Materials
Materials used as replacement parts in the
skeletal system include:
 Silicone
 Ultra
High Weight Polyethylene (UHMWPE)
 Super Alloy
 Cement – to hold replacement parts in place.
9.3.3 d Identify the properties of silicone that
make it suitable for use in bionics.
Vocabulary:
Silicone – is a largely inert, man-made
compound with a wide variety of forms and uses.
Typically heat-resistant, nonstick, and rubber-like,
it is commonly used in cookware, medical
applications, sealants, adhesives, lubricants,
insulation, and breast implants.
Property
High Elongation
High Elasticity
Similar chemical
structure to collagen
Similar density to
natural tissue
High permeability to
oxygen
Low reactivity/toxicity
Why it is useful as a biomaterial
It can stretch without breaking
Will not deform if stretched
It is highly biocompatible
It is highly biocompatible
Does not obstruct the distribution
of oxygen around the body
Will not react with bodily fluids
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 joints would be suitable
substitutes for small joints in the fingers
and toes because they can be made as
strong and as flexible as natural joints.
They are biocompatible, as they allow the
flow of oxygen and do not react with
living tissue. They would last a long time,
as they do not dissolve in water.
9.3.3 f Describe the properties that make ultra-high
molecular weight polyethylene (UHMWPE) a suitable
alternative to cartilage surrounding a ball and socket
joint in terms of its:
• Biocompatibility with surrounding tissue
• Low friction
• Durability
Vocabulary:
Low Friction – is slippery or smooth.
Durability – Lasts along time and is
resistant to wear.
Ultra-high molecular weight polyethylene
(UHMWPE) is biocompatible with surrounding
tissue.
It possesses a
similar density to
living tissue, and
therefore tends not
to cause problems
in the body.
UHMWPE has a low friction coefficient and this,
along with other exhibited characteristics, such
as high hardness, high tensile strength, high
elasticity, add to its suitability of it for use in
joints as artificial cartilage.
Vocabulary:
Tensile Strength – A measurement of the
strength required to pull something until it
breaks.
UHMWPE is very durable. It has no known
effective solvent at mild temperatures. High
temperatures and pressures must be used to
manipulate the material and gain the desired
product. UHMWPE also exhibits a very high
creep resistance.
Creep resistance is the tendency
for polymers to deform when
under constant stress.
9.3.3 g Explain why artificial joints have the
articulating ends covered in polyethylene
Vocabulary:
Articulating Ends – “Jointed Ends”
These are the ends of bones that end in
synovial joints
The articulating ends of a natural joint are
covered by a cartilage which cushions the
bones in the joint and provides ease of
movement when lubricated with synovial fluid.
When constructing artificial joints, it is
important to replicate the natural
structure of the joint.
Polyethylene is used to coat the articulating
ends of artificial joints because:
• it has a similar density to living tissue
• it is relatively elastic (especially high-density
polyethylene)
• it has a low coefficient of friction
• it has low creep properties i.e. it will not
deform under stress.
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
A superalloy is an alloy
that exhibits excellent
mechanical strength and
creep resistance at high
temperatures, good
surface stability, and corrosion and
oxidation resistance.
Many alloys are very strong.
Co-Ni-Cr-Mo alloy is used for making stems of
prostheses for heavily loaded joints like the
knee.
Super alloys needs to be very strong
due to the stress placed on the alloy in
load bearing joints such as the knee
and hip joints
Many metals are not suitable for
implantation because they are heavy, and
may lead to damage of surrounding living
tissue. Titanium-based alloys are very
lightweight (lighter than stainless steel and
Co-alloys), as are the polymer components of
the artificial joint.
Carbon-based alloys show a
high level of biocompatibility.
For this reason, carbon or
carbon-based alloys are
integrated into the
development of new materials.
Corrosion is a major concern, as it may lead
to the breakage of load-bearing joints like the
artificial hip joint. Metal alloys generally have
much lower corrosion factors than stainless
steel. Co-Cr alloys are inert in the body,
remaining literally unchanged under
physiological conditions.
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 a
bone.
Vocabulary:
Cement – Cement is used as a
biomaterial to fix artificial joints
into place.
1. Use the Venn Diagram supplied to organise
the information gathered from the
PowerPoint presentation.
2. Using the completed Venn Diagram to justify
your response, compare the strengths of
UHMWPE and Super Alloy (write 1/3 to ½
page).
Bone cement for the fixation of artificial hip joints was
introduced in the 1950s. Once the diseased bone is removed,
the medullar canal is filled with a doughy bone cement, and the
implant is inserted. Alignment of the implant with the other
components of the joint is verified before the bone cement sets.
The cement used in a cemented implant serves many purposes:
 It allows the initial fixation of the implant to the bone.
 It acts as a shock absorber for the joint.
 It helps to spread the load more evenly over a large area
and reduces the stress concentrated on the bone by the
prosthesis.
Cement-free implants are coated with a porous
layer, which comes into contact with the bone. This
places less pressure on the bone. The porous layer
of cement-free implants allows the bone to grow
into the implant, creating a dynamic interface of
bone and implant.
Cement-free implants are more vulnerable to
loosening than cemented implants, and it is
expected that the time required before the patient
can walk will be longer than for cemented implants.
Cemented
Cement-Free
9.3.3 p Analyse secondary information to
compare the strength of UHMWPE and
“superalloy” metal.
Analyse:
Make and justify generalisations.