. Identify what is being measured or observed in this experiment?
6. Explain why this experiment is reliable.
[1 mark]
[2 marks]
7. Predict possible issues that may arise during the course of this investigation [2 marks]
8. Discuss if a valid conclusion can be drawn about the suitability of silicone in bionics?
[4 marks]
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Marking Criteria
Factors
Mark Allocation
H13 Present information by
 Writing a heading
3–0
/3
3-0
/3
2-0
/2
 Steps outlined in logical order
1–0
/1
 Investigation can be repeated
1–0
/1
 Clearly describe what is being measured
2–0
/2
 Demonstrate use of terms dependent & independent
2–0
/2
 Identify variables that need to be kept constant
2–0
/2
 Method contains control and experimental groups
1–0
/1
 Results are reliable (large number of trials suggested)
1–0
/1
 Complete a risk assessment
2–0
/2
 Identify safe working practices
2-0
/2
b. Using the report scaffold
e. Correctly draw and labelled diagrams to present information
H11.3 choose equipment
bIdentify equipment needed
H11.2 plan first –hand investigations
12.1
Perform first-hand investigation
Discussion questions
/14
Total
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9.3.3 v
Plan, choose equipment or resources for and perform a first-hand investigation
to demonstrate properties of silicone such as acid resistance, flexibility and
imperviousness to water that make it suitable for use in bionics
Planning
Information
What to do
The statements below are a list of instructions
that may be used in designing the 1.
investigation.
2.
To demonstrate the properties of silicone will
involve three tests. One involving salt water, 3.
one involving acid and one, involving
flexibility of hardened gel.
Write a heading for the investigation.
Write an aim for the investigation.
Write a method for the investigation.
a. Discuss the instructions listed.
b. Select appropriate instructions.
c. Add others that may be necessary.
d. Write separate instructions to be
followed for each test, in point form.
e. Draw at least one labelled diagram.
Handle the hardened gel and observe its
flexibility and elasticity.
Place the covered iron in acid.
Squeeze a thick line of silicone gel onto a
piece of rigid plastic.
4. Make a list of equipment needed.
Place an uncovered nail in salt water.
Observe the differences in covered and 5. Perform the investigation using the
uncovered nails.
method you have written.
Leave overnight.
Completely cover a piece of iron with silicone. 6. Write down what you observe in the
Pour 150mL of acid into a bag that has been
results section (see below)
placed inside a beaker.
7. List possible risks and other safety factors
In writing the method, note the following:
V The independent variable affects the 8. Write a conclusion describing why the
dependent variable.
silicone gel would be suitable for use in
G Two groups are being compared – the
bionics.
experimental group and the control group.
M A statement of what needs to be observed 9. Complete the discussion questions.
or measured needs to be made.
A The instructions must be able to be
followed.
N A number of repeats ensures reliability
S Safety precautions must be listed.
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Discussion and discussion questions
1. What is the independent variable?
2. What is the dependent variable?
3. What are two variables that could be controlled?
4. What is the control?
5. What is being measured or observed in this experiment?
6. Is this experiment reliable? Explain your answer.
7. Can a valid conclusion be drawn about the suitability of silicone in bionics? Explain.
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9.3.3 d
Identify the properties of silicone that make it suitable for use in bionics
9.3.3 e
Explain why silicone joints would be suitable substitutes for small joints in the
fingers and toes that bear little force
Silicone and bionics
Silicon is an element found in trace quantities in the body. Silicon is used to make a rubber-like
substance called silicone. Silicone has many applications including oven door seals, heart
valves and mouldings. It can be used in this wide variety of situations because of its properties
that include:
 Inert – will not react chemically
 Retains flexibility at high or low temperatures
 Resists attack from acids
 Impervious to water
In arthritis, joints become less flexible, painful and even deformed. In osteoarthritis the cartilage
between the bones breaks down. This causes the bone ends to rub against each other. The
disease can be treated with a prosthetic material. Prosthetic devices are ideally made of
materials with similar properties to the real body part (cartilage).
Ssilicone is used to replace the cartilage. The silicone is used to coat the end of small bones in
a joint where the cartilage has been destroyed. Silicone is a useful prosthetic material to treat
arthritis because it has similar properties to cartilage.
 Like cartilage, silicone provides a smooth, low friction surface that will not be broken down
by the disease. The smooth surface protects the bone ends when movement occurs.
 As well silicone is also deformable like elastic. When a force is applied to a joint the silicone
will deform and then recover its original shape quickly when the deforming force is removed.
