Total Hip Arthroplasty Bearing Surfaces

Bearing surface selection is one of the most important decisions in total hip arthroplasty, directly impacting longevity, wear, particle disease, and revision risk. The evolution of bearing surfaces reflects advances in materials science and hard-earned lessons from clinical failures.

Historical evolution:

• 1960s-1990s: Charnley's metal-on-conventional polyethylene (MoP) established the gold standard
• 1970s-1980s: First ceramic bearings introduced (alumina-on-alumina)
• 1990s-2000s: Metal-on-metal (MoM) resurgence for large heads and young patients
• 2000s: Introduction of highly cross-linked polyethylene (HXLPE)
• 2010-2012: MoM catastrophic failure - MHRA alerts, widespread abandonment
• 2010s-present: HXLPE becomes standard, ceramic refinements, dual mobility expansion

Current landscape (2024):

• Metal-on-HXLPE: 70-80% of primary THAs globally (most common)
• Ceramic-on-HXLPE: 10-15% (growing in young patients)
• Ceramic-on-ceramic: 5-10% (selected young patients)
• Dual mobility: 5-10% in primary THA, higher in revision (20-30%)
• Metal-on-metal: Less than 1% (legacy cases, essentially abandoned)

Australian context (AOANJRR data): The AOANJRR provides world-leading registry data on bearing surface performance. Key findings inform Australian practice patterns and exam answers.

Tribology fundamentals:

Tribology is the science of friction, wear, and lubrication. In THA, the bearing surface operates under:

• Loads: 2-8x body weight during gait
• Cycles: 1-2 million cycles per year for active patient
• Lubrication: Synovial fluid (boundary and fluid film lubrication)

Wear mechanisms:

1. Adhesive wear: Material transfer between surfaces
2. Abrasive wear: Hard particles (cement, bone, metal) scratch softer surface
3. Fatigue wear: Cyclic loading causes subsurface crack propagation
4. Corrosion: Electrochemical degradation (especially in MoM)

Material properties critical for bearings:

Head size effects:

• Small heads (22-28mm): Low volumetric wear, high dislocation risk, limited ROM
• Large heads (36-40mm+): Low dislocation risk, high ROM, increased volumetric wear in MoP
• Optimal balance: 32-36mm for most patients with HXLPE

The relationship between head size and wear is complex - linear wear rate may increase slightly but dislocation risk decreases significantly.

Pre-operative assessment for bearing choice:

The selection of bearing surface is a critical decision that should be individualized based on comprehensive patient assessment. This is typically done during pre-operative consultation once THA is indicated.

Patient factors to assess:

History elements:

• Age and life expectancy: Critical for longevity planning
• Activity goals: What activities does patient want to return to?
• Prior joint replacements: Experience with previous bearings
• Allergy history: Metal sensitivity, implant reactions
• Neuromuscular conditions: Parkinson's, stroke, dementia (dual mobility indication)
• Falls risk: Cognitive impairment, balance disorders
• Expectations: What has patient heard about different bearings?

Physical examination considerations:

• BMI: Obesity affects liner thickness calculations
• Hip pathology: Dysplasia, bone loss may affect cup size
• Abductor function: Weakness indicates dual mobility consideration
• Neuromuscular examination: Spasticity, tremor, weakness
• Spinopelvic alignment: Fixed deformities affect stability (dual mobility)

Imaging assessment:

• X-rays: Bone quality, acetabular anatomy, dysplasia
• CT if complex anatomy: For accurate component sizing
• Acetabular dimensions: Estimate cup size for liner thickness calculations

Bearing selection framework:

Step 1: Identify contraindications:

• Metal allergy → Exclude MoP
• High dislocation risk → Dual mobility
• Very young (under 40) → Avoid conventional PE
• MoM → Never use (obsolete)

Step 2: Assess longevity requirements:

• Under 50 years → Ceramic options (CoC or CoP)
• 50-65 years → Ceramic-on-HXLPE or MoP
• Over 65 years → MoP standard (cost-effective)

Step 3: Discuss risks and benefits:

• Ceramic: Squeak risk (1-5%), fracture risk (under 0.1%), best wear
• HXLPE: Proven standard, no special risks, excellent outcomes
• Dual mobility: IPD risk (under 1%), excellent stability

