THA Wear and Osteolysis

Polyethylene wear and osteolysis represent the primary mode of late failure in total hip arthroplasty. Understanding the mechanisms, prevention, and management of particle disease is fundamental to modern arthroplasty practice.

Historical context:

• First-generation THA (Charnley) used conventional ultra-high molecular weight polyethylene (UHMWPE)
• Linear wear rates of 0.1-0.2mm per year were common
• By 10-15 years, significant osteolysis developed in 10-30% of cases
• Revision for osteolysis became the leading indication in many series

Modern evolution:

• Introduction of highly cross-linked polyethylene (HXLPE) in late 1990s
• Wear reduction of over 90% compared to conventional PE
• Dramatic reduction in osteolysis rates (under 5% at 10-15 years)
• HXLPE is now the standard bearing surface for primary THA

Current epidemiology:

• Conventional PE THAs still in situ from 1980s-2000s continue to present with osteolysis
• Surveillance is mandatory - many patients asymptomatic until catastrophic failure
• HXLPE has shifted the failure mode from wear to instability/infection
• Australian data (AOANJRR): Revision for wear/osteolysis has declined significantly since HXLPE adoption

Risk factors for accelerated wear:

• Young, active patients (high activity level)
• Larger femoral heads (greater linear distance traveled)
• Thin polyethylene liners (under 6mm minimum thickness)
• Malpositioned components (impingement, edge loading)
• Third-body debris (PMMA, metal, bone cement fragments)

Polyethylene wear mechanisms:

The generation of polyethylene wear particles occurs through three primary mechanisms:

1. Adhesive wear:

• Molecular bonding between bearing surfaces during contact
• Material transfer from polyethylene to metal/ceramic head
• Continuous loading and unloading creates particle release
• Contributes 30-40% of total wear

2. Abrasive wear:

• Hard surface (femoral head) plows through soft surface (PE liner)
• Creates scratches, grooves, and deformation
• Roughened femoral heads accelerate abrasive wear
• Contributes 40-50% of total wear

3. Third-body wear:

• Interposed particles (PMMA cement, metal debris, bone) act as abrasives
• Dramatically accelerates wear rates (can increase 10-100 fold)
• Common sources: cement from acetabular preparation, metal from impingement
• Contributes 10-30% but highly variable

Particle disease cascade:

Effective joint space concept:

The "effective joint space" describes the pathways by which particles access bone:

• Screw holes in acetabular shells (most common pathway)
• Gaps in press-fit fixation or at bone-implant interface
• Osteotomies (greater trochanter, femoral neck cut)
• Capsular defects allowing synovial fluid migration
• Metaphyseal calcar region in uncemented femoral stems

Osteolysis patterns:

Wear classification:

Polyethylene wear is not formally "classified" but is measured and characterized:

Linear wear measurement:

• Total head penetration into liner measured from radiographs
• Calculated as mm of linear penetration
• Annualized wear rate = total penetration / years in situ
• Conventional PE: 0.1-0.2mm/year typical
• HXLPE: under 0.02mm/year expected

Volumetric wear estimation:

• Calculated from linear wear using geometric formulas
• More accurate representation of total material loss
• Accounts for head size (larger heads travel more distance)
• Typically reported as mm³/year

Wear patterns:

• Concentric: uniform wear around entire liner (normal pattern)
• Eccentric: focal wear in one area (suggests malposition or impingement)
• Accelerated: increasing wear rate over time (suggests third-body or component issues)

Manufacturing process:

HXLPE is created through gamma or electron beam irradiation of conventional UHMWPE:

• Radiation dose: 50-100 kGy (vs under 25 kGy for sterilization)
• Cross-linking: Radiation creates free radicals that form covalent bonds between polymer chains
• Increased cross-link density: Restricts polymer chain mobility
• Result: Dramatically increased wear resistance

Post-irradiation processing:

To eliminate free radicals (which cause oxidative degradation):

First-generation (1990s-early 2000s):

