Total Hip Arthroplasty Bearing Surfaces

The working conditions. Tribology is the science of friction, wear and lubrication. A hip bearing carries loads of 2-8 times body weight during gait, runs through 1-2 million cycles a year in an active patient, and is lubricated by synovial fluid under boundary and fluid-film lubrication.

Wear mechanisms.

1. Adhesive wear: material transfer between surfaces
2. Abrasive wear: hard particles (cement, bone, metal) scratch the softer surface
3. Fatigue wear: cyclic loading causes subsurface crack propagation
4. Corrosion: electrochemical degradation, especially in MoM

The materials. The bearing materials compare as follows. Note the low fracture toughness of alumina, brittle where polyethylene is ductile, and the improvement in Biolox Delta.

Two measures of wear. Two measures are quoted throughout, and they behave very differently with head size. Linear wear is roughly independent of head diameter, though the linear wear rate may increase slightly with a larger head; volumetric wear is not.

The geometry. For a given linear penetration, volumetric wear is proportional to the square of the femoral head radius, because the swept worn area scales with radius squared. Doubling the head size therefore generates far more debris for the same depth of penetration. Osteolysis is driven by the volume and number of particles, so this single geometric fact explains the central head-size dilemma.

What that meant for head size.

• With conventional polyethylene, large heads produced prohibitive volumetric wear and osteolysis. Historic practice therefore favoured small (22-28mm) heads, with their low volumetric wear, despite their high dislocation risk and limited range of motion.
• With HXLPE, linear wear is so low (roughly 0.01 to 0.05mm per year) that even a 36mm head generates an acceptable debris volume. That is why modern surgery can use 32-36mm heads to gain stability without paying a wear penalty.
• Large heads (36-40mm and above) give a low dislocation risk and a high range of motion, at the price of increased volumetric wear in MoP. For most patients with HXLPE, 32-36mm is the optimal balance.

Always frame head-size selection as a stability-versus-volumetric-wear trade-off.

Jump distance. If volumetric wear is the cost of a larger head, stability is the benefit, and it too is geometric. Jump distance is the distance the head centre must travel to escape the cup, approximately the head radius (about half the diameter) for a hemispherical cup. A larger head must therefore sublux further before it dislocates, which is the primary reason larger heads, and the large effective outer head of a dual-mobility bearing, resist dislocation.

Head-neck ratio. Head diameter divided by neck diameter governs the primary arc of motion, the range of movement before the neck impinges on the rim. A higher ratio allows more impingement-free motion before the neck levers the head out, the secondary, impingement mechanism of dislocation. Larger heads and skirtless, reduced-diameter necks raise the ratio.

The compromise. Dual mobility exploits both mechanisms by creating a very large effective outer head. They are the stability counterpart to the volumetric-wear penalty, and the reason 32-36mm HXLPE heads are the modern compromise.

Trunnionosis, the third consequence. Wear and stability are the two effects of head diameter that get taught. The third is the one that reined in large heads even after metal-on-metal had been abandoned: corrosion at the head-neck taper, called trunnionosis.

Mechanical, then electrochemical. Every modular head sits on a trunnion, and frictional torque at the articulation is transmitted through that junction. Torque rises with head radius, so a larger head applies a greater moment to the taper. Micromotion abrades the passive oxide layer, the exposed metal repassivates in a crevice of stagnant, acidified fluid, and the cycle repeats: mechanically assisted crevice corrosion.

Recognising it. Trunnionosis releases cobalt, chromium and particulate debris from a bearing that may itself be perfectly intact. Suspect it in a painful THA with a large metal head, a raised cobalt disproportionate to chromium (the reverse of the pattern typical of bearing wear), and an effusion or soft-tissue mass on metal-artefact-reduction MRI, with the bearing surfaces looking normal.

Why it changed head selection. It is a substantial part of why practice settled on 32-36mm, and on ceramic heads where a large head is wanted. A ceramic head does not corrode at the taper, which is why ceramic-on-polyethylene has displaced metal-on-polyethylene in younger patients.

