Research

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

An important early outcome from this effort is the distinction between damage and wear of knee bearing inserts. Damage of joint arthroplasty bearings can be visually striking, can be described semi-quantitatively according to published techniques, and can impact mechanical performance and kinematics of the implant. Wear that occurs by abrasive/adhesive processes can be very challenging to discern and quantify, yet can produce large volumes of small debris particles that can lead to osteolysis (see figure below). Damage and wear are important but distinct phenomena that can have different impacts on clinical performance.

Highly cross-linked (HXL) polyethylene has proven to 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 HXL polyethylene, as compared to conventional, non-HXL bearings. To date, radiographic studies of head penetration represent the state-of-the-art in determining clinical wear of polyethylene hip liners. However, as the amount of wear drops to very low levels, it becomes important to develop a precise and reliable method for measuring wear, facilitating a comparison of clinical results to laboratory expectations. Fixed-magnitude errors associated with digital imaging necessitate increasingly large studies to statistically elucidate the low wear rates. Retrieval analysis provides much better precision, but is subject to different sources of error.

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

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

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 most successful procedures of the last century, issues of wear, oxidation, and fatigue failure remain obstacles to the longevity of joint replacements. As a failure mode, wear is biologically compounded, because wear debris can trigger a series of reactions leading to osteolysis, a condition resulting in long term resorption of the bone around the implant.

Crosslinking and Heat Treatment of UHMWPE

Radiation crosslinking of UHMWPE significantly improves its wear resistance, as evidenced by in vivo clinical studies and in vitro hip simulator studies. During irradiation, crosslinks are formed between polymer chains through homolytic cleavage of C-H and C-C bonds. However, ionizing radiation also produces free radicals randomly throughout UHMWPE as part of the crosslinking process. These long-lived species can react with oxygen, triggering a cyclic complex cascade of chemical reactions. While free radical oxidation involves a number of possible pathways with different mechanisms and end products, the overall outcome includes polymer chain scissions which reduce the molecular weight, and various oxidative products such as hydroperoxides, ketones, alcohols, and carboxylic acids. Overall, this cascading oxidative reaction is responsible for progressive embrittlement of the material. Oxidative degradation thus manifests as a reduction of wear resistance and mechanical properties.

In the late 1990’s, oxidation of UHMWPE was identified as a serious concern as it limits the overall lifetime and success of a joint replacement. Hence, the elimination of free radicals in UHMWPE has been an important focus of orthopaedic manufacturers ever since the industry’s response to shelf storage oxidation occurring in gamma sterilized devices. In collaboration with materials research laboratories, device manufacturers have developed a variety of post-irradiation thermal treatments with the dual goals of promoting oxidative stability and enhancing the crosslink density of bearing materials.

One thermal treatment approach utilizes heating above the melting point of the cross-linked polymer following irradiation. This melts the crystalline regions and allows recombination of trapped free radicals in these domains. After the polymer recrystallizes, the residual free radicals have been quenched and the material is both wear resistant and more oxidatively stable in shelf-aging and artificial aging according to current ASTM Standards. However, this improved wear and oxidation resistance comes at a cost because post-irradiation melting further decreases the fatigue strength of UHMWPE, already reduced by radiation crosslinking; the melting step results in a decrease in crystallinity and ductility. Thus, upon cooling, the extent of recrystallization for cross-linked UHMWPE is inferior to that of UHMWPE without crosslinks.

An alternative method of thermal treatment is post-irradiation annealing below the melting point of the cross-linked polymer. In the absence of a recrystallization step, annealed materials possess superior mechanical properties in comparison to fully remelted materials with the same radiation dose. However, this approach reduces but does not completely eliminate free radicals as achieved by melting. As a result, this material is still susceptible to in vivo oxidation. Our lab and others have reported oxidation in irradiated and annealed UHMWPE both in retrieval analysis and in vitro accelerated aging studies (see below).

Recent retrieval analyses conducted by our laboratory showed that despite the absence of free radicals (prior to implantation), highly cross-linked (HXL) remelted acetabular liners and tibial inserts showed signs of oxidation occurring in vivo, with greater oxidation rates occurring in TKA in comparison to THA. The crosslinking radiation dose significantly impacted the material’s oxidation potential. Additionally, the in vivo oxidation rate significantly correlated with trans-vinylene bond concentration (also referred to as unsaturations), and potentially to contact stress. Others have observed similar results, and have further shown potential connection between absorbed species and in-vivo oxidation. 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 without compromising mechanical properties through thermal treatments, manufacturers have examined the use of antioxidants to prevent oxidation of free radicals. The most 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, and diffusion of vitamin E into already cross-linked UHMWPE. Adding vitamin E intothe resin prior to crosslinking reduces the crosslink efficiency since the antioxidant scavenges free-radicals during irradiation, thereby reducing the effective amount of crosslinking. Diffusion of vitamin E into the polymer following cross-linking does not inhibit crosslinking. More recently, solid-state pentaerythritol tetrakis has been marketed in bearing materials for TKA. Due to the novelty of these antioxidant materials, long duration clinical andr etrieval studies haven’t yet been published, particularly with respect to in vivo oxidation prevention.

The documentation of unexpected in vivo oxidation of thermally stabilized UHMWPE may be of concern to clinicians, industry, and patients, due to the potential for polymer oxidation to lead to increased incidence of fatigue- and wear-related failures. Moreover, the shift in recipient demographic to a younger, heavier, more active patient places greater mechanical demands on bearing surfaces. Thus, attaining a better understanding of the in vivo oxidation process and rate is crucial to ensure device longevity. With this understanding, it is important to develop a predictive capability to accelerate aging under controlled laboratory conditions to test new and existing materials before they are employed in humans.

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