The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 89, Issue 6

Scientific Articles | June 01, 2007

Factors Associated with the Loss of Thickness of Polyethylene Tibial Bearings After Knee Arthroplasty

Matthew B. Collier, MS¹; C. Anderson EnghJr., MD¹; James P. McAuley, MD, FRCSC¹; Gerard A. Engh, MD¹

¹ Anderson Orthopaedic Research Institute, P.O. Box 7088, Alexandria, VA 22307. E-mail address for all authors: research@aori.org

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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (DePuy, a Johnson and Johnson Company). Also, a commercial entity (Inova Health System) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Investigation performed at the Anderson Orthopaedic Research Institute, Alexandria, Virginia

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2007 Jun 01;89(6):1306-1314. doi: 10.2106/JBJS.F.00667

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Abstract

Background: Wear of the polyethylene tibial bearing is a leading cause of failure of knee replacements done prior to the current decade. The objective of this study was to determine how patient-related factors, implant-related factors, and limb or tibial component alignment influenced the amount of thickness loss in polyethylene tibial bearings that were retrieved at the time of revision surgery or after the death of the patient.

Methods: We retrieved polyethylene tibial bearings from eighty-one unicondylar and eighty-nine total knee replacements that had been performed because of osteoarthritis with varus deformity from 1984 to 1998. All of the polyethylene bearings had been sterilized with gamma radiation in air. Polyethylene loss was quantified as the change in the minimum bearing thickness per years in vivo (the mean time in vivo [and standard deviation] was 8 ± 4 years). Multiple linear regression was used to assess whether polyethylene loss was associated with age, weight, gender, varus angle of the tibial component, postoperative hip-knee-ankle angle, initial thickness of the polyethylene, shelf age of the polyethylene, and either the type of polyethylene (for total knee replacements, which were of one posterior cruciate ligament-retaining design) or the manufacturer (for unicondylar knee replacements), and to determine the magnitude by which polyethylene loss would change if any of the significant risk factors were changed.

Results: The mean loss (and standard deviation) of polyethylene thickness in the medial compartment of total knee replacements (0.33 ± 0.28 mm/yr) and that in medial unicompartmental knee replacements (0.49 ± 0.40 mm/yr) were significantly (p < 0.05) associated with the same three variables: patient age, postoperative hip-knee-ankle angle, and shelf age of the polyethylene. A total knee bearing with a one-year increase in shelf age, a unicondylar knee bearing with a six-month increase in shelf age, a patient who was ten years younger at the time of operation, or a limb that was aligned in 5° more varus (less valgus) had similar effects on the loss of polyethylene thickness in the medial compartment; the coefficients of the linear regression equations indicated that any one of these changes would increase polyethylene loss by 0.11 to 0.14 mm/yr.

Conclusions: The wear-related loss of thickness in gamma-irradiated-in-air polyethylene bearings from unicondylar and total knee replacements implanted in osteoarthritic knees with varus deformity is influenced mainly by the shelf age of the polyethylene, the age of the patient, and the postoperative angulation of the knee in the coronal plane.

Clinical Relevance: Although polyethylene bearings have not been sterilized with gamma radiation in air since the end of the last decade, many arthroplasty patients have polyethylene bearings that were sterilized with use of this method. An understanding of the findings of this study may be of value as these patients return for follow-up care. Whether the study findings have relevance to bearings sterilized with other methods is unclear and will remain so for many years.

Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.

Figures in this Article

Wear of the polyethylene tibial component is a leading contributor to the failure of unicondylar^1-8 and total knee replacements^9-16, and it has been attributed to many factors, including those related to the patient (a young age or increased activity^{8,11,14,16-22}, weight^9,11,23, and male gender⁹), the surgical technique (postoperative knee malalignment^{9,12,19,21,23-26} and soft-tissue imbalance^25,27,28), and the polyethylene implant (fabrication method^{16,18,20,21,24,28-36}, sterilization with gamma radiation in air^4,9,30,37-39, gamma-in-air sterilization and subsequent aging on the shelf^7,8,40-42, thinness^{3,4,9,11,17,18,24,27,30,31}, and limited articular conformity^{2,4,9,12,18,19,24,28,30,31}).

The objectives of this study were to determine which patient, alignment, and implant factors were associated with in vivo loss of thickness of polyethylene tibial bearings that had been retrieved from unicondylar and total knee replacements, and to quantify the relative effect of each significant variable on the loss of bearing thickness. It was hypothesized that polyethylene loss would increase with younger patient age, increased weight, male gender, more varus postoperative alignment of the knee, more varus alignment of the tibial component, thinner initial polyethylene thickness, and greater shelf age of the polyethylene.

Materials and Methods

Materials and Methods | Results | Discussion | Appendix | References

Materials

Polyethylene tibial bearings were retrieved from eighty-one unicondylar and eighty-nine total knee replacements at the time of revision or autopsy (see Appendix). Each had been machined to shape, sterilized with gamma radiation in air (25 to 45 kGy), and implanted at our institution in an osteoarthritic knee that was in varus alignment (a preoperative tibiofemoral valgus angle of <6°). The time from implantation to retrieval was a mean (and standard deviation) of 8 ± 4 years (range, one to eighteen years). In general, the unicondylar tibial components differed from the total knee components in that they had been implanted into younger patients, were placed in more varus alignment, and the polyethylene inserts had less initial minimum thickness (the difference between the composite thickness and baseplate thickness at the center of the compartment) and longer shelf ages.

