Polyethylene Wear Rate Research Articles

Comparison of wear of mobile and fixed bearing knees tested in a knee simulator

Mobile bearing knees have been developed to provide theoretical increases in tibiofemoral contact area and conformity to decrease contact stresses when compared to fixed bearing designs. The decrease in contact stress has been conjectured to reduce the microabrasive/adhesive wear rate of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts. Although mobile bearing knees may provide lower contact stresses, the inferior surface of the insert has the potential to encounter large sliding distances against the tibial base plate. These combinations of factors have raised questions of whether mobile bearing knees exhibit less wear than fixed bearing designs. In this study, when both fixed and mobile bearing knees were tested in a knee wear simulator utilizing load/motion inputs for walking gait, there was no statistically significant difference in wear rates measured between fixed and mobile bearing designs ( p=0.37), although a trend toward lower wear was noted with the mobile bearing design. The wear rates for the two fixed bearing designs (mean±standard errors), the I/B II and the NexGen CR, were similar (15.0±0.8 mg/million cycles (Mc), n=9 and 14.4±1.2 mg/Mc, n=6, respectively) and approximately equivalent to the mobile bearing knee design, the NexGen LPS-Flex Mobile knee, (13.0±1.1 mg/Mc, n=6). A major difference in wear rates was noted when a highly crosslinked polyethylene was tested, where the NexGen CR (using conventional gamma irradiated polyethylene) and NexGen CR Prolong™ (using highly crosslinked polyethylene) designs differed only in the relative amounts of polyethylene crosslinking utilized in the manufacture of the fixed bearing surface. The wear rate of the highly crosslinked NexGen CR Prolong™ was significantly lower than the wear rates of the other three design and material combinations tested (2.71±0.38 mg/Mc, n=12, P<0.0001).

Early Failure of Total Hip Arthroplasty with Hydroxyapatite-Coated Smooth Surface Acetabular Component

Purpose : To evaluate the early intermediate outcomes of total hip arthroplasty using hydroxyapatite (HA)-coated smooth surface hemispherical acetabular component. Materials and Methods : Seventy total hip arthroplasties were performed, in 63 patients, using a HA-coated smooth surface hemispherical acetabular component consecutively between June 1993 and August 1996. Fifty-four cases were followed at an average of 71 months. The mean age at the time of arthroplasty was 49 years old. Clinical results were evaluated using the modified Harris hip score, and the radiologic results were evaluated in terms of stability, polyethylene wear rate, and osteolysis around the acetabular component. Results : The mean preoperative Harris hip score of 50.9 was improved to 82.2 points at the most recent follow-up. Pelvic osteolysis was found at 18 cases (33%) and the mean polyethylene wear rate was 0.15 mm/year. Eight acetabular components (15%) were unstable radiologically and 7 acetabular components (13%) were revised. Conclusion : Total hip arthroplasty using HA-coated smooth surface hemispherical acetabular component showed accelerated polyethylene wear rate, which was accompanied by a high incidence of osteolysis, and a high loosening rate due to failure of bony ongrowth at early midterm.

Nonmodular One-Unit Cementless Acetabular Compoment in Primary Total Hip Arthroplasty: With Alumina to Polyethylene Articulation

Purpose : Modular acetabular components have the potential of fretting and back side wear leading to osteolysis. The present study evaluated the outcomes of total hip arthroplasties (THAS) using a nonmodular cementless acetabular component with those of press-fit titanium mesh coatings fixed. Materials and Methods : Eighty-two primary THAS in 75 patients were performed consecutively between April 1995 and August 1998. Sixtytwo hips were eligible for follow up for an average 62 months. Mean patients age was 59 years. The clinical evaluation was performed using the modified Harris hip score (HHS), inguinal pain and limping. The radiologic evaluation included, component stability, polyethylene wear rate, pelvic osteolysis and the remodeling pattern around the acetabular component. Results : Mean HHS improved from 50.2 points preoperatively to 90.5 points at the most recent follow up. All components were stable radiologically. The linear polyethylene wear rate was 0.07 mm/year. No pelvic osteolysis was found. Polar gaps found in 25 hips (40%) in postoperative radiographs and disappeared during follow up in 22 hips (88%). One hip was revised due to chronic unreduced dislocation. Conclusion : Total hip arthroplasty with a nonmodular cementless acetabular component seems to enhance satisfactory early mid-term results in terms of stability and acetabular remodeling with minimal wear and pelvic osteolysis.

