A slide show of the paper- Tribology of artificial joints, T D Stewart BSc PhD Lecturer in Medical Engineering, Institute of Medical and Biological Engineering, The University of Leeds, Leeds, UK, Journal- ORTHOPAEDICS AND TRAUMA 24:6

1. TRIBOLOGY IN
MEDICINE
Dr. Libin Thomas Manathara

2. What is Tribology?
•
Tribos means “to rub” or “rubbing”
•
Coined by Dr. H. Peter Jost in his Jost Report which noted a potential
savings of over £515 million per year ($800 million) for industry by
better application of tribological principles and practices, published in
1966

3. Dr. H Peter Jost

4. Definition
•
Tribology is the science and engineering of interacting surfaces in
relative motion
•
It includes the study and application of the principles of friction,
lubrication and wear
•
Tribology is a branch of mechanical engineering and materials science

5. Tribology in Medicine
•
The general principles of tribology can be used to understand the
friction, lubrication and wear of natural and artificial joints in the
body.
•
The natural synovial joint is covered in a soft delicate layer of articular
cartilage and is lubricated with synovial fluid.
•
Whilst the cartilage is delicate, the loads experienced by our joints
are high and can exceed many times our body weight during normal
daily activities.

7. Tribology in Medicine
•
When load is applied, the cartilage does not act like a shock absorber,
as it does not absorb impact energy, rather it deforms under the
loads applied to it and acts to distribute the load over a wider area
and thus reduces contact stress.
•
This deformation also makes the contact between the articulating
surfaces more conforming, thus making it easier to achieve fluid film
lubrication that protects the cartilage from direct contact.
•
Fluid film lubrication occurs when there is a continuous film of fluid
separating the articulating surfaces.

8. Squeeze film

9. Squeeze film
•
The theory of fluid film lubrication can be described through the
actions of entrainment or squeeze film.
•
In engineering, entrainment is the entrapment of one substance by
another substance
•
"Air entrainment" - The intentional entrapment of air bubbles into
concrete
•
Fluid entrainment occurs when the relative motion of the articulating
surfaces drags fluid into the contact.

10. 10
Squeeze film
•
The relative motion builds up a fluid pressure that, if significantly
high, may separate the articulating surfaces from contact.
•
This is assisted by any deformation that may occur in the cartilage,
which acts to spread the pressure over a wider area and increase the
surface separation force

11. Squeeze film

12. Squeeze film
•
Squeeze film lubrication occurs when two surfaces that are initially
separated move together very quickly.
•
In this manner pools of lubricant may be trapped between the
contact surfaces, slowly leaking out with time.
•
Once again, as with fluid entrainment, the process is assisted by any
deformation that may occur in the cartilage which acts to restrict
fluid from leaving the contact.

13. Squeeze film

14. Squeeze film
•
These theories of lubrication can be applied to the natural hip during walking.
•
During the stance phase, with variable motion and high loads, squeeze film
formation can occur at heel-strike and this can act to protect the cartilage
surfaces from contact.

15. 15
Squeeze film

16. 16
Squeeze film
•
The swing phase, with low load and high velocity, would produce
ideal conditions to replenish the depleted fluid film through fluid
entrainment.
•
Hence, during walking it is feasible that the soft, delicate cartilage
surfaces of our hip joints do not actually come into contact, but are
protected by a thick film of lubricant.
•
If, however, the properties of the lubricant break down and the
viscosity reduces, such as with arthritis, surface contact can no longer
be avoided and this ultimately leads to cartilage degradation, pain,
and the need for joint replacement.

17. 17
Stribeck Curve- Richard Stribeck
•
The "Stribeck curve" named after
Richard Stribeck, who heavily
documented and established
examples of it, is used to categorize
the friction properties between two
surfaces
•
The research of Professor Richard
Stribeck (1861–1950) was performed
in Berlin at the Royal Prussian
Technical Testing Institute

18. Stribeck Curve
•
The relationship of friction and lubricant film thickness is commonly
shown in a Stribeck curve that relates the friction to the Sommerfeld
number
•
Sommerfeld number (zn/P), the product of the viscosity of the
lubricant (z) and the relative velocity (n) of the surfaces divided by the
load (P)