Step 4: Incorporate patient values:

• Cost considerations (ceramic 2-3x more expensive)
• Tolerance for squeak risk
• Activity goals
• Revision aversion (ceramic best longevity for young)

Pre-operative counseling:

• HXLPE standard: "This is the proven standard bearing with excellent long-term results"
• Ceramic options: "Lower wear for your young age, but 1-5% risk of squeaking which is usually benign"
• Dual mobility: "Dramatically reduces your dislocation risk given your risk factors"
• Activity modification: All bearings require avoiding high-impact sports

Documentation:

• Bearing choice and rationale
• Discussion of alternatives
• Patient understanding and agreement
• Special considerations (allergy, dislocation risk)

Clinical assessment for bearing selection is a systematic process balancing patient factors, surgeon experience, and evidence-based outcomes.

The rise and catastrophic fall of metal-on-metal:

Historical rationale (1990s-2000s):

• Large head sizes possible (up to 60mm+) for stability
• Low volumetric wear (self-polishing with use)
• Young active patients with metal-on-metal resurfacing
• Appeared to solve PE particle disease problem

Why MoM failed - the ALVAL disaster:

Failure mechanisms:

1. Metal ion release: Chromium and cobalt ions from tribocorrosion and wear
2. ALVAL (Aseptic Lymphocytic Vasculitis-Associated Lesion):
   • Type IV delayed hypersensitivity reaction
   • Lymphocytic infiltration, tissue necrosis
   • Pseudotumor formation (fluid-filled or solid mass)
3. Soft tissue destruction: Muscles, tendons, nerves destroyed by pseudotumor
4. Metallosis: Metal staining of tissues

Clinical presentation of MoM failure:

• Pseudotumor on MRI or ultrasound
• Pain disproportionate to radiographic findings
• Elevated metal ions: Cobalt and chromium in blood
• Soft tissue mass may be palpable
• Implant may appear well-fixed on X-ray - soft tissue problem

MHRA (UK) and FDA alerts (2010-2012):

• Medical Device Alert issued warning of high failure rates
• Surveillance protocols mandated for all MoM patients
• Many systems recalled or withdrawn from market
• Class action lawsuits (DePuy ASR, Smith & Nephew BHR)

Surveillance of legacy MoM patients (essential to know):

Revision surgery for MoM failure:

• Complete debridement of all necrotic tissue and metallosis
• Ceramic or polyethylene liner (never metal again)
• Large head removal - may need smaller head for adequate liner thickness
• Soft tissue reconstruction may be needed (abductor repair common)
• Outcomes poor if severe tissue destruction

Why MoM matters for exam:

• Demonstrates importance of long-term surveillance and registry data
• Example of biologic response mattering more than wear volume
• Know ALVAL pathophysiology and surveillance protocols
• Legacy patient management - many still have MoM implants

Current status:

• Less than 1% of new THAs (essentially obsolete)
• Used only in rare circumstances (metal allergy with ceramic unavailable)
• Focus now on managing legacy patients with surveillance and revision

Metal-on-metal is a cautionary tale of technology adoption without adequate long-term data.

Dual mobility concept - articulation within articulation:

Design:

• Small femoral head (22-28mm) articulates with polyethylene liner
• Large outer diameter (typically 36-42mm+) articulates with metal shell
• Two articulations: Small inner (primary), large outer (secondary)
• Results in large effective head size for stability

Mechanism of dislocation resistance:

1. Large head-to-neck ratio: Increased ROM before impingement
2. Two centers of rotation: Impingement causes rotation, not dislocation
3. Effective head diameter: 36-44mm equivalent for jump distance
4. Retained ROM: Better than constrained liners

Indications for dual mobility:

• High dislocation risk patients:
  • Revision THA (50-75% dislocation reduction)
  • Prior dislocation
  • Neuromuscular disorders (Parkinson's, stroke, dementia)
  • Abductor deficiency
  • Tumor resection
  • Spinal deformity or fusion
• Primary THA in selected patients:
  • Elderly with cognitive impairment
  • High fall risk
  • Unable to comply with precautions

Bearing surface options in dual mobility:

1. Standard: Cobalt-chrome head on HXLPE liner
2. Ceramic-on-HXLPE: Ceramic head for lower wear
3. All have metal shell outer bearing (shell-liner interface)

