High Yield Overview

HIGHLY CROSS-LINKED POLYETHYLENE IN THA

Material science and bearing selection | Intermediate

ArthroplastySubspecialty

70-90%Wear Reduction

0.03mm/yrLinear Wear

96%+10yr Survival

Critical Must-Knows

HXLPE is the STANDARD bearing for most primary THA in Australia (AOANJRR)
Cross-linking by irradiation (50-100 kGy) reduces wear 70-90% vs conventional PE
Oxidation is managed by annealing/remelting OR vitamin E stabilisation
Ceramic head on HXLPE produces lowest wear in soft-on-hard bearings

Examiner's Pearls

"
IRRADIATION DOSE: 50-100 kGy for cross-linking. Creates free radicals that must be managed
"
PROCESSING: 1st Gen = remelting (reduced fracture toughness), 2nd Gen = annealing (better properties), 3rd Gen = Vitamin E (best properties)
"
MINIMUM POLY THICKNESS: 6mm (ideally 8mm) to avoid contact stress and accelerated wear
"
WEAR RATES: HXLPE 0.03-0.05 mm/yr vs conventional PE 0.1-0.2 mm/yr
"
HEAD SIZE: 32-36mm typical. Larger heads improve stability but increase volumetric wear and reduce PE thickness

Evolution of Polyethylene in THA

Generation	Processing	Advantages	Disadvantages
Conventional UHMWPE	Gamma sterilised in air	Established track record	High wear (0.1-0.2mm/yr), osteolysis
1st Gen HXLPE	High-dose irradiation + remelting	70-90% wear reduction	Reduced fracture toughness
2nd Gen HXLPE	High-dose irradiation + annealing	Better mechanical properties	Some residual free radicals
3rd Gen HXLPE	Vitamin E stabilisation	Maintains mechanical properties, low oxidation	Limited long-term data

Cross-Linking Mechanism

High-energy irradiation (gamma or electron beam) breaks C-H bonds
Creates free radicals on polymer chains
Free radicals recombine to form C-C cross-links between chains
Cross-linked network resists adhesive and abrasive wear

Irradiation Dose Effects

Higher dose = more cross-links = lower wear
But also = more free radicals = oxidation risk
Optimal range: 50-100 kGy (typically 75-100 kGy for HXLPE)
Conventional PE sterilised at 25-40 kGy

Exam Pearl

Examiner Question: "What is the difference in irradiation dose between conventional polyethylene sterilisation and HXLPE cross-linking?"

Model Answer: "Conventional polyethylene is sterilised at 25-40 kGy - sufficient for sterilisation but creates minimal cross-linking. HXLPE uses 50-100 kGy (typically 75-100 kGy) specifically to create extensive C-C cross-links between polymer chains. This higher dose creates a densely cross-linked network that dramatically improves wear resistance - reducing linear wear from 0.1-0.2 mm/year to 0.03-0.05 mm/year (70-90% reduction). However, the higher dose also creates more free radicals which must be managed through post-irradiation processing (remelting, annealing, or vitamin E stabilisation) to prevent oxidative degradation."

Material Science Pitfalls

Oxidised PE failure - free radicals cause embrittlement and delamination if not managed
Confusing dose ranges - cross-linking is 50-100 kGy, NOT the same as sterilisation (25-40 kGy)
Shelf aging - even packaged PE can oxidise over time; check expiry dates
1st Gen HXLPE trade-off - remelting eliminates oxidation but reduces fracture toughness 20-30%

Mnemonic

CROSS

Create links with irradiation (50-100 kGy dose)

Radicals form - must be managed (oxidation risk)

Oxidation prevention: remelt, anneal, or vitamin E

Seventy to ninety percent wear reduction vs conventional

Six to eight mm minimum poly thickness required

Mnemonic

HEADS

Hardness: Ceramic hardest and smoothest (lowest PE wear)

Eighteen percent lower wear with ceramic vs metal heads

Activity level: Higher activity may favour smaller head (less volumetric wear)

Dislocation risk: Larger heads have greater jump distance

Six mm minimum poly thickness - larger heads reduce available thickness

Key Material Science Concepts

Free Radical Management

Irradiation creates free radicals that cause oxidation if not managed - Oxidised PE becomes brittle and delaminates. All HXLPE must undergo post-irradiation processing. EXAM KEY: Know the three methods - remelting (eliminates radicals but reduces toughness), annealing (partial elimination, preserves properties), vitamin E (quenches radicals, preserves properties).