19. 19
Stribeck Curve
•
The curve can be interpreted using a variable, lambda that
distinguishes the type of lubrication regime present.
•
The Lambda ratio is the ratio of the predicted minimum film thickness
to the combined surface roughness of the articulating surfaces

20. Stribeck Curve

21. 21
Stribeck Curve

22. Stribeck Curve
•
If the film thickness is sufficiently large compared to the surface
roughness it will prevent surface asperity interaction leading to low
friction and theoretically no wear.
•
In materials science, asperity is defined as "unevenness of surface, roughness, ruggedness" (from
the Latin asper — "rough"), e.g. Exasperate which means to infuriate or to annoy exceedingly..
•
This is obviously the ideal condition for a bearing to exist.
•
However, if the film thickness is much smaller than the combined
roughness of the articulating surfaces then surface asperity
interaction cannot be avoided and a boundary lubrication regime
exists.
•
In this region of the graph the curve is flat suggesting that any
changes to a boundary lubricated bearing design will unlikely affect
the friction or wear of the device significantly.

23. Stribeck Curve
•
If the lubricant film thickness is of a similar magnitude to the
combined surface roughness then a mixed lubrication regime will
exist with variable amounts of asperity contact.
•
Bearings designed with mixed lubrication are deemed lubrication
sensitive, as the relationship between friction and film thickness is a
slope suggesting that improvements to the design may allow the
bearing to slide down the curve resulting in reduced asperity contact,
friction and wear

24. Stribeck Curve

25. Stribeck Curve
•
In terms of joint replacement bearing types, metal on polyethylene
(ultra-high molecular weight polyethylene) is considered to be
boundary lubricated as the relatively soft polyethylene surface has a
high roughness, hence surface asperity contact and wear cannot be
avoided.

26. Stribeck Curve
•
Conventional metal on metal bearings are generally considered to
operate in the mixed lubrication regime.
•
This regime is lubrication sensitive, which means that changes to the
design of metal on metal bearings can improve their performance.
•
Increasing the bearing radii, whilst keeping the radial clearance low,
has been shown to increase the predicted film thickness in metal on
metal hip prostheses sliding them down the slope of the Stribeck
curve towards the fluid film lubrication regime.
•
This has been used to promote large diameter metal on metal
bearings with advantages for reduced wear and improved stability.

27. Stribeck Curve
•
Ceramic on ceramic bearings are extremely hard and as such can be
polished to a very fine surface finish when compared to metal or
polyethylene.
•
The superior manufacturing tolerances of these bearings result in a
reduced radial clearance which when combined with the low surface
roughness leads to a predicted fluid film lubrication regime during
walking

28. The Wear Process- Polyethylene
Contacts
•
Wear occurs from the interaction of surface asperities during relative
motion
•
In polyethylene contacts this interaction comes in the form of
abrasion, adhesion and fatigue. Occurs at a microscopic scale
•
Abrasion is the form of wear most are familiar with, where the
surface asperities of the harder surface wear away the softer surface
•
Adhesion and fatigue wear work together, with the surface asperities
of the two surfaces momentarily sticking together causing shear
stresses that, over time, lead to the eventual fatigue of the asperity

29. 29
The WEAR process
Microscopic Macroscopic

30. The Wear Process- Delamination
•
Fatigue wear can also occur on a macroscopic scale in the form of
delamination.
•
Delamination is a mode of failure for composite materials. In laminated materials, repeated cyclic
stresses can cause layers to separate, forming a mica-like structure of separate layers, with
significant loss of mechanical toughness.
•
This occurs under cyclic loading when the stress applied to the
material exceeds the material’s fatigue strength.

