Polyethylene cross-linking by two different methods reduces acetabular liner wear in a hip joint wear simulator

Titanium acetabular cementless cups combined with highly crosslinked polyethylene liner have very low rates of aseptic loosening

BACKGROUND

There is a clear tendency towards cementless acetabular components in primary total hip arthroplasty.

OBJECTIVE

The aim of this study was to assess the long-term clinical and radiological outcome of titanium cementless acetabular cups when combined with a highly crosslinked polyethylene liner.

METHODS

This study is a retrospective follow-up of 67 cups in 64 patients. Clinical outcomes were assessed using the Harris Hip Score. Radiolucent lines, osteolysis and loosening were assessed radiologically. Implant survival was determined using the Kaplan-Meier analysis.

RESULTS

The average Harris Hip Score at follow-up was 80.3 ± 14.5. Signs of osteolysis were observed in 7.1% of the radiographs. No aseptic loosening of the cup was reported. Survival of the cup with aseptic loosening as an endpoint was 100%. A comparison with 19 other studies using the same material combination demonstrated very similar results. All manufacturers with available studies have at least one report of 100% survivorship at 10-year follow-up for their titanium cup and highly crosslinked polyethylene acetabular component combination with aseptic loosening as an endpoint.

CONCLUSIONS

The data suggests that the rate of aseptic loosening of a titanium cup combined with a highly-crosslinked polyethylene liner at 10-year follow-up could be as low as 0%.

Hip simulator testing of the taper-trunnion junction and bearing surfaces of contemporary metal-on-cross-linked-polyethylene hip prostheses

Adverse reaction to metal debris released from the taper-trunnion junction of modular metal-on-polyethylene (MoP) total hip replacements (THRs) is an issue of contemporary concern. Therefore, a hip simulator was used to investigate material loss, if any, at both the articulating and taper-trunnion surfaces of five 32-mm metal-on-cross-linked-polyethylene THRs for 5 million cycles (Mc) with a sixth joint serving as a dynamically loaded soak control. Commercially available cobalt-chromium-molybdenum femoral heads articulating against cross-linked polyethylene (XLPE) acetabular liners were mounted on 12/14 titanium (Ti6Al4V) trunnions. Weight loss (mg) was measured gravimetrically and converted into volume loss (mm³ ) for heads, liners, and trunnions at regular intervals. Additionally, posttest volumetric wear measurements of the femoral tapers were obtained using a coordinate measuring machine (CMM). The surface roughness (Sa) of femoral tapers was measured posttest. After 5 Mc, the mean volumetric wear rate for XLPE liners was 2.74 ± 0.74 mm³ /Mc. The CMM measurements confirmed material loss from the femoral taper with the mean volumetric wear rate of 0.045 ± 0.024 mm³ /Mc. The Sa on the worn area of the femoral taper showed a significant increase (p < 0.001) compared with the unworn area. No other long-term hip simulator tests have investigated wear from the taper-trunnion junction of contemporary MoP THRs. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 108B:156-166, 2020.

Comparison of CMM and Micro-CT Volumetric Analysis of Polyethylene Tibial Knee Inserts in Total Knee Replacement

Ultrahigh molecular weight polyethylene (UHMWPE) bearings are used widely in orthopaedic joint replacement as a prominent material for improving the longevity, which is dramatically influenced by wear of polyethylene. Polyethylene tibial knee components from knee simulators under two different input conditions were analyzed using CMM and Micro-CT measurement techniques for volumetric loss with gravimetric measurement as reference. Based on the coordinates and image slices obtained, the surface curve fitting and image digitization methodology were used for the creation of nominal “original” surface in the case of no prewear data provided. The comparison results indicate that gravimetric remains the gold standard and the CMM measurement took less time and had better precision, accuracy, and repeatability compared to Micro-CT measurement technique.

CMM-Based Volumetric Assessment Methodology for Polyethylene Tibial Knee Inserts in Total Knee Replacement

Total knee replacement is a common surgical procedure in orthopaedics. Accurate volumetric wear assessment of the polyethylene knee inserts has been an essential subject for improving the longevity. A new CMM-based methodology was presented to determine volumetric material loss based on curve surface fitting without prewear data, CAD model, or original design of drawings. Both computational and experimental simulated volume removal tests were run to validate the methodology by comparing with the gravimetric measurements. The volume and linear wear of the tibial inserts were calculated using the presented method based on the coordinates acquired by the CMM. The results indicate that the methodology is adequate for clinically retrieved tibial inserts where no prewear data are provided. This technique can also be used for biotribological study of other polyethylene components, since wear and damage can be assessed visually and volumetrically.