Wear Debris and Osteolysis

Particulate debris activates macrophages causing osteolysis - HXLPE produces fewer but smaller particles. Total particle load reduced by 70-90%. EXAM KEY: Osteolysis threshold approximately 1 billion particles/year. HXLPE typically stays below this threshold, dramatically reducing osteolysis rates (less than 5% at 10 years vs 20-40% conventional).

Fracture Toughness Trade-off

Cross-linking improves wear resistance but reduces fracture toughness - Remelted HXLPE has 20-30% reduced fracture toughness. Clinical significance: Theoretically increased rim fracture risk with thin liners. EXAM KEY: Maintain minimum 6mm (ideally 8mm) poly thickness. Avoid excessive cross-linking of thin liners.

In Vivo Oxidation

Long-term oxidation can occur even in implanted HXLPE - Lipid absorption, cyclic loading, and body fluids can cause in vivo oxidation over decades. EXAM KEY: Vitamin E HXLPE provides ongoing antioxidant protection. Long-term surveillance required for all HXLPE - not immune to failure.

Bearing Surface Selection Algorithm

Patient and Implant Factors

Factor	Consideration	Bearing Recommendation
Age less than 55	High demand, long life expectancy	Ceramic on HXLPE or CoC
Age 55-75	Moderate demand	HXLPE with ceramic or metal head
Age greater than 75	Lower demand, lower activity	HXLPE with metal head acceptable
High activity	Increased wear potential	Ceramic head preferred
Renal impairment	Avoid metal ion accumulation	Ceramic on HXLPE or CoC
Metal sensitivity	Avoid metal debris	Ceramic on HXLPE or CoC
Instability risk	Need larger head	36mm head on HXLPE
Small acetabulum	Limited poly thickness	28mm head, adequate poly check

Component Size Considerations

Shell Size	Maximum Head	Available PE Thickness
48mm	28mm	10mm
50mm	32mm	9mm
52mm	32mm	10mm
54mm	36mm	9mm
56mm	36mm	10mm
58mm	40mm	9mm

Exam Pearl

Critical Rule: NEVER select a head size that results in less than 6mm poly thickness. In small acetabula, use 28mm head to preserve adequate PE.

Liner Options and Selection

Liner Design Options

Liner Type	Design Feature	Indication
Neutral	Standard hemisphere	Default for stable reconstruction
Elevated rim (10°)	10° lip extension	Mild instability risk, posterior approach
Elevated rim (20°)	20° lip extension	Higher instability risk
Lateralised	Increased offset	Abductor tensioning, leg length
Constrained	Capture mechanism	Neuromuscular disease, recurrent dislocation

Liner Positioning

For elevated lip liners:

Posterior approach: Lip positioned posteroinferiorly
Anterior approach: Lip positioned posterosuperiorly
Can use clock-face positioning (e.g., 7 o'clock for posterior approach)

Exam Pearl

Elevated Lip Trade-off: Increases stability in one direction but may cause impingement and dislocation in opposite direction. Position lip to cover most vulnerable arc.

Intraoperative Technique

Liner Insertion

Shell Confirmation
- Confirm shell stable (no toggle)
- Clean shell of blood and debris with pulse lavage
- Inspect locking mechanism
Liner Selection
- Choose appropriate liner type (neutral vs elevated)
- Confirm head size compatibility
- Verify adequate PE thickness
Insertion Technique
- Align liner features with shell (locking tabs, anti-rotation features)
- Insert at correct orientation (elevated lip position)
- Apply firm impaction until audible/tactile click
- Confirm full seating - no gap between liner and shell
Verification
- Visual inspection of seating
- Check locking mechanism engaged
- Test stability by attempting to dislodge liner

Exam Pearl

Examiner Question: "How do you confirm proper liner seating and what is the consequence of incomplete seating?"