31. 31
The Wear Process- Delamination
•
In hip prostheses the surfaces are quite conforming and the overall
contact remains primarily at the pole of the cup, hence delamination
was not a significant issue.
•
In knee prostheses, however, the surfaces are less conforming and the
contact shifts posteriorly during rollback.
•
This cyclic movement of the contact within the polyethylene,
combined with the reduced fatigue strength, cause cracks to develop
at the point of maximum stress (below the surface of the
polyethylene) and resulted in gross delamination of large particles

32. The Wear Process- Creep
•
In vivo, initially wear will take the form of bedding-in, where any
mismatch in the sizes of components is accommodated by creep
(polyethylene) or wear (hard on hard bearings)
•
Polyethylene wear- Normal loading of the polyethylene cup comes up
the femoral shaft, along the femoral neck towards the lumbar spine.
So it is normal to see slight thinning in the area of the weight bearing as
the plastic moulds itself. This remoulding of the cup is called creep
•
In ceramics, the wear volumes are so low as to be considered negligible
during normal motion

33. 33
The Wear Process- Creep
•
Creep is normal remoulding and is
superomedial. Wear is superolateral
and pathologic

34. The Wear Process
•
In the longer term the wear performance of most bearings is classed
as steady state, with linear wear rates over time
•
A well functioning polyethylene acetabular prosthesis will wear in the
region of 0.1 mm of penetration per year, and this has been termed
the osteolytic threshold.

35. The Wear Process
•
Wear rates higher than this are usually attributed to polyethylene degradation,
such as oxidation, seen as yellow discolouration of the polymer at revision
•
This was popular historically due to the sterilization and packaging of
polyethylene in oxygen. Polyethylene is now sterilized in an inert atmosphere,
which should reduce oxidative degradation considerably
•
The presence of oxidation makes interpretation of the performance of existing
products, implanted greater than 10 years ago, difficult as oxidation may have
caused a 2 to 3 fold increase in wear.
•
Additionally there would be a large variation in bearing wear depending upon the
time the polyethylene component was stored on the shelf before implantation,
since there was a greater concentration of free oxygen in the packaging than in
vivo

36. The Wear Process
•
There are many types of polyethylene available for joint replacement,
offering reductions in wear and resistance to oxidative degradation.
•
Cross-linked polyethylene has shown dramatic improvements in
implant wear, with 80% reductions in wear rates in in-vitro testing
compared to conventional polyethylene.

37. 37
The Wear Process
•
Interestingly, the theoretical wear rates
of these highly cross-linked materials and
for hard on hard bearing materials are so
low in magnitude that they pose a
challenge for clinical measurement.
•
The long-term clinical performance of
these materials is yet to be confirmed.

38. Corrosion
•
Corrosion in joint replacement can be both an advantage and a
disadvantage to wear.
•
Firstly when any metallic material comes into contact with air
following manufacture it is immediately coated in a surface layer of
oxide as the air reacts with the metal.
•
This oxide is often softer than the base metal and can be easily worn
away if there is any relative motion present, such as at a modular
junction, or head/neck taper.
•
Once worn away the oxide reforms and a continual wear/formation
cycle may begin that can lead to fretting wear.

39. Corrosion
•
In contrast to this negative side, researchers have shown that within a
bearing contact, such as in metal on metal hip prostheses, the oxide
layer, under localized heat and in the presence of biological fluids,
forms a tribochemical film which can act as a protective layer to the
surface, reducing the wear.
•
This works in a similar manner to oil additives in an automobile
engine that are designed to react with the surface for added
protection.
•
Hence, whilst corrosion in modular junctions can be very harmful in
the presence of micromotion, corrosion in the form of tribochemical
films can be beneficial to metal on metal bearing performance

40. From theory to product
•
Whilst tribological theory can be used to promote improved product
design through enhanced fluid film lubrication, the practical
application of this theory may be more challenging
•
The drive to preserve bone on the acetabular side of large diameter
bearings has led to thinner components that may deform elastically
when implanted, leaving a non-spherical component
•
Whether the bone relaxes around this implant, allowing it to go back
to its original shape or not, remains to be seen.
•
However, if the clearance is small the change in shape due to
implantation may be significant enough, at least in the short term, to
cause circumferential impingement

41. From theory to product

42. From theory to product
•
Attempts to increase the possible range of motion have led
manufacturers to reduce the coverage arc of the acetabular cup in
large diameter bearings.
•
Hence the components are often not a complete hemisphere.
•
The larger diameters and smaller clearances of these bearings lead to
large contact areas (8 to 12 mm diameter) that in theory promote
lubrication

43. From theory to product
•
When large contact areas are combined with a reduction in coverage
of the cup it becomes easier for the contact area to reach the edge of
the bearing and become truncated, producing a stress concentration
•
Edge contacts in any bearing, and particularly hard bearings, lead to
accelerated wear and should be avoided

44. From theory to product
•
The challenge in hard on hard bearing design lies in balancing the
surgical patient and tribological requirements so as not to adversely
affect either.
•
Tribologically, large diameter bearings provide increased sliding
distances, which in turn lead to increased sliding velocity.
•
Refined radial clearances in the geometry lead to large contact areas
and low contact stress.

