Friction | Lubrication | Wear Mechanisms | Osteolysis
- 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
- “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°)
Particles 0.1-10 microns activate macrophages. Release TNF-alpha, IL-1, IL-6 causing periprosthetic bone loss. Most common cause of aseptic loosening.
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).
90% reduction in volumetric wear vs conventional PE. Achieved via gamma/e-beam irradiation (50-100 kGy) creating crosslinks. Trade-off: reduced fracture toughness.
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
Hook:AAF keeps implants failing: Adhesive transfer, Abrasive particles, Fatigue cracking!
STAMPSFactors Increasing Polyethylene Wear
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.
| 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.
- 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
- 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.
| 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.
- 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
- 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
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.
- 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)
- 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
| 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.
- Inclination: 30-50° (40° ideal)
- Anteversion: 5-25° (15° ideal)
- Rationale: Minimizes edge loading and impingement
- Outside zone: Increased wear, dislocation risk
- 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:
| 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
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)
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)
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)
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
| 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
Bearing Couple Selection (Differential)
Choosing a bearing is a trade-off between wear, fracture/noise risk, and cost. The table contrasts the realistic options a candidate must weigh.
| Bearing | Wear behaviour | Main risks | Best suited to |
|---|---|---|---|
| Metal-on-XLPE | Low (head penetration approximately 0.004 mm/year on RSA) | PE oxidation if poorly stabilised; rim fracture if thin/malpositioned | Workhorse bearing for most patients, all ages |
| Ceramic-on-XLPE | Very low (smoother, scratch-resistant head) | Slightly higher cost; ceramic head fracture rare | Younger/active patients wanting lowest soft-bearing wear |
| Ceramic-on-ceramic | Lowest of all (near-zero) | Squeaking (approximately 7.5%), liner chipping/fracture (under 1%) | Young, very active patients in selected centres |
| Metal-on-metal (large head) | Low volumetric but nanometre particles, high particle number | ARMD, pseudotumour, systemic Co/Cr ions | Abandoned for routine use |
| Conventional metal-on-PE | High (0.1-0.2 mm/year linear wear) | Osteolysis, aseptic loosening | Largely historical; cost-driven settings |
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
- 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
- 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)
Controversies and Areas of Uncertainty
Remelting (above the melt transition) eliminates free radicals and oxidation but reduces mechanical strength; annealing (below melt) preserves strength but leaves residual radicals risking late oxidation. The optimal balance, and the role of vitamin E-doped PE, is still debated.
Textbooks quote the phagocytosable range as 0.1-10 microns, but biological-activity work suggests the most osteolytic particles are 0.2-0.8 microns. Exact human thresholds remain uncertain and are model-dependent.
The Lewinnek safe zone (inclination 30-50 degrees, anteversion 5-25 degrees) is widely taught, but many dislocations occur within it. Spinopelvic mobility and functional (not just static radiographic) component position are increasingly emphasised over a single fixed target.
XLPE permits larger heads for stability, but whether 36 mm and above increases long-term wear or trunnionosis/fretting at the head-neck taper is still being studied. RSA data to five years show no excess penetration with 36 mm.
Evidence Base
XLPE vs Conventional PE Wear: RSA Randomized Trial
- Prospective, randomized, blinded RSA study, 46 active patients, four cohorts (cup material x liner material)
- Steady-state head penetration (1-5 years): XLPE 0.004 mm/year vs conventional UHMWPE 0.04 mm/year
- XLPE penetration significantly lower at five years (penetration approximately one order of magnitude less)
- No significant difference in proximal migration between tantalum and titanium cups at 5 years
Wear Debris Biology: Particle Size Drives Osteolysis
- Authoritative review establishing that it is the concentration of debris within the critical size range, not total wear volume, that determines biological response
- Most biologically active (macrophage-stimulating) UHMWPE particles fall in the 0.2-0.8 micron range
- Frustrated phagocytosis of non-degradable polymer drives macrophage cytokine release (TNF-alpha, IL-1, IL-6)
- Pre-clinical testing of any new bearing must characterise particle size and biological reactivity, not just wear volume
Cross-linked vs Conventional PE: Meta-analysis of RCTs
- Systematic review and meta-analysis of 12 randomized controlled trials comparing cross-linked with conventional PE liners
- All trials showed significantly reduced radiological wear (linear, 3D linear, volumetric, and total) for cross-linked PE
- Pooled risk ratio for radiological osteolysis 0.40 (95% CI 0.27-0.58) favouring cross-linked PE
- Follow-up was insufficient to demonstrate a difference in revision rates
Delta Ceramic-on-Ceramic Midterm Survivorship
- Prospective multicentre study of Delta (alumina matrix composite) ceramic-on-ceramic THA, 345 hips (28 mm and 36 mm)
- Kaplan-Meier survivorship 96.9% at 6 years; 3 post-operative liner fractures (0.9%)
- Squeaking reported by 7.5% of subjects; none required revision, only one reproducible in clinic
- Squeaking significantly more frequent with 36 mm than 28 mm bearings (P=0.013)
Metal-on-Metal: Wear Volume Drives ARMD
- Retrieval study of 85 ASR hips revised for adverse reaction to metal debris (ARMD)
- Total bearing wear volume correlated strongly with whole-blood chromium (rho 0.80) and cobalt (rho 0.84)
- Wear volume correlated with periprosthetic macrophage sheet thickness and tissue necrosis (dose-response)
- Whole-blood metal ion levels are a useful surrogate for bearing wear and local tissue reaction
Vitamin E-doped vs Standard XLPE Liners (RCT)
- Multi-arm RCT (2x2 factorial), 116 patients, RSA head penetration at 5 years
- No significant difference in head penetration between vitamin E-doped PE and standard XLPE (-0.084 mm; 95% CI -0.173 to 0.005)
- No significant difference between 32 mm and 36 mm heads (-0.020 mm; 95% CI -0.110 to 0.071)
- No difference in acetabular component migration or patient-reported outcomes
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“Examiner shows X-ray of THA with periprosthetic osteolysis and asks: Explain the biological mechanism of polyethylene wear particle-induced osteolysis.”