Model Answer: "I confirm proper liner seating through a four-step verification: (1) Visual inspection - no visible gap between liner and shell rim circumferentially, (2) Audible/tactile click during impaction indicating locking mechanism engaged, (3) Digital palpation around the liner rim feeling for any step-off or gap, (4) Stability testing - attempting to manually dislodge the liner which should be completely stable. Incomplete seating is a catastrophic error that leads to: micromotion and backside wear generating excessive debris, accelerated PE failure, potential liner dissociation requiring urgent revision, and osteolysis from the wear debris cascade. If there is ANY doubt about seating, I would remove and re-insert the liner after confirming the shell is clean and the liner is the correct size for the shell."

Liner Insertion Errors

Incomplete seating - leaves gap causing micromotion, backside wear, accelerated debris, and dissociation risk
Malpositioned elevated lip - creates impingement in wrong arc, paradoxically increases dislocation risk
Wrong liner size - locking mechanism fails, leading to liner dissociation
Blood/debris in shell - prevents full seating; always lavage shell before liner insertion

Head Selection and Insertion

Material Selection
- Ceramic head preferred for young/active patients
- Metal head acceptable for older/lower demand
- Oxinium for metal sensitivity but renal concerns about ceramic
Size Selection
- 32-36mm standard for most patients
- 28mm for small acetabula (maintain PE thickness)
- Larger heads for instability risk
Neck Length
- Assess leg length and offset with trial
- Standard, +3.5mm, +7mm, -3.5mm options typical
- Final selection after stability testing
Insertion
- Clean and dry taper (moisture causes corrosion)
- Align head on taper
- Single firm impaction (do NOT hammer repeatedly)
- Confirm full seating

Exam Pearl

Examiner Question: "What is the correct technique for head impaction onto the femoral taper?"

Model Answer: "Proper head impaction is critical for preventing trunnionosis (head-taper corrosion). The technique is: (1) Clean the taper - any blood, bone, or debris causes fretting and corrosion, (2) Dry the taper completely - moisture trapped at the interface causes crevice corrosion and accelerated metal ion release, (3) Align the head axially on the taper, (4) Single firm impaction using a head impactor - DO NOT hammer repeatedly as this creates micromotion and damages the taper surface. The 'cold welding' of head to taper occurs with the first impaction; subsequent blows cause fretting damage. After impaction, confirm full seating by attempting gentle axial traction - the head should be completely stable."

Head Insertion Errors

Wet or contaminated taper - trapped moisture causes crevice corrosion and trunnionosis with metal ion release
Multiple impaction attempts - damages taper surface causing fretting, corrosion, and potential head dissociation
Wrong neck length - not checking leg length/offset with trial first; leads to LLD or instability
Ceramic head mishandling - dropping or impacting against metal instruments can cause microfractures

Bearing Surface Complications

Complication	Recognition	Prevention	Management
Polyethylene wear	Gradual migration of head into liner on serial radiographs, eccentric head position	Use HXLPE, ceramic head, appropriate head size, maintain PE thickness greater than 6mm	Monitor if asymptomatic, revise if progressive/symptomatic osteolysis
Osteolysis	Progressive lucency around components on radiographs, often asymptomatic until advanced	HXLPE reduces debris, ceramic head reduces wear, avoid excessive volumetric wear	Revise if symptomatic, bone grafting of defects, component revision if loose
Liner dissociation	Acute hip pain, dislocation, metallic grinding sensation, imaging shows liner out of shell	Confirm full seating, check locking mechanism, use appropriate liner for shell	Revision surgery with liner exchange, check shell for damage, may need shell revision
PE fracture/delamination	Acute pain, locking, catching, loose fragments on imaging	Maintain adequate PE thickness, avoid oxidised PE, proper storage of implants	Liner exchange, remove loose fragments, assess cause
Third body wear	Accelerated PE wear, scratching of head surface visible at revision	Careful cement removal, avoid metallic debris contamination, use ceramic heads	Component exchange, meticulous debridement
Edge loading	Stripe wear on head, accelerated PE wear, often with vertical cup	Optimal cup positioning (45° inclination, 15-25° anteversion), avoid excessive anteversion	Observe if mild, revise cup if malpositioned and symptomatic
Oxidation (shelf aging)	Delamination, white banding in PE, early PE failure	Check implant expiration, proper storage (avoid oxygen, radiation exposure), use vitamin E PE	Liner exchange, report to manufacturer
Head-taper corrosion (trunnionosis)	Elevated serum cobalt/chromium, pain, pseudotumour, metal debris at revision	Ceramic heads have lower corrosion, proper taper assembly (clean, dry, single impaction)	Head exchange to ceramic, debridement, consider stem revision if taper damaged
Dislocation	Acute pain, shortening, external rotation (posterior), patient unable to move leg	Appropriate head size, elevated rim liner, restore offset and length, cup position	Closed reduction, bracing, revise if recurrent (larger head, elevated rim, constrained liner)
Squeaking (with ceramic)	Audible squeak with movement, may not be painful	Avoid vertical cups, maintain lubrication (weight bearing), appropriate cup position	Reassurance if asymptomatic, rarely revision needed, consider liner exchange if severe