Equivalent wear performance of dual mobility bearing compared with standard bearing in total hip arthroplasty: in vitro study

PURPOSE

Osteolysis in total hip arthroplasty (THA) depends on polyethylene wear and dictates the survival of the prosthesis. Dual mobility in THAs, which is claimed to reduce dislocation risk, has very good long-term clinical results. However, little is known about how the liner wears in this design, compared to the standard single mobility model.

METHODS

A comparative study looking at wear of a conventional ultra-high-molecular-weight polyethylene liner, using gravimetric measurement, between dual mobility implants and standard implants, was performed on a simulator in accordance with a normed protocol based on the same dimensions, environmental conditions and stresses. A linear regression test was employed.

RESULTS

Under the same conditions (loading, cycles, sterilization, material and surface roughness), the gravimetric wear (for conventional polyethylene) is comparable between a standard and a dual mobility cup. This correlates to ten year follow-up results of dual mobility cup.

DISCUSSION - CONCLUSION

This in vitro equivalent wear serves to confirm the very good long-term clinical results observed with dual mobility bearing, whose use should not be restricted by concerns about increased polyethylene wear.

The Wear Rate of Highly Cross-Linked Polyethylene in Total Hip Replacement Is Not Increased by Large Articulations: A Randomized Controlled Trial

BACKGROUND

Larger articulations reduce the risk of dislocation following primary total hip arthroplasty, leading to increased use of these articulations. The wear rate of highly cross-linked polyethylene (XLPE) is low in standard-diameter articulations but remains unclear in larger articulations. The aim of this randomized controlled trial was to compare the mean wear rates of 36-mm and 28-mm metal-on-XLPE articulations between 1 and 3 years postoperatively.

METHODS

Fifty-six elderly patients undergoing primary total hip arthroplasty were randomized intraoperatively to receive either a 36-mm or 28-mm metal-on-XLPE articulation. Factors that may affect wear were controlled by study design. Wear was measured using radiostereometric analysis.

RESULTS

Mean annual proximal wear rates between 1 and 3 years were 0.00 and 0.01 mm/yr for the 36 and 28-mm articulation cohorts, respectively. No patient had a proximal wear rate of >0.1 mm/yr. Mean wear was very low in all directions, and the wear rate of 36-mm articulations was not significantly greater than that of 28-mm articulations on the basis of proximal, medial 2-dimensional, and 3-dimensional wear.

CONCLUSIONS

The wear rate of a larger 36-mm metal-on-XLPE articulation between 1 and 3 years following primary total hip arthroplasty was low and no greater than that of a 28-mm articulation. However, before a 36-mm metal-on-XLPE articulation is widely recommended, particularly in young active patients, long-term wear rates and association between wear and periprosthetic osteolysis should be determined.

LEVEL OF EVIDENCE

Therapeutic Level II. See Instructions for Authors for a complete description of levels of evidence.

MicroRNA-mediated immune modulation as a therapeutic strategy in host-implant integration

The concept of implanting an artificial device into the human body was once the preserve of science fiction, yet this approach is now often used to replace lost or damaged biological structures in human patients. However, assimilation of medical devices into host tissues is a complex process, and successful implant integration into patients is far from certain. The body's immediate response to a foreign object is immune-mediated reaction, hence there has been extensive research into biomaterials that can reduce or even ablate anti-implant immune responses. There have also been attempts to embed or coat anti-inflammatory drugs and pro-regulatory molecules onto medical devices with the aim of preventing implant rejection by the host. In this review, we summarize the key immune mediators of medical implant reaction, and we evaluate the potential of microRNAs to regulate these processes to promote wound healing, and prolong host-implant integration.

Head-neck taper corrosion in hip arthroplasty

Modularity at the head-neck junction of the femoral component in THA became popular as a design feature with advantages of decreasing implant inventory and allowing adjustment of leg length, offset, and soft tissue balancing through different head options. The introduction of a new modular interface to femoral stems that were previously monoblock, or nonmodular, comes with the potential for corrosion at the taper junction through mechanically assisted crevice corrosion. The incidence of revision hip arthroplasty is on the rise and along with improved wear properties of polyethylene and ceramic, use of larger femoral head sizes is becoming increasingly popular. Taper corrosion appears to be related to all of its geometric parameters, material combinations, and femoral head size. This review article discusses the pathogenesis, risk factors, clinical assessment, and management of taper corrosion at the head-neck junction.

Effect of microseparation and third-body particles on dual-mobility crosslinked hip liner wear

Large heads have been recommended to reduce the risk of dislocation after total hip arthroplasty. One of the issues with larger heads is the risk of increased wear and damage in thin polyethylene liners. Dual-mobility liners have been proposed as an alternative to large heads. We tested the wear performance of highly crosslinked dual-mobility liners under adverse conditions simulating microseparation and third-body wear. No measurable increase in polyethylene wear rate was found in the presence of third-body particles. Microseparation induced a small increase in wear rate (2.9mm(3)/million cycles). A finite element model simulating microseparation in dual-mobility liners was validated using these experimental results. The results of our study indicate that highly crosslinked dual-mobility liners have high tolerance for third-body particles and microseparation.

