TRIBOLOGY AND WEAR
Friction | Lubrication | Wear Mechanisms | Osteolysis
Wear Mechanisms in Arthroplasty
Critical Must-Knows
- Tribology = study of friction, lubrication, and wear at interacting surfaces under load
- Native cartilage friction coefficient 0.02 (lowest in nature) via boundary and fluid film lubrication
- Polyethylene wear particles 0.1-10 microns activate macrophages causing osteolysis
- Linear wear rate modern XLPE under 0.05mm/year (conventional PE 0.1-0.2mm/year)
- Third-body wear from PMMA, metal, or bone debris significantly accelerates PE wear
Examiner's Pearls
- "Stribeck curve describes friction vs lubrication: boundary, mixed, fluid film regimes
- "Highly crosslinked polyethylene (XLPE) reduces wear 90% vs conventional PE
- "Critical particle size for osteolysis: 0.1-10 microns (phagocytosable by macrophages)
- "Cup inclination greater than 45° increases edge loading and wear (Lewinnek safe zone 30-50°)
Critical Tribology Exam Points
Wear Particle Osteolysis
Particles 0.1-10 microns activate macrophages. Release TNF-alpha, IL-1, IL-6 causing periprosthetic bone loss. Most common cause of aseptic loosening.
Lubrication Regimes
Boundary lubrication: Surface contact, friction 0.1-0.3. Fluid film: No contact, friction under 0.01. Arthroplasty operates in mixed regime (0.05-0.15).
XLPE Wear Reduction
90% reduction in volumetric wear vs conventional PE. Achieved via gamma/e-beam irradiation (50-100 kGy) creating crosslinks. Trade-off: reduced fracture toughness.
Third-Body Wear
PMMA, metal, or bone debris accelerates PE wear 10-100x. Acts as abrasive particles trapped between bearing surfaces. Prevent with meticulous lavage.
At a Glance
Tribology is the study of friction, lubrication, and wear at interacting surfaces under load. Native cartilage has the lowest friction coefficient in nature (0.02) via combined boundary and fluid film lubrication, while arthroplasty bearings operate in the mixed regime (0.05-0.15). The three primary wear mechanisms are adhesive (material transfer), abrasive (third-body particles), and fatigue (cyclic delamination). Polyethylene wear particles 0.1-10 microns are phagocytosed by macrophages, releasing cytokines (TNF-α, IL-1) that cause particle-induced osteolysis—the most common cause of aseptic loosening. Highly crosslinked polyethylene (XLPE) reduces volumetric wear by 90% compared to conventional PE.
AAFThree Primary Wear Mechanisms
Memory Hook:AAF keeps implants failing: Adhesive transfer, Abrasive particles, Fatigue cracking!
STAMPSFactors Increasing Polyethylene Wear
Memory Hook:STAMPS accelerate wear: Sterilization, Third-body, Activity, Malpositioning, Particles, Surface roughness!
Overview and Introduction
Tribology is the science of interacting surfaces in relative motion under load. In orthopaedics, understanding tribology is essential for joint replacement design, bearing surface selection, and predicting implant longevity. Wear particle generation leads to osteolysis, the most common cause of aseptic loosening.
Concepts and Principles
Key Tribological Concepts:
- Friction Coefficient: Resistance to motion (cartilage 0.02, arthroplasty 0.05-0.15)
- Lubrication Regimes: Boundary, mixed, and fluid film (Stribeck curve)
- Wear Mechanisms: Adhesive, abrasive, and fatigue
- Osteolysis: Wear particles 0.1-10 microns activate macrophages causing bone loss
Fundamental Tribology Concepts
Definition and Scope
Tribology is the science of interacting surfaces in relative motion under load. It encompasses:
- Friction: Resistance to motion between surfaces
- Lubrication: Fluid or boundary layer reducing friction
- Wear: Progressive material loss from surface
Native articular cartilage achieves a friction coefficient of 0.02, the lowest in nature. This is due to:
- Hyaluronic acid boundary lubrication
- Fluid film formation under load (weeping lubrication)
- Biphasic material properties (water-collagen matrix)
Arthroplasty bearings cannot replicate this, operating at 0.05-0.15 friction coefficient.
