Aims

Wear of the polyethylene (PE) tibial insert of total knee arthroplasty (TKA) increases the risk of revision surgery with a significant cost burden on the healthcare system. This study quantifies wear performance of tibial inserts in a large and diverse series of retrieved TKAs to evaluate the effect of factors related to the patient, knee design, and bearing material on tibial insert wear performance.

Introduction. In vivo, UHMWPE bearing surfaces are subject to wear and oxidation that can lead to bearing fatigue or fracture. A prior study in our laboratory of early antioxidant (AO) polyethylene retrievals, compared to gamma-sterilized and highly cross-linked (HXL) retrievals, showed them to be more effective at preventing in vivo oxidation. The current analysis expands that early study, addressing the effect of:. manufacturing-variables on as-manufactured UHMWPE;. in vivo time on these initial properties;. identifying important factors in selecting UHMWPE for the hip or knee. Methods. After our prior report, our IRB-approved retrieval laboratory received an additional 96 consecutive AO-retrievals (19 hips, 77 knees: in vivo time 0–6.7 years) of three currently-marketed AO-polyethylenes. These retrievals represented two different antioxidants (Vitamin E and Covernox) and two different delivery methods: blending-prior-to and diffusing-after irradiation cross-linking. Consecutive HXL acetabular and tibial inserts, received at retrieval, with in vivo time of 0–6.7 years (260 remelted, 170 annealed) were used for comparison with AO-retrievals. All retrievals were analyzed for oxidation and trans-vinylene index (TVI) using a Thermo-Scientific iN10 FTIR microscope. Mechanical properties were evaluated for 35 tibial inserts by uniaxial tensile testing using an INSTRON load frame. Cross-link density (n=289) was measured using a previously published gravimetric gel swell technique. Oxidation was reported as maximum ketone oxidation index (KOI) measured for each bearing. TVI was reported as the average of all scans for each material. Cross-link density and mechanical properties were evaluated as a function of both TVI and oxidation. Results. Minimal increase in oxidation was seen in these AO-retrievals, out to almost 7 years in vivo. In contrast, HXL-retrievals showed increasing KOI with time in vivo (annealed-HXL = 0.127/year, remelted-HXL = 0.036/year, p<0.001). HXL oxidation rate was higher in knees (0.091/year) than in hips (0.048/year), p<0.001. Cross-link density (XLD) correlated positively with TVI for both HXL (Pearson's correlation=0.591, p<0.001) and AO (Pearson's correlation=0.598, p<0.001) retrievals. AO-materials had higher TVI for the same or similar XLD than did HXL polyethylene. XLD correlated negatively with KOI for HXL retrievals (Pearson's correlation=−0.447, p<0.001). Mechanical properties varied by material across all materials evaluated, with tensile toughness correlating negatively with increasing TVI (Pearson Correlation=−0.795, p<0.001). Discussion. Irradiation cross-linking has been used effectively to improve wear resistance. Residual free radicals from irradiation are the target of AO-polyethylene, to prevent loss of UHMWPE XLD, resulting from in vivo oxidation of free radicals as seen in HXL retrievals, and toughness, resulting from oxidation or initial remelting. Despite different manufacturing variables, AO-polyethylene retrievals in this cohort had minimal oxidation and no change in XLD or toughness due to oxidation. However, toughness did vary with irradiation dose as did cross-link density. To achieve the same level of cross-linking as HXL-polyethylene required a higher irradiation dose in blended AO-polyethylene. AO-polyethylenes evaluated in this study had toughness that decreased with irradiation dose, but avoided loss of toughness due to remelting. Because AO-polyethylenes did not oxidize, they did not show the decrease of cross-link density, and potential loss of wear resistance, seen in HXL-polyethylene. For any figures or tables, please contact authors directly

