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Knee Arthroplasty: Polyethylene Wear


- See:
       - Polyethylene Wear: Discussion
       - Total Knee Replacement Menu:

- Resistance to Wear:
    - conformation of the bearing surfaces is important because it is factor in the determination of contact stress;
    - increases in material strength, will usually result in increases in stiffness and increases in contact stresses;
             - hence, it is important that gains in gains in strength offset increases in surface contact;
    - increases in the modulus of the polymer (or increases in the density) will have effect of increasing contact stress and may result in increased wear;
    - higher contact stresses between ultra-high molecular wt poly & other biomaterials are thought to result in greater polymeric wear;
             - over the lifetime of an implant, polyethylene becomes stiffer near the surface, and therfore, contact forces will increase w/ time;

- Adhesive / Abrasive Wear:
    - predominate type of polyethylene wear (as compared to delamination);
    - results when interatomic forces between mating wear surfaces become greater than intrinsic forces between molecules of the bulk material;
    - primarily affects ultra-high molecular wt polyethylene;
    - major attempt to solve this problem has been to alter surface that bears on the polyethylene;
    - relatively thin ( < 6 mm) tibial plateau components of relatively nonconforming design are susceptible to a delamination type of wear;
           - thicker components with more conforming articulations are expected to have less wear;
    - thinner components result in higher stresses that lead to surface deformation;
    - incongruent contact:
             - occurs from the "round on flat" contact of some early total knee designs;
             - these systems attempted to re-create normal knee kinematics;
             - these knee systems induce high contact stresses in the polyethylene component which leads to wear;

- Undersurface Wear:
    - occurs between the undersurface of the polyethylene component and the tibial base plate;
    - quality of the locking mechanism (or the lack thereof) either prevents (or promotes) undersurface wear;

- Cautions:
    - following radiographs seemed to demonstrate severe polyethylene wear in the patient's right total knee, 8 years post surgery;
    - it was felt that she would benefit from a liner exchange, but at surgery, the liner was found to be largely intact;
    - the teaching point is to be sure that the radiographs are taken perpendicular to the knee joint


Adventures in mobile bearing knee design: A mid life crisis.      

Backside Wear of Modular Ultra-High Molecular Weight Polyethylene Tibial Inserts