45. From theory to product
•
These combine to provide increased lubrication films that are
generated when the person walks with accompanying reductions in
surface asperity interaction and wear.
•
The sensitivity of the tribological theory is such that compromises
made from theory to design are often not ideal, particularly for the
large diameter metal on metal resurfacing bearings and, if things go
wrong, they have a great capacity for increasing wear due to their
size

46. Micro Separation
•
Laxity in hard on hard bearings has also been shown to lead to micro-
separation edge loading.
•
This is not a dislocation, as the components most likely stay in contact
at all times, but rather a small lateral translation (<0.5 mm) of the
femoral component that leads to an adverse contact condition when
the contact shifts from the normal bearing area (Position A) to a small
chamfer at the edge of the cup (Position B)
•
Micro-separation has led to order-of-magnitude increases in the wear
of metal on metal and ceramic on ceramic bearings both in vitro and
in vivo, however, it has not led to runaway wear and fracture

47. Micro Separation
•
Laxity can also contribute to a greater chance of dislocation, a recent
area of concern in orthopaedics, and has led to surgeons generally
considering the effects of soft tissues more significantly.
•
The drive to keep the soft tissues taut has led the surgeons to
increase femoral offset.
•
This improves the efficiency of the abductors and makes it easier for
the person to walk following surgery

48. Micro Separation

49. Micro Separation
•
However, in some cases this has led to leg length inequality, where
the action of increasing femoral offset has led to a longer implanted
leg
•
Leg length inequality has been shown to lead to reduced flexion and
slower sliding velocities during walking
•
Tribological theory suggests that reduced sliding velocity would
negatively affect lubricant entrainment

50. Macro Separation
•
Macro-scale joint separations include dislocations and impingement
and are generally in the order of at least several millimetres of
motion
•
Macro-separations can be very harmful to alternative bearings, as
they generally result in contact entirely outside of the normal highly
polished and conforming bearing area leading to high stress
concentrations that will cause deformation in conventional
polyethylene, accelerated wear or fracture in hard bearings, and
potential edge cracking in cross-linked polyethylene
•
As with micro-separation conventional polyethylene is very tough and
resilient to adverse conditions

51. Limitations
•
The lubrication theory applied to joint replacements makes many
assumptions
•
The Hamrock and Dowson formula, for instance, was developed for
contacts moving at a constant speed
•
As we walk during less than 10% of our daily activities the bearing
surfaces will come into contact when we stand for a short period with
little motion, hence wear will occur and is unavoidable

52. Limitations
•
However, it is important to consider that in periods completely
without motion of the surfaces there is no wear
•
The aims of tribological theory are not to prevent wear, but to
minimize the interaction of surface asperities when there is the
greatest relative motion occurring between the articulating surfaces
•
Hence, pre-clinical testing of implants has historically been conducted
in joint simulators that reproduce a walking cycle

53. Limitations
•
A further interesting consideration in application of tribological
theory to predict bearing performance is the influence of the wear
debris on the lubrication film
•
Debris particles from hard on hard bearings are generally of a similar
magnitude in size to the lubricant film thickness that is predicted
•
Hence, the debris particles will interfere with the lubricant film
•
It is generally felt in the community that the debris assists in polishing
the surfaces and repairing any scratches than may occur during the
lifetime of the bearing

54. Conclusion
•
Whilst it is impossible to prevent wear in joint replacements, the
theory of bearing lubrication can be applied to the design of joint
replacements to minimize asperity contact and wear
•
The practical application of tribology, is challenging for orthopaedic
implant providers who also have to consider manufacturing and
clinical requirements

55. Conclusion
•
Patient and surgical variables then come into play in vivo to add
further complexity
•
Hence the understanding of tribology and its importance to bearing
performance are vital in order to reach the full potential of our
implants and to ensure success by providing the patient with the
potential for “50 active years after 50”

56. THANK YOU