Introduction. The optimum UHMWPE orthopaedic implant bearing surface must balance wear, oxidation and fatigue resistance. Antioxidant polyethylene addresses free radicals, resulting from irradiation used in cross-linking, that could oxidize and potentially lead to fatigue damage under cycles of in vivo use. Assessing the effectiveness of antioxidant (AO) polyethylene compared to conventional gamma-sterilized or remelted highly cross-linked (HXL) polyethylene is necessary to set realistic expectations of the service lifetime of AO polyethylene in the knee. This study evaluates what short-term antioxidant UHMWPE retrievals can reveal about: (1) oxidation-resistance, and (2) fatigue-resistance of these new materials. Methods. An IRB-approved retrieval laboratory received 25 AO polyethylene tibial insert retrievals from three manufacturers with in vivo time of 0–3 years. These were compared with 20 conventional gamma-inert sterilized and 30 HXL (65-kGray, remelted) tibial inserts of the same in vivo duration range. The retrievals were. (1) analyzed for oxidation and trans-vinylene index (TVI) using an FTIR microscope, and (2) inserts of sufficient size and thickness were evaluated for mechanical properties by uniaxial tensile testing using an INSTRON load frame. Oxidation was reported as maximum oxidation measured in the scan from the articular surface to the backside of each bearing. TVI was reported as the average of all scans for each material. Average ultimate tensile strength (UTS), ultimate elongation (UE), and toughness were the reported mechanical properties for each material. Results. Maximum oxidation values differed significantly across material types (p=0.018, Figure 1). No antioxidant retrieval exhibited a subsurface oxidation peak, in contrast to conventional gamma-sterilized (55%) and highly cross-linked (37%) retrievals that exhibited subsurface oxidation peaks over the same in vivo time (Figure 2). Trans-vinylene index (TVI) correlated positively with nominal irradiation dose (p<0.001). Mechanical properties varied by material, with tensile toughness correlating negatively with increasing TVI (p<0.001, Figure 3). Discussion. AO polyethylene was developed to address the problem of free radicals in polyethylene resulting from irradiation used in cross-linking or sterilization. Each manufacturer used a different antioxidant or method of supplying the antioxidant. However, all of the antioxidant materials appeared to be effective at minimizing oxidation over the in vivo period of this study. The antioxidant materials prevented in vivo oxidation more effectively than both conventional gamma-sterilized and remelted HXL polyethylene, at least over the in vivo period represented. The toughness, or ability of the material to resist fatigue damage, decreased with increasing irradiation cross-linking dose (increasing TVI). The AO polyethylenes evaluated in this study had lower toughness than conventional gamma-sterilized polyethylene, but they avoided the loss of toughness due to remelting. Clinical relevance. Antioxidant polyethylene tibial retrievals showed superior oxidation resistance to conventional gamma-inert and remelted HXL inserts. Material toughness varied with the irradiation dose used to produce the material. Comparison of antioxidant retrieval tensile properties can be used as a guide for clinicians in choosing appropriate materials for the applications represented by their patients

Introduction. Highly cross-linked (HXL) polyethylene has demonstrated clinical advantages as a wear resistant acetabular bearing material in total hip arthroplasty (THA) [1]. In vitro wear testing has predicted a tenfold reduction in the wear rate of HXL polyethylene, as compared to its conventional, non-HXL counterpart [2]. To date, radiographic studies of head penetration represent the state-of-the-art in determining clinical wear of polyethylene hip liners [3]. 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 expectations. This study focuses on locating and quantifying the maximum linear wear of retrieved acetabular poly liners using a coordinate measuring machine (CMM). Specifically, HXL liners are compared to a baseline of conventional, non-HXL bearings. Methods. An IRB-approved retrieval laboratory received 63 HXL acetabular bearing retrievals from 5 manufacturers with in vivo durations of 1.01–14.85 years. These were compared with 32 conventional, non-HXL controls (including gas plasma, gamma-barrier and EtO) from 3 manufacturers with in vivo durations of 1.03–20.89 years. Liners were mounted in a tripod of axial contacts with the liner face positioned in a vertical plane. Each bearing was scanned with a CMM dual-probe head, with one horizontal probe scanning the articular surface and the other scanning the non-articular, sequentially. Surface-normal wall thickness values along each latitude were calculated using a custom developed algorithm (Figure 1). Because the liners are axially symmetric as manufactured, deviation in wall thickness at a given latitude represents linear wear [4]. Results. Total wear penetration for the HXL liners ranged from 0.02 to 1.03 mm, and for the conventional, non-HXL controls ranged from 0.07 to 6.85 mm. The HXL liners had an average linear wear rate of 0.02 mm/year, compared to 0.20 mm/year for the conventional, non-HXL controls (Figure 2). The direction of maximum wear, as measured in degrees from the cup pole, ranged from 8.32 to 73.86 degrees. Differences in wear rates as a function of crosslinking dose, as well as presence/absence of a lip can be identified. Discussion. This wear measurement study of retrievals is the first application of a novel CMM technique to locate and quantify wear in HXL liners compared to conventional polyethylene controls. The study confirms the expectations of a tenfold reduction in wear rates that were based on in vitro testing [2]. The results are consistent with those of radiographic studies that have documented lower wear of HXL polyethylene in the hip compared to conventional polyethylene [3]. However, the current technique offers higher precision and reliability, and eliminates the large proportion of negative wear measurements common amongst radiographic methods. A sufficient number of liners have been measured to begin to differentiate wear between different radiation doses