Highly cross-linked polyethylene has been associated with low in vitro wear, but also has decreased in vitro ultimate yield strength. We therefore asked whether highly cross-linked polyethylene would result in lower outcome scores, wear, or early failure in a young patient population. Seventy THAs in 64 patients were performed using a highly cross-linked (electron beam-irradiated to 9 Mrads) acetabular liner and a cobalt-chrome femoral head. The average age of the patients at surgery was 41 years (range, 19–50 years). The minimum followup was 2.4 years (average, 4 years; range, 2.4–6.5 years). We recorded demographic and clinical data, including Harris hip score. Polyethylene wear measurements were analyzed with a validated, computer-assisted, edge detection method. The average Harris hip score improved from 53 to 92 at last followup. There was no evidence of acetabular or femoral loss of fixation, subsidence, or loosening. Linear wear was undetectable at this followup interval. No patient experienced catastrophic failure or underwent revision surgery. These data show low polyethylene wear rates and no catastrophic failures at early followup in a young patient cohort.
Level of Evidence: Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.
Compared with conventional polyethylene, first-generation highly cross-linked polyethylenes have low wear, but controversy exists regarding their reduced mechanical strength and/or retained free radicals. Second-generation highly cross-linked polyethylenes have been developed to reduce wear, maintain mechanical strength, and have oxidative resistance, but it is unclear whether they do so.
The primary objective of this study therefore was to determine if a second-generation annealed material has low linear wear at 5 years followup. Secondary objectives were to evaluate for overall survivorship, implant fixation, osteolysis, and effect of socket inclination on wear.
In a multicenter prospective study, we radiographically evaluated 155 patients (167 hips) at 3 years, 124 patients (132 hips) at 4 years, and 46 patients (51 hips) at 5 years. The linear head penetration rate was measured at 6 weeks, 1 year, and yearly through 5 years.
The head penetration per year after the first year of bedding-in was 0.024 mm per year at 3 years, 0.020 mm per year at 4 years, and 0.008 mm per year at 5 years. The average wear rate over 5 years was 0.015 mm per year and represents a 58% improvement over a first-generation annealed highly cross-linked polyethylene. The Kaplan-Meier survivorship (revision for any reason) was 97.8%. We revised no hip for bearing surface failure and observed no osteolysis. Socket inclination did not affect linear wear.
These data suggest the linear wear rate for a second-generation annealed highly cross-linked polyethylene is no greater than that for historic controls of first-generation highly cross-linked polyethylenes, and no untoward complications were encountered with this new material.
Level of Evidence
Level II, prognostic study. See the Guidelines for Authors for a complete description of levels of evidence.
Oxidized zirconium, a material with a ceramic surface on a metal substrate, and highly cross-linked polyethylene are two materials developed to reduce wear. We measured in vivo femoral head penetration in patients with these advanced bearings. We hypothesized the linear wear rates would be lower than those published for cobalt-chrome and standard polyethylene. We retrospectively reviewed a select series of 56 THAs in a relatively young, active patient population utilizing oxidized zirconium femoral heads and highly cross-linked polyethylene acetabular liners. Femoral head penetration was determined using the Martell computerized edge-detection method. All patients were available for 2-year clinical and radiographic followup. True linear wear was 4 μm/year (95% confidence intervals, ± 59 μm/year). The early wear rates in this cohort of relatively young, active patients were low and we believe justify the continued study of these alternative bearing surfaces.
Level of Evidence: Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.
Wear of highly cross-linked polyethylene is reportedly independent of head size. To confirm that observation we asked in our population whether head size related to wear with one type of electron beam highly cross-linked polyethylene. Of 146 hips implanted, we evaluated complete clinical and radiographic data for 90 patients (102 hips or 70%). The minimum followup was 5 years (mean, 5.7 years; range, 5–8 years). The head size was selected intraoperatively based on the size of the acetabular component and presumed risk of dislocation. Polyethylene wear measurements were performed in one experienced laboratory using the method of Martell et al. There was no hip with pelvic or femoral osteolysis. The median linear wear rate was 0.028 mm/year (mean, 0.04 mm/year), and the median volumetric wear rate was 25.6 mm3/year (mean, 80.5 mm3/year). Median total volumetric wear was 41.0 mm3 (mean, 98.5 mm3). We found no association between femoral head size and the linear wear rate, but observed an association between larger (36- and 40-mm) head size and volumetric wear rate and total volumetric wear. Although the linear wear rate of polyethylene was not related to femoral head diameter, there was greater volumetric wear (156.6 mm3/year) with the 36- and 40-mm heads. Pending long-term studies of large head sizes, we advise caution in using larger femoral heads in young or active patients and in those with a low risk of dislocation.