A minimum 5-year follow-up of an oxidized zirconium femoral prosthesis used for total knee arthroplasty

BACKGROUND

Bearing surface wear remains a potential concern amongst younger patients undergoing total knee arthroplasty (TKA). Because of the potential for lower prosthetic wear rates, oxidized zirconium is a potentially attractive bearing surface for TKA in young, active patients. This material is also well suited for patients with suspected nickel sensitivities as it has no measurable nickel content, in contrast to standard femoral bearings made of cobalt-chromium alloys. Although in vitro testing on knee wear simulators has demonstrated favorable wear characteristics of oxidized zirconium compared to traditional cobalt-chrome bearings, in vivo clinical outcome data with this novel bearing surface are sparse.

METHODS

We retrospectively reviewed 109 consecutive TKAs in 82 patients at a minimum of five years postoperatively to determine prosthetic survivorship and to assess whether any adverse clinical consequences could be attributed to this bearing. Knees were evaluated with Knee Society scores, UCLA activity scores, clinical examinations, and radiographs. The mean age for this cohort was 58.7 years.

RESULTS

Survivorship free of bearing related complications was 100% at a minimum of five years post-surgery. There were no revisions for loosening, osteolysis, implant failure, or deep infection. There were no knees with radiographic failure, visible wear, loosening, or osteolysis.

CONCLUSION

Oxidized zirconium remains an attractive option for patients with nickel sensitivities and in those patients at risk for prosthetic wear due to young age or high activity levels.

Large femoral heads decrease the incidence of dislocation after total hip arthroplasty: a randomized controlled trial

BACKGROUND

The use of larger femoral heads has been proposed to reduce the risk of dislocation after total hip arthroplasty, but there is a lack of evidence to support this proposal. The aim of this multicenter randomized controlled trial was to determine whether the incidence of dislocation one year after total hip arthroplasty is significantly lower in association with the use of a 36-mm femoral head articulation as compared with a 28-mm articulation.

METHODS

Six hundred and forty-four middle-aged and elderly patients undergoing primary or revision arthroplasty were randomized intraoperatively to receive either a 36 or 28-mm metal femoral head on highly cross-linked polyethylene. Patients who were at high risk of dislocation (including those with dementia and neuromuscular disease) and those undergoing revision for the treatment of recurrent hip dislocation or infection were excluded. Patients were stratified according to other potential risk factors for dislocation, including diagnosis and age. Diagnosis of hip dislocation required confirmation by a physician and radiographic evidence of a dislocation.

RESULTS

Overall, at one year of follow-up, hips with a 36-mm femoral head articulation had a significantly lower incidence of dislocation than did those with a 28-mm articulation (1.3% [four of 299] compared with 5.4% [seventeen of 316]; difference, 4.1% [95% confidence interval, 1.2% to 7.2%]) when controlling for the type of procedure (primary or revision) (p = 0.012). The incidence of dislocation following primary arthroplasty was also significantly lower for hips with a 36-mm femoral head articulation than for those with a 28-mm articulation (0.8% [two of 258] compared with 4.4% [twelve of 275]; difference, 3.6% [95% confidence interval, 0.9% to 6.8%]) (p = 0.024). The incidence of dislocation following revision arthroplasty was 4.9% (two of forty-one) for hips with a 36-mm articulation and 12.2% (five of forty-one) for hips with a 28-mm articulation; this difference was not significant with the relatively small sample size of the revision group (difference, 7.3% [95% confidence interval, -5.9% to 21.1%]) (p = 0.273).

CONCLUSIONS

Compared with a 28-mm femoral head articulation, a larger 36-mm articulation resulted in a significantly decreased incidence of dislocation in the first year following primary total hip arthroplasty. However, before a 36-mm metal-on-highly cross-linked polyethylene articulation is widely recommended, the incidence of late dislocation, wear, periprosthetic osteolysis, and liner fracture should be established.

The John Charnley Award: an accurate and sensitive method to separate, display, and characterize wear debris: part 1: polyethylene particles

BACKGROUND

Numerous studies indicate highly crosslinked polyethylenes reduce the wear debris volume generated by hip arthroplasty acetabular liners. This, in turns, requires new methods to isolate and characterize them.

QUESTIONS/PURPOSES

We describe a method for extracting polyethylene wear particles from bovine serum typically used in wear tests and for characterizing their size, distribution, and morphology.