Concerns with dual mobility:

1. Intraprosthetic dislocation (IPD):

   • Liner dissociates from head (rare, under 1%)
   • Usually from impingement or liner manufacturing issue
   • Requires open reduction (closed fails)
   • May need revision if recurrent

2. Dual wear surfaces:

   • Inner bearing (head-liner): Low wear with HXLPE
   • Outer bearing (liner-shell): Concerns about wear
   • Total wear slightly higher than single bearing
   • Long-term data reassuring (over 20 years European experience)

3. Metallosis from outer bearing:

   • Early designs had metal-on-metal outer bearing issues
   • Modern designs with retentive rim reduce motion at outer bearing
   • Most wear occurs at inner bearing

Australian registry data (AOANJRR):

• Revision THA: Dual mobility significantly lower dislocation and revision rates
• Primary THA: Growing use, excellent outcomes in high-risk patients
• No increase in aseptic loosening or late complications
• Trend toward increased adoption (currently 5-10% of primary, 20-30% revision)

Surgical technique considerations:

• Avoid impingement (main cause of IPD)
• Ensure snap-fit of liner into head (manufacturer-specific technique)
• Component position standard targets (cup 40 degrees abduction, 15-20 anteversion)
• Can't use constrained liner with dual mobility (different concept)

Future directions:

• Some surgeons advocate routine use in all elderly patients
• Ceramic-on-HXLPE dual mobility may further reduce wear
• Improved outer bearing surfaces under development

Dual mobility is a major advance for instability prevention - know indications and IPD complication.

Wear debris generation:

Particle size and biologic response:

The osteolysis cascade:

1. Wear particles enter periprosthetic tissue through joint capsule, screw holes, thin implant-bone interface
2. Macrophage activation: Particles phagocytosed by macrophages
3. Cytokine release: TNF-alpha, IL-1, IL-6, RANKL
4. Osteoclast activation: RANK-RANKL pathway
5. Bone resorption: Progressive osteolysis
6. Implant loosening: Loss of fixation from bone loss

Factors determining osteolysis risk:

1. Particle volume: Total amount of wear debris (HXLPE dramatically reduces this)
2. Particle size: 0.1-10 micrometers most osteolytic
3. Particle shape: Elongated worse than round
4. Patient biology: Some patients more susceptible (genetic factors)
5. Time: Cumulative exposure (why young patients at highest risk historically)
6. Implant design: Access of particles to bone

HXLPE impact on osteolysis:

• 95% reduction in osteolysis at 10-year follow-up vs conventional PE
• Osteolysis rates now under 5% at 15 years (vs 30-40% historical)
• Transformed THA outcomes in young patients
• Most osteolysis now from other sources: cement, metal debris, backside wear

Backside wear (liner-shell interface):

• Occurs when liner micromotion against metal shell
• Locking mechanism critical
• Diagnosis: Increasing metallosis without obvious bearing wear
• Prevention: Adequate liner locking, avoid thin liners (under 3-4mm backside thickness)

Third-body wear:

• Cement particles, bone chips, metal debris from components
• Scratch polyethylene surface
• Accelerate wear dramatically
• Prevention: Meticulous surgical technique, thorough lavage, avoid cement extrusion

Bearing-specific surgical pearls:

HXLPE liner fracture:

• Risk factors: Thin liners (under 6mm), large heads, obese patients, trauma
• Presentation: Acute pain, instability, metallosis from exposed shell
• Prevention: Calculate liner thickness, minimum 6mm rule
• Treatment: Revision, ensure adequate thickness (may need smaller head)

Ceramic squeaking:

• Mechanism: Microseparation, stripe wear, edge loading from malposition
• Risk factors: Thin patients, high activity, component malposition, smaller cups
• Presentation: Audible squeak with specific movements (sitting to standing, stairs)
• Natural history: Most remain stable, not associated with higher revision
• Management: Reassurance if benign, revision only if painful or patient distress severe

Ceramic fracture (modern ceramics):

• Incidence: Under 0.1% with Biolox Delta
• Causes: Impaction technique error, severe trauma, pre-existing damage
• Presentation: Acute pain, grinding sensation, metallosis (embedded particles)
• Treatment: Urgent revision, complete debridement (metal particles), convert to MoP or CoP
• Prevention: Careful impaction, avoid ceramic damage, patient counseling on activity