Primary Total Hip Arthroplasties Using Harris-Galante Porous Coated Cementless Prosthesis: Mean 10-Year Follow-Up Results

Purpose : The purpose of this study was to analyze the intermediate to long-term results of primary total hip arthroplasties with a Harris-Galante prosthesis. Materials and Methods : Among 97 consecutive total hip replacements with Harris-Galante prosthesis in 85 patients, 76 hips in 67 patients were evaluated clinically and roentgenographically. The mean follow-up was for 10.1 years (range 6.2-13 years). Clinical and radiographic evaluations including osteolysis, stability of prosthesis, and wear of polyethylene were performed. Statistical analysis was conducted to find the relevant factors influencing osteolysis. The survivorship of overall and each acetabular and femoral component was analyzed by Kaplan-Meier method. Results : The Harris hip score averaged 44 points at preoperatively and improved to 89 points at the most recent follow-up. Pelvic osteolysis was found in 3 hips (4%) and femoral osteolysis was found in 32 (42%). The linear polyethylene wear rate was averaged 0.13 mm/year. Nine femoral components and two acetabular components were revised. There were 2 recurrent dislocations, 3 periprosthetic fractures and 4 infections. The probability of survival of the femoral and acetabular component at a mean 10 years was 81% (range 71- 93%) and 96% (range 91-100%) by the Kaplan-Meier method. Conclusion : The intermediate to long-term follow up of primary total hip arthroplasty with Harris-Galante prosthesis revealed a low failure rate of acetabular component and a relatively high failure rate of the femoral component which was usually related with loosening and osteolysis.

Assessment of backside wear from the analysis of 55 retrieved tibial inserts.

Fifty-five retrieved tibial inserts with four different locking mechanisms were evaluated for evidence for polyethylene wear between the inferior surface of the tibial insert and metal tray. This type of wear will be referred to as backside wear. Backside wear was assessed by evaluating manufacturer's stamped markings on the inferior polyethylene surface. Because these markings are embossed into the polyethylene surface, they were used as indicators of backside wear. Decreases in the depths of markings indicated that backside wear was clearly evident, regardless of design, in 24 (44%) of the inserts. In eight of these 24 inserts, the manufacturer's stamped markings were removed completely. The amount of polyethylene wear was as a high as 591 mg from the inferior surface. This corresponds to a polyethylene wear rate from the backside of the tibial insert of greater than 100 mg per year, which is two to four times higher than wear rates associated with total hip replacements. The current work provides direct evidence of backside wear in all four tibial insert designs. Backside wear of tibial inserts can be a significant contributor to polyethylene wear in total knee arthroplasty. Close attention should be given to the fixation of tibial inserts to metal trays by manufacturers and surgeons.

Wear Characteristics of Ultrahigh Molecular Weight Polyethylene (UHMWPE)

The wear of ultrahigh molecular weight polyethylene (UHMWPE) bearing against 316 stainless steel or cobalt chromium (Co-Cr) alloy was measured using a 12-channel wear tester especially developed for the evaluation of candidate materials for prosthetic joints. The coefficient of friction and wear rate were determined as a function of lubricant, contact stress, and metallic surface roughness in tests lasting 2–3 million cycles, the equivalent of several years use of a prosthesis. Wear was determined by the weight loss of the polyethylene (PE) specimens corrected for the effect of fluid absorption. The friction and wear processes in blood serum differed markedly from those in saline solution or distilled water. Only serum lubrication produced wear surfaces resembling those observed on removed prostheses. The experimental methods provided accurate reproducible measurement of PE wear. The long-term wear rates were proportional to load and sliding distance. Although the PE wear rate increased with increasing surface roughness, wear was not severe except with very coarse metal surfaces. The data obtained in these studies formed a comparison basis for the subsequent evaluation of potentially superior materials for prosthetic joints.