“Examiner asks: Describe the lubrication regimes in total joint arthroplasty and how they relate to wear. What is the Stribeck curve?”
“A 58-year-old man had a large-head metal-on-metal THA six years ago and now presents with new groin pain and a hip effusion. The examiner asks: How would you investigate and manage this patient, and why has metal-on-metal been largely abandoned?”
MCQ Practice Points
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.
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.
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).
Guidelines, Registries & Global Practice
Global epidemiology. Total hip arthroplasty is one of the most successful operations in medicine ("the operation of the century"), with over 1 million procedures performed annually worldwide and demand rising with ageing populations. Historically, wear-induced osteolysis and aseptic loosening were the leading causes of late revision; since the widespread adoption of cross-linked polyethylene from the early 2000s, the proportion of revisions attributable to wear/osteolysis has fallen markedly, and instability, infection and periprosthetic fracture now dominate revision burden in national registries.
| Source / Registry | Position on Bearings | Key Signal |
|---|---|---|
| NJR (England, Wales, NI) | Ceramic-on-XLPE and metal-on-XLPE dominate; large-head MoM abandoned | Stemmed MoM and resurfacing show high cumulative revision; XLPE reduced wear revision |
| AOANJRR (Australia) | XLPE standard; ceramic heads increasingly favoured; MoM withdrawn | Conventional (non-crosslinked) PE has higher revision than XLPE; flagged early |
| AJRR (USA) / Nordic registries (SHAR, Norway) | Ceramic-on-XLPE rising in younger patients; MoM essentially eliminated | Consistent registry signal of XLPE durability across regions |
| Regulators (MHRA / FDA / EMA-aligned) | Risk alerts and recall (e.g. ASR) for large-head MoM; structured MoM follow-up | Whole-blood Co/Cr surveillance, cross-sectional imaging if symptomatic |
Convergent global guidance (AAOS / BOA / NICE / EFORT):
- XLPE is the default hard-on-soft bearing in primary THA
- Avoid large-diameter metal-on-metal bearings; resurfacing only in selected high-volume centres
- Optimise cup orientation to avoid edge loading
- Risk-stratified radiographic surveillance for wear and osteolysis
Where guidance differs by region (MoM follow-up):
- Whole-blood cobalt/chromium with a commonly cited concern threshold around 7 ppb (2-7 ppb thresholds vary by authority)
- Cross-sectional imaging (MARS MRI or ultrasound) for symptoms or rising ions
- Lower revision threshold for symptomatic ARMD / solid pseudotumour
Well-resourced settings:
- XLPE or ceramic-on-XLPE near-universal; vitamin E-doped PE growing
- Robotics/navigation to refine cup position
- RSA in research; routine serial radiographs clinically
Resource-constrained settings:
- Conventional (non-crosslinked) PE still used on cost grounds, accepting higher wear
- Cemented all-polyethylene cups remain cost-effective and durable in older patients
- Reliable serial radiographic follow-up may be limited
Exam viva point - global picture: Across NJR, AOANJRR, AJRR and Nordic registries the message is consistent: cross-linked polyethylene markedly reduced wear-related revision, large-head metal-on-metal has been abandoned for adverse reaction to metal debris, and modern delta ceramic-on-ceramic gives very low wear with squeaking as a usually benign issue. Frame answers around evidence (registry + RCT/meta-analysis) rather than any single country.
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)