What factors drive polyethylene wear in total knee arthroplasty? : results of a large retrieval series.

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. METHODS: An institutional review board-approved retrieval archive was surveyed for modular PE tibial inserts over a range of in vivo duration (mean 58 months (0 to 290)). Five knee designs, totalling 1,585 devices, were studied. Insert wear was estimated from measured thickness change using a previously published method. Linear regression statistical analyses were used to test association of 12 patient and implant design variables with calculated wear rate. RESULTS: Five patient-specific variables and seven implant-specific variables were evaluated for significant association with lower insert wear rate. Six were significant when controlling for other factors: greater patient age, female sex, shorter duration in vivo, polished tray, highly cross-linked PE (HXLPE), and constrained knee design. CONCLUSION: This study confirmed that knee wear rate increased with duration in vivo. Older patients and females had significantly lower wear rates. Polished modular tibial tray surfaces, HXLPE, and constrained TKA designs were device design factors associated with significantly reduced wear rate. Cite this article: Bone Joint J 2021;103-B(11):1695-1701.

Analysis of Failed Ankle Arthroplasty Components.

BACKGROUND:: Although advances in joint-replacement technology have made total ankle arthroplasty a viable treatment for end-stage arthritis, revision rates for ankle replacements are higher than in hip or knee replacements. The questions asked in this study were what can retrieved ankle devices demonstrate about ankle arthroplasty failures and how can understanding the causes of these failures inform clinical decisions for current and future ankle arthroplasty patients? METHODS:: An IRB-approved retrieval laboratory received retrieved components and surgeon-supplied reason for revision from 70 total-ankles (7 designs, including 5 currently marketed designs) from 2002 to 2018. These retrievals were rated for clinical wear and damage. Metal components were rated by method and effectiveness of fixation. Polyethylene inserts received by the laboratory 6 months or less after retrieval (n = 45) were analyzed for oxidation using Fourier transform infrared spectroscopy. Statistical analysis was performed using IBM SPSS, version 22. RESULTS:: The ankle implants were retrieved most commonly for loosening and polyethylene fracture. Loosening occurred more frequently in fixed-bearing designs (n = 18) than in the mobile-bearing designs (n = 4) and after shorter in vivo time (mean in vivo time to retrieval for loosening: fixed bearing 3.2 ± 2.1 years, mobile bearing 9.7 ± 4.5 years). Gamma-sterilized ankle inserts oxidized at a higher rate than non-gamma (EtO or gas-plasma) sterilized ankle inserts (gamma 0.29 ± 0.22/y, non-gamma 0.07 ± 0.05/y, mean difference=0.215, 95% CI 0.128-0.303, P < .001). The presence of clinical fatigue (cracking and/or delamination) of the polyethylene insert correlated with measured oxidation (Spearman rho = 0.685, P < .001). Nine inserts, all gamma-sterilized, fractured in vivo. CONCLUSIONS:: This study suggests that loosening could be more of a problem in fixed-bearing devices than in mobile bearing devices. Gamma-sterilized polyethylene inserts were found to suffer fatigue damage or fracture in vivo, resulting in the need for revision. Retrieval analysis can provide insight into implant-related reasons for revision, with the goal of understanding the implant-related causes of these failures, informing future ankle design and clinical decisions for current and future ankle arthroplasty patients. LEVEL OF EVIDENCE:: Level III, comparative series.

Effectiveness of anti-oxidant polyethylene: What early retrievals can tell us.

The optimum UHMWPE orthopedic 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. This study evaluates what short-term antioxidant UHMWPE retrievals can reveal about: (1) oxidation-resistance and (2) fatigue-resistance of these new materials. Retrievals of three different antioxidant polyethylene materials (n = 25) were analyzed by FTIR and uniaxial tensile tests and compared to conventional (n = 20) and remelted highly cross-linked (n = 30) polyethylene retrievals of similar in vivo duration. Maximum oxidation values differed significantly across material types (p = 0.018). 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. Trans-vinylene index (TVI) correlated positively with nominal irradiation dose (p < 0.001). Tensile toughness correlated negatively with increasing TVI (p < 0.001). The antioxidant materials in this study prevented in vivo oxidation more effectively than remelted HXL polyethylene at least over the in vivo period represented. The 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. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 353-359, 2018.