METHODS

Serum proteins were completely digested using an optimized enzymatic digestion method that prevented the loss of the smallest particles and minimized their clumping. Density-gradient ultracentrifugation was designed to remove contaminants and recover the particles without filtration, depositing them directly onto a silicon wafer. This provided uniform distribution of the particles and high contrast against the background, facilitating accurate, automated, morphometric image analysis. The accuracy and precision of the new protocol were assessed by recovering and characterizing particles from wear tests of three types of polyethylene acetabular cups (no crosslinking and 5 Mrads and 7.5 Mrads of gamma irradiation crosslinking).

RESULTS

The new method demonstrated important differences in the particle size distributions and morphologic parameters among the three types of polyethylene that could not be detected using prior isolation methods.

CONCLUSION

The new protocol overcomes a number of limitations, such as loss of nanometer-sized particles and artifactual clumping, among others.

CLINICAL RELEVANCE

The analysis of polyethylene wear particles produced in joint simulator wear tests of prosthetic joints is a key tool to identify the wear mechanisms that produce the particles and predict and evaluate their effects on periprosthetic tissues.

Cross-linked ultra-high-molecular weight polyethylene liner and ceramic femoral head in total hip arthroplasty: a prospective study at 5 years follow-up

BACKGROUND

Recent data indicate that enhanced wear resistance can be obtained with new cross-linked ultra-high-molecular weight polyethylene (CL-UHMWPE) liners, in comparison with previous-generation liners. The current prospective, cohort study was undertaken to analyse whether the use of a new CL-UHMWPE (Rexpol) results in a lower wear rate than ultra-high-molecular weight polyethylene (UHMWPE) in a group of similar patients undergoing total hip arthroplasty (THA). This study provides the first clinical data with this particular CL-UHMWPE.

METHODS

Between January 2001 and December 2001, patients underwent THA with biconical threaded cups and ceramic femoral heads using either a CL-UHMWPE liner (n = 50) or a regular UHMWPE liner (n = 57). At the time of the final 5-year evaluation, there were 48 patients available in the CL-UHMWPE and 54 patients available in the UHMWPE group.

RESULTS

After a mean 5.6 years follow-up, there were no significant differences between the groups in Harris Hip Score outcomes. However, there was a 69.1% decrease in wear at 5 years for the CL-UHMWPE group in comparison with the UHMWPE group (p < 0.01).

CONCLUSION

The use of CL-UHMWPE significantly reduced the risk of polyethylene wear in patients undergoing THA with biconical threaded cups and ceramic femoral heads. Further follow-up is needed to determine how this will influence long-term outcomes.

Fracture of a cross-linked polyethylene liner: a multifactorial issue

A limited number of reports have detailed the cause of fracture of a highly cross-linked polyethylene liner. Typically, the fractures have occurred in a region of thin and/or unsupported polyethylene, in association with superiorly directed edge loading conditions secondary to an excessively inclinated acetabular component. This case report details an unusual fracture mechanism of a 5-mrad cross-linked liner caused by horizontal loading conditions. The report details several factors that were felt to be etiologic including the specific liner locking mechanism. The treatment options are discussed.

Wear and Creep of Highly Crosslinked Polyethylene against Cobalt Chrome and Ceramic Femoral Heads

The wear and creep characteristics of highly crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) articulating against large-diameter (36 mm) ceramic and cobalt chrome femoral heads have been investigated in a physiological anatomical hip joint simulator for 10 million cycles. The crosslinked UHMWPE/ceramic combination showed higher volume deformation due to creep plus wear during the first 2 million cycles, and a steady-state wear rate 40 per cent lower than that of the crosslinked UHMWPE/cobalt chrome combination. Wear particles were isolated and characterized from the hip simulator lubricants. The wear particles were similar in size and morphology for both head materials. The particle isolation methodology used could not detect a statistically significant difference between the particles produced by the cobalt chrome and alumina ceramic femoral heads.

Total Hip Wear Assessment: A Comparison Between Computational and In Vitro Wear Assessment Techniques Using ISO 14242 Loading and Kinematics

In the present study a direct comparison was made between in vitro total hip wear testing and a computational analysis considering the effects of time and a nonlinear stress-strain relationship for ultrahigh molecular weight polyethylene (UHMWPE) at 37°C. The computational simulation was made correct through calibration to experimental volumetric wear results, and the predicted damage layout on the acetabular liner surface was compared with results estimated from laser scanning of the actual worn specimens. The wear rates for the testing specimens were found to be 17.14±1.23 mg/106 cycles and 19.39±0.79 mg/106 cycles, and the cumulative volumetric wear values after 3×106 cycles were 63.70 mm3 and 64.02 mm3 for specimens 1 and 2, respectively. The value of the calibrated wear coefficient was found to be 5.32(10−10) mm3/N mm for both specimens. The major difference between the computational and experimental wear results was the existence of two damage vectors in the experimental case. The actual location of damage was virtually the same in both cases, and the maximum damage depth of the computational model agreed well with the experiment. The existence of multiple wear vectors may indicate the need for computational approaches to account for multidirectional sliding or strain hardening of UHMWPE. Despite the limitation in terms of describing the overall damage layout, the present computational model shows that simulation can mimic some of the behavior of in vitro wear.