What it was. Ultra-high molecular weight polyethylene (UHMWPE), gamma-sterilised in air, was the historical standard from the 1960s to the 2000s. Its linear wear rate was 0.1-0.2mm per year, and its volumetric wear increased with head size.

What went wrong. Free radicals from sterilisation caused oxidative degradation, ageing the polyethylene. Its particles, 0.1-10 micrometres, are highly osteolytic, and osteolysis reached 10-40% at 10-15 years. Revision for wear was the leading cause of late THA failure.

Where it stands. Conventional PE is essentially obsolete in primary THA, although it may still be used in constrained liners for its better mechanical properties. It taught us about particle disease and has been superseded by HXLPE.

How it is made.

1. Irradiation, gamma or electron beam at 50-100 kGy, creates cross-links between the polymer chains
2. Post-irradiation treatment, by one of three methods:
   • Remelting eliminates free radicals and gives the lowest wear; the most common method
   • Annealing, below the melting point, preserves mechanical properties better
   • Vitamin E stabilisation is a newer method that combines the benefits
3. The cross-linked chains resist creep and wear

What it bought. A 90% reduction in wear compared with conventional PE, to 0.01-0.05mm per year linear, and a 95% reduction in osteolysis at 10 or more years. It allows 32-36mm heads without excessive wear, and it has replaced conventional PE in over 90% of THAs. HXLPE is one of the greatest advances in THA: it solved the particle disease problem.

What it cost. Cross-linking lowers ultimate strength and fatigue resistance. The liner therefore has a minimum thickness requirement, 6-8mm, to prevent fracture, and thin liners are unsuitable: small cups, and obese patients with large heads, may be problematic.

Registry performance. On AOANJRR data, MoP with HXLPE has the lowest revision rate of the common bearings in primary THA and over 95% survivorship at 15 years, with 32-36mm heads the optimal balance of wear and stability. There has been no increase in liner fracture with modern HXLPE, despite the historical concerns.

How it works. Alpha-tocopherol (vitamin E) acts as an antioxidant and prevents oxidative degradation without remelting, preserving mechanical properties while reducing wear. The potential advantages are better mechanical properties than remelted HXLPE, maintained oxidation resistance, and theoretically improved crack resistance.

The evidence so far. In vitro wear performance is excellent, but long-term clinical data are limited (under 10 years for most), and the Australian registry does not yet distinguish vitamin E from standard HXLPE. It is available from the major manufacturers, increasingly used in primary THA, and may allow thinner liners safely, which is under investigation. It is the newest evolution and promising, but it needs more time to prove superiority.

When and how. Bearing choice should be individualised on a comprehensive assessment of the patient, typically at the pre-operative consultation once THA is indicated. The factors, and what each implies:

History. Beyond age, activity and allergy, ask about previous joint replacements and the patient's experience of those bearings, and about neuromuscular conditions (Parkinson's, stroke, dementia), each a dual mobility indication. Establish the falls risk from cognitive impairment or balance disorders, the activities the patient wants to return to, and what they have heard about different bearings.

Examination. BMI matters because obesity affects the liner-thickness calculation, and dysplasia or bone loss may affect cup size. Examine for spasticity, tremor and weakness. Abductor weakness indicates considering dual mobility, and fixed spinopelvic deformities affect stability, another dual mobility consideration.

Imaging. Radiographs show bone quality, acetabular anatomy and dysplasia, with CT for accurate component sizing when the anatomy is complex. The acetabular dimensions give an estimate of cup size for the liner-thickness calculation.

Four steps.

