Research

Retrieved knee devices sent to us by orthopaedicsurgeons are assessed for damage, and also quantitatively assessed for wear.Dimensions of retrievals are compared to design specifications or shorter in-vivoduration devices to calculate both articular and backside wear. Wear and wear rateare correlated with variables including polyethylene pedigree, articularbearing geometry, device fixation, and patient factors.

An important early outcome from this effort is thedistinction between damage and wear of knee bearing inserts. Damage of jointarthroplasty bearings can be visually striking, can be describedsemi-quantitatively according to published techniques, and can impactmechanical performance and kinematics of the implant. Wear that occurs byabrasive/adhesive processes can be very challenging to discern and quantify, yetcan produce large volumes of small debris particles that can lead to osteolysis(see figure below). Damage and wear are important but distinct phenomena thatcan have different impacts on clinical performance.

Highly cross-linked (HXL) polyethylene has provento be a wear-resistant acetabular bearing material in total hip arthroplasty (THA).In vitro wear testing has predicted a tenfold reduction in the wear rate of HXLpolyethylene, as compared to conventional, non-HXL bearings. To date, radiographicstudies of head penetration represent the state-of-the-art in determining clinicalwear of polyethylene hip liners. However, as the amount of wear drops to verylow levels, it becomes important to develop a precise and reliable method for measuringwear, facilitating a comparison of clinical results to laboratory expectations.Fixed-magnitude errors associated with digital imaging necessitate increasinglylarge studies to statistically elucidate the low wear rates. Retrieval analysisprovides much better precision, but is subject to different sources of error.

Our current work focuses on locating and quantifyingthe maximum linear wear of retrieved acetabular liners using a coordinatemeasuring machine (CMM) and a reverse-engineering algorithm. Specifically, HXLliner wear can be assessed as a function of radiation dose and compared to abaseline of conventional, non-HXL bearings.

Few retrievals studies have comparatively examined wear ofboth reverse and total shoulder arthroplasty. The lack of literature on thistopic prevents organizations from standardizing and publishing methods for weartesting of shoulder components. In order to create relevant wear testing standards,it is crucial to understand how components wear in vivo including the modes andlocations of wear. One goal of our current work is to examine series of reverseand total shoulders to determine the incidence of abrasive and adhesive wearand determine typical locations for these wear patterns on polyethylenecomponents.

UHMWPE Overview

Medical grade ultra-high molecular weight polyethylene(UHMWPE) is the current gold standard for joint bearing materials used in TJA.Although TJA involving UHMWPE as a bearing surface has been one of the mostsuccessful procedures of the last century, issues of wear, oxidation, and fatiguefailure remain obstacles to the longevity of joint replacements. As a failure mode,wear is biologically compounded, because wear debris can trigger a series ofreactions leading to osteolysis, a condition resulting in long term resorptionof the bone around the implant.

Crosslinking and Heat Treatment of UHMWPE

Radiation crosslinking of UHMWPE significantly improves its wearresistance, as evidenced by in vivo clinical studies and in vitro hip simulatorstudies. During irradiation, crosslinks are formed between polymer chainsthrough homolytic cleavage of C-H and C-C bonds. However, ionizing radiation alsoproduces free radicals randomly throughout UHMWPE as part of the crosslinkingprocess. These long-lived species can react with oxygen, triggering a cyclic complexcascade of chemical reactions. While free radical oxidation involves a numberof possible pathways with different mechanisms and end products, the overalloutcome includes polymer chain scissions which reduce the molecular weight, andvarious oxidative products such as hydroperoxides, ketones, alcohols, andcarboxylic acids. Overall, this cascading oxidative reaction is responsible forprogressive embrittlement of the material. Oxidative degradation thus manifestsas a reduction of wear resistance and mechanical properties.

In the late 1990’s, oxidation of UHMWPE was identifiedas a serious concern as it limits the overall lifetime and success of a jointreplacement. Hence, the elimination of free radicals in UHMWPE has been animportant focus of orthopaedic manufacturers ever since the industry’s responseto shelf storage oxidation occurring in gamma sterilized devices. In collaborationwith materials research laboratories, device manufacturers have developed a varietyof post-irradiation thermal treatments with the dual goals of promotingoxidative stability and enhancing the crosslink density of bearing materials.

