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Basic Biomechanics &
Biomaterials for
Orthopaedic
p
Surgeons
g
Tariq Nayfeh, M.D./Ph.D.
Outline

Introduction

Basic Definitions

Joint Mechanics


Mechanics of Materials
Bending Theory

Biomaterials
Why Study Biomechanics
and Biomaterials

To Pass Exams?
Basic Definitions

Statics
S
i is
i the
h study
d off forces
f
on
bodies at rest
 Dynamics is the study of the motion
of bodies and the forces that
produce the motion

Basic Definitions

Kinematics is the study of motion in
terms of displacement, velocity, and
acceleration with reference to the cause
of the motion

Kinesiology is the the study of human
movement and motion
Principle Quantities

• This area is actually a low yield area for time spent
studying and the number of questions asked

Biomechanics is the science of the
action of forces, internal or external
on the living body.
Basic Quantities
• Length
• Time
• Mass
So Why Study It?
Derived Quantities

The basis of all implants and devices we use

The basis for most trauma we see
• Velocity (length/time)

The basis for most of our interventions
• Acceleration (length/time2)

• Force (mass length/time2)
1
Scalars and Vectors

Free Body Diagrams
Scalar quantities have magnitude
but no direction.
• Time, speed (not velocity), mass, volume

Vector quantities have magnitude
and direction.
The forces acting
on a body may be
identified by
isolating that body
part as a free body
diagram
• Velocity, Force, Acceleration
Beer and Johnston, “Mechanics of Materials”
Example Free Body
Diagrams
Vectors

A vector can be resolved
into its individual
components

Vectors can be added to
form a new vector by
adding their components
or graphically by the
parallelogram method
Fy
F
Fx
Basic Laws of Mechanics
Newton’s Laws
Moments
A moment (torque) is the rotational
effect of a force about a point.
F
M
=
d
M=Fxd

First Law:
An object at rest will remain at rest and
an object in motion will continue in
motion with a constant velocity
y unless it
experiences a net externall fforce
• Inertia is the tendency of an object to
either remain at rest or to maintain uniform
motion in a straight line
• The weight of a body is a vector quantity
that is equal to the force of gravity acting
on it
2
Basic Laws of Mechanics
Newton’s Laws

By combining the first and second
laws: For equilibrium to occur the
sum of the forces and moments
must be equal to zero
F  0
Basic Laws of Mechanics
Newton’s Laws

Third Law:
Joint Mechanics

Joints are stabilized by the
action of the muscles,
ligaments and bony
structures.

The muscles are located at a
distance from the joint

Muscle action produce
moments about the joint
center
Joint Mechanics

For every action
there is an equal and
opposite reaction.
Joint reaction
forces occur at the
joint center
These reaction forces can be greater than the
weight of the body segment or the entire body
Joint Mechanics

How do joints
maintain stability?

What produces
joint movement?
Joint Mechanics

When the muscle and joint reaction
forces are balanced equilibrium
occurs and the body segments do
not accelerate

When there is an imbalance of
forces acceleration (or deceleration)
of the body segment occurs
3
Illustrative Problem
F
x
0
y
0
F
W=20 N
B  R  G W  0
R  B  15  20
R  B  35
G= 15 N
M  0
B  3  G  15  W  30  0
Illustrative Problem
If the person bends
forward

Lw = 25 cm

Lp = 40 cm
Mspine = 450x0.25+200x0.4
Mspine =192.5 Nm
20  30  15  15
B
 275 N
3
R  240 N
Illustrative Problem
Hip Reaction forces in single leg
stance
Nordin and Frankel, “Basic Biomechanics of the
Musculoskeletal System”
Forces across the hip and
knee

Hip joint contact forces
• Single leg stance – 2 to 3 x BW
• Walking
- 3 x BW
• Stairs,
Stairs running

- 5 to 7 x BW
Knee Tibiofemoral forces
• Rising from a chair – 4 x BW
• Walking – 3 x BW
• Stairs Ascent – 6 to 7 x BW
• Stair Descent – 7 to 8 x BW
Buckwalter, et al. “Orthopaedic Basic Science”
Illustrative Problem
This person is trying to lift a 20 kg
object.



The force from the upper extremities
is 450 N
Mechanics of Materials
In order to understand how materials
behave we need to define some
basic quantities.
The estimated moment arm of the
upper extremities is Lw = 2cm
The estimated moment arm of the
weight is Lp= 30 cm.
Mspine = 450x0.02 + 200 x0.30
Mspine = 69 Nm
4
In pure tension or compression
Stress
Stress is the intensity
of internal force.