Polyethylene liner exchange for excessive wear and osteolysis

To determine if polyethylene liner exchange of a total hip arthroplasty decreased wear and slowed the progression of osteolysis, we evaluated 10 hips at a mean 6.2 years after component exchange. Head penetration and lesion size were quantified on serial radiographs using computer-assisted techniques. A mean 7.9 years before component exchange, true polyethylene wear rates averaged 0.36 ± 0.19 mm/y. A mean 6.2 years after component exchange, average wear rates decreased to 0.17 ± 0.11 mm/y. Similarly, mean acetabular and mean femoral lesion growth rates decreased. At last follow-up, all components remained radiographically stable. Complications included dislocation (1 patient) and increased wear (2 patients). Modular component exchange with removal of granulomatous tissue has the potential to decrease wear and slow the progression of osteolysis without subsequent component loosening. Copyright 2002, Elsevier Science (USA). All rights reserved.

A COMPARATIVE STUDY ON WEAR BEHAVIOR OF HIP PROSTHESIS BY FINITE ELEMENT SIMULATION

A numerical approach was proposed to investigate the wear behavior occurred in the artificial hip joints in this paper. In the numerical simulations, the wear coefficients taken from pin-on-disk tests were introduced into the wear analysis model to assess the wear rates of polyethylene acetabular cups against metallic or ceramic femoral heads. For the established material combinations, different values of polyethylene wear rates were obtained respectively, which were not necessarily the realistic one as expected in vivo but could be confirmed after further discussion on the wear mechanism involved in wear tests. Current results indicated that the polyethylene/ceramic couples represented better wear performances than the polyethylene/metal couples. Furthermore, the ratio of wear rates for polyethylene cups against alumina and the metallic femoral heads was 0.5, which agreed well with that deduced from clinical studies or laboratory hip simulators. It is obvious that these comparable wear behaviors observed from clinics or laboratory studies also can be found by means of the numerical simulation.

A literature review of the association between wear rate and osteolysis in total hip arthroplasty

The establishment of a polyethylene wear rate threshold for the development of osteolysis at the hip would allow surgeons to identify patients at risk for osteolysis and to implement selective, more frequent follow-up. We reviewed publications that met certain criteria for wear and osteolysis measurement. Based on this review, the incidence of osteolysis increases as the rate of wear increases. The literature indicates that osteolysis rarely is observed at a wear rate of <0.1 mm/y. We suggest that a practical wear rate threshold of 0.05 mm/y would eliminate osteolysis. This wear threshold suggests that the new cross-linked polyethylenes would reduce osteolysis, provided that in vivo wear rates mirror those observed in vitro. To facilitate future comparison of published data, we suggest that longitudinal wear studies adopt consistent edge detection–based wear measurement techniques and uniform osteolytic lesion classification and measurement schema. Copyright 2002, Elsevier Science (USA). All rights reserved.

Rapid polyethylene failure of unicondylar tibial components sterilized with gamma irradiation in air and implanted after a long shelf life.

