Tribology and Wear in Orthopaedic Implants

Tribology is the science of interacting surfaces in relative motion under load. It covers friction, the resistance to motion between surfaces; lubrication, the fluid or boundary layer that reduces friction; and wear, the progressive loss of material from a surface. In orthopaedics it underpins joint replacement design, bearing surface selection and the prediction of implant longevity.

Why it matters. Wear particles cause particle-induced osteolysis, the most common cause of aseptic loosening.

The benchmark. Native articular cartilage achieves a friction coefficient of 0.02, the lowest in nature. It does so through:

• Hyaluronic acid boundary lubrication
• Fluid-film formation under load (weeping lubrication)
• The biphasic properties of its water-collagen matrix

Arthroplasty bearings cannot replicate this and run at 0.05-0.15.

The Stribeck curve. It describes friction as a function of speed, viscosity and load, and it divides lubrication into three regimes.

Where implants sit. Most arthroplasty bearings run in mixed lubrication during normal gait. Boundary lubrication occurs at start-up or with the edge loading of malpositioned components, and it increases wear.

Regimes and mechanisms. The Stribeck regimes describe the friction state. The mechanisms that generate a fluid film are a separate, examinable layer.

The lambda ratio. Whether a bearing achieves fluid-film separation is captured by the lambda ratio: the minimum lubricant film thickness divided by the composite surface roughness of the two surfaces.

• Greater than 3: full fluid film; the surfaces never touch and wear is minimal
• 1-3: mixed
• Less than 1: boundary; asperity contact and high wear

Hard-on-hard versus hard-on-soft. This is why ceramic-on-ceramic and metal-on-metal bearings, with very smooth surfaces, large diameters and low clearance, can run in or near fluid-film or elastohydrodynamic lubrication. Metal-on-polyethylene, rougher and with a more deformable polyethylene, stays in the boundary or mixed range. Optimising film thickness (large head, low clearance, smooth surfaces) and minimising roughness both raise lambda.

Archard's equation. Before the individual mechanisms, it is worth having the relationship that governs all of them. The volume of material removed is

V = k x L x s / H

where V is wear volume, L the applied load, s the sliding distance, H the hardness of the softer surface and k a dimensionless wear coefficient specific to the material pairing and lubrication conditions.

Four terms, four clinical levers.

• Load is patient weight and activity intensity. It is linear, so it matters less than people assume relative to sliding distance.
• Sliding distance is the one that dominates in practice. It scales with head radius and with the number of cycles, which is why a young active patient walking several million cycles a year wears a bearing out faster than a heavy sedentary one, and why a larger head sweeps more polyethylene per step.
• Hardness of the softer surface explains why the femoral head's surface finish matters so much: a scratched hard head raises the effective abrasiveness against unchanged polyethylene.
• k is the term materials science actually changed. Cross-linking polyethylene did not alter the patient's load or step count; it altered k.

The linear-versus-volumetric paradox. Sliding distance is the mechanism behind it: a bigger head penetrates the liner more slowly in millimetres while removing more polyethylene in cubic millimetres. The equation also makes plain why no design change can abolish wear in an active patient, since s keeps accumulating for as long as the patient walks.

Three primary mechanisms. Wear is adhesive, abrasive or fatigue. Corrosive wear, the electrochemical degradation seen as taper and fretting corrosion, is classified alongside them.

Adhesive wear. Asperities, the microscopic peaks on one surface, cold-weld to the opposite surface under pressure. Relative motion then breaks the bonds and material from one surface adheres to the other; metal transfer to polyethylene gives the polished appearance. On the liner it shows as:

• Polishing: a smooth, shiny polyethylene surface
• Burnishing: layers of transferred metal
• Scratching: transferred metal particles scratching the polyethylene

Prevention is a smooth, polished femoral head.

Abrasive wear. Hard particles plough through a softer surface and remove material. In two-body abrasion the hard surface does the ploughing; in three-body abrasion free particles are trapped between the surfaces.

Third-body wear. This is the most clinically significant form. The trapped particles act as abrasives and accelerate wear 10-100 fold. They come from:

• PMMA cement, hardness 100-200 MPa and harder than polyethylene
• Metal debris from taper junctions, screws and instrumentation
• Bone fragments entrapped during impaction or reaming

Fatigue wear. Cyclic loading concentrates stress below the surface, where microcracks initiate and then grow with continued cycling until the surface layer separates as large debris. On the liner it appears as pitting (small craters), delamination (sheet-like debris) and surface or subsurface cracking. The risk factors are thin polyethylene (under 6mm), high stress and gamma sterilisation in air.

Sterilisation and oxidation. Conventional polyethylene sterilised with gamma radiation in air developed oxidation, which reduced its fatigue resistance. Gamma in air is worse than XLPE or inert gas, and modern XLPE or gas sterilisation prevents the oxidation.

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Mechanism versus mode. A classic exam trap is to confuse wear mechanisms (adhesive, abrasive, fatigue: how material is lost) with the McKellop wear modes (which surfaces are articulating). A complete answer names both.

Two ranges, often merged. Particles of roughly 0.1-10 microns are phagocytosable: that is the range a macrophage can ingest. The most biologically reactive subset is far narrower, about 0.2-0.8 microns, and the wide range should not be quoted as though it were the critical one.

Volume within the band. The central principle of wear biology is that the biological response is determined by how much of the wear volume falls within the critical size range, rather than by the wear volume itself. A bearing therefore cannot be judged on wear rate alone, and pre-clinical testing of a new material must characterise the particles it produces, not merely how much it produces.

The cascade.

1. Phagocytosis: macrophages ingest particles in the phagocytosable range
2. Activation: frustrated phagocytosis, because the macrophage cannot digest polyethylene
3. Cytokine release: TNF-alpha, IL-1, IL-6 and prostaglandins
4. Osteoclast activation through the RANKL pathway
5. Bone resorption: periprosthetic osteolysis and aseptic loosening

What cross-linking buys. XLPE achieves a 90% reduction in volumetric wear compared with conventional polyethylene, and with it less osteolysis. Data beyond 15 years are now available and show excellent survivorship.

How it is made.

1. Irradiation with gamma or e-beam radiation at 50-100 kGy
2. Cross-linking: covalent bonds form between the polyethylene chains
3. Remelting: thermal treatment removes free radicals, which prevents oxidation

The result is a highly crosslinked, wear-resistant network. Vitamin E is added as an antioxidant for free-radical scavenging.

The price. Cross-linking reduces fracture toughness, with the potential for rim fracture, so the liner needs a minimum thickness of 6-8mm to avoid fatigue failure.

Cup position. Cup inclination and anteversion significantly affect wear. The Lewinnek safe zone, inclination 30-50° (40° ideal) and anteversion 5-25° (15° ideal), minimises edge loading and impingement. Outside it, wear and dislocation risk increase.

Edge loading. Cup inclination over 45° causes edge contact, with high contact stress at the rim and accelerated wear. Stripe wear, a visible linear wear pattern, appears on the polyethylene liner, and the failures that follow are rim fracture, excessive wear and osteolysis.

Head size. A larger femoral head has competing effects on wear.

With XLPE, larger heads (36-40mm) provide stability without prohibitive wear. Conventional polyethylene was limited to 28-32mm heads.

Surface finish. Femoral head roughness critically affects adhesive wear, and a high polish minimises it. The targets are Ra less than 0.05 microns for cobalt-chrome and less than 0.02 microns for ceramic, which is smoother than metal. Scratches on the head increase polyethylene wear exponentially, and they come from:

• Intraoperative handling with metal instruments
• PMMA contact during cementation
• Metal-on-metal taper debris transfer

Taper junctions. The head-neck junction undergoes fretting and corrosion, and tribocorrosion is that combined mechanical and electrochemical wear. A titanium trunnion with a cobalt-chrome head carries a risk of galvanic corrosion; matched materials or ceramic heads are preferred.

The families. Hard-on-soft bearings are metal-on-XLPE, the standard and most common with an excellent track record, and ceramic-on-XLPE, with lower wear and a combination of benefits. Hard-on-hard bearings are ceramic-on-ceramic, with the lowest wear but a risk of squeaking, and metal-on-metal, abandoned because of adverse reaction to metal debris (ARMD).

Wear rate hierarchy.

1. Ceramic-on-ceramic: less than 0.001mm/year
2. Metal-on-XLPE: less than 0.05mm/year
3. Conventional metal-on-polyethylene: 0.1-0.2mm/year

Choosing a bearing. It is a trade-off between wear, fracture or noise risk, and cost. The table contrasts the realistic options.

By patient.

• Young, active: ceramic-on-ceramic for the lowest wear, or ceramic-on-XLPE as the alternative, with large heads (36mm) for stability
• Older, less active: metal-on-XLPE for its durability, with dual mobility if there is a risk of instability
• Avoid: metal-on-metal (ARMD) and conventional polyethylene (high wear)

Radiographic wear. Serial radiographs measure femoral head penetration. Linear wear is migration of the head centre into the liner; volumetric wear is calculated from linear wear and head size. Osteolysis shows as expanding lucencies and scalloping.

Metal-on-metal concerns. Cobalt and chromium ion levels are measured, and a whole-blood cobalt greater than 7 ppb is a concern. Metal artefact reduction sequence (MARS) MRI images the soft tissues.

Simulator testing. Hip joint simulators follow the ISO 14242 standard protocol. Five million cycles represent approximately five years of use, bovine serum is the lubricant because it mimics synovial fluid, and both gravimetric (weight loss) and linear wear are measured.

Surface analysis.

• Profilometry: surface roughness (Ra)
• Electron microscopy: wear patterns and debris
• Retrieval analysis: study of explanted bearings

Prevention. Wear reduction begins at the primary operation: XLPE, component position within the Lewinnek zone, a smooth femoral head, and the avoidance of third-body debris by lavage and protecting the head. The intraoperative detail is under Surgical Technique, and surveillance under Postoperative Care.

Revision for wear. The indications:

• Progressive osteolysis: expanding lucencies, impending fracture
• Linear wear over 2mm, with its increased osteolysis risk
• Symptoms: pain, instability, loosening

At revision. Remove all polyethylene debris by thorough debridement of granulation tissue, and fill osteolytic defects with bone graft, allograft or autograft. Replace conventional polyethylene with an XLPE liner, and exchange a scratched or worn femoral head.

What to expect. Removing the particles stops osteolysis progressing, and grafted defects incorporate over 6-12 months. Revision to XLPE has excellent 10-15 year survivorship.

Cup position. Aim for the Lewinnek ideals of 40° inclination and 15° anteversion, and avoid the edge loading of high inclination. Navigation and robotics improve accuracy.

Protecting the head.

• Never touch the articulating surface with metal instruments
• Use soft liner trays, never a metal surface
• Avoid PMMA contact, and protect the head during cement insertion
• Inspect for scratches before final reduction

Clearing third bodies. Use copious pulsatile lavage (greater than 3L), and remove all PMMA fragments and any bone debris from the acetabulum.

Seating the liner.

• Clean the locking mechanism
• Confirm full seating, with no gap
• Avoid a malpositioned or proud liner
• Test stability with a trial before the final components

Osteolysis. Progressive bone loss around the implant, which may lead to loosening and periprosthetic fracture. It is treated by revision with XLPE and bone grafting.

Aseptic loosening. The end stage of wear-induced osteolysis, presenting with pain, instability and radiographic loosening. Historically it was the leading cause of late revision; the current picture is under Outcomes.

Bearing-specific complications.

• Ceramic-on-ceramic: squeaking in 1-8%, usually benign; fracture in 0.02-0.1% with modern delta ceramic; stripe wear, an edge-loading pattern
• Metal-on-metal: ARMD, with pseudotumours, metallosis and elevated metal ions, the reason for its abandonment
• XLPE: rim fracture if thin (less than 6mm) or malpositioned; oxidation in older designs without remelting

The Delta ceramic-on-ceramic series in the evidence section recorded 3 liner fractures in 345 hips (0.9%), above the fracture range quoted here.

Routine surveillance. Review at 6 weeks and 1 year, then every 2-5 years, on serial AP pelvis radiographs. Compare head position over time to measure linear wear as head penetration, and watch for osteolysis as expanding lucencies. Concern begins at greater than 0.1mm/year for XLPE or greater than 0.2mm/year for conventional polyethylene, and where osteolysis appears, consider revision before bone loss becomes severe.

Enhanced monitoring is indicated for:

• Young, active patients
• Large head sizes
• Metal-on-metal bearings, with metal ions annually
• Symptoms of instability or pain

Activity. Low-impact activities are preferred. High-impact sports increase wear and are avoided, and weight management reduces load cycles.

Ceramic-on-ceramic. Near-zero wear, with 4.8% revision at 10 years in registry data.

Registries. Across the national joint registries (NJR, AOANJRR, AJRR, Nordic), ceramic-on-ceramic and ceramic-on-XLPE show excellent long-term survival and metal-on-XLPE shows comparable durability. Large-head metal-on-metal has the highest revision rates and has been abandoned.

Wear-related revision. With modern bearings osteolysis is now a small minority of revisions. The leading causes of revision today are infection, instability, periprosthetic fracture and loosening, not wear.

Global epidemiology. Total hip arthroplasty is one of the most successful operations in medicine ("the operation of the century"), with over 1 million procedures performed annually worldwide and demand rising with ageing populations. Historically, wear-induced osteolysis and aseptic loosening were the leading causes of late revision; since the widespread adoption of cross-linked polyethylene from the early 2000s, the proportion of revisions attributable to wear/osteolysis has fallen markedly, and instability, infection and periprosthetic fracture now dominate revision burden in national registries.

Remelted versus annealed XLPE. Remelting above the melt transition eliminates free radicals and oxidation but reduces mechanical strength. Annealing below the melt preserves strength but leaves residual radicals that risk late oxidation. The optimal balance, and the role of vitamin E-doped polyethylene, is still debated.

Critical particle size. Biological-activity work suggests the most osteolytic particles are 0.2-0.8 microns, but exact human thresholds remain uncertain and are model-dependent.

Is there a true safe zone? The Lewinnek zone is widely taught, but many dislocations occur within it. Spinopelvic mobility and functional, not just static radiographic, component position are increasingly emphasised over a single fixed target.

Large heads with XLPE. XLPE permits larger heads for stability, but whether 36mm and above increases long-term wear or trunnionosis and fretting at the head-neck taper is still being studied. RSA data to five years show no excess penetration with 36mm.