Tribology is defined as the science that deals with the interaction between surfaces in motion and consequences of that interaction,

1. Dr. Ajay Manohar
Junior Resident
Department Of Orthopedics
TRIBOLOGY

2. Introduction
 TRIBOLOGY is defined as the science that deals
with the interaction between surfaces in motion
and consequences of that interaction, i.e.
 Friction
 Lubrication and
 Wear.
 Sir William Hunter described the tribological
features of synovial joints succinctly in 1743:
 “Both are covered with a smooth crust, to prevent
natural abrasion; connected with strong ligaments
to prevent dislocation; and enclosed in a bag that
contains a proper fluid deposited there, for
lubricating the two contiguous surfaces”.

3. FRICTION
 Friction is defined as the resistance to sliding
motion between two bodies in contact. Surface
friction comes from the adherence of one surface
to another or the viscosity of the sheared
lubricant film between the two surfaces.
 Following are the friction types which depend on
the types of motion:
 Static Friction
 Sliding Friction
 Rolling Friction
 Fluid Friction

4. LAWS OF FRICTION
 Frictional force (F) is directly proportional to the
applied load (W). i.e. F= μ x W where μ is the
coefficient of friction.
 F is independent of the apparent area of contact.
 Kinetic (sliding) friction is independent of sliding
velocity (V).
So, friction between two solids is independent of
the materials and is dependent upon:
 The size of the contact zone
 Surface Roughness (Asperites)
 Load or Pressure on surfaces

5. Coefficient Of Friction (μ)
 Frictional force (F) is directly proportional to the
applied load (P). i.e. F= μ x P
 Its a unit less ratio, where F represents the
frictional force experienced by the two contacting
bodies in motion & P represents the normal force
pressing the same two bodies together
 Value ranges from 0 to 1.
 Higher the value , higher the frictional force / the
resistance of the contacting bodies towards
motion

7. Synovial fluid : Physiology &
Mechanics
 Synovial fluid is a dialysate of blood plasma,
without clotting factors, erythrocytes or haemoglobin.
 Synovial fluid is clear, and sometimes yellowish and
viscous, and contains hyaluronate and plasma
proteins.
 Non Newtonian Fluid
 Reduces Friction, provides lubrication, lessen shock,
and supplies nutrients.

8. Synovial fluid mechanics
VISCOSITY:
 Viscosity is the measure of the internal friction of a
fluid; when a layer of fluid moves in relation to another
layer
 The greater the friction, the greater the amount of
force required for this movement, known as SHEAR.
 Shear: the rate of deformation of a fluid when
subjected to a mechanical shearing stress.
 Shear stress: an applied force per unit area needed to
produce deformation in a fluid.
 Highly viscous fluids therefore require more force to
move than less viscous materials.

9.  Isaac Newton defined viscosity mathematically by the
formula:
viscosity = shear stress/shear rate
 where shear stress is the force per unit area required
to produce shearing action (measured in dynes/cm2),
shear rate is a measure of the change in speed at
which the intermediate layers of fluid move with
respect to each other.
 Newton’s law of viscosity states that this ratio is a
constant and so fluids that adhere to this law are
called Newtonian.
 Non-Newtonian fluids do not follow this law, and
therefore their viscosity is not constant and changes
as the shear rate changes.

10.  Viscosity of synovial fluid from normal knee joints
decreases from 500 Pa.s to 0.5 Pa.s as the shear
rate increases from 0.001/s to 1000/s. : a.k.a
SHEAR THINNING
 Enzymatic degradation of synovial fluid in
rheumatoid arthritis leads to loss of non-Newtonian
properties, making the fluid a less effective lubricant.
 In contrast, hyaluronates of synovial fluids from
osteoarthritic joints are not degraded and maintain
their non-Newtonian properties.
 These properties allow synovial fluid to act as an
efficient lubricant as well as allowing nutrition of
articular cartilage.