Contribution of micro-motion to backside wear in a fixed bearing total knee arthroplasty.

This study seeks to identify important factors related to backside wear of tibial inserts in vivo and determine an appropriate wear model for backside wear. An IRB approved database was queried for tibial inserts of a single design from one manufacturer that exhibited evidence of rotatory motion on the backside of the polyethylene. These devices were measured for volumetric wear using a previously established protocol. Features including the change in locking lip width and measurement of micro-motion marks were used to describe the motion pattern. Volumetric wear and implant characteristics were compared using linear regressions by modeling wear theories suggested by Archard and Wang to determine the most appropriate model for backside wear. The Wang model showed that duration, adjusted sliding distance, and cross-shear index accounted for approximately 58% of the volumetric wear variation while adjusted sliding distance and duration in vivo accounted for approximately 35% of the volumetric wear variation in the Archard model. Patient weight (p = 0.750), patient BMI (p = 0.680), and backside area (p = 0.784) of the tibial insert were all found to be non-significant in the Wang model. Similarly, patient weight (p = 0.233), patient BMI (p = 0.162), and backside area (p = 0.796) were found to be non-significant in the Archard model. Multidirectional micro-motion appears to contribute significantly to the wear of these components, supporting the Wang theory of cross-shear for polyethylene wear. Cross-shear of polymers on an unpolished titanium tray can lead to an increase in wear debris in the body. Care should be taken when designing locking mechanisms and tray designs. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1933-1940, 2016.

Comparison of Wear and Oxidation in Retrieved Conventional and Highly Cross-Linked UHMWPE Tibial Inserts.

Two groups of retrieved tibial inserts from one manufacturer's knee system were analyzed to evaluate the effect of a highly cross-linked bearing surface on wear and in vivo oxidation. The two groups ((1) conventional gamma-inert sterilized and (2) highly cross-linked, coupled with the same rough (Ra=0.25) Ti-6Al-4V tray) were matched with statistically similar in vivo duration and patient variables. The retrieved inserts were analyzed for ketone oxidation and wear in the form of dimensional change. The difference in oxidation rate between highly cross-linked and conventional gamma-inert sterilized inserts did not reach statistical significance. Observations suggest that the majority of wear can be accounted for by the backside interface with the rough Ti-6Al-4V tray; however, wear measured by thickness-change rate was statistically indistinguishable between the two bearing materials.

Does increased topside conformity in modular total knee arthroplasty lead to increased backside wear?

BACKGROUND: Modular metal-backed tibia components allow surgeons intraoperative flexibility. Although it is known that modular tibia components introduce the possibility for backside wear resulting from relative motion between the polyethylene insert and the tibial baseplate, it is not known to what degree variability in the conformity of the tibial polyethylene liner itself might contribute to backside wear. QUESTIONS/PURPOSES: The purpose of this study was to determine whether a flat, cruciate-retaining tibial polyethylene bearing generates less backside wear than a more conforming (curved) tibial polyethylene bearing in an analysis of specimens explanted during revision surgery. METHODS: The study included 70 total knee inserts explanted at revision surgery, all implanted and explanted by the same surgeon. Two different cruciate-retaining insert options in an otherwise similar knee system were used: one with a curved-on-flat (17) articular geometry and one with a highly conforming curved-on-curved design (53); both groups were sequential cohorts. The composite backside wear depth for the insert as well as the volume of backside wear was measured and compared between groups. RESULTS: The median linear backside-normalized wear for the posterior lipped inserts was 0.0063 mm/year (range, 0-0.085 mm/year), which was lower than for the curved inserts at 0.05 mm/year (range, 0.00003-0.14 mm/year) (p<0.001). The median calculated volumetric backside-normalized wear for the posterior lipped inserts was 14.2 mm3/year (range, 0-282.8 mm3/year) compared with 117 mm3/year (range, 2.1-312 mm3/year) for the curved inserts (p<0.001). CONCLUSIONS: In this retrieval study, more conforming tibial inserts demonstrated more backside-normalized wear than the flatter designs. This suggests that in this modular total knee arthroplasty design, higher articular conformity to address the issues of high bearing contact stress comes at a price: increased torque transmitted to the backside insert-to-tray interface. We suggest further work be undertaken to examine newer insert designs to evaluate if our conclusions hold true with the newer generation locking mechanism, tibial tray finish and polyethylene designs, as more highly conforming tibial inserts are introduced into the market. LEVEL OF EVIDENCE: Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Knee wear measured in retrievals: a polished tray reduces insert wear.