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

EXAMINER

"A 52-year-old active male is undergoing primary THA for osteoarthritis. What bearing surface would you choose and why?"

EXCEPTIONAL ANSWER

For this young, active male patient with a long life expectancy, I would choose a ceramic head on highly cross-linked polyethylene (HXLPE) as my preferred bearing combination. **Rationale for HXLPE liner**: HXLPE is the standard of care and most commonly used bearing surface in Australia according to AOANJRR data. Cross-linking by high-dose irradiation (typically 75-100 kGy) reduces wear by 70-90% compared to conventional polyethylene, from 0.1-0.2 mm/year to 0.03-0.05 mm/year. This dramatically reduces the wear debris burden and the incidence of osteolysis - less than 5% at 10 years with HXLPE compared to 20-40% with conventional PE. **Rationale for ceramic head**: Ceramic (alumina or BIOLOX delta) provides the lowest wear rates when articulating with HXLPE due to its extreme hardness and surface smoothness. Studies show approximately 18% lower wear with ceramic versus cobalt-chrome heads. For this young, active patient who may have this bearing for 30+ years, minimising wear is paramount. Ceramic also avoids metal ion generation, which is relevant for a patient who may need revision surgery in the future. **Head size selection**: I would use a 32mm or 36mm head depending on his acetabular size, ensuring at least 6mm (preferably 8mm) of polyethylene thickness. The larger head provides improved stability with greater jump distance (approximately 10mm for 36mm) without excessive volumetric wear concerns in the HXLPE era. **HXLPE type**: I would use a vitamin E stabilised HXLPE if available, as this maintains both excellent wear resistance and mechanical properties without the fracture toughness reduction seen with remelted first-generation HXLPE.

KEY POINTS TO SCORE

Ceramic on HXLPE = lowest wear in soft-on-hard bearings

HXLPE reduces wear 70-90% vs conventional PE (0.03-0.05 vs 0.1-0.2 mm/yr)

Ceramic heads further reduce wear by approximately 18% vs metal heads

Young active patients benefit from lowest-wear bearing couples

32-36mm head typical, maintain minimum 6mm poly thickness

COMMON TRAPS

✗Not considering patient age and activity level in bearing selection

✗Forgetting that HXLPE is standard of care in Australia (not ceramic on ceramic)

✗Not mentioning the trade-off between head size, stability, and PE thickness

✗Not discussing vitamin E vs remelted HXLPE generations

LIKELY FOLLOW-UPS

"What if this patient had chronic kidney disease stage 3? Would this change your bearing selection?"

VIVA SCENARIOStandard

EXAMINER

"Explain the manufacturing process of highly cross-linked polyethylene and the purpose of each step."