11.  Key facts about synovial fluid
 1. Non-Newtonian (shear stress is not
proportional to shear rate)
 2.Pseudoplastic (undergoes shear thinning)
 3. Thixotropic (undergoes shear thinning with time
when sheared at a constant rate)

12. LUBRICATION
 Lubrication interposes a material between 2 contacting
surfaces to minimize the interaction between them so
that friction is reduced.
 The effectiveness of a lubricant film depends on the
specific film thickness & its uniformity so that it can
equally keep the surfaces away.
 The Film thickness is dependent on :
i. Lubricant viscosity
ii. Force/pressure applied across the moving surfaces
iii. Relative velocity between the interfacing surfaces
iv. Surface roughness (asperite thickness)

14.  BOUNDARY LUBRICATION : Contact bearing
surfaces are seperated only by lubricant of molecular
thickness.
*Fluid film is depleted
* In human diarthrodial joints, these layers contain
the lubricin that adheres to the articular surface. This
type of lubrication is considered to be most effective
at low loads.
* In boundary lubrication, the load is carried by the
surface asperities rather than by the lubricant.

15.  FLUID FILM LUBRICATION
 surfaces are separated by a fluid film that fully
supports the applied load, preventing contact
between the surfaces.
 The minimum thickness of the fluid film must exceed
the surface roughness of the bearing surfaces in
order to prevent asperity contact.
 The biotribological performance of a joint depends
on the lambda (λ) ratio. This is the ratio of fluid-film
thickness to surface roughness: a ratio of 3
represents fluid-film lubrication, while a ratio of less
than 1 represents boundary lubrication.

16. STIRBECK CURVE
 The relationship of coefficient of friction and oil film
thickness to lubricant viscocity (Z), Eqipment
speed(N), and Equipment load or pressure (P) are
graphically represented by the Stribeck curve.

17.  This implies that high Lubricant Viscosity (Z), high
Equipment Speed (N), and low Equipment Load (P),
will allow the formation of a thick lubricant film and
hence the equipment will encounter little or no
friction.
 Conversely, low Lubricant Viscosity (Z), low
Equipment Speed (N), and high Equipment Load (P),
will create a situation where the fluid film thickness
will be inappropriate and equipment will encounter
high friction

18. TYPES OF FLUID FILM LUBRICATION
I. Hydrodynamic lubrication
 Hydrodynamic lubrication is a form of fluid
lubrication in which a wedge of fluid is created
when non parallel opposing surfaces slide on each
other.
 hence HD lubrication can occur only under high
speeds and low loads.
 EG : during the high-speed non-accelerating rotatory
motion of the femur during the swing phase of gait
cycle.

19. II. Elastohydrodynamic lubrication
 fluid film is maintained at an appropriate thickness
by the elastic deformation of the articular surfaces.
 the elastic cartilage deforms slightly to maintain an
adequate layer of fluid between the opposing joint
surfaces.
 The elastohydrodynamic action can maintain a
fluid film under conditions of heavy loading

20. III. Micro-elastohydrodynamic lubrication
(MEHD)
 (MEHD) lubrication model assumes that the
asperities of articular cartilage are deformed under
high loads.
 This smoothes out the bearing surface and creates a
film thickness of 0.5–1 μm, which is sufficient for
fluid-film lubrication.

21. MIXED LUBRICATION
I. Squeeze-film lubrication
 This occurs when parallel bearing surfaces approach
each other without relative sliding motion, trapping
the joint fluid between the approaching bearing
surfaces.
 As the opposing surfaces move close together, they
squeeze the fluid film out of the area of impending
contact.
 The viscosity of the fluid in the gap between the
surfaces produces pressure and this resulting
pressure created by the fluid’s viscosity keeps the
surface separated.
 suitable for high loads maintained for a short

23. II. Boosted lubrication
 under squeeze-film conditions, the water in the
synovial fluid is pressurized into the cartilage,
leaving behind a more concentrated pool of
hyaluronic acid–protein complex to lubricate the
surfaces.
 pools of concentrated hyaluronate molecules which
are filtered out of the synovial fluid are trapped in the
natural undulations and areas of elastic deformation
on the articular surfaces.

24. III. Weeping lubrication
 synovial fluid is squeezed out of the areas of
cartilage that are under load which forms a fluid film
over the articular surfaces.
 load bearing surfaces are held apart by a film of
lubricant that is maintained under pressure.
 When the load is removed the fluid flows back into
the articular cartilage through osmotic pressure.