Normal stress are
perpendicular
p
p
to the
surface
Shear stress are
parallel

F
A
Beer and Johnston, “Mechanics of Materials”
The plane of maximum shear is at 45 degrees to the
axis of loading!!
Strain
Strain (Engineering):
Relative measure of the
deformation (six
components) of a body as a
result of loading.
L

L
Can be normal or shear
AO/ASIF
**A relative quantity with
no units. Often expressed
as a percent
Beer and Johnston, “Mechanics of Materials”
Depending on how you
“slice” the material you can
get combinations of stress
and sheer
Beer and Johnston, “Mechanics of Materials”
AO/ASIF
5
Shear strain
Usually expressed as an angle radians
Hoop Stress
As humans age, the
diameter of their
bones increase, but
the thickness
decreases…
We will see later that this
change is not bad for ordinary
human activity. It matters
most when we as surgeons
intervene.
Material Testing
In order to characterize how materials
behave we have to create standardize
methods to test them and document
the behavior.
behavior


In the US the ASTM standards are the
most widely used
In Europe the most widely used is the
ISO standards
Beer and Johnston, “Mechanics of Materials”
“Hoop” stress
Hoop stress is the stress in a
direction perpendicular to the
axis of an item
***As the
thickness of the
item decreases
the hoop stress
increases***
Why is this
important?
Materials Testing
Materials of standardized sizes and shapes are
placed in testing machines and loaded
following standardized protocols
p
pr
t
pr
2 
2t
1 
Beer and Johnston, “Mechanics of Materials”
6
Stress-Strain Curves
Standardized curves used to help quantify
how a material will respond to a given load.
AO/ASIF
Quantities Derived from
Stress-Strain Curves





Yield Strength: The stress level at which
a material begins to deform plastically
Ultimate Strength: The stress level at
which a material fails
Modulus of Elasticity: The linear slope
of the materials elastic stress-strain
behavior.
Ductility: The deformation to failure
Toughness: Energy to failure (the area
under the stress strain curve)
Elastic vs. Plastic Behavior
AO/ASIF
Types of failure
Ductile
Brittle
Elasticity vs. ductility and
strength
All of these materials
have the same
modulus of elasticity
But they have different
toughness, ductility
and strength.
AO/ASIF
Beer and Johnston, “Mechanics of Materials”
7
Force-deformation curves for materials
having various combinations of structural
properties
Force-Displacement Curves

Similar to stress-strain
curves

Not a material property,
instead a measure of how
the entire structure
behaves

Depends on
• Material
• Geometry
Force-Displacement Curves
Beer and Johnston, “Mechanics of Materials”
Stiffness
L
Buckwalter, et al. “Orthopaedic Basic Science”
Question
Unloaded:
A=cross section area
E=Young’s
E
Young s modulus of elasticity
The linear relationship between an applied stress and the resultant
deformation defines a material's
•1- modulus of elasticity.
•2- brittleness.
•3- yield strength.
•4- ultimate strength.
•5- toughness.
F
u
Longitudinal stiffness Sax = EA
L
F = SEA
ax u = Saxu
L
If the question was changed to applied force, instead
of applied stress. The answer would change to
stiffness.
8
Bending of Beams
Most bones and orthopaedic
implants are subjected to axial,
bending, and torsion loading

Most failures
M
f il
occur secondary
d
to
bending and torsion
compressio
on tension
M
eccentric
load
compressio
on tension
centric
load
Linear bending theory
eccentric
load
compressio
on tension

Bending Theory Definitions

Neutral Axis: The location where a beam
experiences zero stress (this is a
theoretical axis and can actually be
located outside of the structure)

Moment of Inertia: The geometric
property of a beam/s cross section that
determines the beams stiffness
• There is a bending and a torsion moment of inertia
(we will limit our discussion to bending)
Lo w s t res s
High
stress
9
Relative bending
resistance
Bending Resistance


The resistance of a beam to bending
is directly proportional to its
moment of inertia
The moment of inertia depends on
its cross sectional area and shape
Solid rod
1
Flat beam
3.5
I beam edge on
6
I beam flat
0.6
Hollow cylinder
5.3
Identical size
of
cross sectional
area
Gozna et al. 1982
Bending resistance solid cylinder
=  / 64 · diam4
Bending resistance of a hollow cylinder
=  / 64 · (outer diam4 – inner diam4)
or for thin shells
=  / 8 · diam3 · shell thickness
When the diameter of a spinal instrumentation rod is increased from
4 mm to 5 mm, the rod's ability to resist a bending moment is
increased by approximately what percent?
•1- 10%
•2- 25%
•3- 50%
•4- 100%
•5- 300%
R1 

