TRIBOLOGY IN MEDICINEDr. Libin Thomas Manathara

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

Dr. H Peter Jost

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

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.

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.

Squeeze film

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.

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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

Squeeze film

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.

Squeeze film

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.

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Squeeze film

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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.

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Stribeck Curve- Richard Stribeck• The "Stribeck curve" named after Rich

ard 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

Stribeck Curve• The relationship of friction and lubricant film thickness is commonly s

hown 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)

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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

Stribeck Curve

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Stribeck Curve

Stribeck Curve• If the film thickness is sufficiently large compared to the surface roug

hness 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 rou

ghness 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.

Stribeck Curve• If the lubricant film thickness is of a similar magnitude to the combine

d 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

Stribeck Curve

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.

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.

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

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

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The WEAR process

Microscopic Macroscopic

The Wear Process- Delamination• Fatigue wear can also occur on a macroscopic scale in the form of del

amination.• 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.

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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

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

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The Wear Process- Creep• Creep is normal remoulding and is

superomedial. Wear is superolateral and pathologic

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.

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

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.

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The Wear Process• Interestingly, the theoretical wear rates o

f 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.

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.

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

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

From theory to product

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

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

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.

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

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

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

Micro Separation

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

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

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

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

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

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

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”

THANK YOU