Stribeck Curve and Lubrication Regimes
The Stribeck curve describes friction as a function of speed, viscosity, and load.
Lubrication Regimes
| Regime | Friction Coefficient | Characteristics | Implant Example |
|---|---|---|---|
| Boundary lubrication | 0.1-0.3 | Surface contact, molecular film, high wear | Start-up, edge loading |
| Mixed lubrication | 0.05-0.15 | Partial surface contact, some fluid film | Most THA/TKA bearings during gait |
| Fluid film lubrication | Under 0.01 | No surface contact, full fluid separation | Native cartilage, ideal bearing |
Clinical implication: Most arthroplasty bearings operate in mixed lubrication during normal gait. Boundary lubrication occurs at start-up or with edge loading (malpositioned components), increasing wear.
Wear Mechanisms
Adhesive Wear
Adhesive wear occurs when asperities (microscopic peaks) on one surface bond to the opposite surface and material transfers.
Mechanism
- Contact: Surface asperities cold-weld under pressure
- Shear: Relative motion breaks bonds, transfers material
- Result: Material from one surface adheres to other
- Example: Metal transfer to polyethylene creates polished appearance
Clinical Manifestation
- Polishing: Smooth, shiny PE surface
- Burnishing: Metal transfer layers on PE
- Scratching: Transferred metal particles scratch PE
- Prevention: Smooth, polished femoral heads (Ra under 0.05 microns)
Abrasive Wear
Abrasive wear occurs when hard particles plough through a softer surface, removing material.
Two-Body vs Three-Body Abrasive Wear
| Type | Mechanism | Particles | Prevention |
|---|---|---|---|
| Two-body abrasive | Hard surface (femoral head) ploughs soft (PE) | Surface asperities or embedded particles | Polished femoral heads, avoid scratches |
| Three-body abrasive | Free particles trapped between surfaces | PMMA, metal debris, bone fragments | Meticulous lavage, avoid PMMA on bearing |
Third-body wear is the most clinically significant. Sources of third-body particles:
- PMMA cement: Hardness 100-200 MPa (harder than PE)
- Metal debris: From taper junctions, screws, instrumentation
- Bone fragments: Entrapped during impaction or reaming
Third-body particles accelerate wear 10-100 fold by acting as abrasives.
Fatigue Wear
Fatigue wear results from cyclic loading causing subsurface crack initiation and propagation.
Subsurface Fatigue
- Mechanism: Cyclic stress concentrates below surface
- Initiation: Microcracks form at stress concentration
- Propagation: Cracks grow with continued cycling
- Delamination: Surface layer separates, creating large debris
Clinical Features
- Pitting: Small craters on PE surface
- Delamination: Sheet-like PE debris
- Cracking: Surface or subsurface cracks
- Risk factors: Thin PE (under 6mm), high stress, gamma sterilization in air
Historical problem: Conventional PE sterilized with gamma radiation in air developed oxidation, reducing fatigue resistance. Modern XLPE or gas sterilization prevents oxidation.
Polyethylene Wear and Osteolysis
Wear Particle Size and Biological Response
Not all wear particles cause osteolysis. The critical size range is 0.1-10 microns.
Biological cascade:
- Phagocytosis: Macrophages ingest particles 0.1-10 microns
- Activation: Frustrated phagocytosis (cannot digest PE)
- Cytokine release: TNF-alpha, IL-1, IL-6, prostaglandins
- Osteoclast activation: RANKL pathway stimulated
- Bone resorption: Periprosthetic osteolysis and aseptic loosening
Volumetric Wear Threshold for Osteolysis
Clinical osteolysis risk increases above 0.1-0.2mm linear wear per year. Conventional PE: 0.1-0.2mm/year (high risk). XLPE: under 0.05mm/year (low risk). This is why XLPE has dramatically reduced osteolysis rates.
Highly Crosslinked Polyethylene (XLPE)
XLPE achieves 90% wear reduction compared to conventional PE.