The effect of oxidative aging on the wear performance of highly crosslinked polyethylene knee inserts under conditions of severe malalignment

First-generation crosslinked polyethylene developed for total hip arthroplasty has not gained wide acceptance for knee arthroplasty because of the increased potential for failure under high stresses and the increased risk for oxidative damage caused by free radicals. Sequentially crosslinked polyethylene (SQXL) is a second-generation crosslinked polyethylene that is reported to reduce the level of free radicals and preserve mechanical properties. Three groups of ultrahigh molecular weight polyethylene inserts were wear tested after artificial aging as per ASTM F2003: gamma-irradiated in air (GA-aged), sequentially crosslinked (SQXL-aged), and electron-beam irradiated and remelted (EBeam-aged). Inserts were tested in an AMTI knee wear simulator under malalignment conditions that were two standard deviations from the mean reported for computer-navigated and conventionally aligned techniques. GA-aged inserts delaminated by 500,000 cycles, and were severely damaged after 1 million cycles. None of the highly crosslinked inserts (SQXL-aged or EBeam-aged) delaminated or showed any signs of severe wear. Mean wear rate for GA-aged inserts was 124.6 +/- 49.4 mg/million cycles. Mean wear rates for SQXL-aged and EBeam-aged inserts were significantly lower (1.74 +/- 0.3 and 4.72 +/- 0.7, respectively). These results support the low levels of free radicals and preservation of mechanical properties reported in second-generation crosslinked ultrahigh molecular weight polyethylene.

Wear resistance of highly cross-linked and remelted polyethylenes after ion implantation and accelerated ageing

Ion implantation may provide medical polyethylenes with excellent mechanical and tribological properties, helping to lower the risk of long-term osteolysis. Highly crosslinked and remelted polyethylenes, materials currently used as soft components in artificial joints, were implanted with N+ and He+ ions at different ion fluences. The mechanical and tribological properties under distilled water lubrication at body temperature were assessed after ion implantation by means of microhardness and pin-on-disc tests respectively. Thus, the influences of the ionic species and implantation dose on surface hardness, friction coefficient, and wear factor were fully characterized. Furthermore, the tribological behaviour was evaluated after an accelerated ageing protocol (120 degrees C for 36h). Ion implantation increased the surface hardness, as well as friction coefficients, and decreased the wear factors especially at the highest doses. Also, even though all artificially aged materials showed a worse wear behaviour, polyethylenes implanted with either N+ or He+ at the highest doses maintained a relatively good wear factor in comparison with the aged non-implanted material. The origins of these modifications are discussed according to the effects of ion implantation on the microstructure of the polymer.

Highly cross-linked polyethylene in total knee arthroplasty: in the affirmative

Polyethylene bearing failure has been cited as one of the leading causes of knee arthroplasty revision surgery. In 1998, highly cross-linked polyethylene was introduced for clinical use in total hip arthroplasty. Altered mechanical properties in first-generation highly cross-linked polyethylene did exhibit clinical failures, such as post fractures in total knee arthroplasty. Remelting alters the integrity of polyethylene. Some approaches to mitigate this include mechanical deformation, vitamin E incorporation, and sequential irradiation and annealing. Forces and stresses in total knee arthroplasty differ substantially from the wear mechanisms and forces seen in total hip arthroplasty. There is now considerable interest in the clinical use of highly cross-linked polyethylene for the knee. The use of sequentially annealed, highly cross-linked polyethylene, based on bench top data, appears to be promising for use in total knee arthroplasty. One should be aware that all highly cross-linked polyethylenes are not manufactured, nor processed, in the same manner. Marked and significant differences may exist between products.

A comparison of the penetration rate of two polyethylene acetabular liners of different levels of cross-linking. A prospective randomised trial

We carried out a prospective randomised study designed to compare the penetration rate of acetabular polyethylene inserts of identical design but different levels of cross-linking at a minimum of four years follow-up. A total of 102 patients (102 hips) were randomised to receive either highly cross-linked Durasul, or contemporary Sulene polyethylene inserts at total hip replacement. A single blinded observer used the Martell system to assess penetration of the femoral head. At a mean follow-up of 4.9 years (4.2 to 6.1) the mean femoral head penetration rate was 0.025 mm/year (SD 0.128) in the Durasul group compared with 0.106 mm/year (SD 0.109) in the Sulene group (Mann-Whitney test, p = 0.0027). The mean volumetric penetration rate was 29.24 mm(3)/year (SD 44.08) in the Durasul group compared with 53.32 mm(3)/year (SD 48.68) in the Sulene group. The yearly volumetric penetration rate was 55% lower in the Durasul group (Mann-Whitney test, p = 0.0058). Longer term results are needed to investigate whether less osteolysis will occur.