The mechanical toughness of polyethylene that has been sterilized by gamma irradiation in air decreases after a long shelf life. The purpose of the present study is to report the high failure rate after unicondylar knee replacements performed with polyethylene bearings that had been sterilized with gamma irradiation in air and implanted after a shelf life of < or = 4.4 years. Between December 1997 and January 2000, seventy-five unicondylar knee replacements were performed in sixty-two patients. All patients were followed both clinically and radiographically. A revision operation was offered when the patient had pain, swelling, and radiographic evidence of rapid polyethylene wear. The effect of aging of the polyethylene during storage was evaluated by dividing the knees into three groups on the basis of shelf life and comparing them with regard to the rate of revision and the observed wear of the polyethylene. Four retrieved components were examined for the presence of oxidation. At a mean of eighteen months after the arthroplasty, thirty knees had been revised and seven were scheduled for revision. The rate of polyethylene wear increased as the shelf life increased. There was a significant inverse linear correlation between the shelf life of the polyethylene and the time to revision (p < 0.01, r (2) = 0.64). All retrieved components had greater-than-expected wear with pitting and delamination of the surface. Seven components had fractured, and ten had both fractured and fragmented. Analysis of four components confirmed severe oxidation of the polyethylene. The present study demonstrated early, severe wear of tibial polyethylene bearings that had been sterilized by gamma irradiation in air and stored for < or = 4.4 years. This risk can be minimized by ensuring that implants have not been sterilized with gamma irradiation in air and stored for several years.

Survival and polyethylene wear of porous-coated acetabular components in patients less than fifty years old: results at nine to fourteen years.

Younger patients (those who are less than fifty years old) have been shown to have a high rate of failure of cemented acetabular components following total hip arthroplasty. In this report, we present the results associated with the use of an uncemented acetabular component in young patients who were evaluated at a minimum of nine years postoperatively. Between December 1984 and December 1989, the senior author performed 174 primary total hip arthroplasties with use of a single design of porous-coated acetabular component. Seventy-one of these procedures were performed in fifty-six patients who were younger than fifty years old. Fifty-six of the seventy-one hips were available for radiographic and clinical analysis after a mean duration of follow-up of eleven years. All hips had been treated with a Harris-Galante-I porous-coated acetabular component that had been placed with a line-to-line fit and fixed with a mean of four screws. Clinical analysis was performed with use of the Harris hip score. Standardized anteroposterior radiographs were analyzed with regard to migration, radiolucent lines, pelvic osteolysis, and two-dimensional linear wear of the polyethylene. No metal shell was revised because of aseptic loosening, and no shell was loose at the time of the latest follow-up. A nonprogressive radiolucent line was seen in one zone in ten hips (18%) and in two zones in six hips (11%). No hip had a radiolucent line in all three zones. Pelvic osteolysis was noted in thirteen hips (23%); the osteolysis was observed in the ischium in eleven hips and around the screws in two. Survivorship analysis revealed that the probability of survival of the metal shell was 98% (95% confidence interval, 96.9% to 99.9%) at ten years. The mean rate of linear polyethylene wear (and standard deviation) was 0.15 +/- 0.10 mm/yr (range, 0.02 to 0.59 mm/yr). The wear rate was significantly increased in patients with an excellent Harris hip score (p = 0.004) and a younger age (less than thirty-eight years) (p = 0.026). With the numbers available, no relationship could be detected between the wear rate and the gender or weight of the patient, the polyethylene thickness, the abduction angle, or the femoral neck length. The fixation and survival of porous-coated acetabular metal shells in patients less than fifty years old was excellent after a mean duration of follow-up of eleven years. The high rate of linear polyethylene wear and the high prevalence of pelvic osteolysis are of serious concern in this patient population. Continued follow-up will be necessary to evaluate the influence of these findings on the longevity of the fixation of this prosthesis.

Midterm outcome of the plasma cup in total hip arthroplasty.

One hundred and twenty eight primary total hip arthroplasties (THA) in 104 patients were reviewed to assess mid-term survivorship and clinical outcome of the Plasma cup. Outcome was evaluated clinically, radiographically, and by self-administered questionnaires. Patient mean age at surgery was 51 years, and 52 THAs (41%) were performed for secondary arthritis. Prior to surgery the median Merle DAubigne score was 8. At 59 (standard deviation 18) months this score had improved to 17, and the median Harris hip score was 85. The mean annual polyethylene wear rate was 0.14mm/year. There were no instances of aseptic loosening but 2 cups had small, focal osteolytic lesions at the site of screw holes. Three cups were revised, two for recurrent dislocations, and one for infection. Cup survivorship at 5 years was 97% (Kaplan-Meier).This data suggests that the Plasma cup performs well in the mid-term and may be used safely in a young population with a high incidence of secondary osteoarthritis. (Hip International 2002; 2: 119-25).