Manufacturing Process
- Irradiation: Gamma or e-beam radiation (50-100 kGy)
- Crosslinking: Creates covalent bonds between PE chains
- Remelting: Thermal treatment removes free radicals (prevents oxidation)
- Result: Highly crosslinked network (wear resistant)
Trade-Offs
- Advantages: 90% wear reduction, less osteolysis
- Disadvantages: Reduced fracture toughness, potential for rim fracture
- Thickness: Minimum 6-8mm to avoid fatigue failure
- Follow-up: 15+ year data now available, excellent survivorship
Conventional PE vs XLPE Performance
| Property | Conventional PE | XLPE | Clinical Impact |
|---|---|---|---|
| Linear wear rate | 0.1-0.2 mm/year | Under 0.05 mm/year | XLPE: 90% reduction in wear |
| Osteolysis rate (15 years) | 10-30% | Under 5% | XLPE: dramatic reduction in osteolysis |
| Fracture toughness | Higher (less crosslinking) | Lower (trade-off) | XLPE: requires minimum 6-8mm thickness |
Factors Affecting Wear
Component Positioning
Cup inclination and anteversion significantly affect wear.
Lewinnek Safe Zone
- Inclination: 30-50° (40° ideal)
- Anteversion: 5-25° (15° ideal)
- Rationale: Minimizes edge loading and impingement
- Outside zone: Increased wear, dislocation risk
Edge Loading Consequences
- Mechanism: Cup inclination over 45° causes edge contact
- Result: High contact stress at rim, accelerated wear
- Stripe wear: Visible linear wear pattern on PE liner
- Failure: Rim fracture, excessive wear, osteolysis
Head Size Effects
Larger femoral head sizes have competing effects on wear:
Small vs Large Femoral Heads
| Head Size | Advantages | Disadvantages | Modern Practice |
|---|---|---|---|
| Small (28mm or under) | Lower volumetric wear (less linear distance per cycle) | Higher dislocation risk, lower ROM, higher linear wear | Historical, rarely used |
| Medium (32-36mm) | Balanced wear and stability, most common | Moderate volumetric wear | Standard in most THA (32-36mm) |
| Large (over 40mm) | Lower dislocation (higher head:neck ratio), greater ROM | Higher volumetric wear, thinner PE (fatigue risk) | XLPE enables large heads safely |
Modern trend: With XLPE, larger heads (36-40mm) provide stability without prohibitive wear. Conventional PE limited to 28-32mm heads.
Surface Finish
Femoral head surface roughness critically affects adhesive wear.
Scratched femoral heads dramatically increase PE wear. Causes:
- Intraoperative handling (metal instruments)
- PMMA contact during cementation
- Metal-on-metal taper debris transfer
Prevention: Protect femoral head from scratches, never place on metal tray, avoid PMMA contact.
Surface Structure and Topography
Femoral Head Surface
Surface Requirements:
- Roughness (Ra): less than 0.05 microns for CoCr
- High polish minimizes adhesive wear
- Scratches increase wear exponentially
- Ceramic: Ra less than 0.02 microns (smoother than metal)
Polyethylene Liner
Structure:
- Conventional: gamma sterilized, oxidation prone
- XLPE: crosslinked network, oxidation resistant
- Vitamin E: antioxidant for free radical scavenging
- Minimum thickness: 6-8mm (avoid fatigue failure)
Bearing Couples
Modern Options:
- Metal-on-XLPE: Most common, excellent track record
- Ceramic-on-ceramic: Lowest wear, squeaking risk
- Ceramic-on-XLPE: Combination of benefits
- Metal-on-metal: Abandoned (ARMD concerns)
Taper Junctions
Trunnion Tribology:
- Head-neck junction undergoes fretting/corrosion
- Ti trunnion with CoCr head: galvanic corrosion risk
- Matched materials or ceramic heads preferred
- Tribocorrosion = combined mechanical + electrochemical wear
Classification
Wear Mechanism Classification
Wear Types Summary
| Type | Mechanism | Clinical Example |
|---|---|---|
| Adhesive | Material transfer between surfaces | Head polishing, metal transfer to PE |
| Abrasive (Two-body) | Hard surface scratches soft surface | Scratched head on PE liner |
| Abrasive (Three-body) | Free particles trapped between surfaces | PMMA/bone debris wear |
| Fatigue | Cyclic loading causes subsurface cracks | PE delamination, pitting |
| Corrosive | Electrochemical degradation | Taper corrosion, fretting corrosion |