64

d14 

64

4 4
5
d 24 
64
64
4
4
R2  R1 5  4 
625  256


 1.44  100%
4
R1
256
4 
R2 
4
850Kg. 800Kg.
60Kg.
20Kg.
The bending stiffness of a half pin is
proportional to one half the radius of
the pin to what power?
• 2
• 3
• 4
• One third
• One fourth
Bone-implant composite
AO/ASIF
10
Tension band principle
Example of tension bands
A properly done
tension band shifts
the neutral axis to
the surface of the
beam so that
compression
occurs across the
entire cross
section
torque
shear
Example of tension bands
Mechanical Properties of
Materials

Isotropy
• Material properties
do not depend on
direction
• Steel
• Aluminum

Anisotropy
• Material properties
depend on the
direction of loading
•
•
•
•
Bone
Tendons
Ligaments
Cement
11
Anisotropy

Bone is an
anisotropic
material

Hence failure
depends on load
direction and
loading type
Bending forces in the long bones most commonly result in what type of fracture
pattern?
•1- Short oblique
•2- Transverse with butterfly
•3- Linear shear of 45°
•4- Spiral
•5- Segmental
What type of loading is most likely to cause a pure spiral fracture?
•1- Crush
•2- Bending
•3- Tensile
•4- Compression
•5- Torsion
Buckwalter, et al. “Orthopaedic Basic Science”
Bone Mechanics
Cortical bone is
weakest in directions
that cause tensile
stresses.
In the transverse
direction the bone is
acting as a brittle
material
AO/ASIF
Three-point bending produces a
predominantly transverse fracture because
1.
2.
3.
4
4.
a compression crack begins at the fulcrum.
bone is weaker in tension than in compression.
bone is weaker in compression than in tension.
the forces are equally resolved between tension and
compression.
5. the forces are resolved into pure tension.
Bending forces in the long bones most commonly result in what type of fracture
pattern?
•1- Short oblique
•2- Transverse with butterfly
•3- Linear shear of 45°
•4- Spiral
•5- Segmental
What type of loading is most likely to cause a pure spiral fracture?
•1- Crush
•2- Bending
•3- Tensile
•4- Compression
•5- Torsion
12
Clinical Example
A 27-year-old patient sustains the
closed femoral fracture shown. This
fracture pattern is most likely the
result of which of the following
f
forces?
?
1.
Pure torsion
2.
Pure bending
3.
Pure compression
4.
Four-point bending
5.
Torsion plus bending
Why are Long Bones
Hollow?

For the same total cross sectional area a hollow
tube has higher bending and torsional resistance
than a solid tube

Most bones are loaded in bending and torsion

Bone responds to Wolfe’s law and tries to maximize
the bone density where stress is highest and
minimize it where stress is lowest

The thinner a bone is the easier it is for nutrients to
reach the osteocytes

Less energy is required to maintain the bone
Clinical Question
Case 1: A 75 year old female
with osteoporosis falls and
sustains a supracondylar
femur fracture. The patient
undergoes ORIF with a
locked supracondylar plate.
plate
She is allowed to increase
her weight bearing to full
weight bearing at 6 weeks.
Two weeks later she
presents with increasing
pain, swelling and can not
bear weight.
Case 2: A 75 year old female with
osteoporosis falls and sustains an
intertrochanteric hip fracture. She
undergoes ORIF with an
intramedulary device and is allowed
to weight bear as tolerated the next
day. Her fracture goes on to heal
without complications.
Clinical Example
Case 3: An 83 year old male
with multiple medical
problems presents with
severe right hip pain and
the
h inability
i bili to b
bear
weight. He had undergone
a revision of his right total
hip 10 years ago to a
cementless stem.
Clinical Case
Why did the patient in Case 2 do well
while the patient’s in Case 1 and
Case 3 have their implants fail?
13
Fatigue
Fatigue Life
Fatigue testing is done using the same type
of samples and machines that are used to
create stress-strain curves. However, the
samples
p
are loaded cyclically
y
y to failure.
The goal of testing is to determine how
many loading cycles at a given load a
material can withstand before failing.
In a fatigue test, the maximum stress
under which the material will not fail,
regardless of how many loading cycles are
applied, is defined as
•1- endurance limit.
•2- failure stress.
•3 critical stress.
•3stress
•4- yield stress.
•5- elastic limit.
**The failure stress levels are not the
same as the yield stress and ultimate
stress.**
Fatigue
Fatigue testing generated fatigue
life curves.