EXCEPTIONAL ANSWER

The manufacturing of highly cross-linked polyethylene involves several sequential steps, each with specific purposes: **Step 1: Starting Material - UHMWPE** Ultra-high molecular weight polyethylene (molecular weight greater than 2 million g/mol) is the base material. This is consolidated from powder under heat and pressure to form bar stock or direct-moulded components. **Step 2: Cross-Linking by Irradiation** The UHMWPE is exposed to high-dose ionising radiation - either gamma radiation or electron beam irradiation at doses of 50-100 kGy (compared to 25-40 kGy for sterilisation of conventional PE). This energy breaks carbon-hydrogen bonds on the polymer chains, creating highly reactive free radicals. These free radicals recombine with adjacent polymer chains, forming carbon-carbon covalent cross-links. The cross-linked network dramatically improves resistance to adhesive and abrasive wear by restricting molecular chain mobility. **Step 3: Free Radical Management** The irradiation process leaves residual free radicals that will cause oxidative degradation if not managed. Three approaches exist: *Remelting (1st Generation)*: Heating above the melt temperature (approximately 150°C) allows complete recombination of free radicals, eliminating oxidation risk. However, this destroys crystalline structure, reducing fracture toughness by 20-30%. *Annealing (2nd Generation)*: Heating below melt temperature (120-130°C) preserves crystalline structure while reducing (but not eliminating) free radicals. Better mechanical properties but some residual oxidation risk. *Vitamin E Stabilisation (3rd Generation)*: Alpha-tocopherol (vitamin E) is incorporated as an antioxidant, either by diffusion into finished components or blending before consolidation. This quenches free radicals chemically without thermal processing, maintaining both mechanical properties and oxidation resistance. **Step 4: Machining and Sterilisation** Components are machined to final dimensions and sterilised. Sterilisation is typically by gamma radiation at lower dose (25-40 kGy) in an inert atmosphere (nitrogen or vacuum packaging) to avoid further oxidation.

KEY POINTS TO SCORE

High-dose irradiation (50-100 kGy) creates cross-links between polymer chains

Cross-linking reduces wear by 70-90% but creates free radicals

Free radicals must be managed to prevent oxidation and embrittlement

Three methods: remelting (1st Gen), annealing (2nd Gen), vitamin E (3rd Gen)

Trade-off: remelting eliminates oxidation but reduces fracture toughness

COMMON TRAPS

✗Not knowing the dose range for cross-linking (50-100 kGy) vs sterilisation (25-40 kGy)

✗Confusing remelting (above melt temp) with annealing (below melt temp)

✗Not understanding why free radical management is essential

✗Not knowing the mechanical property trade-offs of each processing method

LIKELY FOLLOW-UPS

"What clinical evidence supports the use of HXLPE over conventional polyethylene?"

VIVA SCENARIOStandard

EXAMINER

"You are reviewing a 48-year-old woman 8 years post-THA who has conventional polyethylene. Her radiograph shows 3mm of linear wear and small areas of osteolysis around the acetabular component. How do you manage this?"

EXCEPTIONAL ANSWER

This patient has significant polyethylene wear (approximately 0.4mm/year, consistent with conventional PE) with early osteolysis. My management approach would be structured and evidence-based. **Immediate Assessment**: I would take a detailed history including pain, function, and activity level. Many patients with radiographic osteolysis are asymptomatic. Examination would assess for signs of loosening (pain with loading) and baseline function. **Investigations**: - Inflammatory markers (ESR, CRP) to exclude low-grade infection - AP and lateral radiographs for comparison with previous films - Consider CT scan to better characterise osteolytic lesions if surgery contemplated **Management Options**: *Option 1: Observation with Enhanced Surveillance* (if asymptomatic, small stable lesions) If the osteolysis is less than 1cm, non-progressive, and the patient is asymptomatic with a stable cup, I would recommend close surveillance with 6-monthly or annual radiographs. The wear rate will continue, so this is a holding strategy. *Option 2: Liner Exchange Only* (if cup well-fixed, lesions accessible) If the acetabular component is well-fixed (no migration, good bone ingrowth on CT) and I can access the osteolytic lesions through the liner, I would consider liner exchange to modern HXLPE with a ceramic head. The osteolytic lesions would be curetted and bone grafted. Success depends on cup fixation and lesion accessibility. *Option 3: Acetabular Revision* (if cup loose or lesions behind cup) If the cup shows evidence of loosening (migration, circumferential lucency) or if the lesions are behind the cup where they cannot be accessed, I would revise the acetabular component. This would involve cup removal, debridement and bone grafting of defects, and reimplantation with a new cup and HXLPE liner. **My Recommendation**: Given she is only 48 years old (long life expectancy), has established wear and early osteolysis, and has conventional PE that will continue to wear, I would recommend revision rather than observation. My preferred approach would be liner exchange if the cup is solidly fixed, or cup revision if there is any concern about fixation. Either way, I would convert her to ceramic head on HXLPE to dramatically reduce future wear. **Counselling Points**: - Osteolysis typically progresses without intervention - Earlier intervention before catastrophic bone loss is preferred - HXLPE will reduce future wear by 70-90% - Revision carries surgical risks but offers long-term solution