BACKGROUND: Polyethylene wear is often cited as the cause of failure of TKA. Rotating platform (RP) knees show notable surface damage on the rotating surface raising concerns about increased wear compared to fixed bearing inserts. QUESTIONS/PURPOSES: We therefore addressed the following questions: Is wear in RP inserts increased compared to that in fixed bearing inserts? Does the surface roughness of the tibial tray have a measurable impact on in vivo wear of modular knees? And does wear rate differ between posterior stabilized (PS) and cruciate retaining (CR) knees? METHODS: We compared wear in two series of retrieved knee devices: 94 RP mobile bearings with polished cobalt-chrome (CoCr) trays and 218 fixed bearings with both rough titanium (Ti) and polished CoCr trays. Minimum implantation time was 0.4 months (median, 36 months; range, 0.4-124 months) and 2 months (median, 72 months; range, 2-179 months) for the RP and fixed bearing series, respectively. RESULTS: Wear rate was lower for RP inserts than for fixed bearing inserts. Backside wear rate was lower for fixed bearing inserts mated to polished CoCr trays than for inserts from rough Ti trays. Inserts against polished trays (RP or fixed bearing) showed no increase in wear rate increase over time. Wear rate of PS knees was similar to that of CR knees. CONCLUSIONS: We found mobile bearing knees have reduced wear rate compared to fixed bearings, likely due to the polished CoCr tibial tray surface. Fixed bearing inserts in polished CoCr trays wear less than their counterparts in rough Ti trays, and the wear rate of inserts from polished CoCr trays does not appear to increase with time.

Femoral stem fracture and in vivo corrosion of retrieved modular femoral hips.

A series of 78 retrieved modular hip devices were assessed for fretting and corrosion. Damage was common at both the head-neck junction (54% showing corrosion; 88% showing fretting) and at the stem-sleeve junction (88% corrosion; 65% fretting). Corrosion correlated to in vivo duration, patient activity, and metal (vs ceramic) femoral heads but did not correlate to head carbon content. Femoral stem fatigue fracture was observed in seven retrievals; all had severe corrosion, were under increased stress, and were in vivo longer than the non-fractured cohort. This study emphasizes the potential for stem fracture when small diameter femoral stems with large offsets are used in heavy and active patients. Designs which reduce fretting and corrosion in modular implants is warranted as patients demand longer lasting implants.

Metal transfer on a ceramic head with a single rim contact.

A notable feature of retrieved ceramic-on-ceramic hips is metal transfer on the femoral head, which is an important alteration of the bearing surface. This report documents metal transfer streaks on a ceramic femoral head resulting from discrete subluxations, which occurred intraoperatively during reduction and stability testing. An important implication is that metal transfer can occur whenever a femoral head is reduced into the liner during surgery or from in vivo subluxation/dislocation. If a ceramic liner is recessed below a raised metal rim, care should be taken to prevent head-to-rim contact during intraoperative reductions and stability testing. If metal transfer occurs during final surgical reduction of the hip, its presence may remain undetected, and detrimental effects are present from the time of surgery.

The effect of radiation dose on the tensile and impact toughness of highly cross-linked and remelted ultrahigh-molecular weight polyethylenes.

Several highly cross-linked and remelted ultrahigh-molecular weight polyethylenes (UHMWPE) were introduced in 1998 as bearing materials for orthopaedic implants to achieve superior wear performance. However, gamma radiation and the subsequent postirradiation thermal treatment are associated with decreased mechanical properties such as ductility, toughness, and fatigue strength compared to noncross-linked materials. The purpose of this study was (a) to characterize the toughness (tensile and impact) of highly cross-linked and remelted UHMWPE of varying doses (0-255 kGy) and (b) determine whether a correlation exists between both toughness measures. As radiation dose increased, tensile toughness and impact toughness were shown to decrease in a nonlinear fashion; largely a result of the decrease in ductility, which was also observed. Impact toughness and tensile toughness were also found to be strongly correlated to one another (R(2) = 0.97).

Evaluating the suitability of highly cross-linked and remelted materials for use in posterior stabilized knees.