Fatigue Endurance Limit – The
stress level below which a material
does not fail (usually must last
greater than 10 million cycles)
Bone Fatigue

Bone has no in vitro endurance
limit!

In vivo bone heals

If bone fails to heal when subjected
to cyclic loads we get stress
fractures
Fatigue life – The number of cycles
that a material can withstand at a
given stress level
Fatigue Life
Clinical Examples

In Case 1 above the patient was
allowed to weight bear before her
fracture healed. In this case the
stress from walking on the bone
resulted in rapid failure with
relatively few cycles.
Endurance
Limit
14
At higher rates of loading, bone absorbs
more energy prior to failure because
Case 3

The applied stress
to the small
diameter implant
again resulted in
fatigue failure of
the stem
Stress Concentration
When a structural member contains a
discontinuity, such as a hole or a sudden
change in cross section, high localized
stresses may occur near the discontinuity.
1.
the modulus of elasticity
decreases.
2.
bone is anisotropic.
3.
bone is viscoelastic.
4.
bone deforms plastically.
5.
bone is stronger in compression
than in tension.
Viscoelasticity
Viscoelasticty is a term used to describe
materials that demonstrate time-dependant
behavior to loading.
• Visco is derived from viscocity (fluid like)
• Elastic come from elasticity (solid like)


Most normal temperature metals are elastic
Most biologic materials (bone, tendon,
ligaments), glass, polymers, and metals at
high temperature exhibit viscoelastic behavior
Beer and Johnston, “Mechanics of Materials”
Stress Concentration
The highest stress
concentration occurs
near a sharp point

 y max   n 1  2

a

p 
Viscoelasticity

A simple model for an elastic
material is a simple spring in which
instantaneous displacement occurs
to an applied load.
load
The energy of
displacement is stored as
potential energy and
recovered when the load
is removed.
Buckwalter, et al. “Orthopaedic Basic Science”
15
Viscoelasticity

Viscous behavior can be modeled as
a dashpot (shock absorber).
Deflection occurs in response to the
rate of force application
At higher rates of loading, bone absorbs more energy prior to failure
because
•1- the modulus of elasticity decreases.
•2- bone is anisotropic.
•3- bone is viscoelastic.
•4- bone deforms plastically.
•5- bone is stronger in compression than in tension.
In this case the
energy produced
from loading is
dissipated as heat.
Buckwalter, et al. “Orthopaedic Basic Science”
Viscoelasticity

Viscoelastic Behavior is modeled as
a combination of elastic and viscous
materials.
The change in strain of a material under a constant load that occurs
with time is defined as
•1- creep.
•2- relaxation.
Time
•3- energy dissipation.
•4- plastic deformation.
•5- elastic deformation.
The energy from
loading is
partially stored
and partially
dissipated
Buckwalter, et al. “Orthopaedic Basic Science”
Stress Relaxation
The biomechanical properties of ligaments
and bone demonstate
1.
a time-dependent behavior.
2.
a rate-independent
t i d
d tb
behavior.
h i
3.
a straight-line load-deformation behavior.
4.
modeling with linear elastic-spring
elements.
5.
similar stress-stretch curves.

Stress relaxation is the decrease of
stress with time under constant
strain.
Time
16
Examples of Materials Used
for Implants
Stress Shielding

Wolff’s law
“If you don’t use it, you lose it!”

Stress shielding occurs when an
implant carries most of the stress
and effectively unloads the bone

Examples are the proximal femur
with an ingrown implant and loss of
bone under a plate.
Assume the plate is stainless steel with
E=190 GPa
Stress
Shielding
Assume the bone is all cortical bone with
E=17 GPa
Which of the following properties is most commonly associated with titanium
alloy implants when compared with cobalt-chromium alloys?
Both the bone and the plate must deform
the same
•1- Lower elastic modulus
•2- Lower corrosive resistance
•3- Better wear characteristics
P
F  A
P  Fp  Fb
  E
 p
Ess
p

