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Updated: Jun 19 2023

TKA Polyethylene Wear & Manufacturing

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  • Summary
    • TKA polyethylene wear refers to macroscopic premature failure of polyethylene (PE) due to excessive loading and mechanical loosening
    • Diagnosis is generally made with plain radiographs of the knee showing narrowing of the tibiofemoral implant interface
    • Treatment generally involves revision TKA or isolated polyethylene exchange depending on the stability of the femoral and tibial implants
  • Epidemiology
    • Incidence
      • Catastrophic failure is most commonly seen in TKA
        • in contrast to osteolytic failure that is usually seen in THA
        • catastrophic failure may occur in TSA and THA replacement, but less common
  • Etiology
    • Pathophysiology
      • Primary variables that lead to catastrophic wear include
        • PE thickness
        • articular surface design and geometry
          • flat PE should be avoided as knee loads exceed yield strength of UHMWPE in a flat design
          • goals of PE design
            • maximize contact area
            • minimize contact loads (force/area)
            • biplanar congruency is the best design
              • congruent in both coronal and sagittal planes
        • kinematics
          • sliding wear is bad for PE
            • occurs when ACL is sacrificed and PCL remains
              • most pronounced with CR knee design with a flat PE insert
              • least pronounced with PS or AS knee design with a congruent PE insert
        • PE sterilization
        • PE manufacturing
        • surgical technique
          • tight flexion gap hastens sliding wear effect
          • tight PCL and anterior tibial slope amplify stress
  • Polyethylene thickness
    • Introduction
      • PE insert thickness can be variable depending on manufacturer definition (e.g. some may list PE thickness as the combined thickness of the insert + tibial tray)
        • PE insert labeled as 8 mm, may only have a "true" PE thickness of 4-5 mm at the thinnest point with a ~3 mm thick metal tray
    • Cause of failure
      • PE thickness <8 mm
        • leads to loads transmitted to localized area of PE which exceed PE's inherent yield strength (12-20 mPA)
        • thickness of <8 mm associated with catastrophic PE failure
        • data based on older studies/PE generations, may not be as applicable with modern manufacturing
    • Solution
      • maintain thinnest portion of PE >8 mm
        • a more aggressive tibial cut may avoid having to use a PE insert of <8 mm
          • in younger patients, increased activity combined with thinner PE will increase risk of catastrophic failure
  • Articular surface design and geometry
    • Introduction
      • two general designs in total knee prosthesis include
        • a deeper congruous joint (deeper cut PE) without rollback
          • less anatomic
          • maximizes contact loads
          • decreases contact stress
        • a flat tibial PE that improves femoral rollback and optimizes flexion
          • more anatomic
          • PCL sparing
          • increases contact stress and catastrophic failure
    • Cause of failure
      • flat designs of tibia PE
        • low contact surface area leads to high contact stress loads in areas of contact
    • Solution
      • increase congruency of articular design
        • higher contact surface area leads to lower contact stress load
        • newer prosthesis designs sacrifice rollback and have a more congruent ("dished") fit between the femoral condyle and the tibial insert in both the sagittal and coronal plane in order to decrease the contact stress
  • Kinematics
    • Introduction
      • variables that affect kinetics include
        • knee alignment
          • varus alignment of knee associated with catastrophic PE failure
        • femoral rollback
          • optimizes flexion at the cost of increasing contact stress and increased risk of catastrophic failure
    • Cause of failure
      • excessive femoral rollback
        • dyskinetic sliding movements of femur on tibia causes surface cracking and wear
    • Solution
      • perform adequate bone cuts and/or releases to avoid varus malalignment
      • decrease contact stress by minimizing femoral rollback
        • use a more congruous joint design
        • increase posterior slope of tibia
        • use PCL substituting knee for incompetent PCL or dyskinetic femoral rollback
        • to compensate for the lack of rollback, newer designs move the point of contact (where femoral condyle rests) more posterior and have a steeper posterior slope to aid with flexion
  • Polyethylene sterilization
    • Radiation
      • gamma radiation is the most common form of polyethylene sterilization
        • results in oxidized PE which wears poorly and results in osteolysis
      • oxidation vs. cross linking
        • presence of oxygen determines pathway following free radical formation
          • oxygen rich environment
            • PE becomes oxidized
              • leads to early failure due to
                • subsurface delamination
                • pitting
                • fatigue strength/cracking
          • oxygen depleted environment
            • PE becomes cross linked
              • improved resistance to adhesive and abrasive wear
              • decrease in mechanical properties (decreased ductility and fatigue resistance)
              • greater risk of catastrophic failure under high loads
            • methods to obtain
              • packing via argon, nitrogen
              • packing in vacuum environment
      • removal of free radicals
        • thermal stabilization/remelting
          • removes free radicals formed during the radiation sterilization process for cross-linking
            • most effective means of removing free radicals as it occurs above the PE melting point
          • changes the PE from its partial crystalline state to its amorphous state
            • disadvantage is that it reduces the mechanical properties of the material
        • annealing
          • maintains its mechanical property
          • less effective at removing free radicals as it occurs below the PE melting point
            • leaves the PE more susceptible to oxidation
    • Solution
      • irradiate PE in inert gas or vacuum to minimize oxidation
  • Polyethylene manufacturing
    • Introduction
      • cutting tools can disrupt chemical bonds of PE
    • Fabrication methods
      • ram bar extrusion and machining
        • UHMWPE powder fed into heated chamber, ram pushes powder into heated cylinder barrel forming a cylindrical rod, cut into 10 ft lengths for sale
        • implants are machined from the cylindrical bar stock
        • leads to variations in PE quality within the bar
      • calcium stearate additive
        • leads to fusion defects in PE
      • sheet compression molding
        • UHMWPE powder introduced into large 4' x 8' rectangular container to make sheets up to 8" thick
        • implants are machined from these molded sheets
      • direct compression molding/net shape
        • best PE fabrication process
        • UHMWPE powder placed into a mold the shape of the final component, which is heated
        • the net shape implant is removed and packaged
        • no external machining involved, implants have high gloss surface finish
        • lower wear rates (50% wear rate of machined products)
          • slow, expensive
    • Cause of failure
      • machining shear forces cause subsurface region (1-2 mm) stretching of PE chains
        • especially in amorphous regions > crystalline regions
      • PE chains are more susceptible to radiation resulting in greater oxidation in this region
        • leads to subsurface delamination and fatigue cracking
          • can show classic white band of oxidation in subsurface (1-2 mm below articular surface) 
      • "Perfect storm" scenario for catastrophic wear
        • metal-backed tibial baseplate with bone-conserving tibial bone cut (thin PE)
        • flat bearing design in coronal plane (low contact area with high contact load)
        • PCL retention with flat PE insert (high sliding wear)
        • ram bar PE with calcium stearate additive (fusion defects in PE)
        • gamma radiation sterilization in air (weakened mechanical properties of PE)
        • machined PE surface (cutting-tool stretch effect upon the PE)
    • Solution
      • use direct-compression molding of PE
        • performed by molding directly from PE powder to the desired product
        • results in less fatigue crack formation and propagation compared to ram bar extrusion
      • avoid machining of articular surface
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