KEY POINTS TO SCORE

3mm wear at 8 years = approximately 0.4mm/year (typical for conventional PE)

Osteolysis indicates particle disease - will progress without intervention

Options: observe (small stable lesions), liner exchange (fixed cup), cup revision (loose cup)

Younger patients with established wear warrant earlier intervention

Convert to HXLPE with ceramic head to reduce future wear by 70-90%

COMMON TRAPS

✗Ignoring osteolysis in a young patient and waiting for catastrophic bone loss

✗Not excluding low-grade infection before revision

✗Performing liner exchange when cup is loose

✗Not counselling about progressive nature of osteolysis

LIKELY FOLLOW-UPS

"At revision, you find a well-fixed cup but one osteolytic lesion extends behind the cup into the ilium. How do you manage this?"

References

Australian Orthopaedic Association National Joint Replacement Registry. Annual Report 2023. Adelaide: AOA; 2023.
Kurtz SM, Gawel HA, Patel JD. History and systematic review of wear and osteolysis outcomes for first-generation highly crosslinked polyethylene. Clin Orthop Relat Res. 2011;469(8):2262-2277.
Bragdon CR, Doerner M, Martell J, et al. The 2012 John Charnley Award: Clinical multicenter studies of the wear performance of highly crosslinked remelted polyethylene in THA. Clin Orthop Relat Res. 2013;471(2):393-402.
Oral E, Muratoglu OK. Vitamin E diffused, highly crosslinked UHMWPE: a review. Int Orthop. 2011;35(2):215-223.
Engh CA Jr, Hopper RH Jr, Huynh C, et al. A prospective, randomized study of cross-linked and non-cross-linked polyethylene for total hip arthroplasty at 10-year follow-up. J Arthroplasty. 2012;27(8 Suppl):2-7.
Devane PA, Horne JG, Ashmore A, et al. Highly cross-linked polyethylene reduces wear and revision rates in total hip arthroplasty: a 10-year double-blinded randomized controlled trial. J Bone Joint Surg Am. 2017;99(20):1703-1714.
Glyn-Jones S, Thomas GE, Garfjeld-Roberts P, et al. The John Charnley Award: Highly crosslinked polyethylene in total hip arthroplasty decreases long-term wear: a double-blind randomized trial. Clin Orthop Relat Res. 2015;473(2):432-438.
Muratoglu OK, Bragdon CR, O'Connor DO, et al. A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. J Arthroplasty. 2001;16(2):149-160.
Harris WH. The problem is osteolysis. Clin Orthop Relat Res. 1995;311:46-53.
Callary SA, Solomon LB, Holubowycz OT, et al. Wear of highly crosslinked polyethylene acetabular components: a review of RSA studies. Acta Orthop. 2015;86(2):159-168.

HXLPE in THA - Exam Summary

High-Yield Exam Summary

Cross-Linking Basics

•Irradiation dose: 50-100 kGy (vs 25-40 kGy for conventional PE sterilisation)
•Creates C-C cross-links between polymer chains
•Reduces wear by 70-90% (0.03-0.05 mm/yr vs 0.1-0.2 mm/yr)
•Free radicals created must be managed to prevent oxidation

Processing Generations

•1st Gen: Remelting above 150°C - eliminates radicals, reduces fracture toughness
•2nd Gen: Annealing at 120-130°C - preserves properties, some residual radicals
•3rd Gen: Vitamin E - chemical quenching, best of both worlds

Head Selection

•Ceramic head = lowest wear (18% less than metal)
•Metal head acceptable for lower demand patients
•32-36mm typical head size
•Larger heads improve stability but increase volumetric wear

Critical Numbers

•Minimum PE thickness: 6mm (ideally 8mm)
•HXLPE wear rate: 0.03-0.05 mm/year
•Conventional PE wear: 0.1-0.2 mm/year
•Osteolysis threshold: approximately 1 billion particles/year
•AOANJRR 15-year survival: greater than 93%

Osteolysis

•Conventional PE at 10 years: 20-40% osteolysis
•HXLPE at 10 years: less than 5% osteolysis
•Caused by macrophage activation to particulate debris
•HXLPE produces fewer particles overall