Remelting of highly cross-linked polyethylene worn under laboratory conditions

It has been suggested that apparent wear damage in highly cross-linked polyethylene acetabular liners can be removed with subsequent remelting of retrieved liners. To test this hypothesis, we remelted liners that had been previously tested under controlled laboratory conditions and that had experienced nonzero wear rates and visible wear damage. Five liner groups were examined: three with a range of irradiation doses without heat treatment, and two irradiated to 100 kGys, one with remelting, and the other annealing. All groups had been worn in a hip simulator under impingement conditions that produced nonzero wear rates and loss of machining marks. Each liner was cut into quadrants that were graded for wear damage before and after posttest remelting. Cross-linked liners not previously heat-treated lost all prior damage and all machining marks. Remelted liners and three of six annealed liners experienced only a slight return of machining marks at the interface of the burnished area and the remaining intact machining marks. Our experiments represent a severe wear case but demonstrate removal of material from the surface through measurable wear prevents the return of identifiable machining marks despite remelting.

Analysis of retrieved acetabular components of three polyethylene types

The polyethylene used in total hip arthroplasty has gone through many changes over the past several decades, including consolidation processes, resin types, method of sterilization, packaging, and the extent of crosslinking. To isolate the in vivo performance of material changes from implant system design changes, we assessed the postretrieval surface wear and damage of components made from three different polyethylene types used in a single implant system. The polyethylene types investigated are representative of the sequentially available bearing materials that have dominated use in total hip arthroplasty over the last several decades. Forty-six components with implantation durations of 12 to 96 months were assessed for surface wear and damage and for socket wear and creep volume change. Acetabular components made from highly crosslinked polyethylene had a 50% lower total damage score than components made from polyethylene that was either gamma-sterilized in air or in nitrogen. The wear and creep socket volume change was 80% and 90% lower for the highly crosslinked components compared with the gamma-sterilized in air and nitrogen groups, respectively. These data of direct component measurement are consistent with earlier predictions that recent changes in polyethylene material processing can lead to clinically improved bearing performance.

The biology of aseptic osteolysis

Total hip arthroplasty is one of the most commonly performed and successful elective orthopaedic procedures. However, numerous failure mechanisms limit the long-term success including aseptic osteolysis, aseptic loosening, infection, and implant instability. Aseptic osteolysis and subsequent implant failure occur because of a chronic inflammatory response to implant-derived wear particles. To reduce particulate debris and their consequences, implants have had numerous design modifications including high-molecular-weight polyethylene sockets and noncemented implants that rely on bone ingrowth for fixation. Surgical techniques have improved cementation with the use of medullary plugs, cement guns, lavage of the canal, pressurization, centralization of the stem, and reduction in cement porosity. Despite these advances, aseptic osteolysis continues to limit implant longevity. Numerous proinflammatory cytokines, such as interleukin-1, interleukin-6, tumor necrosis factor-alpha, and prostaglandin E2, have proosteoclastogenic effects in response to implant-derived wear particles. However, none of these cytokines represents a final common pathway for the process of particle-induced osteoclast differentiation and maturation. Recent work has identified the fundamental role of the RANKL-RANK-NF-kappaB pathway not only in osteoclastogenesis but also in the development and function the immune system. Thus, the immune system and skeletal homeostasis may be linked in the process of osteoclastogenesis and osteolysis.

Wear and biological activity of highly crosslinked polyethylene in the hip under low serum protein concentrations

Crosslinked ultra-high molecular weight polyethylene (UHMWPE) has been developed and introduced into clinical practice in order to reduce wear in the hip. Zero wear of highly crosslinked UHMWPE in vitro has been reported by some groups using lubricants with high concentrations of serum proteins in hip simulators. In contrast, some clinical studies have reported finite wear rates. The aim of this study was to compare the wear rates, wear surfaces, and wear debris produced by UHMWPE with different levels of crosslinking in a hip joint simulator, with lower, more physiologically relevant concentrations of protein in the lubricant.

The UHMWPEs were tested in the Leeds ProSim hip joint simulator against cobalt-chromium (CoCr) femoral heads. The wear particles were isolated and imaged using a field emission gun scanning electron microscope (FEGSEM) at high resolution. The highly crosslinked UHMWPEs had significantly lower wear volumes than the non-crosslinked UHMWPEs. No significant difference was found in the percentage number and percentage volume of the particles in different size ranges from any of the materials. They had similar values of specific biological activity. The functional biological activity (FBA), which takes into account the wear volume and specific biological activity, showed that the highly crosslinked UHMWPEs had lower FBAs due to their lower wear volume.