Osteoconductive coatings for total joint arthroplasty.

Osteoconductive calcium phosphate coatings for total joint arthroplasty have been in clinical use since the mid1980s. The basic principles involved and basic science evidence for the efficacy of osteoconductive coatings were examined. Hydroxyapatite coatings provide consistent and better filling with bone of the gaps and spaces around cementless joint components after surgery as compared with porous-coated implant surfaces, resulting in better implant stability. Of all the calcium phosphate coatings, hydroxyapatite coatings have had the most widespread application in hip arthroplasty. Their clinical advantages over more conventional implant surfaces are evident in primary and revision hip arthroplasties. A clinical survival rate in the author's series of 97% at a minimum of 11 years followup for the femoral component in a young active patient population (average age, 53 years) was obtained with no mechanical failures. The average polyethylene wear rate in this group was 0.129 mm/year. In a similar group of young patients with revision arthroplasty using hydroxyapatite-coated femoral components, an 11-year survival rate of 93% was obtained. Histologic analysis of specimens retrieved at autopsy confirmed the excellent bony fixation of components. Advantages of the more recent biomimetic hydroxyapatite coatings were examined.

Pattern of osteolysis around two different cementless metal-backed cups: Retrospective, radiographic analysis at minimum 10-year follow-up

In this retrospective study, we analyzed the formation of osteolysis with 2 cementless cups to evaluate if surface holes in metal shells influence the radiographic pattern of osteolysis. At a minimum of 10 years' follow-up, serial radiographs of 112 hips with Arthropor cups (having multiple shell holes) and 126 hips with Anatomic Medullary Locking (AML) cups (with no holes) were reviewed. Despite differences in variables other than cup design that can affect polyethylene wear and osteolysis rates, the incidence of osteolysis between the groups was coincidentally similar (47.3%, Arthropor; 47.6%, AML). The time of onset of osteolysis (mean, 7.5 and 7.4 years) also was similar. The radiographic pattern of the osteolytic lesions , which is largely a factor of cup design, differed. The Arthropor group had significantly larger lesions in Charnley zone I and significantly more hips with retroacetabular lesions (Charnley zones I and II); however, these lesions did not compromise cup stability. Patients with AML cups had more and larger lesions in the greater trochanter.

Ceramic-on-polyethylene bearing surfaces in total hip arthroplasty. Seventeen to twenty-one-year results.

Polyethylene wear debris, and the resulting inflammatory response leading to osteolysis and loosening, is the primary mode of failure limiting the longevity of total hip replacements. Alternative bearing surfaces, including ceramic-on-polyethylene, have been investigated in an effort to decrease the amount of polyethylene wear debris. The purpose of this study was to evaluate the seventeen to twenty-one-year results of the use of ceramic-on-polyethylene total hip prostheses. Sixty-four total hip prostheses were implanted with cement, by one surgeon, in fifty-six patients from 1978 to 1981. The average age at the index arthroplasty was sixty-nine years (range, fifty-one to eighty-four years). The components consisted of a cemented Charnley-Müller stem with a 32-mm modular alumina femoral head and a cemented all-polyethylene acetabular component. All patients who retained the index prosthesis were assessed clinically with use of Harris hip scores and were evaluated radiographically at the time of the latest follow-up. At the time of this latest follow-up, of the original sixty-four implants, eighteen (28%) were still in place and five (8%) had been revised. The remaining forty-one implants were in patients who had died and were functioning well until the patient's death. No patient was lost to follow-up. Of the eighteen hips with an intact prosthesis in the surviving patients, seven had an excellent clinical result; nine, a good result; and two, a fair result. One asymptomatic hip had definite radiographic evidence of femoral loosening. No hip had definite signs of acetabular loosening or evidence of osteolysis. Survivorship analysis revealed that the probability of survival of the prostheses without revision was 95% at five years, 95% at ten years, 89% at fifteen years, and 79% at twenty years. The mean linear and volumetric polyethylene wear rates were 0.034 mm/yr and 28 mm(3)/yr, respectively. There were no fractures of the ceramic heads. Outstanding long-term clinical and radiographic results were attained despite the use of what are now considered substandard techniques (an inferior stem design, a 32-mm head, and first-generation cementing techniques). The wear rates in this study are lower than previously reported metal-on-polyethylene wear rates and are consistent with the lowest reported in vivo ceramic-on-polyethylene wear rates. These findings support the consideration of ceramic-on-polyethylene bearing surfaces in total hip arthroplasty.