• Remelting at over 150°C
• Eliminated free radicals but reduced crystallinity
• Lower fracture toughness and mechanical properties
• Some oxidation in vivo

Second-generation (mid 2000s-present):

• Annealing below melting temperature (130-150°C)
• Preserves more crystallinity
• Better mechanical properties
• Vitamin E doping (antioxidant stabilization)
• Superior oxidation resistance

Third-generation (2010s-present):

• Sequential irradiation and annealing
• Antioxidant (Vitamin E) blended before cross-linking
• Mechanical properties closer to conventional PE
• Excellent oxidation resistance

Clinical evidence:

Limitations and contraindications:

While HXLPE is standard, there are scenarios where caution is warranted:

Australian context (AOANJRR data):

• HXLPE adoption in Australia over 90% for primary THA
• Revision rates for wear/osteolysis have declined dramatically since 2005
• Metal-on-metal (MoM) largely abandoned due to adverse reactions
• Ceramic-on-ceramic used selectively in young patients

Presentation patterns:

Osteolysis typically presents in one of four ways:

1. Asymptomatic (most common - 70-80%):

• Discovered on routine surveillance radiographs
• Patient has no complaints
• THA functioning well clinically
• Critical to detect at this stage before progression

2. Pain (15-20%):

• Gradual onset groin or thigh pain
• Worse with activity
• May have mechanical symptoms (clunking, instability)
• Often indicates implant loosening or impending failure

3. Instability (5-10%):

• Recurrent dislocations
• PE liner wear can reduce effective head size
• Abductor damage from acetabular osteolysis
• Malposition from component migration

4. Catastrophic failure (under 5%):

• Periprosthetic fracture through osteolytic bone
• Acute presentation with severe pain, inability to weight-bear
• Worst-case scenario - often massive bone loss

Physical examination:

In early osteolysis (asymptomatic):

• Examination is completely normal
• Full range of motion, no pain
• No instability
• Abductor strength intact
• This is why imaging is essential

In advanced osteolysis with loosening:

• Antalgic gait
• Groin or thigh pain with range of motion
• Pain with internal/external rotation (acetabular) or axial loading (femoral)
• Leg length discrepancy (if subsidence)
• Instability signs (Trendelenburg, apprehension)

Laboratory workup:

Rule out infection before attributing symptoms to osteolysis:

• ESR and CRP - should be normal in pure osteolysis
• Joint aspiration if elevated inflammatory markers
• Cell count, differential, cultures
• Alpha-defensin or other synovial biomarkers

Plain radiography (mandatory baseline and annual):

Standard views:

• AP pelvis - bilateral comparison, cup position, acetabular osteolysis
• AP hip - close-up of affected hip
• Lateral hip - frog-leg or cross-table lateral for femoral osteolysis

Radiographic signs of osteolysis:

• Radiolucent lines at bone-implant interface (wider than 2mm progressive)
• Expanding lesions - scalloped, geographic bone loss
• Implant migration - change in position compared to baseline
• Component loosening - radiolucent line over 2mm, migration over 2mm
• Thin or absent polyethylene - eccentric head position indicating wear

Radiographic zones for documentation:

Advanced imaging:

CT scanning:

• Gold standard for quantifying osteolysis extent
• 3D reconstructions show true bone loss volume
• Guides surgical planning (bone graft needs, structural support)
• Reduces metal artifact with modern protocols
• Recommended for: large lesions, surgical planning, unclear plain film findings

MRI:

• Metal artifact reduction sequences (MARS protocol)
• Excellent for soft tissue assessment
• Can detect early osteolysis not visible on plain films
• Useful for: abductor tears, fluid collections, soft tissue masses
• Not routine but valuable in selected cases

Nuclear medicine:

• Bone scan (Tc-99m MDP) - non-specific, shows increased uptake
• White cell scan (In-111 WBC) - differentiates infection from aseptic loosening
• Mainly used when infection is suspected

Wear measurement techniques:

Annual wear rate calculation:

• Measure total linear penetration
• Divide by years since implantation
• Conventional PE: 0.1-0.2mm/year is typical
• HXLPE: under 0.02mm/year expected
• Accelerating wear rate (increasing over time) is concerning

