This document discusses the tribology of total hip replacements (THRs). It covers topics like wear, lubrication, friction, bearing materials, and concerns with different bearings. Regarding bearings, it describes hard-on-soft bearings like metal-on-polyethylene, and hard-on-hard bearings like ceramic-on-ceramic and metal-on-metal. It also discusses factors like head size, noting that large heads can decrease dislocation but increase wear.

1. TRIBOLOGY OF THR
DR. MOHIT PABARI (R3)
DEPARTMENT OF ORTHOPEDICS,
SKH HOSPITAL.

2. TRIBOLOGY-the study of friction, wear, lubrication, and the design of bearings; the science
of interacting surfaces in relative motion.

3. Tribology
◦ Wear – types and mode
◦ Lubrication
◦ Friction
Types of bearing
◦ Polyethlyene
◦ Metal
◦ Ceramic
Concerns with different bearing
Introduction

4. Ideal Bearing
An Articulating Surface That Has
◦ Virtually no wear
◦ Accommodates large head
◦ Debris evokes no host immune response
◦ Exhibits low friction
◦ Generate no noise
◦ Chemically stable in vivo
◦ Tough (resists fracture)
◦ Less susceptible to scratching and 3rd body wear

5. History
Carnochan – first surgeon who thought hip can be replaced – used wooden
blocks (18th century)
Mould arthroplasty – Smith Peterson (1925)
◦ Used glass
◦ Restore congruous articular surfaces
◦ Glass put on Bleeding cancellous bone
◦ Metaplasia of fibrin clot to fibrocartilage
◦ Very poor longevity

6. MOM THR
Philip Wilis performed first MOM THR (1938)
Mckee, Wateson and Farrar (1951) used Thompson type of femoral
component and metallic acetabular cup of cobalt chrome alloy
Muller (1951) introduced 1st generation MOM hip arthroplasty
◦ Quality of metal was poor, hence
◦ Friction metal wear
◦ High incidence of loosening and pain

7. John Charley
Revolutionized Hip Replacement in 1962
Some of his concepts are in practice even today.
He introduced concepts of-
◦ Low friction Torque Arthroplasty
◦ Surgical Alteration of Hip Biomechanics
◦ Lubrication
◦ Material Design
◦ Operating Room Environment
◦ PMMA

8. Low Frictional Arthroplasty
•Stainless steel head on Polythene Cup
•Lateralization of Trochanter
•Medialization of Acetabulum
•22.2 mm Femoral Head-much smaller then which we usually
remove-hence biggest problem was higher incidence of hip
dislocation

10. Tribology
Science of interactive surfaces in relative motion
Study of
◦Friction
◦Lubrication
◦Wear
◦Design of bearing material we use in hip
replacement

11. What is Wear?
It is the removal of material from opposing and moving
surfaces under an applied load
Biggest reason for failure of hip replacement is wear and
aseptic loosening

12. Step 1 : Wear and Osteolysis
Motion between articulating surfaces
Unintended impingement or motion
Leads to formation of Wear Particles

13. Step 2 : Macrophage Activated
Osteoclastogenesis and Osteolysis
Wear partials ->Macrophage activation and recruitment
Osteolysis by osteolytic factors (cytokines)
◦ TNF – alpha , TGF – beta
◦ Oxide radicals, hydrogen peroxide
◦ Acid phosphatase, Prostaglandins
◦ Interlukins (IL – 1, IL – 6)
Increase RANK and RANKL activation
◦ RANKL mediated bone resorption

14. Step 3 : Prosthesis Micromotion
Osteolysis surrounding the prosthesis leading to
micromotion
◦Increases particle wear and further prosthesis loosening

15. In Vivo wear rates
CUP HEAD LINEAR WEAR mm/year
Polyethylene Stainless steel 0.1-0.3
Polyethylene CoCrMo – metal 0.1-0.3
Polyethylene Al2O3 - Ceramic 0.05-0.15
Al2O3 - Ceramic Al2O3 - Ceramic 0.003-0.01

16. Types of wear *
1. Adhesive wear
2. Abrasive wear ( erosive, fretting)
3. Surface fatigue
4. Corrosion and oxidation wear

17. Adhesive wear
•When bonding of microcontacts exceeds inherent strength of either material
•Weaker material is then torn off and adhere to the stronger material

18. Abrasive wear
The two surface materials have different hardness and the harder material cuts
into the softer material
Types
◦ Two body
◦ Three body

19. Surface fatigue
Fatigue wear occurs as a result of repetitive stressing of a
bearing material

20. Modes of wear
Mode 1
◦ Results from motion that occurs between two primary bearing surface
◦ Eg Wear from femoral prosthetic head against acetabular liner