26. LUBRICATION MECHANISMS IN
PROSTHETIC JOINTS
I. Metal-on-polyethylene articulations
 The fluid film is too thin in metal on ultra-high-
molecular weight polyethylene (UHMWPE)
articulations, therefore boundary lubrication
predominates

27. II. Hard-on-hard bearing couples
(Metal on Metal; Ceramic on Ceramic and Ceramic
on Metal)
 Mixed lubrication; part of the time there is boundary
lubrication and part of the time HD lubrication
occurs.
 The factors determining the lubrication state:
1.Surface roughness
2.Sphericity
3.Radial Clearance : defined as the difference in the
radius of the head and cup.

28. *If the radius of the head is smaller than the cup, then the
head will contact at the apex of the head or polar region
* Conversely, if the diameter of the head is larger than the
cup
then contact occurs at the equatorial region of the cup; in
this situation fluid cannot ingress and jamming of the head
can occur.
*The ideal situation lies in between these two extremes
giving mid-polar contact. In mid-polar contact, there is an

29. WEAR
 Wear of bearings is a progressive loss of bearing
substance from the material secondary to
mechanical or chemical action.
 4 MODES OF WEAR depending on the mechanical
conditions under which prosthesis is functioning
when wear occurs
 MODE 1 : produced by intended relative motion
between two primary bearing surfaces.
Predominant wear in a newly implanted joint

30.  MODE 2 wear (Subluxation Wear) :
Due to mechanically imperfect moving surfaces
Eg : mechanically loose / unstable implant
 MODE 3 wear (Third Body Abrasion):
Refers to mode 1 wear occuring in the presence of
third body particles that are interposed between
surfaces
 MODE 4 wear
Due to two secondary surfaces (not primarily involved
in relative motion) rubbing together
Eg : wear between implant cement / cement bone
interface
Backside wear, Trunnionosis

31.  TRUNNIONOSIS
 Trunnionosis is defined as wear of the femoral
head-neck interface
 recently been acknowledged as a growing cause of
total hip arthroplasty failure.
 The exact cause of trunnionosis is currently
unknown; however, postulated etiologies include
modular junction wear, corrosion damage, and
metal ion release.

34. TYPES OF WEAR
 3 TYPES : Abrasive ; Adhesive ; Fatigue wear
(Delamination)

35. ABRASIVE WEAR :
 Two-body abrasive wear occurs when a soft material
(e.g. UHMWPE) comes into contact with a
significantly harder material (e.g. metal).
 Third-body abrasive wear (e.g. a stone in one’s
shoes) occurs when extraneous material such as
metallic, ceramic (bearing or coating), bone or
cement particles, or even products of corrosion,
enter the interfacial region.
 Abrasive wear can be minimized by manufacturing a
hard and smooth femoral head and avoiding
extraneous material in the interface.

36. ADHESIVE WEAR :
 occurs when a junction is formed between the two
opposing surfaces as they come into contact.
 The junction is held by intermolecular bonds between
solids, and this force is responsible for friction.
 fragments of the weaker material may be torn off and
adhere to the stronger material.
 Eg : UHMWPE adheres to metal, especially if dry,
leading to shearing of UHMWPE.

37. FATIGUE WEAR (DELAMINATION)
 This is a form of failure that occurs in structures
subjected to dynamic and fluctuating stresses.
 Fatigue life : This is the number of cycles needed to
cause failure at a specified stress level, as taken
from the S–n plot

38.  The endurance limit is defined as the stress level
below which a specimen will withstand cyclic stress
indefinitely without exhibiting fatigue failure. In
practice, this limit is usually arbitrarily set at 10
million cycles.
 Volumetric wear is the volume of material detached
from the softer material as a result of wear and is
expressed in mm3/year.
 Linear wear is the loss of height of the bearing
surface and is expressed in mm/year.

39. CHEMICAL WEAR (CORROSION)
 Corrosion is defined as the unwanted dissolution of a
metal in a solution, resulting in its continued
degradation
 It is the reaction of metal with environment that
results in continuous degradation to oxides,
hydroxides and other compounds.
 The most commonly used metals for implants are
titanium, cobalt-chrome and stainless steel alloys.
These are normally considered to be highly
corrosion-resistant due to the formation of a thin
passive oxide film that spontaneously forms on their
surface and serves as a protective barrier to further
corrosion (PASSIVATION).