Clinical Applications
Revision for Osteolysis and Wear
Indications for revision:
- Progressive osteolysis: Expanding lucencies, impending fracture
- Linear wear over 2mm: Increased osteolysis risk
- Symptomatic: Pain, instability, loosening
Surgical Principles
- Remove all PE debris: Thorough debridement of granulation tissue
- Bone graft: Fill osteolytic defects (allograft or autograft)
- XLPE liner: Replace conventional PE with XLPE
- Head exchange: Replace scratched or worn femoral head
Expectation
- Osteolysis arrest: Removal of particles stops progression
- Bone regeneration: Grafted defects incorporate over 6-12 months
- Wear reduction: XLPE reduces future wear 90%
- Durability: Revised with XLPE has excellent 10-15 year survivorship
Investigations
Wear Assessment
Radiographic Measurement:
- Serial radiographs: Measure femoral head penetration
- Linear wear: Head center migration into liner
- Volumetric wear: Calculated from linear wear + head size
- Osteolysis: Expanding lucencies, scalloping
Laboratory Testing:
- Metal ion levels (Co, Cr): For metal-on-metal concerns
- Serum cobalt greater than 7 ppb = concern
- MARS MRI: Metal artifact reduction sequences for soft tissue
Management
Wear Reduction Strategies
Primary Prevention:
- Use XLPE (90% wear reduction)
- Optimal component positioning (Lewinnek zone)
- Smooth femoral head (Ra less than 0.05 microns)
- Avoid third-body debris (lavage, protect head)
Surveillance:
- Serial radiographs (annual initially, then 2-yearly)
- Monitor for osteolysis
- Measure head penetration
Surgical Technique
Intraoperative Wear Prevention
Cup Positioning:
- Inclination: 40° (range 30-50°)
- Anteversion: 15° (range 5-25°)
- Navigation/robotics improve accuracy
- Avoid edge loading (high inclination)
Head Handling:
- Never touch articulating surface with metal instruments
- Use soft liner trays, never metal surface
- Avoid PMMA contact during cementation
- Inspect for scratches before final reduction
Complications
Wear-Related Complications
Osteolysis:
- Progressive bone loss around implant
- May lead to loosening, periprosthetic fracture
- Treatment: Revision with XLPE, bone grafting
Aseptic Loosening:
- Most common cause of THA revision
- End-stage of wear-induced osteolysis
- Pain, instability, radiographic loosening
Postoperative Care
Surveillance for Wear
Standard Follow-up:
- 6 weeks, 1 year, then every 2-5 years
- Serial AP pelvis radiographs
- Compare head position over time
- Watch for osteolysis (expanding lucencies)
Activity Advice:
- Low-impact activities preferred
- Avoid high-impact sports (increases wear)
- Weight management (reduces load cycles)
Outcomes
Bearing Outcomes
XLPE Performance:
- Linear wear: less than 0.05mm/year
- Osteolysis: less than 5% at 15 years
- Excellent survivorship
Ceramic-on-Ceramic:
- Near-zero wear
- 4.8% revision at 10 years (registry data)
- Squeaking: 1-8% (usually benign)
Evidence Base
XLPE Wear Reduction in THA
- Randomized trial: XLPE vs conventional PE in THA
- XLPE steady-state wear rate: 0.004 mm/year (90% reduction)
- Conventional PE: 0.05 mm/year (historical control)
- No differences in osteolysis at 5-year follow-up (too early)
Wear Particle-Induced Osteolysis Mechanism
- Particles 0.1-10 microns activate macrophages (phagocytosable range)
- Frustrated phagocytosis releases TNF-alpha, IL-1, IL-6
- Cytokines stimulate osteoclast differentiation via RANKL
- Volumetric wear threshold for osteolysis: approx 40-50 mm³/year
Ceramic-on-Ceramic Bearing Longevity
- 10-year follow-up: ceramic-on-ceramic revision rate 4.8% vs 5.1% for metal-on-XLPE
- No statistical difference in survivorship between modern bearings
- Ceramic squeaking occurs in 1-8% (usually benign)
- Alumina matrix composite (BIOLOX delta) reduces fracture risk vs pure alumina
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Wear Mechanisms and Osteolysis
"Examiner shows X-ray of THA with periprosthetic osteolysis and asks: Explain the biological mechanism of polyethylene wear particle-induced osteolysis."