21. Modes of wear
Mode 2
◦ Occurs when a primary bearing surface articulates with a non
bearing surface but is not intended
◦ Eg – prosthetic femoral head penetrating through polyethylene and articulating
with the metallic articulating shell

22. Modes of wear
Mode 3 ( Third – body wear )
◦ Occurs from entrapped abrasive particles between primary bearing surfaces
◦ Cement, bone, polyethylene or metallic particulates

23. Modes of wear
Mode 4
◦ Occurs from motion at two secondary or non bearing surface
◦ Eg –
◦ Impingement of prosthetic femoral neck onto the rim of acetabular
component
◦ Fretting at a Morse taper between the prosthetic femoral neck and head
◦ Backside Wear between acetabular shell and backside of polyethylene line
insert

24. Lubrication
•The thickness of the lubrication film as well as the surface roughness determine
type of lubrication
•Improves longevity of implant

25. Fluid film lubrication

29. Bearing Surfaces

30. Hard on Soft Bearing
Metal on Polyethylene
Ceramic on Polyethylene

31. Polyethylene
Discovered in 1935
Viscoelastic and thermoplastic polymer
Long chains of the monomer ethylene
◦ Low density PE – Density 0.910 to 0.940 gmc3 (high volumetric
wear)
◦ High density PE – Density > 0.941 g/cm
◦ UHMWPE – molecular weight > 3 million g/mol
Mainstay of joint replacement over last 40 years

32. Polyethylene Sterilization
•Historically sterilized by 2.5 mrad of either beam or gamma radiation
•These processes produce free radicals in material, predisposing the
polyethylene to oxidation and rendering it more susceptible to wear.

33. HXLPE
Preparation
◦ Gamma or electron beam radiation – 10mrad
◦ Subsequent annealing at 150 degrees.
Resulting polymer highly resistant to wear and oxidative degradation
Muratoglu et al.
◦ Wear rate not related to the size of femoral head within 22 to
46mm in diameter->so we started using higher diameter head to
reduce risk of dislocation

34. HXLPE
Risk
◦ Fatigue cracking, delamination
◦ Lower fracture toughness and tensile strength
◦ Increase bioactivity of wear particles
◦ Implant fracture when a thin liner is used to accommodate a large diameter
head
Modification of HXLPE
◦ Anti-oxidant diffusion, Mechanical deformation
◦ Low dose irradiation with interspersed annealing
◦ Vitamin E

35. Metal – On - Polyethylene
Benefits
◦ Longest track record of bearing surfaces
◦ Lowest cost
Disadvantages
◦ Higher wear and osteolysis rates compared to MOM and ceramics
◦ Smaller head compared to MOM leads to higher risk of
impingement

36. MOP vs COP
ELDERLY PATIENTS – NO DIFFERENCE
YOUNG ACTIVE PATIENTS – COP BETTER

37. Does Vitamin E PE reduce wear
•Short term results are favourable
•Long term ?

38. Hard on Hard Bearings
Metal on Metal
Ceramic on Ceramic
Ceramic on Metal

39. Metal on Metal

40. Tribology MOM
•Produces lower and smaller wear particles as compared to
MOP bearings (ranging from 2.5 – 5.0 micro / year)
•Has an initial run – in phase of increased wear followed by
steady state wear phase

41. Benefits of metal on metal
Favors larger diameters ( hence lower wear )
Better wear properties than Metal-on-polyethylene
◦ Lower linear wear rate
◦ Decreased volume of particles
Long in vivo experience

42. MOM bearing concerns
Increase metal ion level
ALVAL or pseudotumor
?Carcinogens
Contradictions
◦ Pregnant women
◦ Renal disease
◦ Metal hypersensitivity due to metal ions
◦ Increased rate of revision for MOM

43. Metal on Metal Bearings
The lack of clinical advantage with metal bearings and the
significant downsides to use of MOM mean that this option
should be used in great caution, if at all
◦ The Journal Of Artroplasty Vol 25 No 1 2010 Editorial

44. Ceramics in THR
1st generation (1970)
◦ Alumina pierre boutin
◦ Low density and very coarse microstructure
2nd generation (1980)
◦ Reduced microstructure and grain size
3rd generation (1990)
◦ Improved mechanical strength
◦ Reduced grain size
◦ Prepared by isostatic pressuring
4th generation (2000)
◦ New alumina – Zirconia – Strontium oxide matrix composition

45. Tribology of CoC
Smaller grains
◦ Low surface roughness
◦ Reduced friction
High hardness
◦ Low wear rate
◦ Resistance to scratch
High Wettability
◦ Fluid film lubricartion
Inert

46. Ceramic on Ceramic
Benefits
◦ Best wear properties
◦ Lowest coefficient of friction
◦ Inert particles
Disadvantages
◦ Expensive
◦ Less modularity available
◦ Low fracture toughness
◦ Unique complications