Posterior stabilized (PS) knee designs are a popular choice for cruciate sacrificing knee arthroplasty procedures. The introduction of PS inserts fabricated from highly cross-linked and remelted Ultra High Molecular Weight Polyethylene (UHMWPE) has recently generated concern as these materials have been shown to possess reduced mechanical properties. This study investigated whether highly cross-linked and remelted UHMWPE material (referred to as XRP) can be expected to perform similarly to historical gamma-air polyethylene, which has suffered few reported incidences of tibial post failure. Never-implanted gamma-air PS tibial inserts shelf-aged 14 years were examined and compared to XRP materials. Evaluation of oxidation levels, impact toughness, and fatigue strength demonstrated never-implanted gamma-air PS tibial inserts to possess nonuniform mechanical properties. Despite severe oxidation along the exterior of gamma-air tibial posts, comparatively low oxidation levels at the center of the tibial posts corresponded to sufficiently high mechanical properties. XRP material (75 kGy) showed superior impact toughness over shelf aged gamma-air material; however, tibial post fatigue testing demonstrated XRP material (100 kGy) to be less resistant to fatigue failure than historical gamma-air material. Results from this study indicate that XRP materials (100 kGy) may demonstrate an inferior resistance to tibial post failure than historical polyethylene.

Clinical wear measurement on low contact stress rotating platform knee bearings.

Whereas fixed-bearing total knee arthroplasty (TKA) designs secure the polyethylene bearing to the tibial tray, mobile-bearing TKAs allow the bearing to move relative to the tray. This study evaluated wear performance of the rotational articulation of the Low Contact Stress Rotating Platform mobile-bearing TKA (DePuy, Warsaw, Ind) by analyzing 100 retrievals. All retrieved bearings showed rotation surface damage, but severity of the damage did not correlate with duration. Rotation surface damage appeared to be caused by contaminant particles, which produced curvilinear scratches that were longer than the normal rotational excursion of the knee. Wear measurement indicated that wear was relatively uniform, long-term wear rates were low (mean, 54 mm(3)/y for durations >2 years) and decreased with longer duration, and damaged appearance did not correspond to high wear.

Rim cracking of the cross-linked longevity polyethylene acetabular liner after total hip arthroplasty.

BACKGROUND: Studies have suggested that cross-linked polyethylene bearings reduce wear rates from 40% to 100% compared with conventional polyethylene. However, the reduced mechanical properties of highly cross-linked polyethylene have the potential to be a limiting factor in device performance. We reviewed a series of retrieved acetabular liners with a fracture of the superior rim to assess the factors that played a role in their failure. METHODS: Four Longevity acetabular bearings, which had been retrieved from two patients after seven to twenty-seven months in vivo, were visually examined for clinical damage, were assessed with use of Fourier transform infrared spectroscopy to determine the level of oxidation, and were analyzed for mechanical properties and fracture surface characterization. Control data were obtained from never-implanted devices and from global reference ultrahigh molecular weight polyethylene bar stock as an industry calibration material. RESULTS: All four retrieved liners demonstrated articular surface wear modes, which in most cases were rated as moderate, and none were rated as severe. All showed cracking or rim failure of the liner at the superior aspect along the groove in the polyethylene that engages the locking ring of the shell. The retrieved liners had no measurable oxidation, and the mechanical properties were comparable with those of never-implanted material. CONCLUSIONS: There was no notable in vivo degradation of the retrieved liners. Important factors related to failure appear to be thin polyethylene at the cup rim, relatively vertical cup alignment, and the material properties of the highly cross-linked polyethylene that are decreased relative to conventional polyethylene. The critical dimension with respect to rim failure in modular liners appears to be the minimum thickness at the equatorial region.

Evaluation of oxidation and fatigue damage of retrieved crossfire polyethylene acetabular cups.