Level of Evidence: Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
Wear and osteolysis continue to be major reasons for revision surgery in THA. Ceramic-on-ceramic bearings eliminate polyethylene wear debris. The newest generation of these bearings incorporate nanosized, yttria-stabilized tetragonal zirconia particles producing an alumina matrix composite. We asked whether this new material would perform as well as a conventional bearing in terms of functional hip scores, radiographic migration and osteolysis, complications and survival. As part of a US FDA investigational device exemption study (G000075), we conducted an initial prospective safety study of 21 alumina matrix composite femoral heads articulating on alumina liners followed by a prospective, randomized study with 44 more of these articulations and 45 zirconia femoral heads on polyethylene liners. The minimum followup for all patients was 26 months (mean, 73 months; range, 26–108 months). Harris hip scores and radiographic findings were similar in the two groups as was survivorship (trial 95% versus control 93%). There were three reoperations in the trial group and three in the control group. A fractured head retrieval showed a 33% monoclinic transformation with an increase in surface roughness from 3 to 5 nm at the main wear zone. While our numbers were insufficient to compare device-related complications, the trial device performed as well as the control device in terms of reoperation, and clinical and radiographic outcome. The alumina matrix composite femoral head on an alumina liner provided high survivorship.
Level of Evidence: Level II, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
First-generation modular titanium fiber-metal-coated acetabular components had high rates of wear, pelvic osteolysis, and liner dissociation. Second-generation components were designed to reduce the incidence of these problems but it is unclear whether the changes achieved these goals.
We asked the following questions: (1) Is the risk of revision surgery for loosening, wear, or liner dissociation low with the second-generation acetabular component? (2) Is the rate of pelvic osteolysis low? (3) Can the liner be exchanged without bone cement?
We retrospectively reviewed prospectively collected data from 99 patients (118 hips) undergoing THAs with one second-generation modular titanium acetabular component with routine screw fixation and conventional polyethylene. The minimum followup was 10 years (mean, 12 years; range, 10–16 years). We obtained Harris hip scores and examined radiographs for loosening and osteolysis.
At last followup, all acetabular components were well fixed and no titanium shell had been revised or removed. No liner had dissociation. At most recent followup, the mean Harris hip score was 89. We observed pelvic osteolysis in eight hips (7%). There were three reoperations for dislocation (head-liner exchange only) and three loose femoral components revised. Two liners (at 11 and 14 years) were exchanged for wear-pelvic osteolysis.
This second-generation modular titanium fiber-metal-coated acetabular component with screw fixation had no loosening, no liner dissociation, and a low rate of pelvic osteolysis at 10 to 16 years. Liner exchange is practical without use of cement. We continue to use this component with highly crosslinked polyethylene liners.
Level of Evidence
Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
Highly crosslinked and thermally treated polyethylenes were clinically introduced to reduce wear and osteolysis. Although the crosslinking process improves the wear performance, it also introduces free radicals into the polymer that can subsequently oxidize. Thermal treatments have been implemented to reduce oxidation; however, the efficacy of these methods with regard to reducing in vivo oxidative degradation remains to be seen. Polyethylene oxidation is a concern because it can compromise the ultimate strength and ductility of the material.
We analyzed the oxidation, oxidation potential, and mechanical behavior of thermally treated highly crosslinked polyethylene retrieved acetabular liners.
Three hundred seven acetabular liners were collected from consecutive revision surgeries at six institutions over a 10-year period. Twenty-four were sterilized using nonionizing methods, 43 were sterilized in an inert environment, 80 were highly crosslinked and annealed, and 160 were highly crosslinked and remelted. Oxidation and oxidation potential were assessed by Fourier transmission infrared spectroscopy. Mechanical behavior was assessed by the small punch test.