47. Unique complications
Liner fracture
◦ Old generation CoC fracture rate high – 20%
◦ Titanium shell is thinner than ceramic and has only 30% stiffness
of ceramic
◦ On impaction of liner, shell expands
◦ Failure to seat the liner increases chance of chipping
◦ Risk of fracture with newer CoC is approximately 0.02% to 0.1%
◦ - Hamilton WG, McAukey JP 2010

48. Any unique complication?
Stripe wear
◦ Caused by contact between femoral head and rim of the cup
during partial subluxation
Recurrent dislocations of incidental contact of femoral head with
metallic shell can cause ‘ led pencil like markings ‘ that lead to
increased femoral head roughness and polyethylene wear rates

49. Unique Complication - Squeaking
Cause ?
◦ Localised ‘striped wear’
◦ Changes of fluid film lubrication conditions
◦ Femoral head micro – separation
◦ Neck rim impingement
◦ Clinically Often Minor And Revision Indicated Only Occasionally

50. Unique Complications – Difficulty During
Revision
•Total synovectomy needed
•Liner exchange may not be sufficient as removal may
damage the shell or taper

51. Unique Complications – Less chance to
customize
•CoC systems have only one head size per cup diameter
•No lateralized liners, elevate rims etc
•Equalizing leg length and maximizing stability is more challenging
Loss of all these options may currently be the most substantial
disadvantage of CoC THA

52. Oxinium (oxidised zirconium)
•It is expected to provide wear resistance of ceramic without
its fracture risk
•Oxidized layer harder and more scratch resistant than
untreated alloy
•However, long term clinical results are yet to prove its
advantages

53. Head Size

54. Large diameter head
Advantage
◦ Better range of motion
◦ Decrease dislocation
◦ Less impingement
Disadvantage
◦ More volumetric wear
◦ Psoas impingement
◦ Wear debris generation at trunnion – bone interface
◦ Increase frictional torque and stress over component – bone interface

55. Conclusion on Head Size
Hard on Soft bearing
◦ Limit be 32/36mm as wear increases with head diameter
Hard on Hard bearings
◦ Head diameter should be limited to < 40mm
◦ Wear not influenced by head diameter
◦ But larger heads found – greater noise

56. PE tear With LDH
UHMWPE
◦ Wear / Fatigue / Osteolysis
◦ PE Liner < 6mm -- more cup failure
◦ Minimum thickness of poly – 9mm
XLPE
◦ Increased head size doesn’t significantly increase wear of HPE

57. Fixation options

58. Primary THR
Cemented Stem
◦ Collar
◦ Surface finish
◦ Polished
◦ Matte
◦ Grit blasted

59. Cemented Stem – Advantages
•Better fixation ( especially useful in poor bone )
•Proximal seal from debris better
•Less risk of intraoperative fracture
•Revision easier than fully coated non cemented
•Less expensive
•Can alter femoral version
•Less stress shielding
•Less thigh pain
•Local antibiotic delivery

60. Cemented stems - Disadvantages
•Increased operative time
•Technique development
•Greater embolization of fat and marrow elements

61. Primary THA – Femoral Fixation
Uncemented stem
◦Proximal coated
◦Extensively coated
Hydroxyapatite

62. Most common uncemented stem shapes

63. Cementless stem - Advantages
•Technically easier (v/s cemented) in most cases
•Excellent fixation durability
•Avoids cement mess and ‘THE WAIT’ for cement heading
•More amenable to small incisions
•More familiar to most as experience with cement decreases.

64. Cementless stems - Disadvantages
•Stiffness
•Stress Shielding
•Greatest if already osteoporotic
•Marked bone loss possible if removal required
•Thigh plane ( likely mechanical mismatch )
•Intraoperative fracture – Risk management vs cemented
•Fibrous fixation – Possible->Pain and early failure

65. Cementless Stems
Tapered femoral stems represent gold standard for
cementless stems
•Durability
•Less tight pain
•Less stress shielding
•Ease of insertion
•HA Unecessary

66. Implant selection – femur (Dorr)

67. Cemented or uncemented
Cemented versus uncemented fixation through
systematic review and meta-analysis concluded that
cemented THR is similar if not superior to
uncemented THR

68. Cemented stems - indications

69. Implant selection - Acetabulum
•Primary stable fixation necessary
•Cemented / Uncemented

70. Continuum cup (Zimmer)
Trabecular metal coated
Acetabular shells high primary stability

71. Delta Motion cups
•Monobloc cementless metal shell with preassembled
ceramic liner.
•Allow larger heads in small diameter acetabulum
•Permits increased range of motion and stability
•High risk of squeaking > 60mm cup with 48mm heads

72. Dual mobility cups / constrained liners
•May be used to reduce risk of dislocation in patients with risk of instability
•Rate of polyethylene wear in these cups not yet clear, should be examined further.