Volumetric computerized tomography as a measurement of periprosthetic acetabular osteolysis and its correlation with wear.

Osteolysis, which is considered to be a major source of morbidity following total hip joint replacement, has been notoriously difficult to measure accurately, particularly in the acetabular area. In order to study periacetabular osteolysis, specialized software for computerized tomography (CT) scan image analysis has been developed. This software (3D-CT) eliminates metal artifacts, allows three-dimensional segmentation of the CT image, and reconstructs the segmented image to provide an accurate representation and measurement of volume for osteolytic lesions. In the present study, 20 patients underwent periacetabular osteolytic volume determination using 3D-CT, functional assessment (using the Harris Hip Scale, the Western Ontario and McMaster University Osteoarthritis Index, and the short form 36 questionnaire), and two-dimensional analysis of volumetic polyethylene wear using digitalized plain films. Periacetabular osteolysis correlated directly with the polyethylene wear rate (relative risk [RR] = 0.494, P = 0.027). If one patient with an acetabular revision, one patient with recurrent dislocation, and one patient with a Biomet prosthesis are excluded, then the correlation between wear and osteolysis is improved (RR = 0.685, P = 0.002). In summary, the current study demonstrates both the feasibility of CT imaging of periacetabular osteolysis and the correlation between polyethylene wear and osteolytic volume, providing a potential outcome measure for clinical trials that are designed to examine interventions in this complex disease process.

A hip simulator study of the influence of patient activity level on the wear of crosslinked polyethylene under smooth and roughened femoral conditions

Over the last three decades, tribological studies of polyethylene total hip replacements have been undertaken using a simplified model of normal walking. As hip prostheses are being implanted in younger and more active patients, coupled with the increased wear resistance of crosslinked polyethylene, such in vitro approximations in activity are very limiting. Using a hip joint simulator, the influence of a significant increase in patient activity was studied by applying a series of simulated walking, stumbling and jogging sequences, at varying cycle speeds, using crosslinked UHMWPE/CoCrMo components, with both smooth and roughened femoral heads. All tests were performed using 25% bovine calf serum, and all components were positioned physiologically. The effects on wear, frictional torque, counterface roughness, lubricant temperature, material deformation and particle morphology were measured and analysed. It was found that with smooth heads and non-constraining socket fixtures, the occurrence of excessive stumbling at 1Hz (5kN max) had a negligible effect on the wear rate of polyethylene, whilst simulated jogging at 1.75Hz (4.5kN max) only showed a median increase in wear volume of 40% compared to normal walking. Fast walking showed the largest wear rate, and was consistently greater than for simulated jogging, thus, suggesting that short periods of increased load and speed have a relatively small effect on polyethylene wear. However, increasing the femoral roughness Ra to 0.38μm under simulated jogging, 1.75Hz, led to a massive increase in wear and frictional torque, generating wear rates >3000mm3/106cycles for crosslinked polyethylene. Surface topography, sliding speed and the type of socket fixturing were shown to be the most influential factors when simulating increased patient activity.

Analysis of survivorship after total hip arthroplasty using a ceramic head.