Oxidation and hydroperoxide (oxidation potential) indices were elevated in the annealed and gamma inert sterilized groups compared with those of the remelted liners and uncrosslinked gas sterilized controls, particularly at the rim. We also detected an increase in oxidation over time at the bearing surface of the remelted group. Ultimate strength of the polyethylene at the bearing surface was negatively correlated with implantation time for the annealed liners.
Within the first decade of implantation, the clinical outlook for first-generation highly crosslinked polyethylene remains promising. However, ongoing research continues to be warranted for first-generation highly crosslinked polyethylene bearings to monitor the implications of elevated oxidation at the rim of annealed liners as well as to better understand the subtle changes in oxidation at the bearing surface of remelted liners that occur in vivo.
Introduction of highly crosslinked polyethylene has increased interest in large femoral heads, because thin acetabular liners can be used while maintaining low wear rates and larger heads decrease the incidence of instability. However, crosslinking and subsequent thermal treatments can cause decreased mechanical properties that might obviate the reduced wear under extreme conditions. To examine whether increased contact pressures would adversely affect wear in thin liners, we tested thin and thick highly crosslinked liners (3.8 mm thickness/44-mm head and 7.9 mm thickness/36-mm head, respectively) to 5 million cycles on a hip simulator under near impingement conditions. Conventional polyethylene liners (7.9 mm thickness/36-mm head) served as controls. Large femoral heads with highly crosslinked polyethylene liners as thin as 3.8 mm in thickness do not wear at a higher rate than a thicker liner of the same material, even when subjected to large contact pressures such as occur under near-impingement conditions. Crosslinked polyethylene may allow for liners that are thinner than has been traditionally accepted. This conclusion, however, is based solely on wear test results with idealized cup position, no intentional edge loading, no head subluxation, and no artificial aging. Continued monitoring will be necessary to elucidate the clinical efficacy of these devices.
Wear particle-induced osteolysis is a major cause of aseptic loosening in THA. Increasing wear resistance of polyethylene (PE) occurs by increasing the cross-link density and early reports document low wear rates with such implants. To confirm longer-term reductions in wear we compared cross-linked polyethylene (irradiation in nitrogen, annealing) with historical polyethylene (irradiation in air) in a prospective, randomized clinical study involving 48 patients who underwent THAs with a minimum followup of 7 years (mean, 8 years; range, 7–9 years). The insert material was the only variable. The Harris hip score, radiographic signs of osteolysis, and polyethylene wear were recorded annually. Twenty-three historical and 17 moderately cross-linked polyethylene inserts were analyzed (five patients died, three were lost to followup). At 8 years, the wear rate was lower for cross-linked polyethylene (0.088 ± 0.03 mm/year) than for the historical polyethylene (0.142 ± 0.07 mm/year). This reduction (38%) did not diminish with time (33% at 5 years). Acetabular cyst formation was less frequent (39% versus 12%), affected fewer DeLee and Charnley zones (17% versus 4%), and was less severe for the cross-linked polyethylene. The only revision was for an aseptically loose cup in the historical polyethylene group. Moderately cross-linked polyethylene maintained its wear advantage with time and produced less osteolysis, showing no signs of aging at mid-term followup.
Level of Evidence: Level I, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
Highly crosslinked UHMWPE is associated with increased wear resistance in hip simulator and clinical studies. Laboratory and case studies, however, have described rim fracture in crosslinked acetabular liners. Controversy exists, therefore, on the relative merits of crosslinked liners over conventional liners in terms of wear performance versus resistance to fatigue cracking.
We asked whether crosslinked liners would show less surface damage than conventional liners but would be more susceptible to fatigue damage.
We examined 36 conventional UHMWPE and 39 crosslinked UHMWPE retrieved implants with similar patient demographics and identical design for evidence of wear damage, including articular surface damage, impingement, screw-hole creep, and rim cracks.
We observed no difference in wear damage scores for the two liners. Conventional liners more frequently impinged but were more often elevated with smaller head sizes. We observed creep in approximately 70% of both types of liners. Incipient rim cracks were found in five crosslinked liners, and one liner had a rim fracture. Only one conventional liner had an incipient rim crack.
