Wear

Wear in Bearing Surfaces

Definition of Wear

Wear is the progressive loss of a bearing substance caused by mechanical or chemical actions, including corrosion.

Modes of Wear

  1. Normal Motion of Articulating Surfaces: The main type of wear in a well-functioning joint replacement.
  2. Intended Articulating Surface vs. Non-Articulating Surface: Example includes a femoral head against a metal cup after erosion through polyethylene.
  3. Third Body Wear: Caused by debris, such as cement particles.
  4. Wear Debris from Non-Articulating Surfaces: For instance, neck impingement on a cup edge or trunnion.

Mechanisms of Wear

Wear can occur through mechanical or chemical processes:

Mechanical Wear

  1. Abrasive Wear
  2. Adhesive Wear
  3. Fatigue Wear
    • All three types co-exist to some degree.

Chemical Wear

  1. Corrosion

Types of Wear Mechanisms

Abrasive Wear

  • Occurs between a hard and soft bearing couple.
  • Asperities on the hard bearing carve ridges into the soft bearing.
  • This action generates third bodies that embed into and scratch the hard bearing, increasing surface roughness and accelerating abrasive wear of the soft surface.
  • Single scratches on metal can increase wear by tenfold, though ceramics have better scratch profiles and do not form a heaped edge, preventing debris accumulation.

Adhesive Wear

  • Involves a hard and soft bearing couple.
  • The two surfaces bond with each other, forming a junction held by intermolecular bonds, generating friction.
  • If these bonds are stronger than the cohesive strength of the soft material, the surface shears off, resulting in a steady low rate of wear.

Fatigue Wear – Delamination

  • Repetitive cyclical loading leads to micro-delamination.
  • This occurs in the subsurface white layer of polyethylene.
  • Cracks form and propagate, generating large volumes of third body wear debris.
  • Plastic deformation occurs, leading to material fracture at stress levels below its ultimate tensile strength (UTS).
  • Fatigue wear is represented by the S-n curve.
  • More prevalent in total knee replacements (TKR) but can occur in misaligned total hip replacements (THR).
  • Accelerated by:
    1. Low joint conformity leading to stress concentration.
    2. Low polyethylene thickness causing surface stress concentration.
    3. Malalignment increasing stress concentration.
    4. Subsurface faults or oxidation from manufacturing and storage.

Measurement of Wear

Wear can be quantified in two ways: - Volumetric Wear: The volume of material cleaved per year (mm³/year). A critical volume is 140 mm³/year. - Linear Wear: The penetration of the bearing surface per year (mm/year).

Measurement Techniques

  • In Vitro Testing: Utilizes load cycling machines but tends to underestimate wear. One million cycles take around six days, simulating a year of in vivo use.
  • In Vivo Testing: Involves measuring explanted cups for volumetric wear and using X-ray measurements on standardized radiographs for linear wear.

Radiostereometric Analysis

  • While it does not measure wear directly, it assesses implant movement, which is a consequence of wear and osteolysis.
  • Tiny tantalum beads are implanted during surgery, and X-rays in two planes provide accurate images, with computer analysis precise up to 0.2 mm.

Laws of Wear

  • Volume of removed material (V) increases with:
    • Applied load (L)
    • Sliding distance (S)
    • Decreases with the hardness of the softer material (H)
  • The formula is represented as: V proportional to LS/H.
  • Larger diameter heads yield greater sliding distances, leading to:
    • Increased volumetric wear with larger heads (due to sliding distance).
    • More loading cycles, especially in young, active patients.
    • Increasing the hardness of bearing surfaces counteracts wear.

Wear in Total Hip Replacement (THR)

  • Penetration into an acetabular cup occurs due to both creep and wear.

Creep

  • Occurs in the initial years in a superomedial direction, following the direction of compressive joint contact force.

Wear

  • Typically occurs in a superior or superior-lateral direction, influenced by sliding actions.

Summary of Factors Affecting Wear

  1. Implant Size: Larger heads increase sliding distance, thus increasing volumetric wear.
  2. Surface Roughness: Higher roughness contributes to abrasive wear.
  3. Material Toughness: Affects abrasive and adhesive wear.
  4. Material Hardness: Enhances scratch resistance and reduces volumetric wear.
  5. Load Magnitude and Type: Influences weight, activity, and type of joint.
  6. Coefficient of Friction: Affects wear rates.
  7. Presence of Third Bodies: Contributes to wear.
  8. Implant Orientation: Can influence wear patterns.

Wear and Wear Particles

Consequences of Wear Particles

  1. Synovitis
  2. Osteolysis leading to aseptic loosening
  3. Immune reaction resulting in granuloma formation
  4. Systemic dissemination of wear particles
  5. Vicious cycle leading to malalignment, dislocation, and implant failure

Factors Affecting Osteolysis

  1. Size of Particles: Smaller particles are more active.
  2. Shape of Particles: Influences the immune response.
  3. Volume of Particles: Critical volume is 140 mm³/year.
  4. Total Number of Particles: Higher numbers can exacerbate issues.
  5. Immune Response: Affects how the body reacts to particles.

Particle Size

  • Most active particles are submicron-sized (0.1-0.5 µm).
  • Larger particles are often too big for macrophages to process, while high volumes of small particles can be more detrimental.
  • Metal bearings generate large volumes of particles, most of which are too small to be biologically active but may still trigger an immune response.

Osteolysis Cascade

  • Particles are phagocytosed by macrophages, leading to an inflammatory cascade involving cytokines and prostaglandins.
  • This results in osteoclastic bone resorption, with macrophages potentially resorbing bone directly.
  • A self-sustaining vicious cycle occurs:
    • Osteolysis causes implant loosening, increased micromotion, and increased surface roughness, further accelerating component wear.
  • Osteolysis can occur throughout the effective joint space, where joint fluid communicates.
  • High wear leads to increased intra-articular volume and pressure, causing wear particles to dissipate along the path of least resistance, often around the stem and cup, resulting in aggressive osteolysis.
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