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 login to view 6 more bullets kinematics login to view 4 more bullets PE sterilization PE manufacturing surgical technique login to view 2 more bullets Polyethylene thickness Introduction PE insert thickness can be variable depending on manufacturer definition (i.e. 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 that 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 login to view 1 more bullet Articular surface design and geometry Introduction two general designs in total knee prosthesis include: a deeper congruous joint (deeper cut PE) without rollback login to view 3 more bullets a flat tibial PE that improves femoral rollback and optimizes flexion login to view 3 more bullets 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 planes to decrease the contact stress Kinematics Introduction variables that affect kinetics include knee alignment login to view 1 more bullet femoral rollback login to view 1 more bullet 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 that wears poorly and results in osteolysis oxidation vs. cross-linking presence of oxygen determines pathway following free radical formation login to view 14 more bullets removal of free radicals thermal stabilization/remelting login to view 4 more bullets annealing login to view 3 more bullets 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) login to view 1 more bullet 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 login to view 1 more bullet "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 on 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 the articular surface