Tribology of alternative bearings

The tribological performance and biological activity of the wear debris produced has been compared for highly cross-linked polyethylene, ceramic-on-ceramic, metal-on-metal, and modified metal bearings in a series of in vitro studies from a single laboratory. The functional lifetime demand of young and active patients is 10-fold greater than the estimated functional lifetime of traditional polyethylene. There is considerable interest in using larger diameter heads in these high demand patients. Highly cross-linked polyethylene show a four-fold reduction in functional biological activity. Ceramic-on-ceramic bearings have the lowest wear rates and least reactive wear debris. The functional biological activity is 20-fold lower than with highly cross-linked polyethylene. Hence, ceramic-on-ceramic bearings address the tribological lifetime demand of highly active patients. Metal-on-metal bearings have substantially lower wear rates than highly cross-linked polyethylene and wear decreases with head diameter. Bedding in wear is also lower with reduced radial clearance. Differential hardness ceramic-on-metal bearings and the application of ceramic-like coatings reduce metal wear and ion levels.

The role of macrophages in osteolysis of total joint replacement

The osteolysis associated with conventional polyethylene on metal total joint replacements is associated with the formation of an inflamed periprosthetic membrane rich in macrophages, cytokines and implant-derived wear particles. There is a wealth of evidence to indicate that the presence and activation of macrophages in the periprosthetic tissues around joint replacements is stimulated by UHMWPE particles. Particles within the size range 0.1-1.0 microm have been shown to be the most reactive. Animal studies have provided increasing evidence that, of the milieu of cytokines produced by particle-stimulated macrophages, TNF-alpha is a key cytokine involved in osteolysis. Recent advances in the understanding of the mechanisms of osteoclastogenesis and osteoclast activation at the cellular and molecular level have indicated that bone marrow-derived macrophages may play a dual role in osteolysis associated with total joint replacement. Firstly, as the major cell in host defence responding to UHMWPE particles via the production of cytokines and secondly as precursors for the osteoclasts responsible for the ensuing bone resorption.

Validation of a micro-CT technique for measuring volumetric wear in retrieved acetabular liners

In this study, a novel micro-CT-based technique for evaluating wear in retrieved acetabular liners was introduced and validated. Six UHMWPE acetabular components ranging in implantation time from 2.7 to 14.4 years were collected and evaluated with the use of a high-resolution micro-CT scanner. The components were scanned with a uniform volumetric resolution of 74 microns (16-bit precision) with the use of a 1,024 x 1,024 in-plane image matrix. Manual rigid 3D image registration of the interior hemispherical portion of the acetabular cup with geometric primitives by trained observers allowed for isolation, visualization, and measurement of the wear volume. Results for these six components indicated an average wear rate of 65 mm(3)/year. Overall scanner error was quantified gravimetrically and associated with a maximum uncertainty of 0.6%. Intra-- and interobserver uncertainty analysis showed the method to be both accurate and repeatable.

Wear, debris, and biologic activity of cross-linked polyethylene in the knee: benefits and potential concerns

Cross-linked polyethylene currently is being introduced in knee prostheses. The wear rates, wear debris, and biologic reactivity of non cross-linked, moderately cross-linked, and highly cross-linked polyethylene have been compared in multidirectional wear tests and knee simulators. Multidirectional pin-on-plate wear studies of noncross-linked, moderately cross-linked (5 Mrad), and highly cross-linked (10 Mrad) polyethylene showed a 75% reduction in wear with the highly cross-linked material under kinematics found in the hip, but only a 33% reduction under wear in kinematics representative of the knee. In knee simulator studies, with the fixed-bearing press-fit, condylar Sigma cruciate-retaining knee under high kinematic input conditions, the wear of 5 Mrad moderately cross-linked polyethylene was 13 +/- 4 mm per 1 million cycles, which was lower (p < 0.05) than the wear of clinically used, gamma vacuum foil GUR 1020 polyethylene (23 +/- 6 mm/1 million cycles). For the low-contact stress mobile-bearing knee, the wear of moderately cross-linked polyethylene was 2 +/- 1 mm per 1 million cycles, which was lower (p < 0.05) than GVF GUR 1020 polyethylene (5 +/- 2 mm/1 million cycles). The wear debris isolated from the fixed-bearing knees showed the moderately cross-linked material had a larger percentage volume of particles smaller than 1 mum in size, compared with GVF GUR 1020 polyethylene. Direct cell culture studies of wear debris generated in sterile wear simulators using multidirectional motion showed a increase (p < 0.05) in tumor necrosis factor-alpha levels and reactivity for GUR 1050 cross-linked polyethylene debris compared with an equivalent volume of noncross-linked GUR 1050 polyethylene. The use of cross-linked polyethylene in the knee reduces the volumetric wear rate. However, the clinical significance of reduced fracture toughness, elevated wear in abrasive conditions, and the elevated tumor necrosis factor-alpha release from smaller more reactive particles warrant further investigation.