Eb
b
 b
190
p 
b 
 10 b
17
Eb
Ep
P
•4- Lower notch sensitivity
•5- Greater hardness
P  Ap p  Ab b  (10 Ap  Ab ) b
b 
•ELASTIC MODULUS
cancellous bone
polyethylene
PMMA (bone cement)
cortical bone
titanium alloy
stainless steel
cobalt-chromium alloy
•ULTIMATE TENSILE STRENGTH
cancellous bone
polyethylene
PMMA (bone cement)
cortical bone
stainless steel
titanium alloy
cobalt-chromium alloy
P
(10 Ap  Ab )
 p  10
Elastic modulus and ultimate tensile strength of the
most common orthopedic biomaterials, listed in order
of increasing modulus or strength:
P
(10 Ap  Ab )
Stainless Steel
In a 77-year-old woman who
underwent total hip arthroplasty 10
years ago. What is the predominant
cause of the proximal femoral bone
loss?
1.
Stress shielding
2.
Polyethylene debris-induced
osteolysis
3.
Senile osteoporosis
4.
Modulus of elasticity of the femoral
stem
5.
Diffuse osteopenia





Used for fracture fixation and spinal
implants
Most common is 316L
Contains chromium,
chromium nickel,
nickel molybdenum
The chromium forms an oxide layer on
the out side of the implant that acts as a
corrosion resistant layer and forms the
“stainless” quality to it
Strong material but can get stress or
crevice corrosion with time
• Caused by “cracking” the Cr-oxide layer with loading
17
Cobalt-Chromium Alloys
Polymers

Polymers are large molecules made from
combinations of smaller molecules
Consists mostly of cobalt with chromium added
for corrosion resistance


Like stainless steel the chromium forms a
surface oxide layer
Their mechanical and biologic properties
depends on their micro and macro-structure


Used for joint replacements, bearing surfaces
and occasionally for fracture fixation devices
A polymers molecular weight depends on
the number of molecules in its chains

Not all Co-Cr is the same and the mechanical
properties are a function of which alloy is used
and how the alloy is processed

There are a number of different alloys used for
implants depending on what type of
manufacturing is used

Titanium


• Nylon, PMMA, Polyethylene
Polymers
One of the most biocompatible metals
Very good corrosion resistance
• Resistance is generated by a rapidly formed oxidized layer on its
surface and this layer makes the titanium implant more corrosion
resistant that Stainless steel or CoCr implants

Most commonly used alloy is Ti-6Al-4V
Ti 6Al 4V
• 6% aluminum and 4% vanadium
• Initially developed as a high strength to weight ratio material for
aircraft



Its modulus of elasticity is around half of that of stainless
steel or CoCr, hence using titanium implants my reduce
the stress sheilding
Very notch sensitive – leads to crack formation and
decreased fatigue life
Not a good bearing surface in joint arthroplasty because it
gets rough with time
Buckwalter, et al. “Orthopaedic Basic Science”
Ceramics

Ceramics are materials are inorganic materials formed
from metallic and nonmetallic materials held together by
ionic and covalent bonds
• Examples include silica, alumina, zirconia

Mechanical properties are very process dependant and can
vary from manufacturer to manufacturer
• Ceramtec a few years ago changed a single step in their process
of making femoral heads (they did not change the material)
which resulted in fracture of the heads in vivo



Ceramics are very stiff, very hard, demonstrate very little
wear
Can be very brittle
Very biocompatible if manufactured to a high purity level
Polyethylene

Semi-crystalline polymer


Basic momer is CH2 with a molecular
weight of 28
It mechanical
Its
h i l and
d wear properties
ti
depend on its molecular weight,
structure, oxidation, cross linking,
processing method, and sterilization

**Not all polyethylene is the same**
18
Highly cross-linked ultra-high molecular weight
polyethylene has what effect on tensile and fatigue
strength when compared with ultra-high molecular
weigth polyethylene?
1.
Increased
d tensile
il and
d ffatigue
i
strength
h
2.
Increased tensile strength and decreased fatigue
strength
3.
Decreased tensile and fatigue strength
4.
Decreased tensile strength and no change in
fatigue strength
5.
No change in tensile or fatigue strength
Crosslinking

Crosslinking is done to create larger
molecular polyethylene molecules
that can theoretically be more wear
resistant

There are two common methods for
crosslinking
• Irradiation
• Free radical generating chemical
Crosslinking
Crosslinking