1. Identify contraindications. Metal allergy excludes MoP; high dislocation risk means dual mobility; the very young (under 40) should avoid conventional PE; MoM is never used.
2. Assess longevity requirements. Under 50 years, ceramic options (CoC or CoP); 50-65 years, ceramic-on-HXLPE or MoP; over 65 years, MoP as the cost-effective standard.
3. Discuss risks and benefits. Ceramic: squeak risk 1-5%, fracture risk under 0.1%, best wear. HXLPE: the proven standard, no special risks, excellent outcomes. Dual mobility: IPD risk under 1%, excellent stability.
4. Incorporate patient values. Cost (ceramic is 2-3 times as expensive), tolerance for squeak risk, activity goals, and revision aversion, since ceramic offers the best longevity for the young.

Counselling. How each option might be put to the patient:

• HXLPE: "This is the proven standard bearing with excellent long-term results"
• Ceramic: "Lower wear for your young age, but a 1-5% risk of squeaking, which is usually benign"
• Dual mobility: "Dramatically reduces your dislocation risk given your risk factors"

Every bearing requires the patient to avoid high-impact sports. Record the bearing choice and its rationale, the alternatives discussed, the patient's understanding and agreement, and any special considerations such as allergy or dislocation risk.

Four generations. Modern-generation ceramics (alumina matrix composite, Biolox Delta) have a lower fracture risk than older alumina:

• 1st generation (1970s): pure alumina, fracture risk up to 1%
• 2nd generation (1990s): improved alumina, 0.2-0.5%
• 3rd generation (2000s): high-purity alumina (Biolox Forte), under 0.1%
• 4th generation (2010s on): alumina matrix composite (Biolox Delta)

Head and liner are not the same. In National Joint Registry data on 223,362 bearings (PMID 28768777), Delta heads fractured in 0.009%, but Delta liners in 0.126%, no better than Forte's 0.112%. The under-0.1% figure quoted for modern ceramics holds for the head, not the liner.

Biolox Delta. An alumina matrix with zirconia platelets for crack resistance, 20-25% stronger than pure alumina, while maintaining the low wear.

What it offers. CoC has the lowest wear rate of any couple, 4-5 micrometres per year, 10-20 times less than HXLPE. Ceramic particles are less biologically active, so particle disease is minimal, and the hydrophilic surface gives better fluid-film lubrication. Together these give excellent longevity potential, ideal for young patients (under 50).

What it costs. Squeaking in 1-5%, multifactorial; fracture, rare with modern ceramics but catastrophic if it occurs; stripe wear with component malposition or microseparation; a price 2-3 times that of MoP; and a demanding technique, in which impaction is critical and the ceramic must not be damaged. Squeaking and fracture are covered under Complications.

Indications and contraindications. CoC offers the best wear performance but demands perfect technique and patient selection.

• Indicated: young patients (under 50) with high longevity requirements, a high activity level but not extreme impact sports, who accept the squeak risk after counselling
• Contraindicated: high-impact sports (rugby, parachuting, martial arts), significant component malposition anticipated, or a patient unwilling to accept the squeak risk

The rationale. A ceramic head on an HXLPE liner combines ceramic hardness with HXLPE's wear resistance. It has 30-50% less wear than cobalt-chrome on HXLPE: the smoother ceramic surface reduces polyethylene shear stress, and its scratch resistance prevents third-body wear. There is no squeaking risk, and less fracture concern, because only the head is ceramic.

Against CoC and MoP. Compared with CoC it does not squeak, is easier to revise if the head does fracture (very rare), costs less and is more forgiving of minor malposition. Compared with MoP it wears less, is better for metal-allergy concerns and has a smoother surface finish.

Where it is used. Popularity is growing, especially in Europe and Asia. It is the middle ground between MoP and CoC and the choice for young patients who want ceramic benefits without squeak risk, and some surgeons use it as the routine bearing for all primary THAs. On AOANJRR data it has lower revision rates than MoP under 55, a revision rate similar to CoC but without squeak revisions, and a trend towards increased use in primary THA.