To determine the clinical efficacy of an alumina ceramic head, 119 cemented total hip arthroplasties in 97 patients using an alumina head coupled with ethylene oxide gas sterilized polyethylene were reviewed. Eighty-two patients (101 hips) with greater than 10 years followup were evaluated clinically and radiographically (range, 10-17.6 years), and 97 patients (119 hips) were evaluated for survivorship analysis (range, 0.6-17.6 years). The average functional hip scores according to Merle d'Aubigné and Postel improved from 8.6 preoperatively to 15.0 at the final followup, and 57 patients (64 hips) had no pain. The average polyethylene wear rate was 0.15 mm/year (range, 0.04-0.34 mm/year). Patients with polyethylene wear greater than 3 mm showed significantly higher rates of acetabular loosening. Fifteen-year survival rates (with 95% confidence intervals) with radiographic evidence of aseptic loosening as the end point were 46.8% +/- 13.4% in acetabular components and 91.9% +/- 6.6% in femoral components. Fifteen-year survival rates of hip arthroplasties with revision because of aseptic loosening as the end point were 75.3% +/- 10.2% and 97.9% +/- 3.0%, respectively. Results of the current study suggest that using an alumina head instead of a metal head may not be beneficial when coupled with ethylene oxide gas sterilized polyethylene.

The Harris-Galante Porous Acetabular Component at Intermediate Follow-Up

Outcome of the acetabular component in 90 consecutive primary noncemented total hip arthroplasties (THAs) was prospectively studied. The acetabular cup consisted of a hemispherical titanium alloy shell with a titanium fiber-mesh porous coating and a modular polyethylene liner (Harris-Galante Porous-1, Zimmer, Warsaw, Ind). The cup was implanted using line-to-line reaming with adjunctive dome screw fixation. The femoral component consisted of a titanium alloy stem with titanium fiber-mesh porous coating and a 28-mm cobalt-chrome modular head. Mean patient age was 53 years (range: 27-75 years); male:female ratio was 48:42; and mean follow-up was 6 years (range: 4.5-8 years). One acetabular component was revised for aseptic loosening. Of 81 unrevised hips available for follow-up, mean Harris hip score was 57 preoperatively and 96 at final follow-up (72% excellent, 15% good, 1 3% fair, and none poor). Of 61 unrevised hips with adequate radiographic follow-up, radiographic failure (complete periprosthetic radiolucency) was evident in 3 (4.9%) and periacetabular osteolysis in none. Radiographic failure did not correlate with poor clinical outcome. Linear polyethylene wear rate (mean: 0.13 mm/year) did not correlate with age, gender, weight, outcome, or cup abduction angle, but did correlate with the presence of femoral periprosthetic osteolysis (0.18 mm/year with femoral osteolysis versus 0.11 mm/year without; P= .01). This series of porous-coated hemispherical cups demonstrated excellent intermediate-term clinical and radiographic outcome, comparable with similarly favorable results reported by the prosthesis designers. A potentially adverse effect of polyethylene wear on the longevity of a THA was supported by a positive correlation between polyethylene wear rate and femoral osteolysis.

Wear of fixed bearing and rotating platform mobile bearing knees subjected to high levels of internal and external tibial rotation.

In order to extend the lifetime of total knee replacements (TKR) in vivo, reduction of the volumetric wear rate of ultra high molecular weight polyethylene (UHMWPE) bearings remains an important goal. The volume of wear debris generated in fixed bearing total knee devices increases significantly when subjected to higher levels of internal-external rotation and anterior-posterior displacement. Six PFC Sigma fixed bearing TKR were compared with six LCS rotating platform mobile bearing knees using a physiological knee simulator with high rotation kinematic inputs. The rotating platform polyethylene inserts exhibited a mean wear rate which was one-third of that of the fixed bearing inserts despite having increased femoral contact areas and additional tibial wear surfaces. The rotating platform design decouples knee motions, by allowing unidirectional motion at the tray-insert articulation, which reduces rotation at the femoral-insert counterface. This translation of complex knee motions into more unidirectional motions results in molecular orientation of the UHMWPE and reduced volumetric wear.