Contrary to our expectation, damage was similar between crosslinked and conventional UHMWPE liners. Moreover, the 15% occurrence (six of 39) of incipient or complete fractures in crosslinked liners as compared with a 3% occurrence (one of 36) in conventional liners may have implications for the long-term performance of crosslinked liners. Longer-term studies will be necessary to establish the fate of rim cracks and thus the overall clinical fatigue performance of crosslinked liners.
Osteolysis is a major mode of hip implant failure. Previous literature has focused on the amount of polyethylene wear comparing highly crosslinked polyethylene (HXPLE) with conventional liners but has not clarified the relative incidence of osteolysis with these two liners.
We determined (1) the incidence of osteolysis in HXLPE versus conventional polyethylene (CPE), (2) the ability to detect and evaluate the size of lytic lesions using radiographs compared with CT scans, (3) head penetration in hips without and with lysis, and (4) determined whether acetabular position, head size, and UCLA activity score contributed to lysis.
We compared head penetration and osteolysis on plain radiographs and presence and volume of osteolysis on CT scans in 48 patients with HXLPE (mean, 46.5 years) and 50 patients with CPE (mean, 43.2 years). The minimum followup was 5 years (average, 7.2 years; range, 5.1–10.9 years),
Osteolysis was apparent on CT in a larger number of patients with CPE liners than HXLPE liners: 12 of 50 (24%) versus one of 48 (2%), respectively. We found no correlation between head penetration and volume of osteolytic lesions. Head penetration was greater in patients with osteolysis. Smaller head sizes were associated with greater wear and those with osteolysis had smaller head sizes; however, there was no difference in acetabular component position or UCLA activity in those with lysis compared with those without.
HXLPE diminished the incidence of osteolysis, but the lack of correlation between penetration and volume of osteolysis suggests other factors other than wear contribute to the development of osteolysis.
Level of Evidence
Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
Aseptic loosening of total hip arthroplasties is generally caused by periprosthetic bone resorption due to tissue reactions on polyethylene wear particles. In vitro testing of polyethylene cups incorporated with vitamin E shows increased wear resistance. The objective of this study is to compare vitamin E-stabilized highly cross-linked polyethylene with conventional cross-linked polyethylene in “reversed hybrid” total hip arthroplasties (cemented all-polyethylene cups combined with uncemented femoral stems). We hypothesize that the adjunction of vitamin E leads to a decrease in polyethylene wear in the long-term. We also expect changes in bone mineral density, less osteolysis, equal functional scores and increased implant survival in polyethylene cemented cups incorporated with vitamin E in the long-term.
A double-blinded randomized controlled trial will be conducted. Patients to be included are aged under 70, suffer from non-inflammatory degenerative joint disease of the hip and are scheduled for a primary total hip arthroplasty. The study group will receive a reversed hybrid total hip arthroplasty with a vitamin E-stabilized highly cross-linked polyethylene cemented cup. The control group will receive a reversed hybrid total hip arthroplasty with a conventional cross-linked polyethylene cemented cup. Radiological follow-up will be assessed at 6 weeks and at 1, 3, 5, 7 and 10 years postoperatively, to determine polyethylene wear and osteolysis. Patient-reported functional status (HOOS), physician-reported functional status (Harris Hip Score) and patients’ physical activity behavior (SQUASH) will also be assessed at these intervals. Acetabular bone mineral density will be assessed by dual energy X-ray absorptiometry (DEXA) at 6 weeks and at 1 year and 2 years postoperatively. Implant survival will be determined at 10 years postoperatively.
In vitro results of vitamin E-stabilized polyethylene are promising, showing increased wear resistance. However, controlled clinical follow-up data are not available at this moment.
This randomized controlled trial has been designed to determine wear, bone mineral density, functional outcome and survival in reversed hybrid total hip arthroplasty comparing cemented vitamin E-stabilized highly cross-linked polyethylene cups with cemented conventional cross-linked polyethylene cups.
Dutch Trial Registry NTR3049
This is a retrospective study comparing polyethylene wear between ceramic ball heads and metal ball heads in total hip arthroplasty.