The technology. Oxidised zirconium (Oxinium) is a zirconium alloy substrate heated to create a 5-micrometre ceramic (zirconia) surface layer: ceramic surface hardness with metal toughness underneath, marketed as "best of both worlds". In theory it is harder than cobalt-chrome and scratches less, wears less than metal-on-HXLPE, has no fracture risk because the core is metal, and releases fewer metal ions than cobalt-chrome.

The clinical reality. The wear reduction is modest, 20-30% against cobalt-chrome on HXLPE, and ceramic-on-HXLPE performs better. Registry numbers are smaller than for other bearings and the market share is small (under 5%). Some surgeons use it routinely; most reserve it for specific indications, its niche being the metal-allergy patient who does not want a full ceramic bearing. It is not clearly superior to ceramic-on-HXLPE, and it has not achieved widespread adoption.

Why it was adopted. In the 1990s and 2000s metal-on-metal allowed large heads, up to 60mm and more, for stability. It had low volumetric wear because it self-polished with use, it suited young active patients as metal-on-metal resurfacing, and it appeared to solve the polyethylene particle disease problem.

How it failed.

1. Metal ion release: chromium and cobalt ions from tribocorrosion and wear
2. ALVAL (aseptic lymphocytic vasculitis-associated lesion), with pseudotumour formation, a fluid-filled or solid mass
3. Soft tissue destruction: muscles, tendons and nerves destroyed by pseudotumour
4. Metallosis: metal staining of the tissues

ALVAL as usually taught. Histologically ALVAL is a lymphocyte-dominated perivascular infiltrate with tissue necrosis, and it is conventionally described as a type IV delayed hypersensitivity reaction. Do not stop at "allergy", because the Langton study argues against it as the main driver.

What Langton found. The revised hips had greater measured surface wear on explant analysis and were associated with smaller components and higher acetabular anteversion, both of which increase wear through poorer fluid-film lubrication and edge loading. Post-revision lymphocyte transformation testing showed no reactivity to chromium or cobalt at all.

The working model is dose-dependent. Excess wear generates the particle and ion burden, and the lymphocytic reaction follows it. True idiosyncratic hypersensitivity at low wear exists but is the minority. That is why cup position and component design predict failure, and why a painful MoM hip with a well-positioned cup and low ions needs a different explanation.

Presentation. Pain disproportionate to the radiographic findings, raised blood cobalt and chromium, and a pseudotumour on MRI or ultrasound, which may be palpable as a soft tissue mass. The implant may appear well fixed on X-ray, because this is a soft tissue problem. Pseudotumours are graded by size and tissue involvement.

The regulators. MHRA (UK) and FDA alerts in 2010-2012, including a Medical Device Alert, warned of high failure rates and mandated surveillance protocols for all MoM patients. Many systems were recalled or withdrawn from the market, and class action lawsuits followed (DePuy ASR, Smith & Nephew BHR).

The MHRA 2012 protocol.

1. Annual clinical assessment: pain, function, soft tissue mass
2. Annual metal ion levels: cobalt and chromium, with ions over 7 ppb (parts per billion) the threshold for action
3. MARS MRI (metal artefact reduction sequence) if symptomatic or ions are over 7 ppb, and with a lower threshold for imaging in an ASR or other recalled system and in large heads (over 36mm), both of which the protocol lists as MRI indications. The finding that matters is a pseudotumour, present or growing.
4. Revision if symptomatic with pseudotumour, if asymptomatic but with a large or growing pseudotumour, or if metal ions are persistently over 20 ppb

Revision for MoM failure. Debride all necrotic tissue and metallosis completely and change to a ceramic or polyethylene liner, never metal again. The large head comes out, and a smaller head may be needed for adequate liner thickness. Soft tissue reconstruction may be needed; abductor repair is common, as is abductor deficiency, and outcomes are poor when tissue destruction is severe.

Where it stands. MoM is examined because it demonstrates the importance of long-term surveillance and registry data, and because many patients still have MoM implants. It is used only in rare circumstances, and the focus now is managing legacy patients with surveillance and revision: a cautionary tale of technology adoption without adequate long-term data.

