Wear Mechanisms in Orthopaedic Implants

Adhesive Wear

Definition: Wear resulting from adhesion (cold welding) between asperities of two surfaces, followed by material transfer.

Mechanism:

1. High local pressure at asperity contact points
2. Local adhesion (micro-welding) between surfaces
3. Relative motion shears the junction
4. Material transfers from weaker to stronger surface
5. Transferred material may detach as debris

Factors:

• Surface finish quality
• Lubrication (synovial fluid)
• Material hardness mismatch
• Contact pressure

Clinical Examples:

• Metal-on-metal bearings (historic)
• Poorly lubricated interfaces
• Run-in wear in new implants

Prevention:

• Good surface finish
• Adequate lubrication
• Material selection (CoCr or ceramic heads)

Abrasive Wear

Definition: Wear caused by hard particles or asperities cutting or ploughing through a softer surface.

Two Types:

• Two-body: Rough hard surface scratches smooth soft surface
• Three-body: Hard particles trapped between surfaces

Mechanism:

1. Hard material contacts soft material
2. Cutting, ploughing, or scratching occurs
3. Material removed as wear debris
4. Surface roughening accelerates further wear

Clinical Examples:

• Scratched CoCr head on polyethylene
• Cement particles between bearing surfaces
• Bone fragments in joint space

Prevention:

• Excellent surface finish on hard surfaces
• Avoid intraoperative scratching
• Remove loose bodies and debris

Fatigue Wear (Delamination)

Definition: Wear resulting from cyclic subsurface stress causing crack initiation and propagation.

Mechanism:

1. Cyclic contact stress creates subsurface shear
2. Cracks initiate at peak shear stress zone
3. Cracks propagate parallel to surface
4. Surface layer delaminates as sheet-like debris
5. Characteristic of gamma-sterilized PE

Historic Problem:

• Gamma irradiation in air caused oxidation
• Oxidation degraded polyethylene
• Subsurface white band formation
• Delamination and catastrophic wear

Modern Solution:

• Cross-linking by irradiation
• Remelting or annealing to remove free radicals
• Vitamin E stabilization
• Inert packaging (nitrogen, vacuum)

Third-Body Wear

Definition: Accelerated wear caused by hard particles trapped between bearing surfaces.

Sources of Third-Body Particles:

• Cement fragments
• Bone particles (impingement)
• Metal debris (fretting corrosion)
• Hydroxyapatite particles
• Surgical debris

Mechanism:

1. Hard particles enter joint space
2. Particles trapped between bearing surfaces
3. Act as abrasive agents
4. Scratch and damage both surfaces
5. Generate additional wear debris

Clinical Implications:

• Accelerates polyethylene wear 3-10x
• Can damage ceramic or metal surfaces
• Creates cascade of debris generation

Prevention:

• Thorough lavage before closure
• Avoid cement extra-vasation
• Proper component positioning (avoid impingement)
• Manage trunnion corrosion

Articulating Surfaces

Hip Arthroplasty:

• Femoral head (CoCr, ceramic, or oxinium)
• Acetabular liner (PE, ceramic, or metal)
• Modular junction (head-neck trunnion)

Knee Arthroplasty:

• Femoral component (CoCr)
• Tibial insert (polyethylene)
• Patellofemoral articulation

Material Properties

Surface Characteristics:

• Surface roughness (Ra value)
• Hardness (harder scratches softer)
• Wettability affects lubrication

Trunnion (Head-Neck Junction):

• Site of fretting corrosion
• Potential third-body debris source
• Taper design affects stability

Wear Mechanism Types (AAFT)

Four Primary Mechanisms:

Sub-Classifications

Abrasive Wear:

• Two-body: Hard surface scratches soft
• Three-body: Particles between surfaces

By Material:

• Polyethylene wear (most common concern)
• Metal ion release (MoM, trunnion)
• Ceramic particles (fracture)

Particle-Induced Osteolysis

Pathophysiology:

1. Wear particles generated at bearing surfaces
2. Macrophages attempt to phagocytose particles
3. Particles too large or resistant to digestion
4. Frustrated phagocytosis activates macrophages
5. Release of pro-inflammatory cytokines (IL-1, TNF-α, IL-6)
6. Cytokines stimulate osteoclast differentiation (RANK-RANKL)
7. Osteoclastic bone resorption around implant
8. Progressive osteolysis leads to loosening

Particle Characteristics:

• Size 0.1-1μm most biologically active
• Larger particles less inflammatory (cannot be phagocytosed)
• Smaller particles - billions generated, cumulative effect
• Material: PE > metal > ceramic (in terms of volume)