The ceramic-on-polyethylene bearing option has been introduced as an alternative to metal-on-polyethylene to minimize polyethylene wear debris and reduce subsequent osteolysis and aseptic loosening. However, the reported data were debatable. We designed this retrospective study to compare polyethylene wear between alumina ceramic ball heads and cobalt-chrome ball heads.
Bilateral simultaneous primary total hip arthroplasty was performed in 22 patients between January 2002 and December 2002, with one side using metal-on-polyethylene bearing surface and the other side using alumina ceramic-on-polyethylene bearing surface. After 10 years of follow-up, the wear rate of polyethylene liner on both sides was measured using the Dorr method and compared.
The annual wear rate of the polyethylene liner was 0.133 mm with a standard deviation of 0.045 in the metal-on-polyethylene group and 0.056 mm with a standard deviation of 0.032 in the ceramic-on-polyethylene group. The wear rate per year was significantly lower in the ceramic-on-polyethylene group (p < 0.001).
Although the implication is still controversial, our study showed that the use of ceramic head lowered the liner wear rate.
Ceramic is harder and more resistant to scratching than cobalt-chrome. By increasing polyethylene liner survivorship and decreasing potential osteolytic response and aseptic loosening, ceramic head is a better alternative than cobalt-chrome head.
Background and purpose
Excessive wear of acetabular liners in hip replacements may lead to osteolysis and cup loosening. Different head materials are currently used. We measured differences in wear between alumina and cobalt-chrome heads with the same polyethylene liner.
Patients and methods
39 patients (43 hips) with osteoarthritis were included in a study with 10-year follow-up. Wear was measured as proximal and 3D penetration of the head in the liner with radiostereometry (RSA). All the patients were followed clinically with Harris hip score (HHS) for up to 10 years. Radiolucent lines and osteolytic lesions were assessed on plain radiographs.
With alumina heads, proximal wear (95% CI) after 10 years was 0.62 (0.44–0.80) mm as compared to 1.40 (1.00–1.80) mm in the cobalt-chrome group. For 3D wear, the results were 0.87 (0.69–1.04) mm for alumina heads and 1.78 (1.35–2.21) mm for cobalt-chrome heads. Median (range) HHS was 98 (77–100) in the alumina group and it was 93 (50–100) in the cobalt-chrome group (p = 0.01). We found no difference in osteolysis between the groups.
We found better wear properties with alumina heads than with cobalt-chrome heads. We recommend the use of alumina heads in patients in whom a high wear rate might be anticipated.
Modular acetabular systems have several advantages; however, increased polyethylene wear has been recognized as a disadvantage. In our study, an acetabular shell design with a highly polished inner surface, a locking mechanism that minimizes micromotion, and a high polyethylene liner to shell conformity was evaluated. A prospective cohort of 50 consecutive hips was followed up for a mean of 3.6 years (range, 2–7 years). One patient required an early revision for recurrent instability. Mean linear head penetration rates were 0.242 mm/year (range, 0.048–0.655 mm/year). The true linear wear rate was calculated after subtracting the linear wear in the first follow-up year from the total wear at the end of the follow-up. The true linear wear rate was 0.173 mm/year (range, 0.03–0.423 mm/year). A positive correlation was found between wear rate and male gender. The low head penetration rates suggest that a polished liner with an improved locking mechanism and increased liner conformity can reduce polyethylene wear.
total hipar throplasty; polyethylene wear; backside wear; clinical outcomes; radiographic outcomes
Highly cross-linked polyethylene (HCLPE) has been used extensively to decrease osteolysis and related implant failure in THA. We compared the wear rate of HCLPE and noncross-linked conventional PE (CPE) liners and the rate of radiographic calcar resorption and osteolysis in young patients (35–60 years of age) who underwent THA by one surgeon. Thirty-four patients (41 THAs) who received a hybrid THA using a HCLPE liner were match-paired for age, gender, body mass index, and diagnosis with a group of patients who underwent THA with identical implants but with a CPE liner. The minimum followup was 4 years (average, 5.3; range, 4–8 years). Using the Livermore measurement technique, the averages of total wear of the HCLPE and CPE liners were 0.01 mm (range, −0.23–0.4) and 0.64 mm (range, 0–1.7), respectively. The average annual wear was less for the HCLPE than the noncross-linked PE (0.002 mm, range, −0.05–0.1 versus 0.12 mm, range, 0–0.29, respectively). Four hips in the HCLPE group and 23 in the CPE group had calcar resorption measuring averages of 2.5 mm (range, 2–3) and 7.5 mm (range, 1.8–23.8), respectively. Periprosthetic osteolysis occurred in two and eight hips in the HCLPE and CPE groups, respectively. Longer followup is needed to determine if these findings will result in improved implant survivorship.