Cervical arthroplasty: material properties

Discectomy, decompression, and fusion are traditionally used to manage cervical disc disease accompanied by neural element compression that is refractory to conservative management. Concerns regarding stress at levels adjacent to fusion and possible adjacent-level degeneration as well as a desire to maintain a more normal biomechanical environment have led to investigation of cervical disc replacement as an alternative to fusion procedures. Cervical disc prostheses currently under investigation are constructed of predominantly metal-on-polyethylene or metal-on-metal bearing surfaces, and use roughened titanium surfaces and osteoconductive coatings to facilitate fixation. The unique anatomy and biomechanics of the cervical spine must be considered when extrapolating from the experience of appendicular arthroplasty and lumbar disc replacement.

Comparison of the wear rates of twenty-eight and thirty-two-millimeter femoral heads on cross-linked polyethylene acetabular cups in a wear simulator

BACKGROUND

The use of larger femoral head sizes in total hip arthroplasty has been shown to reduce the rate of dislocation and to increase the range of motion; however, such components have been associated with unacceptably high polyethylene wear rates. Studies have shown dramatic differences in wear rates between nominally cross-linked polyethylene (i.e., polyethylene that is cross-linked during radiation sterilization) and elevated cross-linked polyethylene (i.e., polyethylene that is cross-linked to a higher degree than that obtained by radiation sterilization alone). The aim of this study was to test the effect of increased cross-linking and of increased head size on polyethylene wear rates.

METHODS

Four groups of acetabular liners obtained from a single manufacturer, including 28-mm-diameter nominally cross-linked, 32-mm-diameter nominally cross-linked, 28-mm-diameter elevated cross-linked, and 32-mm-diameter elevated cross-linked polyethylene liners, were tested. Three implants from each group were tested in a twelve-station hip wear simulator with use of 90% bovine serum as a lubricant. The liners were articulated with the appropriately sized cobalt-chromium femoral head. Additional liners from each design were subjected only to the same load without motion to serve as load-soak controls to account for any weight gain due to fluid absorption. Gravimetric analysis was performed every 500,000 cycles for a total of five million cycles.

RESULTS

Nominally cross-linked liners demonstrated mean wear rates of 14.97 and 16.92 mg per million cycles for the 28-mm and 32-mm head sizes, respectively. Both of the elevated cross-linked liners had significantly lower wear rates than the nominally cross-linked liners, with a mean of 1.51 and 2.57 mg per million cycles for the 28-mm and 32-mm head sizes, respectively (p < 0.001).

CONCLUSION

The dramatic reduction in wear rates with polyethylene cross-linking, even with the larger head size, may increase the potential for use of 32-mm head components in total hip arthroplasty.

Polyethylene wear and acetabular component orientation

BACKGROUND

Polyethylene wear contributes substantially to both periprosthetic osteolysis and aseptic loosening after total hip arthroplasty. Acetabular component orientation has been shown to affect the range of motion of the hip as well as contact stresses. A series of studies was designed to test the hypothesis that acetabular component orientation can affect the magnitude and direction of polyethylene wear.

METHODS

A finite-element model was used to compute contact stresses during a normal gait cycle. Wear at the end of each gait cycle was calculated with use of the sliding-distance-coupled finite-element formulation. The wear that was calculated with use of finite-element analysis was validated by comparison with the findings of hip wear simulator studies with the acetabular liner oriented to simulate 45 degrees and 55 degrees of abduction. In a clinical study, fifty-six patients who underwent sixty hip arthroplasties with use of a single prosthetic design were followed for as long as five years. Radiographs were analyzed to measure the abduction angle of the acetabular component and polyethylene wear.

RESULTS

The finite-element analysis predicted increased peak contact stresses with an increased abduction angle and reduced peak contact stresses with an increased anteversion angle. Linear wear rates ranging from 0.036 to 0.045 mm/million cycles were also predicted, and increased acetabular abduction angles were predicted to be associated with higher linear wear rates. In the hip wear simulator studies, significantly different wear rates were found between the cups with acetabular abduction angles of 45 degrees and 55 degrees (mean, 17.2 compared with 21.7 mg/million cycles; p < 0.01). In the clinical study, radiographic analysis revealed significant correlation between the acetabular abduction angle and the linear polyethylene wear rate. A 40% increase in mean linear polyethylene wear was seen in cups with an abduction angle of >or=45 degrees. The direction of wear was more medial (by 9.4 degrees ) in cups with an abduction angle of <45 degrees.

CONCLUSION

All three studies presented here underlined the importance of optimizing the position of the acetabular component. Careful attention to acetabular position may help to minimize wear.