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 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 pronouced 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 8mm, may only have a "true" PE thickness of only 4-5 mm at the thinnest point with a ~3mm thick metal tray Cause of Failure PE thickness <8mm leads to loads transmitted to localized area of PE which exceed PE's inherent yield strength (12-20 mPA) thickness of < 8mm 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 >8mm a more aggressive tibial cut may avoid having to use a PE insert of <8mm 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 load 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 or "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 maintaines 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 powed into heated cylinder barrel, forming a cylindrical rod, cut into 10ft 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 UHMWPE powder placed into a mold the shape of the final component, which is heated best PE fabrication process the net shape implant is removed and packaged no external machining involved, implants have highly glossy surface finish lower wear rates (50% wear rate of machined products) slow, expensive Cause of failure machining shear forces cause subsurface region (1-2mm) 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 1-2mm below articular surface "Perfect storm" scenario for catastophic 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