Bearing Couples

•Ceramic on HXLPE = lowest wear in soft-on-hard
•Metal on HXLPE = standard, slightly higher wear
•CoC = lowest overall wear but squeaking/fracture concerns
•HXLPE is most common bearing in Australia (AOANJRR)

Liner Types

•Neutral: Standard for stable reconstruction
•Elevated rim (10-20°): Instability risk, position lip to cover vulnerable arc
•Lateralised: Increased offset
•Constrained: Neuromuscular disease, recurrent dislocation

Generation

Processing

Advantages

Disadvantages

Conventional UHMWPE

Gamma sterilised in air

Established track record

High wear (0.1-0.2mm/yr), osteolysis

1st Gen HXLPE

High-dose irradiation + remelting

70-90% wear reduction

Reduced fracture toughness

2nd Gen HXLPE

High-dose irradiation + annealing

Better mechanical properties

Some residual free radicals

3rd Gen HXLPE

Vitamin E stabilisation

Maintains mechanical properties, low oxidation

Limited long-term data

Factor

Consideration

Bearing Recommendation

Age less than 55

High demand, long life expectancy

Ceramic on HXLPE or CoC

Age 55-75

Moderate demand

HXLPE with ceramic or metal head

Age greater than 75

Lower demand, lower activity

HXLPE with metal head acceptable

High activity

Increased wear potential

Ceramic head preferred

Renal impairment

Avoid metal ion accumulation

Ceramic on HXLPE or CoC

Metal sensitivity

Avoid metal debris

Ceramic on HXLPE or CoC

Instability risk

Need larger head

36mm head on HXLPE

Small acetabulum

Limited poly thickness

28mm head, adequate poly check

Shell Size

Maximum Head

Available PE Thickness

48mm

28mm

10mm

50mm

32mm

9mm

52mm

32mm

10mm

54mm

36mm

9mm

56mm

36mm

10mm

58mm

40mm

9mm

Liner Type

Design Feature

Indication

Neutral

Standard hemisphere

Default for stable reconstruction

Elevated rim (10°)

10° lip extension

Mild instability risk, posterior approach

Elevated rim (20°)

20° lip extension

Higher instability risk

Lateralised

Increased offset

Abductor tensioning, leg length

Constrained

Capture mechanism

Neuromuscular disease, recurrent dislocation

Liner Insertion

Shell Confirmation
- Confirm shell stable (no toggle)
- Clean shell of blood and debris with pulse lavage
- Inspect locking mechanism
Liner Selection
- Choose appropriate liner type (neutral vs elevated)
- Confirm head size compatibility
- Verify adequate PE thickness
Insertion Technique
- Align liner features with shell (locking tabs, anti-rotation features)
- Insert at correct orientation (elevated lip position)
- Apply firm impaction until audible/tactile click
- Confirm full seating - no gap between liner and shell
Verification
- Visual inspection of seating
- Check locking mechanism engaged
- Test stability by attempting to dislodge liner

Exam Pearl

Examiner Question: "How do you confirm proper liner seating and what is the consequence of incomplete seating?"

Liner Insertion Errors

Incomplete seating - leaves gap causing micromotion, backside wear, accelerated debris, and dissociation risk
Malpositioned elevated lip - creates impingement in wrong arc, paradoxically increases dislocation risk
Wrong liner size - locking mechanism fails, leading to liner dissociation
Blood/debris in shell - prevents full seating; always lavage shell before liner insertion

Head Selection and Insertion

Material Selection
- Ceramic head preferred for young/active patients
- Metal head acceptable for older/lower demand
- Oxinium for metal sensitivity but renal concerns about ceramic
Size Selection
- 32-36mm standard for most patients
- 28mm for small acetabula (maintain PE thickness)
- Larger heads for instability risk
Neck Length
- Assess leg length and offset with trial
- Standard, +3.5mm, +7mm, -3.5mm options typical
- Final selection after stability testing
Insertion
- Clean and dry taper (moisture causes corrosion)
- Align head on taper
- Single firm impaction (do NOT hammer repeatedly)
- Confirm full seating

Exam Pearl

Examiner Question: "What is the correct technique for head impaction onto the femoral taper?"