The major problem with crosslinking is that
usually higher doses of radiation which produce
the greatest amount of crosslinking also may
cause a degradation in the materials mechanical
properties. Specifically a decrease in fracture
toughness
h
and
d fatigue
f i
strength
h and
d life.
lif
Newer “versions” of highly crosslinked
polyethylene are being released that are being
treated by a combination of lower dose radiation
and post irradiation melting and or annealing.
These processes are showing promise for low
wear rates and small changes to the mechanical
properties of the polyethylene
Tribology
The study of Friction, Lubrication,
and Wear.
Lewis, Biomaterials 22 (2001) 371-401
19
The natural joint
Lubrication.
Elements that influence the
tribological function of a joint are:
Lubrication



reduces Wear
reduces Friction
The articular cartilage
The synovial fluid
And to a lesser extent the subcondral
bone, capsule, soft tissues and
ligaments.
Friction
Ability of a bearing to support a fluid
film will inevitably influence the
friction and wear of the bearing
surfaces during articulation
Lubrication modes
“is
the resistance to motion that is
experienced whenever one solid body
Slides over another”
 Boundary
Lubrication
 Hydrodynamic
Lubrication
 Hydrostatic
Lubrication
LOAD
FRICTION FORCE
DIRECTION OF
MOTION
STRIBECK CURVE
Lubrication.
“materials
applied
to the interface
reducing friction and
wear”.
Coefficient
of Friction
(μ)
BL
ML
FFL
Sommerfeld Number
(viscosity x sliding speed x radius / load)
20
Hydrodynamic Lubrication
Boundary Lubrication

Bearings are supported by a thin
layer of fluid which is pulled into the
bearing through viscous
entrainment compressed
entrainment,
compressed, creating a
sufficient hydrodynamic pressure to
support load.
h
EHD h ~0.025μm
- 2.5 μm
High Friction and Wear
Boundary Lubrication

No pressure build up in the
lubricant.

Loading is 100% carried by the
asperities
i i iin the
h contact area.
The contact area is protected by
absorbed molecules of the lubricant
and / or a thin oxide layer.


HD h >0.25μm
Generation of fluid film
As the ball rotates, fluid is drawn
into the converging wedge and
builds up a pressure which carries
the load
The characteristics for boundary
lubrication is the absence of
Hydrodynamic pressure.
Fluid Film Lubrication
Hydrodynamic Lubrication
Pressure builds as
speed increases.

 The surface
asperities are
completely separated
by a lubricant film.
No Friction or Wear
 The load and
Hydrodynamic
pressures are in
equilibrium.
21
Hydrostatic Lubrication
Clearance
 Bearings
are supported on a thick
film of fluid supplied from an
external pressure source.
P
h
P
What is Radial Clearance?
Artificial joint surfaces

Metal / Ceramic bearing on UHMWPE do
not benefit from fluid film lubrication
they operate in a mixed fluid film regime.

Unavoidable wear results at a rate of
approximately 200µm of linear
penetration per year giving a life
expectancy of a 4mm thick cup about 20
years.

M-on-M and Ceramic on Ceramic perform
in a fluid film regime therefore the
resultant wear rate is significantly
reduced.
Which of the following features improved
fluid film lubrication in a metal-on-metal
total hip arthroplasty?
1.
2.
3.
4.
5.
Smaller diameter femoral head, a completely
congruent fit between the socket and the
head, and sufficient roughness to allow for
some microseparation between the head and
socket
Smaller diameter femoral head, a slight
clearance between the socket and the head,
and no surface roughness
Larger diameter femoral head, a completely
congruent fit between the socket and the
head, and minimal surface roughness
Larger diameter femoral head, a slight
clearance between the socket and the head,
and minimal surface roughness
Larger diameter femoral head, a slight
 Radius Cup
R2
-- Radius
Head R1
-= Radial
Clearance
-Clearance
allows a fluid
Is there an optimal
clearance?
 No
one clearance, it
is a ratio to head
diameter
 The bigger the head
gets, the bigger the
clearance gets
 We must consider
manufacturing
capabilities Nominal
and Ranges etc…
 The lubricant fluid in
Vivo
V
ti ht
22
Effect of clearance with bovine
serum all tests to date
Too tight
and too high
clearances
may end up
in high wear
rates due to
an increase in
friction
0
Does reduced clearance
make a difference?
BOA Manchester 2004
McMinn presented early
results of 20 controlled
clearance cases
implanted in 2004.
 Radiolucencies
observed in superior
acetabulum in 10% of
the cases to date.

 Reduced
Clearance
bearings need further
assessment.
24 hour Cobalt output in
Regular and low clearance BHR£
Regular BHR vs Controlled Clearance BHR
Urine Cobalt Output
90
80
Regular BHR
Low Clearance BHR
Urine Output µg/24hr
70
60
50
40
30
20
10
0
Pre Op
5 day
2 month
6 month
1 year
4 year
23