Scenario 2: Tribology and Lubrication Regimes
"Examiner asks: Describe the lubrication regimes in total joint arthroplasty and how they relate to wear. What is the Stribeck curve?"
MCQ Practice Points
Critical Particle Size Question
Q: What is the critical particle size range for polyethylene wear-induced osteolysis? A: 0.1-10 microns - This is the phagocytosable range for macrophages. Smaller particles (under 0.1 microns) are cleared without activation. Larger particles (over 10 microns) cannot be phagocytosed.
XLPE Wear Reduction Question
Q: By what percentage does highly crosslinked polyethylene (XLPE) reduce wear compared to conventional polyethylene? A: 90% - XLPE achieves approximately 90% reduction in volumetric wear through increased crosslinking from high-dose radiation (50-100 kGy). Steady-state wear rate is under 0.05mm/year vs 0.1-0.2mm/year for conventional PE.
Lubrication Regime Question
Q: What is the friction coefficient of native articular cartilage and what lubrication regime does it represent? A: 0.02 (fluid film lubrication) - Native cartilage has the lowest friction in nature due to hyaluronic acid boundary lubrication and fluid film formation. Arthroplasty bearings operate at 0.05-0.15 (mixed lubrication).
Australian Context
AOANJRR Data
Registry Evidence:
- XLPE dominant bearing in Australian THA
- Ceramic-on-ceramic: Excellent outcomes
- Metal-on-metal: High revision rates, abandoned
- Osteolysis rates dramatically reduced with XLPE
Practice Patterns
Australian Trends:
- XLPE standard for all THA
- Ceramic heads increasingly used
- 36mm head most common size
- Dual mobility for instability risk
Metal-on-Metal Issues
Australian Experience:
- ASR recall led to significant litigation
- TGA increased implant surveillance
- Annual metal ion monitoring if MoM
- Many patients revised to MoP or CoC
Exam Relevance
Exam Points:
- Know XLPE wear reduction (90%)
- Particle size for osteolysis (0.1-10μm)
- Lubrication regimes (Stribeck curve)
- Third-body wear prevention
Clinical Pearl
Exam Viva Point - Australian Practice: AOANJRR data strongly supports XLPE as standard bearing. Ceramic-on-ceramic provides lowest wear but squeaking occurs in 1-8%. Metal-on-metal has been abandoned in Australia due to ARMD and high revision rates. Know that osteolysis requires particles 0.1-10 microns (phagocytosable range) and XLPE reduces this by 90%.
TRIBOLOGY AND WEAR
High-Yield Exam Summary
Wear Mechanisms
- •Adhesive: material transfer (polishing, scratches)
- •Abrasive: hard particles plough soft (third-body PMMA/metal debris)
- •Fatigue: cyclic loading causes delamination (PE pitting)
- •Third-body wear accelerates PE wear 10-100x
Lubrication Regimes
- •Boundary: friction 0.1-0.3 (surface contact, high wear)
- •Mixed: friction 0.05-0.15 (most THA/TKA during gait)
- •Fluid film: friction under 0.01 (no contact, ideal)
- •Native cartilage: friction 0.02 (lowest in nature)
Osteolysis
- •Critical particle size: 0.1-10 microns (phagocytosable)
- •Frustrated phagocytosis releases TNF-alpha, IL-1, IL-6
- •RANKL pathway activates osteoclasts
- •Volumetric wear threshold: 0.1-0.2mm/year linear wear
XLPE Benefits
- •90% wear reduction vs conventional PE
- •Irradiation: 50-100 kGy gamma or e-beam
- •Steady-state wear: under 0.05mm/year
- •Trade-off: reduced fracture toughness (minimum 6-8mm thickness)
Positioning Effects
- •Lewinnek safe zone: 30-50° inclination, 5-25° anteversion
- •Cup inclination over 45° causes edge loading
- •Edge loading increases wear and rim fracture risk
- •Ideal: 40° inclination, 15° anteversion
Wear Prevention
- •Use XLPE (90% wear reduction)
- •Optimal cup positioning (avoid edge loading)
- •Polished femoral head (Ra under 0.05 microns)
- •Prevent third-body debris (lavage, avoid PMMA on bearing)