Like cartilage, this allows the silicone to spread the forces on the joint over a wider area. This
decreases the stresses on the contacting surfaces.
 The human body is well designed to enable the movements of the joints to occur continuously
over a lifetime with few problems. A type of silicone rubber used to make artificial finger joints
can maintain its flexibility, and it can be bent 90 million times without breaking.
 As well, silicone is biocompatible, inert, withstands radiation used to sterilize implants
and resilient.
Although silicone can withstand the low forces in small joints, it is not able to withstand the much
larger forces found in larger joints such as the hip. Therefore it is used in small joints such as
the fingers and toes.
9.3.3 f
Describe the properties that make ultrahigh molecular weight polyethylene
(UHMWPE) a suitable alternative to cartilage surrounding a ball and socket joint in
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terms of its
– Biocompatability with surrounding tissue
– Low friction
– Durability
9.3.3 h
Describe the properties of materials, including ‘superalloy’ that make a ball and
stem made for the bone components of a large joint including:
– High strength
– Low weight
– Good compatibility with body tissue
– Inertness
Ball and Socket Joint
Joints allow motion to occur between two bones. Each year hundreds of thousands of people
have affected hips, knees, shoulders, elbows, fingers, knuckles, ankles and toes replaced by
artificial implants.
The hip and shoulder are both ball and socket
joints.
The oldest and most common joint
replacement is the artificial hip. The artificial hip
has two moving parts. It consists of:
The cup (socket): An artificial, moulded cup
implanted into the natural worn-out socket of the
hip joint. It is used to house the ball portion of the
joint.
The ball & stem: The thighbone component of
the artificial hip consists of a metal stem or rod
with a metal ball on top. This is attached at an
angle to mimic the shape of the top end of the
human thighbone.
The first significant attempts at total joint
replacement were hip implants in 1938.
A
stainless steel cup and femoral component were held in place by screws and a bolt. Before
stainless steal was introduced, other materials such as gold, silver, lead, steel, iron, and ferrous
alloys were used to try and find durable, biocompatible components. Some of the metals, such
as gold and silver, had excellent biocompatibility; but they were too soft to withstand the
tremendous loads exerted on them. Scientists then turned to materials with better mechanical
properties, such as lead and steel; however, these materials displayed adverse reactions in the
body, and in some cases such as lead produced toxic reactions. Stainless steel was expected to
be an ideal material because of its corrosive resistance and durability. However, the metalon-metal stainless steel joints had a tendency to disintegrate and corrode early.
The choice of materials is extremely important. Whatever the material choices used, the goal is
to select combinations that have good wear properties and low coefficients of friction.
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Today, the most widely used combination is cobalt-chromium-molybdenum (Co-Cr-Mo) alloy for
the femoral component and ultra-high molecular weight polyethylene (UHMWPE) for the ball.
Titanium alloys (Ti-6Al-4V) are being tested. When applied in combination with UHMWPE, it
displays high wear properties. Wear improves again when the alloy is coated with another
substance, such as titanium-nitride (Ti-N). Ceramics on ceramics, and ceramics on polyethylene
are gaining interest. Ceramics such as Alumina have been found to have low wear rates when
coupled with UHMWPE, but when paired with each other the wear will increase greatly if the
components are not closely matched.
Mechanical properties of implant materials
There are three main factors that will influence the performance of biomaterials in the human
body: biocompatibility, mechanical properties and degradation. At present we do not have any
materials that can mimic perfectly the mechanical property of bone. The table shows the
stiffness (Young’s modulus), strength and fracture resistance of a number of materials used for
implants. Metals (stainless steel, Co-Cr-Mo, Ti-6Al-4V) have sufficient strength and fracture
toughness but have relatively high stiffness, which can lead to weight shielding problems.
Ceramics (alumina, hydroxyapatite-HA) are generally very hard materials; they are strong in
compression but exhibit low fracture resistance. Polymers (polymethylmethacylate-PMMA and
polyethylene-PE) have low stiffness values, reasonable fracture toughness but poor strength.
Material
Stiffness – Young’s modulus
Strength
Fracture resistance
(Giga Pascals)
(Mega Pascals)
(Mega Newton’s m-3)
Bone
7 - 25
100 – 150
2 - 12
Steel
210
230 – 1160
~ 100
Co-Cr-Mo
230
430 – 1028
~100
Ti – 6Al – 4V
106
780 – 1050
~ 80
Aluminium
365
6 - 55
~3
PMMA
3.5
70
1.5
Polyethylene
1
30
~ 1.5
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The ball and stem of a hip joint – Properties of alloys used
The ball and stem are generally made of metal alloys. Large forces are exerted at the hip joint.