One thermal treatment approach utilizes heatingabove the melting point of the crosslinked polymer following irradiation. Thismelts the crystalline regions and allows recombination of trapped free radicalsin these domains. After the polymer recrystallizes, the residual free radicals havebeen quenched and the material is both wear resistant and more oxidativelystable in shelf-aging and artificial aging according to current ASTM Standards.However, this improved wear and oxidation resistance comes at a cost becausepost-irradiation melting further decreases the fatigue strength of UHMWPE, alreadyreduced by radiation crosslinking; the melting step results in a decrease in crystallinityand ductility. Thus, upon cooling, the extent of recrystallization forcrosslinked UHMWPE is inferior to that of UHMWPE without crosslinks.

An alternative method of thermal treatment is post-irradiationannealing below the melting point of the crosslinked polymer. In the absence ofa recrystallization step, annealed materials possess superior mechanicalproperties in comparison to fully remelted materials with the same radiationdose. However, this approach reduces but does not completely eliminate freeradicals as achieved by melting. As a result, this material is stillsusceptible to in vivo oxidation. Our lab and others have reported oxidation inirradiated and annealed UHMWPE both in retrieval analysis and in vitroaccelerated aging studies (see below).

Recent retrieval analyses conducted by our laboratory showedthat despite the absence of free radicals (prior to implantation), highlycrosslinked (HXL) remelted acetabular liners and tibial inserts showed signs ofoxidation occurring in vivo, with greater oxidation rates occurring in TKA in comparisonto THA. The crosslinking radiation dose significantly impacted the material’s oxidationpotential. Additionally, the in vivo oxidation rate significantly correlatedwith transvinylene bond concentration (also referred to as unsaturations), andpotentially to contact stress. Others have observed similar results, and havefurther shown potential connection between absorbed species and in-vivooxidation. There thus exist several potential oxidation mechanism pathways,including the following:

1. Free radical mediated mechanism (conventionally accepted)

2. Stress induced chain scission mechanism

3. Absorbed pro-oxidative species mechanism

4. Irradiation-energy, chemical bond-related mechanism  

Antioxidant Polyethylene

In an effort to improve oxidation resistance withoutcompromising mechanical properties through thermal treatments, manufacturershave examined the use of antioxidants to prevent oxidation of free radicals. Themost prevalent antioxidant available today is a liquid-based alpha-tocopherol (VitaminE). Two methods of adding vitamin E to the UHMWPE have been examined: combiningliquid antioxidant into UHMWPE resin powder prior to compression molding, anddiffusion of vitamin E into already cross-linked UHMWPE. Adding vitamin E intothe resin prior to crosslinking reduces the crosslink efficiency since theantioxidant scavenges free-radicals during irradiation, thereby reducing the effectiveamount of crosslinking. Diffusion of vitamin E into the polymer followingcrosslinking does not inhibit crosslinking. More recently, solid-statepentaerythritol tetrakis has been marketed in bearing materials for TKA. Due tothe novelty of these antioxidant materials, long duration clinical andretrieval studies haven’t yet been published, particularly with respect to invivo oxidation prevention.

The documentation of unexpected in vivo oxidationof thermally stabilized UHMWPE may be of concern to clinicians, industry, andpatients, due to the potential for polymer oxidation to lead to increasedincidence of fatigue- and wear-related failures. Moreover, the shift inrecipient demographic to a younger, heavier, more active patient places greatermechanical demands on bearing surfaces. Thus, attaining a better understandingof the in vivo oxidation process and rate is crucial to ensure devicelongevity. With this understanding, it is important to develop a predictive capabilityto accelerate aging under controlled laboratory conditions to test new andexisting materials before they are employed in humans.

Over the last decade, our understanding ofoxidation has advanced through systematic review of retrieval oxidation in thelaboratory. Specifically, we have observed exponential oxidation rates ingamma-air, gamma-barrier, annealed, and remelted materials. These rates appearto be influenced by several different factors, including stress, free radicalconcentration, absorbed species, and radiation source. We believe that thesefactors may lead to oxidation through a number of competing pathways, each ofwhich is a subject of research in our laboratory.