Head Insertion Errors

Wet or contaminated taper - trapped moisture causes crevice corrosion and trunnionosis with metal ion release
Multiple impaction attempts - damages taper surface causing fretting, corrosion, and potential head dissociation
Wrong neck length - not checking leg length/offset with trial first; leads to LLD or instability
Ceramic head mishandling - dropping or impacting against metal instruments can cause microfractures

Complication

Recognition

Prevention

Management

Polyethylene wear

Gradual migration of head into liner on serial radiographs, eccentric head position

Use HXLPE, ceramic head, appropriate head size, maintain PE thickness greater than 6mm

Monitor if asymptomatic, revise if progressive/symptomatic osteolysis

Osteolysis

Progressive lucency around components on radiographs, often asymptomatic until advanced

HXLPE reduces debris, ceramic head reduces wear, avoid excessive volumetric wear

Revise if symptomatic, bone grafting of defects, component revision if loose

Liner dissociation

Acute hip pain, dislocation, metallic grinding sensation, imaging shows liner out of shell

Confirm full seating, check locking mechanism, use appropriate liner for shell

Revision surgery with liner exchange, check shell for damage, may need shell revision

PE fracture/delamination

Acute pain, locking, catching, loose fragments on imaging

Maintain adequate PE thickness, avoid oxidised PE, proper storage of implants

Liner exchange, remove loose fragments, assess cause

Third body wear

Accelerated PE wear, scratching of head surface visible at revision

Careful cement removal, avoid metallic debris contamination, use ceramic heads

Component exchange, meticulous debridement

Edge loading

Stripe wear on head, accelerated PE wear, often with vertical cup

Optimal cup positioning (45° inclination, 15-25° anteversion), avoid excessive anteversion

Observe if mild, revise cup if malpositioned and symptomatic

Oxidation (shelf aging)

Delamination, white banding in PE, early PE failure

Check implant expiration, proper storage (avoid oxygen, radiation exposure), use vitamin E PE

Liner exchange, report to manufacturer

Head-taper corrosion (trunnionosis)

Elevated serum cobalt/chromium, pain, pseudotumour, metal debris at revision

Ceramic heads have lower corrosion, proper taper assembly (clean, dry, single impaction)

Head exchange to ceramic, debridement, consider stem revision if taper damaged

Dislocation

Acute pain, shortening, external rotation (posterior), patient unable to move leg

Appropriate head size, elevated rim liner, restore offset and length, cup position

Closed reduction, bracing, revise if recurrent (larger head, elevated rim, constrained liner)

Squeaking (with ceramic)

Audible squeak with movement, may not be painful

Avoid vertical cups, maintain lubrication (weight bearing), appropriate cup position

Reassurance if asymptomatic, rarely revision needed, consider liner exchange if severe

Property	Remelted	Annealed	Vitamin E
Wear resistance	Excellent	Excellent	Excellent
Fracture toughness	Reduced	Moderate	Preserved
Oxidation resistance	Excellent	Good	Excellent
Fatigue strength	Reduced	Moderate	Preserved

Head	Wear with HXLPE	Advantages	Disadvantages
Ceramic (Alumina/BIOLOX)	Lowest	Hardest, smoothest, no metal ions	Fracture risk (rare), cost
CoCr Metal	Low	Established, cheaper	Metal ion generation, scratching
Oxinium	Very low	Scratch resistant surface	Limited long-term data, cost

Head Size	Stability	Volumetric Wear	PE Thickness	Typical Use
28mm	Lower	Lowest	Most	Small acetabula, young active
32mm	Moderate	Moderate	Moderate	Standard choice
36mm	Higher	Higher	Less	Instability risk, posterior approach
40mm+	Highest	Highest	Least	High dislocation risk only

Property	Remelted	Annealed	Vitamin E
Wear resistance	Excellent	Excellent	Excellent
Fracture toughness	Reduced	Moderate	Preserved
Oxidation resistance	Excellent	Good	Excellent
Fatigue strength	Reduced	Moderate	Preserved

Head	Wear with HXLPE	Advantages	Disadvantages
Ceramic (Alumina/BIOLOX)	Lowest	Hardest, smoothest, no metal ions	Fracture risk (rare), cost
CoCr Metal	Low	Established, cheaper	Metal ion generation, scratching
Oxinium	Very low	Scratch resistant surface	Limited long-term data, cost

Highly Cross-Linked Polyethylene in THA