Clinical Manifestations:

• Often asymptomatic until advanced
• Radiolucent lesions on radiographs
• Progressive loosening
• Pathological periprosthetic fracture

Bearing Material Considerations

Highly Cross-Linked Polyethylene

Manufacturing:

• Gamma or electron beam irradiation (50-100 kGy)
• Creates cross-links between polymer chains
• Remelting (above melt temperature) or annealing (below) to eliminate free radicals
• Vitamin E addition as alternative antioxidant

Benefits:

• Wear reduction 50-90% vs conventional PE
• Dramatically reduced osteolysis
• Proven long-term outcomes (greater than 15 years)

Concerns:

• Reduced mechanical properties (toughness, fatigue)
• Potential for rim fracture in thin liners
• Oxidation if not properly processed

Clinical Application:

• Standard of care for hip arthroplasty
• Increasingly used in knee arthroplasty
• Caution with thin liners or constrained designs

Assessment of Wear

Radiographic Evaluation:

• Serial radiographs for PE wear measurement
• Measure from femoral head center to acetabular rim
• Compare to baseline post-op films

Key Findings:

• Linear wear rate (mm/year)
• Eccentric head position
• Osteolytic lesions (radiolucent areas)

Advanced Assessment

Wear Thresholds:

• Historic: Greater than 2 mm/year = high osteolysis risk
• Modern HXLPE: 0.01-0.05 mm/year

Metal Ion Testing:

• Cobalt and chromium levels
• Indicated for MoM or suspected trunnion corrosion
• Greater than 7 ppb suggests adverse reaction

Management Approach

Surveillance:

• Asymptomatic wear: Serial monitoring
• Annual or biannual radiographs
• Assess for progressive osteolysis

Intervention Thresholds:

• Symptomatic loosening
• Progressive osteolysis
• High wear rate with HXLPE available

Surgical Decision Making

When to Revise:

• Symptomatic with radiographic changes
• Progressive osteolysis threatening bone stock
• Large lytic lesions (greater than 2 cm)

Bearing Selection at Revision:

• HXLPE for most revisions
• Ceramic-on-ceramic in young patients
• Address trunnion if corroded

Revision Approach

Options:

• Isolated liner exchange: Well-fixed cup, adequate bone
• Cup revision: Loose cup or insufficient bone
• Stem revision: If loose or corroded trunnion

Liner Exchange:

• Remove liner, debride membrane
• Bone graft osteolytic lesions
• Insert new HXLPE liner

Technical Considerations

Osteolysis Management:

• Curette all membrane (inflammatory tissue)
• Bone graft contained defects
• Augment with structural graft if needed

Trunnion Corrosion:

• Consider stem revision if corroded
• Titanium sleeve adapters available
• Match new head to clean trunnion

Wear-Related Complications

Biological:

• Particle-induced osteolysis
• Aseptic loosening
• Periprosthetic fracture

Material-Specific:

• Polyethylene: Osteolysis, loosening
• Metal: ALVAL, pseudotumor, metallosis
• Ceramic: Fracture, squeaking

Prevention Strategies

Surgical:

• Proper component positioning
• Avoid intraoperative scratching
• Thorough lavage before closure

Material Selection:

• HXLPE standard for most patients
• Ceramic consideration in young patients
• Avoid MoM and large metal heads

After Revision Surgery

Immediate:

• Standard THA precautions
• Protected weight-bearing if bone grafted
• VTE prophylaxis

Follow-up:

• Serial radiographs to assess bone healing
• Monitor for osteolysis resolution
• Long-term surveillance

Long-Term Surveillance

Monitoring Protocol:

• Annual radiographs
• Assess for new osteolysis
• Clinical assessment for symptoms

Registry Participation:

• AOANJRR data submission
• Outcomes tracking
• Early warning for implant issues

Modern Bearing Outcomes

HXLPE Results:

• 95-98% survivorship at 10-15 years
• Dramatic reduction in osteolysis
• Lower revision rates than conventional PE

Ceramic-on-Ceramic:

• 97-99% survivorship at 10 years
• Lowest wear rates
• Rare fracture/squeaking

Evidence for Modern Bearings

AOANJRR Data:

• HXLPE lower revision rates than conventional
• Ceramic has lowest revision for wear
• MoM has highest revision rate (abandoned)

Long-Term Studies:

• 15+ year HXLPE data now available
• Sustained low wear rates
• No late mechanical failures