As well the normal patient cycles their hip at least one million times per year doing the normal
activities of daily living. As such, the hip prothesis is subject to a wearing out process.
 All the alloys used are biocompatible (ie they are not seen by the body as foreign objects
and do not cause an immune response).
 As well the alloys have high strength that means they can withstand the compression forces
exerted when people stand, walk and jump. High fracture resistance allows metal alloys to
withstand sideways forces, such as a bump on the side of the hip. These mechanical
properties make the metal alloys very durable - they don't wear out easily, and are not brittle,
so shouldn't break.
 The inside of the body is a very moist environment. Biomaterials such as the alloys used in
joint replacements are non-reactive in this environment – relatively inert.
Stainless steel is an alloy of iron, chromium, nickel and molybdenum. It has extremely high
resistance to corrosion (inert), and thus does not degrade in the body. It can be shaped
easily which is an important consideration for implant manufacturers who want to minimise
production costs. However, problems may arise because of its relatively high stiffness, and the
fact that some people may develop an allergic reaction to the nickel content.
An alternative to stainless steel is cobalt chromium alloy (27-30% Cr, 5-7% Mo.; rest Co). It has
good wear properties and is more resistant to scratching. The fact that it contains no nickel
means that it can be used in patients who have nickel sensitivity.
An advantage of titanium alloys is they are lightweight (light enough not to hinder the patient's
movement). Titanium alloys are lighter than both stainless steel and Cobalt alloys. Also it is
easy to fasten to the existing bone because it is easy to shape and drill.
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Properties of UHMWPE
Ultra high molecular weight polyethylene is a physically inert material, with outstanding
combination of properties for indoor and outdoor environments:
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Very high impact resistance
Continuous working temperature 60°C (max 80°C)
Good abrasive wear resistance
Suitable for food contact
Excellent chemical resistance
Excellent electrical properties
Natural translucent white colour
The socket of a hip joint – Properties of UHMWPE used
A ball and socket prosthesis normally consists of a metal thigh section attached to an ultra high
molecular weight polyethylene (UHMWPE) socket. UHMWPE is used to replace the function of
cartilage in high load joints, such as the hip and knee. The UHMWPE reduces wear and acts as
a cushion at the joint.
 Like cartilage, UHMWPE provides a smooth, low friction surface that will not be broken
down by the disease. The smooth surface protects the bone ends when movement occurs.
It is so smooth and tough that you can ice skate on a sheet of the plastic with out much
damage to the material.
Friction and wear are a major concern in joint prostheses. Friction is a force that is present
between two moving surfaces and is required in order to produce movement. If the friction
force is high enough, it can cause loosening of the prosthetic and cause fatigue failure.
The highest friction forces are seen in metal on metal joints, next in metal on UHMWPE, and
they are even lower in ceramic on UHMWPE. Although metal on plastic does not have the
lowest friction forces, the force produced is not believed to play a role in the failure of a
prosthetic.
 UHMWPE has a higher molecular weight; over one million; and the polymer chains have very
few branches. Because of the limited branches and symmetry of the chains, the chains
undergo partial crystallization and are surrounded by non-crystallized, amorphous materials.
This results in a polymer that has high abrasive resistance and good chemical resistance.
As a consequence UHMWPE is very durable.
Durability also results because UHMWPE deforms like elastic. When a force is applied to a
joint the UHMWPE will deform and then recover its original shape quickly when the deforming
force is removed. Like cartilage, this allows the UHMWPE to spread the forces on the joint
over a wider area. This decreases the stresses on the contacting surfaces.
 As well, silicone is biocompatible, (ie they are not seen by the body as foreign objects and
do not cause an immune response).
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9.3.3.i
Identify that artificial implants can be either cemented or uncemented into place
9.3.3.j
Describe the properties of the cement that is used in implants and discuss how
an uncemented implant forms a bond with bone
Attaching implants to the body
Artificial hip joints are broadly of two types; those that require bone cement to anchor it in the
human body and those that do not.
Properties of the cement
Rather than glue or cement, implant adhesive works more like
the grout in your bathroom tile. It's a space-occupying material that has some minor adhesive
capability, but its real function is to fill the space between the prosthesis and the bone. The
liquid cement flows into minute cavities and cracks in the bone surface, creating a finely bonded,
rigid enclosure for the stem of the prosthesis. To attach the components the cavity is first filled
with bone cement. Then the component (stem or cup) is pushed into place and held still until the
cement sets. As such the cement would be relatively quick setting. Unfortunately, over time,
this cement dries and cracks, causing the shaft to loosen. Patients then need to undergo
(revision) surgery to re-glue the shaft.