Post-revision protocol:

Key rehabilitation principles:

Weight-bearing protocols:

• Standard revision (no major graft): weight-bearing as tolerated from day 1
• Morselized allograft cavitary defects: weight-bearing as tolerated (graft is impacted)
• Structural allograft segmental defects: toe-touch 6 weeks, progressive to full by 12 weeks
• Pelvic discontinuity repair: toe-touch 8-12 weeks minimum
• Periprosthetic fracture fixation: depends on construct stability, often 6-12 weeks protected

Activity restrictions:

• Hip precautions for 6-12 weeks (posterior approach)
• No high-impact activities indefinitely (running, jumping)
• Low-impact activities encouraged (swimming, cycling, golf)
• Return to work: desk job 4-6 weeks, manual labor 3-6 months

Outcomes by intervention:

Factors affecting prognosis:

Positive prognostic factors:

• Early detection (small lesions, stable components)
• Adequate bone stock or successful grafting
• Well-fixed revision components
• Young age (better healing, longer life expectancy for benefit)
• Absence of comorbidities
• Patient compliance with restrictions

Negative prognostic factors:

• Delayed detection (massive bone loss)
• Pelvic discontinuity or severe defects
• Prior failed revisions
• Infection
• Medical comorbidities (diabetes, smoking, osteoporosis)
• Obesity

Australian context (AOANJRR):

• Revision for wear/osteolysis has declined from 30% of revisions in 1990s to under 10% in 2020s
• This reflects widespread HXLPE adoption
• Metal-on-metal (MoM) had high failure rates due to metallosis and pseudotumor
• Ceramic-on-ceramic has very low wear but squeaking and fracture concerns
• Metal-on-HXLPE is now the standard bearing (over 80% of primary THAs)

AOANJRR (Australian Orthopaedic Association National Joint Replacement Registry) Data:

The AOANJRR provides world-leading registry data on bearing surface performance:

Bearing surface trends:

• Metal-on-HXLPE: over 80% of primary THAs (standard of care)
• Ceramic-on-HXLPE: increasing use in younger patients (approximately 10%)
• Ceramic-on-ceramic: selective use (approximately 5%), concerns about squeaking
• Metal-on-metal: abandoned (under 1% currently, down from 10% peak in 2000s)

Revision rates by bearing surface (AOANJRR 2023):

• Metal-on-HXLPE: lowest revision rates for wear/osteolysis (under 2% at 15 years)
• Ceramic-on-ceramic: very low wear but higher revision for squeaking and fracture
• Metal-on-metal: highest revision rates (15-20% at 10 years) due to adverse reactions
• Conventional PE: historical data shows 10-20% revision for wear at 15 years

Clinical implications:

• HXLPE adoption in Australia has virtually eliminated wear as a major failure mode
• Infection and instability are now the leading causes of THA revision
• Surveillance remains important for early detection of complications

Management considerations:

• Subspecialty arthroplasty referral often appropriate for complex revision with osteolysis
• Major centers have expertise in bone grafting techniques, structural reconstruction, and custom implants
• Availability of allograft bone through tissue banks (state-based systems)
• Multidisciplinary approach involving surgeons, radiologists, and sometimes vascular surgeons for complex cases

Medication and prophylaxis:

• Indomethacin may be used for heterotopic ossification prophylaxis (not PBS-listed for this indication)
• Surgical antibiotic prophylaxis follows eTG guidelines (cefazolin is standard choice)
• DVT prophylaxis with enoxaparin or rivaroxaban as per institutional protocols
• Extended antibiotic prophylaxis not routinely recommended unless specific risk factors

Australian injury epidemiology:

• THA is predominantly for primary osteoarthritis (approximately 90%)
• Post-traumatic arthritis (fracture sequelae) accounts for 5-10%
• Inflammatory arthritis, AVN, dysplasia account for remainder
• Osteolysis was a major problem in 1990s-2000s but has declined with HXLPE