BACKGROUND: Crossfire cross-linked polyethylene is produced differently from other cross-linked polyethylene materials; a below-melt-temperature annealing process is used with the goal of avoiding compromised mechanical properties. The present study was performed to evaluate retrieved Crossfire acetabular cups to determine whether they had oxidized and to what extent oxidation might have influenced their clinical performance. METHODS: Eleven acetabular cups were received at retrieval and a twelfth acetabular cup was received two years post-retrieval over a period of four years. None were retrieved because of polyethylene wear or fatigue. The cups had been in vivo from 0.1 to 5.3 years. Each was examined visually, clinical fatigue damage was rated, and oxidation was measured with use of Fourier transform infrared spectroscopy. RESULTS: The cups exhibited oxidation that varied with its location on the cup: the oxidation value was generally low on the articular surface but more than an order of magnitude higher value on the rim. Maximum rim oxidation correlated significantly with the time in vivo (Spearman rho = 0.734, p = 0.010). Oxidation was identified visually by a white band in thin sections on the rim of seven of the cups and on the articular surface of one of these seven cups. Six of the seven cups also exhibited clinical fatigue damage. Eight of the twelve cups exhibited evidence of impingement or dislocation. CONCLUSIONS: Acetabular cups made of Crossfire polyethylene oxidized to a measurable degree. The oxidation-related reduction of polyethylene mechanical properties was sufficient to allow the fatigue damage seen in these retrieved cups.

In vivo oxidation of gamma-barrier-sterilized ultra-high-molecular-weight polyethylene bearings.

gamma-Barrier packaging is shown to be effective in preventing oxidation of polyethylene during shelf storage and in addressing the problem of early fatigue failure seen in gamma-air-sterilized bearings with long shelf-storage before implantation. The series of gamma-barrier retrievals studied suggests that oxidation occurs in the body via the same mechanism as seen in gamma-air-sterilized bearings. A critical oxidation level is identified above which polyethylene bearings are susceptible to fatigue damage after sufficient cycles of use. Although critical oxidation was not reached in the majority of the retrieved gamma-barrier bearings studied, in vivo oxidation appears to follow an exponential increase with time. The result of in vivo oxidation is expected to be loss of mechanical properties, and susceptibility of polyethylene bearings to eventual fatigue failure.

Analysis of wear asymmetry in a series of 94 retrieved polyethylene tibial bearings.

Knee joint kinematics is the focus of a significant amount of experimental study for the purpose of knee prosthesis design and for testing the wear of current and prospective bearing materials. This study reports the wear assessment of a series of 94 explanted tibial bearings of various designs and manufacturers and focuses on the extent to which clinical wear is symmetric in the medial-lateral aspect, or is indicative of a systematic asymmetry that would be informative to the design and testing of knee prostheses or surgical practice. Wear assessment of the series of retrievals indicates that, statistically, there was more clinical wear on the medial side. Patterns of wear varied greatly among individual knees; a majority showed very similar extents of wear on the medial and lateral sides, however there were cases with significantly more wear on one condylar articulation than the other. Evidence of edge loading, whereby the femoral component articulates at the margin of the tibial bearing, was common. It was seen most frequently in the central zone of the medial condylar area, and, like the overall wear, edge loading was significantly more frequent on the medial side of bearings. Total bearing wear was seen to generally increase with time over the 208 months of in vivo duration covered by the retrievals in the study. The medial-lateral asymmetry of the wear does not appear to be significantly dependent on duration, however.

Comparison of cross-linked polyethylene materials for orthopaedic applications.

Cross-linked polyethylenes are being marketed by orthopaedic manufacturers to address the problem of osteolysis caused by polyethylene particulate wear debris. Wear testing of these cross-linked polyethylenes in hip simulators has shown dramatic reduction in wear rate compared with standard ultrahigh molecular weight polyethylene, either gamma irradiated in air or nitrogen - or ethylene oxide-sterilized. However, this reduction in wear rate is not without cost. The cross-linking processes can result in materials with lower mechanical properties than standard ultrahigh molecular weight polyethylene. To evaluate the effect of the various cross-linking processes on physical and mechanical properties of ultrahigh molecular weight polyethylene, commercially available cross-linked polyethylenes from six orthopaedic manufacturers were tested. This study was the culmination of collaboration with these manufacturers, who provided cross-linked polyethylene for this study, wear characteristics of the material they provided, and review of the physical and mechanical properties measure for their polyethylene. Cross-linked materials were evaluated as received and after an accelerated aging protocol. Free radical identity and concentration, oxidation, crystallinity, melt temperature, ultimate tensile strength, elongation at break, tensile stress at yield, and toughness are reported for each material. By comparing these physical and mechanical properties, surgeons can evaluate the trade-off that results from developing materials with substantially lower wear rates.