Level of Evidence: Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
Multiple factors have been identified as contributing to polyethylene wear and debris generation of the acetabular lining. Polyethylene wear is the primary limiting factor in the functional behavior and consequent longevity of a total hip arthroplasty (THA). This retrospective study reviewed the clinical and radiographic data of 77 consecutive THAs comparing in vivo polyethylene wear of two similar acetabular cup liners. Minimum follow-up was 7 years (range 7–15). The incidence of measurable wear in a group of machined liners sterilized with ethylene oxide and composed of GUR 1050 stock resin was significantly higher (61%) than the compression-molded, GUR 1020, O2-free gamma irradiation sterilized group (24%) (P = 0.0004). Clinically, at a 9-year average followup, both groups had comparable HHS scores and incidence of thigh or groin pain, though the machined group had an increased incidence of osteolysis and annual linear wear rate.
We analyzed 20 retrieved gamma-sterilized polyethylene liners (Biomet Hexloc). The long-term durability varied significantly depending on shelf-life time before implantation. Liners with a shelf-life time of 3 years or more evinced significantly (P 0.002) higher volumetric wear than those with a shelf life time less than 3 years. Infrared spectroscopy and scanning calorimetry showed that all explanted implants underwent substantial in vivo oxidation and crystallization. The oxidative ageing of polyethylene renders the polyethylene liner susceptible to severe wear. Scanning electron microscopy of the bearing surface of the liner revealed abrasive wear as a dominant mechanism. Moreover, poor acetabular design produces excessively thin liners, substandard locking mechanism, and backside wear of the liner. The primary reason for severe wear in the Hexloc liner was poor modular design and oxidative degradation of the polyethylene.
We assessed the clinical and radiographic results of 40 porous-coated acetabular cups with an Acetabular Cup System polyethylene liner over a minimum 14-year follow-up. Femoral head penetration was estimated using a software package. Fifteen cups were revised, 11 due to polyethylene liner rupture. All cups but two were radiographically stable, and 11 hips showed acetabular osteolysis. The overall femoral head penetration rate in hips without liner fracture with reference to the early penetration point was 0.1188±0.070 mm per year. Polyethylene liner fractures were associated with higher early femoral head penetration (P<0.0001) and a vertical cup position (P=0.0016). The 14-year survival without cup revision for any reason was 63.9%, 71.8% with no ACS polyethylene liner fracture and 65.3% with no acetabular osteolysis. Most cups showed a good clinical outcome in general, but major Acetabular Cup System liner failure and osteolysis were frequent. Patients with the ACS cups still in place should be monitored closely.
Sixty Crossfire (Stryker Orthopaedics, Mahwah, NJ) liners were consecutively revised after an average of 2.9 years (range: 0.01 – 8.0 years) for reasons unrelated to wear or mechanical performance of the polyethylene. Femoral head penetration was measured directly from 42 retrievals implanted for over 1 year. Penetration rate results (0.04 mm/y, on average; range: 0.00-0.13 mm/y) confirmed decreasing wear rates with longer in vivo times. Overall, we observed oxidation levels at the bearing surface of the 60 liners (0.5, on average; range: 0.1-1.7) comparable to those of non-implanted liners (0.5, on average; range: 0.3-1.1) and preservation of mechanical properties. We also measured elevated oxidation of the rim (3.4, on average; range: 0.2-8.8) that was correlated with implantation time. Rim surface damage, however, was observed in only 3/60 (5%) cases. Retrieval analysis of the three rim-damaged liners did not reveal an association between surface damage and the reasons for revision.