ALVAL/Pseudotumor (MoM legacy):

• Pathophysiology: Type IV hypersensitivity to metal ions, tissue destruction
• Presentation: Pain, soft tissue mass, elevated metal ions, abnormal MRI
• Diagnosis: MRI MARS protocol, metal ion levels (Co, Cr)
• Grading: Pseudotumor size and tissue involvement
• Treatment: Revision with complete debridement, bearing change (ceramic or PE), soft tissue reconstruction
• Outcomes: Poor if severe tissue destruction, abductor deficiency common

Intraprosthetic dislocation (dual mobility):

• Mechanism: Liner disengages from femoral head (not traditional dislocation)
• Causes: Impingement, liner design/manufacturing, inadequate snap-fit
• Presentation: Dislocation that cannot be closed reduced (key feature)
• Diagnosis: X-ray shows liner outside head
• Treatment: Open reduction required, assess for impingement, may need revision
• Prevention: Avoid impingement, ensure proper liner seating intraoperatively

Standard bearing surveillance:

MoM-specific surveillance (MHRA 2012 guidelines):

MHRA protocol summary:

1. Annual clinical assessment - pain, function, soft tissue mass
2. Annual metal ion levels - cobalt and chromium
3. MRI if:
   • Symptomatic (pain, mass)
   • Metal ions over 7 ppb
   • ASR or other recalled system
   • Large head (over 36mm)
4. Revision if:
   • Symptomatic with pseudotumor
   • Asymptomatic but large/growing pseudotumor
   • Metal ions persistently over 20 ppb

Activity restrictions by bearing:

Patient education key points:

• HXLPE: Proven standard, excellent longevity, no special restrictions
• Ceramic: Avoid high trauma risk, report squeaking (usually benign)
• Dual mobility: Excellent stability, rare IPD risk
• MoM legacy: Lifetime surveillance required, report any symptoms immediately

Long-term outcomes monitoring:

• Registry data (AOANJRR in Australia) provides population-level outcomes
• Individual patient: Clinical symptoms most important
• Radiographic loosening: Progressive radiolucent lines, migration
• Osteolysis: Expansile lucencies, cortical thinning

Long-term outcomes by bearing surface:

Metal-on-HXLPE (most data):

• 15-year survivorship: 94-96% (AOANJRR data)
• Wear rate: 0.01-0.05mm/year linear
• Osteolysis: Under 5% at 15 years (vs 30-40% conventional PE)
• Excellent outcomes across all age groups
• Most predictable long-term performance

Ceramic-on-HXLPE (emerging data):

• 15-year survivorship: 95-96% (highest of common bearings)
• Wear rate: 30-50% less than metal-on-HXLPE
• Best outcomes in under 55 age group (AOANJRR)
• Growing body of 10-15 year data
• May become future standard

Ceramic-on-Ceramic:

• 15-year survivorship: 92-95%
• Lowest wear rate: 4-5 micrometers/year
• Squeak: 1-5%, most benign (not associated with higher revision)
• Fracture: Under 0.1% with modern ceramics (Biolox Delta)
• Young patients: Best wear performance over decades
• Revision for squeak: Under 1% (rare indication)

Dual Mobility:

• Dislocation reduction: 50-75% vs conventional bearings
• Revision THA: 1-2% dislocation rate (vs 5-15% standard)
• Primary THA high-risk: 1-3% dislocation rate
• IPD rate: Under 1%
• No increase in loosening vs standard bearings
• Excellent outcomes in appropriate indications

Metal-on-Metal (historical - for comparison):

• 10-15 year revision rate: 15-30% (catastrophic failure)
• ALVAL/pseudotumor: 10-30% at 10 years
• Essentially abandoned globally
• Legacy patients require ongoing surveillance

Prognostic factors for bearing longevity:

Age-specific outcomes (AOANJRR data):

Under 55 years:

• Ceramic-on-HXLPE: Lowest revision rate (4-5% at 15 years)
• Metal-on-HXLPE: Acceptable (5-7% at 15 years)
• Longevity critical (may need 40+ year implant life)
• Recommendation: Ceramic options when possible

55-65 years:

• Ceramic-on-HXLPE and Metal-on-HXLPE: Similar excellent outcomes
• Individual factors guide choice (activity, preference, cost)
• Both excellent options

Over 65 years:

• Metal-on-HXLPE: Standard of care, excellent outcomes
• Cost-effective
• Longevity less critical (20-25 year life expectancy)
• Ceramic options reasonable but not necessary for most

Patient satisfaction:

• Overall satisfaction: Over 90% with modern bearings
• Squeak impact: Variable - some patients unbothered, others distressed
• Function: Excellent with all modern bearings
• Pain relief: 90-95% significant improvement
• Return to activities: Most patients achieve desired activity level

Bearing-specific patient-reported outcomes:

• HXLPE: High satisfaction, no bearing-specific concerns
• Ceramic: High satisfaction if no squeak, decreased if squeak present
• Dual mobility: Excellent satisfaction, confidence in stability
• MoM: Low satisfaction due to surveillance burden and complications

Registry data confirms:

• HXLPE revolution transformed THA outcomes
• Ceramic options further improve outcomes in young patients
• Dual mobility solves instability problem
• Modern bearings achieve 95%+ survivorship at 15 years

These excellent outcomes are why THA is considered one of the most successful surgical procedures in medicine.

AOANJRR - world's most comprehensive joint registry:

The Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) tracks over 95% of joint replacements performed in Australia since 1999. This provides Level 1 evidence for bearing performance.

Key AOANJRR findings on bearing surfaces (2023 Annual Report):

Primary THA revision rates at 15 years:

1. Ceramic-on-HXLPE: Lowest revision rate (approximately 4-5%)
2. Metal-on-HXLPE: Second lowest (approximately 5-6%)
3. Ceramic-on-Ceramic: Similar to MoP (5-7%)
4. Metal-on-Metal: Highest revision rate (15-30% at 15 years) - obsolete

Age-specific findings:

• Under 55 years: Ceramic-on-HXLPE and Ceramic-on-Ceramic show lower revision rates than MoP
• 55-65 years: Ceramic-on-HXLPE advantage narrows
• Over 65 years: MoP (HXLPE) excellent outcomes, cost-effective choice

Dual mobility outcomes:

• Revision THA: Significantly lower dislocation rate vs conventional bearings
• Primary THA in high-risk patients: Lower revision for dislocation
• No increase in aseptic loosening or other complications
• Rapidly increasing usage (now 5-10% of primary THAs)

Head size findings:

• Optimal: 32-36mm for most bearings
• Under 28mm: Higher dislocation rate (avoid)
• Over 40mm: No additional benefit, potential increased wear in MoP

Bearing trends in Australia (AOANJRR data):

• HXLPE dominance: Over 90% of primary THAs use HXLPE liner (MoP or CoP)
• Ceramic growth: Ceramic heads increasing (now 15-20% of primaries)
• Dual mobility expansion: Growing from under 1% (2010) to 5-10% (2023)
• MoM abandonment: Essentially zero new implants (under 0.1%)

Current Australian practice patterns (2024):

Bearing distribution in primary THA:

• Metal-on-HXLPE: 70-80% (most common, standard of care)
• Ceramic-on-HXLPE: 10-15% (growing, especially young patients)
• Ceramic-on-Ceramic: 5-10% (selected young, active patients)
• Dual Mobility: 5-10% (high-risk instability patients)
• Metal-on-Metal: Under 0.1% (essentially abandoned)

AOANJRR recommendations:

• HXLPE is standard - conventional PE obsolete
• 32-36mm heads optimal for most patients
• Ceramic options for young patients (under 55) show lower revision rates
• Dual mobility excellent for instability risk in primary and revision
• MoM surveillance continues for legacy patients

Regulatory framework:

• TGA approval required for all bearing surfaces
• MoM surveillance mandated for legacy patients (MHRA 2012 equivalent)
• Registry participation over 95% of Australian THAs
• Informed consent must include bearing-specific risks

Australian epidemiology: THA is one of the most common elective procedures in Australia, with over 30,000 primary procedures performed annually. The average patient age is 65-70 years, with younger patients (under 55) comprising approximately 15% of cases. Age-standardized rates are increasing due to population aging and expanded indications.

Management considerations in Australian practice: