Modes of Lubrication - Bearing Surfaces in THR

Overview

1. Boundary Lubrication
2. Fluid Film Lubrication (with sub-types: Hydrodynamic, Elastohydrodynamic, Squeeze Film)

1. Boundary Lubrication

• A single monolayer of lubricant molecules is adsorbed onto each bearing surface
• The lubricant molecules physically attach to the surface and carry the load directly
• In natural joints, this is achieved by a macromolecular monolayer (lubricin / surface-active phospholipids) attached to the articular surface
• These layers directly contact each other, carrying loads and reducing friction
• Film thickness ~equals the roughness of the bearing surface (~0.3 mm)
• Operates predominantly at low velocities and high loads (when fluid film cannot be maintained)

Key feature: surfaces are essentially in contact; the lubricant prevents direct metal-on-metal or cartilage-on-cartilage adhesion.

2. Fluid Film Lubrication

• Viscosity of the lubricant
• Shape of the gap between the two bearing surfaces
• Relative velocity of the surfaces

Sub-types of Fluid Film Lubrication

a) Hydrodynamic Lubrication

• Motion at sufficiently high velocity tilts the bearing and forms a wedge shape of entrained lubricant
• Viscous properties of the fluid create pressure within the film to support the load
• Surfaces are fully separated - no solid-solid contact
• Works well in rigid bearing surfaces (e.g., metal-on-metal at high speeds)

b) Elastohydrodynamic (EHD) Lubrication

• The primary lubrication mechanism for articular cartilage during dynamic function
• Occurs when bearing materials are not rigid and stiff (as with cartilage)
• As surfaces move and entrain synovial fluid, the fluid pressure causes surface deformation (AC flattening)
• This deformation:
  • Increases the contact area
  • Increases load-carrying capacity
  • Reduces escape of lubricant from between surfaces
  • Generates a longer-lasting film
  • Produces lower stress concentration within joint surfaces
• Both hydrodynamic (sliding) and squeeze film (impact) actions can drive EHD lubrication

c) Squeeze Film Lubrication

• Occurs when two surfaces approach each other rapidly (e.g., at heel strike)
• Fluid is squeezed out from between the surfaces but the viscous resistance of the fluid generates a transient pressure that carries the load
• Very important at the start of motion and during high-impact loading before hydrodynamic film is established
• In deformable (EHD) bearings, surface deformation spreads the load over a larger area, prolonging the squeeze film effect

3. Mixed Mode Lubrication

• Fluid film pressure supports loads in areas of non-contact (troughs between surface asperities)
• Boundary lubrication operates in areas of contact (at surface asperities)

Comparison: Hydrodynamic vs. Squeeze Film (Rigid vs. Deformable)

Hydrodynamic Lubrication - Principle

Coefficient of Friction - Comparative Data

Role of Synovial Fluid

• Synovial fluid is thixotropic: becomes fluid when agitated, settles when at rest
• This property allows it to serve boundary lubrication needs at low velocities and fluid film needs at high velocities
• Joint effusion disrupts this velocity-dependent property, reducing lubrication quality and increasing surface damage
• Key molecular lubricants: hyaluronan, lubricin, and phospholipids - each plays a different role at different stages of motion, and none alone can explain the extremely low friction of natural joints

Summary Table

Bearing Surfaces in THR - Structured Answer (6 Marks)

A. THE FOUR BEARING COUPLES - Wear Rates

1. Metal-on-Polyethylene (MoP) - Conventional UHMWPE

• Backside wear - additional PE debris from PE insert micromotion against metal shell
• PE particles (submicron size) stimulate osteolysis when count exceeds 10 billion particles per gram of tissue
• Osteolytic lesions typically appear after 10 years with conventional PE

2. Metal/Ceramic-on-Highly Cross-Linked Polyethylene (HCLPE) - Most Popular Today

• Rates of osteolysis have fallen dramatically with widespread HCLPE adoption
• HCLPE allows larger-diameter femoral heads without excessive linear wear
• Wear rate largely independent of head diameter
• Disadvantage: Reduction in mechanical properties (fracture toughness, tensile strength) - catastrophic failure is extremely rare
• Free radical problem: Residual free radicals after irradiation cause oxidative degradation; addressed by:
  • Remelting (>135°C) - eliminates free radicals but reduces crystallinity
  • Annealing (below melting point) - preserves crystallinity but less effective
  • Vitamin E doping - scavenges free radicals without remelting stage
• Currently Ceramic-on-HCLPE is favored over Metal-on-HCLPE due to trunnion corrosion concerns

3. Metal-on-Metal (MoM)

• Run-in wear: Higher wear in first 1 million cycles (~1 year of high activity) - polishes out high points, carbide asperities, and areas out-of-round
• After run-in: lower steady-state wear rate
• Now largely abandoned due to ALTR (see Section C)

4. Ceramic-on-Ceramic (CoC)

• 1st generation alumina - head fracture rate up to 13.4% (due to neck impingement, adverse head-neck ratio, poor manufacturing)
• Modern AMC (delta ceramic): fracture rate ~1 in 100,000 (0.001%)
  • Compare to pure alumina: 1 in 5000 (0.02%)
• Ceramic head change: Any new ceramic head placed on a used femoral neck must use an internal titanium adapter sleeve - high points on roughened neck cause burst fracture
• Stripe wear - area of roughness from repetitive subclinical subluxation (head rotates on cup edge)
• After ceramic fracture: microscopic shards remain, severely abrasive - must replace with another CoC (use HCLPE if unavailable)

B. PSEUDOTUMOR AND ALTR in Metal-on-Metal (MoM)

Definition

1. Metal-on-metal articulating bearing surfaces
2. Modular neck femoral components
3. Stems with modular femoral heads (trunnion/taper corrosion)

Pathogenesis

• Nanometer-sized MoM particles dissolve to generate cobalt (Co²⁺) and chromium (Cr³⁺) ions
• Normal wear: serum Co and Cr levels mildly elevated (1-3 µg/L)
• ALVAL (Aseptic Lymphocyte-Dominated Vasculitis-Associated Lesion): Delayed hypersensitivity reaction to metal ions; CD4+ T-lymphocyte mediated
• Local tissue necrosis + cystic masses = pseudotumor
• More common in women (smaller head diameter, higher inclination, higher wear)

Metal Ion Levels and ALTR Risk (Campbell's Table 4.10)

Diagnosis

• Serum Co and Cr levels - first-line screening
• ESR, CRP - may be elevated in ALTR alone (mimics infection)
• Ultrasound or MARS MRI (Metal Artifact Reduction Sequence) - imaging gold standard for pseudotumor characterization

Contraindications to MoM

• Women of childbearing age (metal ions cross placenta)
• Renal failure (metal ions no longer eliminated)
• Metal hypersensitivity

Outcomes

• Results of ALTR surgery are generally poor - high rates of infection, instability, reoperation
• Refer to centers with required implant availability and surgical expertise

C. DUAL MOBILITY - THE BOUSQUET CONCEPT

Historical Origin

• Designed by Prof. Gilles Bousquet in France in 1974 (first implanted 1976)
• Concept: use two articulating interfaces to increase effective head-to-neck ratio and range of motion without increasing head size proportionally

Design Principle (The "Double Mobility" Concept)

Small metal head (22-28 mm)
         ↕  Inner articulation (small head in large PE ball)
Large PE outer ball ("mobile bearing" or "retentive cup")
         ↕  Outer articulation (large PE ball in metal shell)
Metal acetabular shell

• A small-diameter femoral head articulates with a large outer polyethylene liner (forming a large-diameter bipolar construct)
• The outer PE ball then articulates with the metal acetabular shell
• Two articulating surfaces = dual mobility

Biomechanical Advantages

• Provides greater impingement-free range of motion
• Larger head-to-neck ratio reduces dislocation risk dramatically

Clinical Applications

• Primary THA in high-risk dislocation patients: Poor abductors, neuromuscular disease, cognitive impairment
• Revision THA - historically the main indication
• Conversion of failed hemiarthroplasty - convert MoM monoblock to dual mobility on femoral side
• Low dislocation rates reported in both primary and revision settings

Complications

• Intraprosthetic dislocation (IPD): Dislocation of the inner head from the outer PE ball - requires open reduction - unique to dual mobility
• Outer bearing PE wear - long-term concern (outer articulation is metal-on-PE)
• Acetabular component: monoblock (no screws) or modular (ensure metal liner is properly seated)

D. TRUNNION CORROSION ("TRUNNIONOSIS")

Definition

Mechanism

• Fretting = micromotion at taper junction under cyclic loading
• Corrosion = electrochemical degradation of metal at the fretting site
• Combined process releases Co and Cr ions and metallic debris
• Can cause ALTR identical to MoM bearing failure

Contributing Factors (not fully elucidated)

• Large femoral head diameter (greater moment at taper)
• CoCr head on titanium stem (dissimilar metal galvanic corrosion)
• Increased patient activity / body weight
• Longer neck length (increased lever arm)
• Modular femoral neck junctions (additional taper = additional corrosion site)

Clinical Significance

• Concerns about trunnion corrosion have reduced use of CoCr femoral heads in favor of ceramic heads worldwide
• Metal debris from trunnion corrosion may be responsible for osteolysis appearing within first 2-3 years (vs. PE wear osteolysis which appears after 10 years)
• Management: revision to ceramic head with titanium adapter sleeve

E. CLINICAL APPLICATIONS OF TRIBOLOGY IN THR