Bonding uncemented prostheses to the bone
The ability of the human body to repair
itself is used to keep the prosthesis in place. The implant has a specially designed surface on its
outer side that feels porous like a sponge. It is designed to "fool" the body into mistaking it for
bone and causes your bone to actually grow into the implant over time. The implant is
hammered into the bone, causing some trauma. The bone responds to the trauma by gradually
growing into the surface of the implant, in a similar way a bone repairs itself when broken. The
healing process takes 6-12 weeks and bond gets stronger over time. It is important that the
implant is held firmly in place during this time. Hammering the implant into an accurately
machined cavity achieves the initial fit. Further fixation can be obtained if necessary by using
additional screws.
Which implant – cemented or uncemented
The choice of implant is made by the specialist taking into account your age, lifestyle, how active
you are, whether the hip replacement is being done for the first time or is a replacement for a
previously carried out artificial hip and also the specialists own experience and training. The
uncemented implant is usually chosen for a younger patient (under 65), who presumably will
need at least one revision, because it provides better fixation (and potentially longer 'life') than
the cemented model. It also takes longer for the bond to become secure, though a younger
patient's bone would generally grow more quickly into the prosthesis (up to a year). Most people
over 65 would have a cemented prosthesis, at least a cemented femoral stem.
Elderly people have softer, more osteoporotic bone. As well, for the elderly, the basic bone
structure on which the prosthesis is attached isn't as strong or as good to begin with, so there's a
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greater risk that it may crack. The ingrowth of bone into an uncemented prosthesis may not be
as great, and it may be more fibrous soft tissue that doesn't calcify effectively.
Of course, it depends on a person’s physical condition: If you're 65 and osteoporotic, and you've
lived a very inactive life without much exercise, there's a good chance your bone quality will be
relatively poor. In this case a cemented prosthesis would be recommended. On the other hand,
if you're 70, and you've jogged every day of your life, you may have excellent bone quality. In
this case, your surgeon might recommend that you have an uncemented prosthesis.
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9.3.3 g
Explain why artificial joints have the articulating ends covered in polyethylene
Reducing friction and wear at joints
The articulating ends of artificial joints need to be covered in polyethylene because it reduces
friction. A reduction in friction means that there is a reduction in wear of the biomaterial.
Joint prostheses are bearings. One function of bearings is to reduce friction and wear of the
moving parts. In machines, some bearings use fluids to reduce friction. Friction is a force that is
present between two moving surfaces and is required in order to produce movement.
Friction and wear are a major concern in joint prostheses. If the friction force is high enough, it
can produce shearing stresses that can cause loosening of the prosthetic at the bone-prosthesis
interface and cause fatigue failure in the prosthesis itself. The highest friction forces are seen in
metal on metal joints, next in metal on UHMWPE, and they are even lower in ceramic on
UHMWPE. There are many causes and results of wear and the wear debris, and any one will
cause prosthetic failure.
There are different types of wear. One example is called abrasive
wear. It results from the direct contact between metal and plastic
components. Even highly polished surfaces have a microscopic
roughness. When the plastic and metal come into direct contact,
the peaks of the metal will cut into the plastic.
To reduce this problem of surface roughness in a machine bearing
a lubricant is used. The lubricant is placed between the moving
surfaces and this results in one part actually floating over the other. The two articulating
surfaces do not come into contact if the parts are moving. Also, the bearing is usually moving at
a constant velocity and in a unidirectional motion. Under these conditions, abrasive wear is
negligible. In the body, the motion is oscillating and the liquid film cannot be maintained.
A special type of abrasive wear is called third-body wear. This occurs when foreign substances
such as bone cement, metal beads, bone debris, and wear particles are present. The harder
substances become embedded into the softer plastic bearing. The embedded bodies can then
quickly deteriorate the metal surface.
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9.3.3 vi
Analyse secondary information to compare the strength of UHWPE and
‘superalloy’ metal
Material
Stiffness – Young’s modulus
Strength
Fracture resistance
(Giga Pascals)
(Mega Pascals)
(Mega Newton’s m-3)
Bone
7 - 25
100 – 150
2 - 12
Steel
210
230 – 1160
~ 100
Co-Cr-Mo
230
430 – 1028
~100
Ti – 6Al – 4V
106
780 – 1050
~ 80
Aluminium
365
6 - 55
~3
PMMA
3.5
70
1.5
Polyethylene
1
30
~ 1.5
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