Mechanical properties; oxidation; wear; hip arthroplasty; highly crosslinked ultra-high molecular weight polyethylene
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.
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.
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).
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.
The new protocol overcomes a number of limitations, such as loss of nanometer-sized particles and artifactual clumping, among others.
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.
Electronic supplementary material
The online version of this article (doi:10.1007/s11999-011-2057-x) contains supplementary material, which is available to authorized users.
The hypothesis of this study was that acetabular liner vulnerability to scratching from femoral heads, roughened by third bodies embedded in the liner, is not significantly lower for highly crosslinked polyethylene (HXPE) than for conventional polyethylene (CPE). Six CPE and six HXPE acetabular liners were each reproducibly embedded with five CoCrMo beads, then run for 10,000 cycles in a joint simulator. By visual rank-ordering, there was low association between liner scratch severity and polyethylene type. The CPE and HXPE liner scratches were not significantly different in scratch peak-valley height or width, or in liner roughness in the vicinity of the embedded beads. This model indicated that high crosslinking of polyethylene does not offer appreciable protection against severe scratching induced by large embedded third body particles.
The ABG I uncemented hip prosthesis has demonstrated an unacceptably high failure rate and poor functional outcome in our patients. A prospective review was performed of 38 implanted hips with a mean follow-up of 11 years. Sixteen hips have been revised or are candidates for revision hip surgery, due principally to polyethylene wear and pelvic osteolysis. Contributing factors were likely to be the poor wear characteristics of the polyethylene liners—which were gamma irradiated in air—and the thinness of these liners. Of further concern were the poor pain and function scores in the remaining hips [mean Harris Hip Scores (HHS) of 70 (SD 22) at 11-year follow-up]. The influence of the surgical learning curve on clinical results may have been an important factor. Undersized femoral components were associated with poor pain and functional scores in five patients with HHS < 60. All ABG I hip implants must remain under careful, long-term review.
Hylamer polyethylene was introduced in the 1990s as an alternative to conventional polyethylene. Its chemical and physical properties, and especially its high crystallinity, were claimed to improve resistance to wear. Initially Hylamer devices were sterilized by gamma radiation in air, then the technique was changed and gamma radiation was performed in the absence of oxygen. Clinical experience has shown the early loosening of some devices made from Hylamer. The aim of this study was to ascertain whether early wear and consequent osteolysis was linked to the sterilization method. We retrospectively compared 31 patients with hip prostheses with Hylamer liners sterilized by gamma radiation in air (group 1) with 30 patients with the same prosthesis, but sterilized in the absence of oxygen (group 2). The groups were similar for sex, age, disease, head diameter and material. Mean follow-up was 84 months and no clinical signs of failure were present. Radiographic measurement revealed that wear of group 1 was significantly greater than that of group 2 (0.23 mm/year vs 0.09 mm/year, p=0.001). Periacetabular and femoral osteolysis in group 1 was significantly greater than group 2. In conclusion, the method of sterilization might have influenced the outcome of Hylamer polyethylene liners.
In total joint arthroplasty, third body particle access to the articulating surfaces results in accelerated wear. Hip joint subluxation is an under-recognized means by which third body particles could potentially enter the otherwise closely conforming articular bearing space. The present study was designed to test the hypothesis that, other factors being equal, even occasional events of femoral head subluxation greatly increase the number of third body particles that enter the bearing space and become embedded in the acetabular liner, as compared to level walking cycles alone. Ten metal-on-polyethylene hip joint head-liner pairs were tested in a multi-axis joint motion simulator, with CoCrMo third body particles added to the synovial fluid analog. All component pairs were tested for two hours of level walking; half also were subjected to twenty intermittent subluxation events. The number and location of embedded particles on the acetabular liners were then determined. Subluxation dramatically increased the number of third body particles embedded in the acetabular liners, and it considerably increased the amount of scratch damage on the femoral heads. Since both third body particles and subluxation frequently occur in contemporary total hip arthroplasty, their potent synergy needs to be factored prominently into strategies to minimize wear.
Hip arthroplasty; impingement; mechanical testing; subluxation; third body wear