Bearing Surfaces in Total Hip Arthroplasty

HistoryInterpositional arthroplasty

1912- Sir Robert Jones- Gold foil

Mould arthroplasty 1923-Smith Peterson

Restore congruous articular surfacesBleeding cancellous bone of femoral head

and acetabulum Metaplasia of fibrin clot to fibrocartilage

Glass

Pyrex – viscalloid - celluloid derivative

Bakelite

Fragility and foreign body reaction

1937-Venable and Stuck- Vitallium- results encouraging

Aufranc

Judet brothers- heat curved acrylic femoral head prosthesis-fragmentation of acrylic with wear- severe tissue reaction-bone destruction

Thompson and Moore – metallic endoprosthesis with medullary stems- erosion of bone on pelvic side

Metal on metal Urist, Ring and McKee Farrar

Friction metal wear

High incidence of loosening and pain.

Sir John Charnley -Low friction torque arthroplasty-Surgical alteration of hip biomechanics-Lubrication-Material design-Operating room environment-PMMA

1 st Charnley prosthesis Moore prosthesis stainless steel femoral

component

Thin polytetrafluroethylene shell for acetabulum

Tribology Surfaces interacting under an applied load

and in relative motion

Study of: -Friction -Lubrication -Wear

Wear Types of wear:

1.Adhesive wear2.Abrasive wear3.Third body wear4.Fatigue wear

Adhesive wear Adhesion during contact of opposing bearing

surfacesSliding breaks these contactsStrength of adhesion exceeds strength of

materialParticles are pulled from the material

Abrasive wear

Hard projection on one surface cuts into the opposing surface

Third body wear

Hard particles such as bone or PMMA if trapped between bearing surfaces cause abrasive damage

Fatigue wear

Repetitive loading of the bearings during articulation

Wear modes

Conditions under which the prosthesis was functioning when the wear occurred

Mode 1

Motion of 2 primary bearing surfaces against each other

Mode 2

Primary bearing surface moving against a secondary surface that was not intended to come into contact with the first

Mode 3

Contaminant particles directly abrade one or both of the primary bearing surfaces

Third body abrasion or wear

Mode 4 2 secondary surfaces rubbing together

Backside wear

Wear debris causes osteolysis Compromise fixation

Complicate revision procedure.

Wear threshold value

0.1 mm/year

Threshold is modified by

-Intracapsular pressure-Bone interface access-Patient reaction to debris

Types of bearings Hard

Soft

Femoral Head material Metallic alloys that can be used with

UHMWPE liner:-Stainless steel 316L-Cobalt chromium alloy-Titanium alloys

Ceramics with UHMWPE Alumina

Zirconia

Conventional UHMWPE Ram extrusion

Compression molding

Sterilization Ethylene oxide Gas plasma Gamma radiation in air (2.5 to 4 mrad)Gamma radiation in inert atmosphere

(nitrogen, argon or vacuum)

Gamma radiationCross-linking of polyethylene molecules

Interaction of free radicals formed during irradiation

Improved wear resistance

Highly cross-linked UHMWPE Higher doses of radiation

Heat

Remelting: Heating above the melting range of polyethylene

Annealing: Heating below the melting range

UHMWPE- Semi crystalline polymer Mechanical behavior - crystalline morphology

Benefits of cross linked polyethylene -High wear resistance-No toxicity-Relatively low cost-Multiple liner options (elevated rim etc)

Risks of cross-linked polyethylene -Reduction in other material properties -gross

material failure

-Increased bioactivity of wear particles

Metal on metal bearings Muller and Weber Wear depends on:1.Type of cobalt chromium alloy2.Surface finish3.Bearing clearance4.Sphericity

Fluid film lubrication 1. Bearing size2. Clearance3. Sphericity4. Surface finish

Benefits of Metal on metal Very high wear resistance

Favors larger diameters (lowers wear)

Long in vivo experience

Risks of Metal on metal Increased ion levels

Delayed type hypersensitivity

Carcinogenesis

Ceramic on ceramic bearings Alumina:1.Hardness2.High wear resistance3.Chemical inertness

Benefits of ceramic on ceramicHighest wear resistance

No toxicity

Long in vivo experience

Risks of ceramic on ceramic

1.Position sensitivity2.Liner chipping3.Fracture risk

Material properties

CoC MoM MoHCLUHMWPE

HardnessMPa

2300 350 Low

# reported no +remelted

TribologyCoC MoM HCLUHM

WPE

Wear 1 25 100

Particle size

<0.02and >0.2

0.05 0.4

Metal ion Not increased

increased Not increased

Biologic effectsCoC MoM MoHCLUHWPE

Cell toxicity

No Yes No

Local tissue reaction

Low Low Low

Chromo. changes

NR R NR

Hypersensitivity

NR R NR

Carcin. NR * NR

Coc MoM MoHCLUHMWPE

Squeaking + + -

Clicking + + -

Seizing - + -

Future

*IDEAL*

Next generation polyethylenes 1. Sequential irradiation and annealing 2. Irradiation and solid state deformation by

extrusion below melting temperature3. Vit E antioxidant containing polyethylene

Metal on metal Increase in femoral head size –greater sliding

velocity and increased probability of fluid film lubrication – decrease in wear

Reduced risk of impingement and dislocation 32 mm and bigger

Ceramic on metal Alumina head Cobalt chromium alloy cup Reduced wear rate 100 foldClinical studies underway

Ceramics Alumina – Zirconia combination: for

increasing toughness of alumina75 % alumina 24 % Zirconia 1 % chromium

oxide-         Greater bending strength-         Lower wear rate-         Added manufacturing complexity-         Cost

Take home message There is no 100 % ideal bearing surface Hard/hard and hard/soft Tribology –friction, lubrication and wear Wear causes osteolysis

Modern bearings: - all have low wear

Young and active patients Hard/hard bearings will produce less wear debris but noise remains a problem

For hard on hard bearings cup placement is important to reduce risk of impingement, excessive wear and fracture.

Metal on metal bearings release metal ions and corrosion products and probably should not be used for patients with impaired kidney function or women of child bearing age.