40. TYPES OF CORROSION
 Two types : GENERALIZED CORROSION :
uniformly affecting the entire surface of the implant
 LOCALIZED CORROSION : affecting either
seemingly random sites on the surface
Generalised Corrossion : GALVANIC
CORROSION
• This form of corrosion occurs when two dissimilar
metals are electrically coupled together.
 The difference in potential causes electron flow
between the two metals

41. LOCALISED CORROSION
1. PITTING CORROSION
 localized corrosion attack in which small pits or
holes form
 Pitting results in damage to the implant with a
substantial amount of metal ion release
 Human body fluids contain approximately 0.9%
sodium chloride, and so pitting can occur on metal
implants

42. 2. Crevice corrosion
 caused by the formation of a cavity or crevice where
exchange in material with the surrounding bulk
solution is limited
 High concentrations of H+ and Cl– ions have been
found in crevices, which can be particularly
damaging to the passive films on metal implants.
3. Fretting corrosion
 synergistic combination of wear and crevice
corrosion of two materials in contact.
 It results from micro-motion between the two, which
disrupts the protective film of a metal.

43. 4. Stress corrosion (fatigue)
 Metals that are repeatedly deformed and stressed in
a corrosive environment show accelerated corrosion
and fatigue damage.
5. Intergranular corrosion
 Occurs due to granular structure of metals at
boundaries of grains due to included impurities
6. Intragranular (leaching) corrosion
 This is granular corrosion due to electrochemical
difference in the grain patterns

44. 7. Inclusion corrosion
 This occurs due to the inclusion of impurities, cold
welding or metal transfer, e.g. Metal fragments from
a screwdriver.

45. FATE OF WEAR
Consequences of wear particles:
 Synovitis.
 Aseptic osteolysis and loosening.
 Systemic distribution.
 Immune reaction.
 Increased friction of the joint.
 Misalignment of the joint and catastrophic failure.

46. OSTEOLYSIS
 Osteolysis causes Aseptic loosening of the implanted
prosthesis and is the biggest cause of revision
surgery and failure of arthroplasty
 Osteolysis results from resorption of bone as a result
of biological reaction to wear particles

47. RADIOLOGY OF OSTEOLYSIS
 DeLee and Charnley, and Gruen zones of the hip are
distinct regions used in assessment of aseptic loosening
in total hip joint replacements.
 Acetabulum: DeLee and Charnley zones
zone 1: superior 1/3
zone 2: middle 1/3
zone 3: inferior 1/3
 Femur: Gruen zones
AP 1-7
 zone 1: greater trochanter
 zone 4: tip
 zone 7: lesser trochanter
lateral 8-14
 zone 8: anterior-superior
 zone 11: tip
 zone 14: posterior-superior

49. BIOLOGY OF OSTEOLYSIS
 Osteolysis is induced by the migration of wear
particles into periprosthetic space
 Phagocytosis by macrophages
 Stimulates release of proinflammatory cytokines
(Particles in the size range 0.1–10 μm are biologically active,
with those in the size range 0.1–0.5 μm being the most
potent.)
 Mediators released near to bone stimulate bone
resorption by osteoclasts leading to osteolysis,
aseptic loosening and ultimate failure of the
prosthesis

51. WEAR ON DIFFERENT BEARING
COMBINATIONS

52. 1.METAL ON POLY (Highly Crosslinked UHMWPE)
 Most widely acceped bearing couple
 Less wear rates
 Incerased tensile strength
2.CERAMIC ON CERAMIC(Alumina/Zirconia/delta
ceramic/ silicon nitride)
 Ceramics are very strong, stiff, biocompatible and
bioinert and do not corrode.
 High wettability and scratch resistant > so low
friction & wear
 But susceptible to abrasive wear, edge loading and
brittle fracture

53. 3.METAL ON METAL
 Was used in first gen implants
 Good fracture toughness and resist abrasive wear
 Metal ions can be toxic / carcinogenic

55. FACTORS THAT DETERMINE WEAR
1. Patient factors:
 weight (applied load);
 age and activity level (applied rate of load and number of
cycles over time).
2. Implant factors
 – coefficient of friction of materials;
 – roughness (surface finish);
 – toughness (abrasive wear);
 – hardness (scratch resistance, adhesive wear).
 Surface damage
 Presence of third bodies