First choice: metal-on-HXLPE with a 32-36mm head. For most patients over 65 of average activity this is the gold standard. It has the most proven long-term data (HXLPE over 15 years), it is cost-effective, and it forgives minor malposition.

Head size. 32mm is a good balance and the most common; 36mm gives slightly more stability with acceptable wear on HXLPE. Avoid heads under 28mm (dislocation risk) or over 40mm (potential wear concerns).

Alternative: ceramic-on-HXLPE. At the younger end of the range (65-70) or with a long life expectancy, for slightly better wear than MoP and without the squeak risk of CoC.

Not for this patient. Conventional PE (obsolete), MoM (failed technology), CoC (unnecessary in this age group), and dual mobility unless there are instability risk factors.

First choice: ceramic-on-ceramic or ceramic-on-HXLPE. For a patient under 50 with high activity, the key is minimising wear to prevent particle disease over decades of use.

Ceramic-on-ceramic. The best longevity potential, critical when a 40-year implant life is needed, and modern ceramics (Biolox Delta) have a very low fracture risk. The patient must accept the squeak risk after counselling about the 1-5% chance.

Ceramic-on-HXLPE. No squeak risk, its major advantage, and more forgiving than CoC. Its evidence base is growing, and it is many surgeons' preference for young patients.

Alternative: large-head MoP (36-40mm HXLPE). If ceramic is unavailable or cost prohibitive. It is still acceptable with modern HXLPE, but ensure a liner thickness of at least 6mm and counsel about higher wear than with the ceramic options.

Avoid. Small heads (under 32mm), for the dislocation risk over a long life; MoM, a failed technology especially in young patients; and conventional PE, which will definitely fail.

First choice: dual mobility. For the high-risk patients listed under Dual Mobility Bearings, with a cobalt-chrome or ceramic head on an HXLPE liner and a complete snap-fit of liner to head.

If dual mobility is unavailable. A large head (36-40mm) with a constrained liner. The constrained liner adds a posterior lip but reduces range of motion, has a higher dislocation rate than dual mobility, and brings increased liner wear and fracture risk.

Technique still decides. Component position is critical, because impingement leads to IPD. Repair the soft tissues (capsule, abductors) and select the approach, posterior with repair or anterior. Counsel about the dislocation reduction and the IPD risk, and that dual mobility is not a substitute for proper technique and positioning.

Metal allergy or sensitivity. Ceramic-on-ceramic or ceramic-on-HXLPE, avoiding all metal bearing surfaces; the ceramic head eliminates metal contact. Consider skin patch testing if the history is unclear. Titanium shells are available and are less allergenic than cobalt-chrome.

Obesity. High loads and thick soft tissues are the challenge. Prefer HXLPE (standard or vitamin E), proven under high loads, and calculate liner thickness carefully, since adequate thickness is essential and may need a smaller head. Ceramic-on-HXLPE is the option for the best wear; avoid CoC if the BMI is over 40, given the higher fracture concern with impact.

Revision THA. The reason for revision decides the bearing:

• Instability: dual mobility, first choice
• Wear or osteolysis: HXLPE, with a ceramic head option for the young
• Infection: depends on bone loss and stability concerns
• After MoM failure: HXLPE or ceramic-on-HXLPE, never metal again

Bone loss and dysplasia. Dual mobility is common because of the instability risk. Larger cups (dysplasia) may limit liner options, and augments or structural graft may affect the bearing choice.

Cost constraints. The standard HXLPE bearing is cost-effective and excellent, while ceramic premium pricing may be prohibitive in some systems. Prioritise longevity in young patients despite cost; dual mobility is cost-effective if it prevents revision for dislocation.

Liner thickness. Standard metal-on-HXLPE technique is straightforward and forgiving, but the liner must be thick enough. Calculate it: (cup ID - head OD) / 2 must be over 6mm.

• 54mm cup, 36mm head: (54-36)/2 = 9mm, safe
• 48mm cup, 36mm head: (48-36)/2 = 6mm, marginal

If the thickness is under 6mm, choose a smaller head or a larger cup.

Liner insertion. Ensure complete seating, listening for the snap and feeling for a stable rim lock, and check the integrity of the locking mechanism. Avoid backside damage, because scratches accelerate backside wear. Some systems require a specific impaction technique.

Head impaction. Clean the Morse taper thoroughly of blood and debris, align the head with the taper and seat it with a single firm strike. Avoid repeated impaction, which damages the taper, and test stability before closure.

When it goes wrong.

• Liner will not seat: check for debris or shell deformation
• Head will not lock: check for taper damage or blood contamination
• Backside scratches: consider liner replacement if severe

Plan for fracture. Ceramic technique demands perfect execution, and any error risks fracture. Ceramic heads and liners can fracture during insertion, so have a backup plan with an alternative bearing (MoP components) available, and review the manufacturer's guidelines, since technique varies by system.

Liner insertion. Inspect the liner carefully for cracks and chips, and discard it if there is any damage. Use the manufacturer-specific impactor and a single, firm, centred strike, since multiple strikes risk fracture, and never hit the edge of the liner. Listen for a solid seating sound.

Head impaction. Inspect the head and discard it if there are any chips. Clean the taper meticulously, because debris causes point loading and fracture, and dry it before inserting the head. Align carefully and seat with a single centred strike, never on the lateral edge of the head and without excessive force.

Protect the ceramic. Do not impinge ceramic on ceramic during trial reduction, keep metal instruments off the ceramic articulating surfaces, and protect them from impact during surgery. Assess stability with trial components, and never reduce the actual ceramic liner and head before final implantation. If the ceramic cracks during insertion, remove all debris and convert to MoP.

Cup and liner. Acetabular preparation is standard, with no specific concerns for dual mobility over a standard cup, but ensure adequate seating for the locking mechanism. The liner goes into the cup first (manufacturer-specific), and a complete snap-fit is critical to prevent IPD. Test the locking by attempting to remove the liner, which should resist; some systems have visual or tactile indicators.

Head-liner assembly. The critical step is engaging the small femoral head into the polyethylene liner, and the technique varies by manufacturer: some compress the liner, insert the head and release it (snap-fit), others use direct impaction. Verify complete engagement by feeling or hearing the snap and inspecting the rim. The head must be fully captured, because any exposure risks dissociation.

Reduction. Reduce the large outer bearing into the metal shell. It should reduce smoothly, and resistance means checking for head-liner dissociation. Check stability through a full range of motion.

Intraoperative IPD. If the head separates from the liner during trial or closure, do not accept it; it requires correction:

1. Stop immediately
2. Assess for impingement: component position, bone, osteophytes
3. Re-assemble the liner-head complex
4. Correct the source of impingement

Position. Dual mobility does not compensate for malposition. The standard targets apply, cup 40 degrees abduction and 15-20 degrees anteversion, and impingement, the primary cause of IPD, must be avoided. A constrained liner cannot be used with dual mobility, which is a different concept.

The painful or noisy THA. The bearing-related causes, and the infection and loosening that mimic them:

HXLPE liner fracture. The risk factors are thin liners (under 6mm), large heads, obesity and trauma. It presents with acute pain, instability and metallosis from the exposed shell. Prevention is calculating liner thickness to the minimum 6mm rule; treatment is revision with adequate thickness, which may need a smaller head.

Ceramic squeaking: mechanism. Microseparation during gait, stripe wear, and edge loading from component malposition produce it, as does impingement causing lever-out. The risk factors are thin patients, high activity, component malposition and smaller cups.

Ceramic squeaking: course. The squeak is audible with specific movements, such as rising from sitting and climbing stairs. Most squeaking is benign and remains stable, and it is not associated with higher revision rates. Reassure if it is benign, and revise only if the hip is painful or the patient's distress is severe.

Ceramic fracture. The causes are impaction technique error, severe trauma and pre-existing damage. It presents with acute pain, a grinding sensation and metallosis from embedded particles. Treatment is urgent revision with complete debridement of the metal particles, and the revision bearing is as in the alert under Ceramic Bearing Options; prevention is careful impaction, avoiding ceramic damage and counselling the patient about activity.

ALVAL and pseudotumour (legacy MoM). It presents with pain, a soft tissue mass, raised metal ions and an abnormal MRI, and is diagnosed with MARS MRI and cobalt and chromium levels. The pathophysiology, surveillance protocol and revision are set out under Investigations.

Intraprosthetic dislocation (dual mobility). The liner disengages from the femoral head, which is not a traditional dislocation. The causes are impingement, liner design or manufacturing, and inadequate snap-fit.

• Key feature: a dislocation that cannot be closed reduced
• Diagnosis: the X-ray shows the liner outside the head
• Treatment: open reduction, assessing for impingement; may need revision, including if recurrent
• Prevention: avoid impingement and ensure proper liner seating intraoperatively

Surveillance by stage. Rehabilitation is independent of the bearing; follow-up varies by bearing.

Activity by bearing.

What to tell the patient. HXLPE is the proven standard, with excellent longevity and no special restrictions. Ceramic patients should avoid high trauma risk and report squeaking, which is usually benign. Dual mobility gives excellent stability with a rare IPD risk. Legacy MoM patients need lifetime surveillance and should report any symptoms immediately.

Long-term monitoring. Registry data (the AOANJRR in Australia) provide population-level outcomes, but for the individual patient clinical symptoms matter most. Radiographic loosening shows as progressive radiolucent lines and migration, and osteolysis as expansile lucencies and cortical thinning.

Metal-on-HXLPE. The most data: 94-96% 15-year survivorship (AOANJRR data), excellent outcomes across all age groups and the most predictable long-term performance.

Ceramic-on-HXLPE. Emerging data, with a growing body of 10-15-year results: 95-96% 15-year survivorship and the best outcomes in the under-55 age group (AOANJRR).

Ceramic-on-ceramic. 92-95% 15-year survivorship and the best wear performance over decades in young patients. Revision for squeak is under 1%, a rare indication.

Dual mobility. Its dislocation and registry figures are given under Dual Mobility Bearings; outcomes are excellent in appropriate indications.

Metal-on-metal. A 10-15-year revision rate of 15-30%, a catastrophic failure, and legacy patients require ongoing surveillance.

What drives longevity.

By age (AOANJRR data).

• Under 55: ceramic-on-HXLPE has the lowest revision rate, 4-5% at 15 years, against an acceptable 5-7% for metal-on-HXLPE. Longevity is critical, since the implant may need to last 40 years or more, so use ceramic options when possible.
• 55-65: ceramic-on-HXLPE and metal-on-HXLPE give similar excellent outcomes, and individual factors (activity, preference, cost) guide the choice.
• Over 65: metal-on-HXLPE is the standard of care, excellent and cost-effective. Longevity is less critical with a 20-25-year life expectancy, and ceramic options are reasonable but not necessary for most.

Patient satisfaction. Over 90% of patients are satisfied with modern bearings and 90-95% have significant pain relief; function is excellent with all modern bearings, and most patients achieve their desired activity level. HXLPE brings high satisfaction with no bearing-specific concerns, ceramic high satisfaction if it does not squeak and less if it does, since the impact of a squeak varies from patients who are unbothered to those who are distressed, and dual mobility excellent satisfaction with confidence in stability. MoM satisfaction is low, because of the surveillance burden and the complications.

The registry verdict. Modern bearings achieve 95% or better survivorship at 15 years. HXLPE transformed THA outcomes, ceramic options further improve them in young patients, and dual mobility solves the instability problem. These outcomes are why THA is considered one of the most successful surgical procedures in medicine.