The total knee bearings were of a single, unconstrained, posterior cruciate ligament-retaining design (Anatomic Modular Knee with a standard Enduron insert; DePuy, Warsaw, Indiana)¹⁷, and they had been machined from five polyethylene types (compression-molded sheet formed from 1900 resin; compression-molded sheet, from 412 resin; compression-molded sheet, from 415 resin; ram-extruded bar, from 412 resin; ram-extruded bar, from 415 resin)⁴³. Five unicondylar designs that employed a fixed-bearing, metal-backed tibial component were examined. Each unicondylar component manufacturer used a unique combination of implant articular geometries. The designs from Osteonics (Allendale, New Jersey), which included thirteen Omnifit and thirty-one Single Compartment Replacement⁸ knee prostheses, had curved-on-slightly-curved articulations. The four Miller-Galante knees^5,44 from Zimmer (Warsaw, Indiana) all had curved-on-flat articulations. The designs from Johnson and Johnson (Raynham, Massachusetts), which included thirty Robert-Brigham^3,4 and three Microloc² knee replacements, had flat-on-flat articulations. Original machining stock was confirmed for thirty-seven of eighty-one unicondylar bearings and included compression-molded sheets formed from 1900 or 412 resin (twenty-seven knees), compression-molded sheets formed from 1000 resin (six knees), and ram-extruded bars formed from 415 resin (four knees). Both the polyethylene bearing and metal backing of the tibial component had been retrieved in seventy-three of eighty-one unicondylar knee replacements and in sixty-two of eighty-nine total knee replacements. All unicondylar and total knee femoral components had polished cobalt-chromium-molybdenum alloy articular surfaces.

The components we studied were selected because of their prevalence within our collection of retrieved implants. Of the primary knee replacements that had been performed at our institution and subsequently had the polyethylene tibial component retrieved, an overwhelming number featured a metal-backed tibial component with a polyethylene bearing that had been sterilized with gamma irradiation in air and placed into an osteoarthritic knee with a varus deformity. We did not examine other styles or designs of polyethylene tibial components or any of the studied designs that were placed for other reasons because of their limited availability. The eighty-nine total knee bearings represent all of those that we had retrieved from patients who had a primary total knee replacement with an Anatomic Modular Knee, in which a gamma-irradiated-in-air, standard cruciate ligament-retaining insert was placed at our institution from 1987 to 1996, for the treatment of osteoarthritis with a varus deformity. The eighty-one unicondylar components represent all of those that we had retrieved from knees with a metal-backed unicondylar tibial component with a gamma-irradiated-in-air polyethylene bearing that had been placed by us from 1984 to 1998 for osteoarthritis with varus angulation.

Retrieval Analysis

The polyethylene articular surfaces were inspected to qualify (rather than quantify) the extent of fatigue damage. Three stages of fatigue were seen (Fig. 1). In stage I, fatigue failure was limited to the region below the articular surface with perhaps an isolated pit. In stage II, a focal area of the surface pitting or delamination had developed. In stage III, surface fatigue had spread to involve at least one edge of the tibiofemoral compartment.

For total knee replacements, the minimum initial thickness of the polyethylene was considered the labeled composite thickness of the tibial component (composite thickness = metal baseplate thickness + minimum thickness of the polyethylene insert) less the standard 4-mm baseplate thickness (the quality control department of the manufacturer does not accept the part if the actual minimum thickness deviates by >0.2 mm from this value, according to a representative of the manufacturer). The final minimum polyethylene thickness of each tibiofemoral compartment of the insert was measured with calipers on two occasions by one observer, and the initial and final values were averaged (the two measurements from the same compartment varied by 0.1 ± 0.1 mm). The minimum compartmental thickness was recorded as 0.0 mm when full-thickness wear was present (full-thickness loss was observed in nine compartments in eight knees). Loss of polyethylene thickness in each compartment was defined as the difference between the initial and the final minimum polyethylene thicknesses. In each weight-bearing compartment, the studied polyethylene insert had a flat articular surface (except for the antisubluxation rises at its extreme medial and lateral edges and near its central intercondylar eminence) and a flat distal surface that rested against a flat metal baseplate surface.

Because the polyethylene part of each unicondylar tibial component was inset into a recess in the metal backing, the polyethylene bearing tended to be thinner peripherally than centrally and could not always be removed. Both before implantation and after explantation, we chose to use the minimum composite thickness of each unicondylar tibial component in our measurements. Examination of undamaged implants or undamaged regions of worn implants was done to confirm that the labeled composite thickness matched the minimum composite thickness of the tibial component before implantation (we found that, in one design, the implant was 0.5 mm thinner centrally compared with the labeled composite thickness, and we subtracted 0.5 mm from the labeled thickness when this design had been used). The loss of polyethylene thickness in a unicondylar replacement was defined as the difference between the labeled composite thickness of the tibial component and the mean of two measurements of minimum composite thickness made with calipers. (For the eight knees in which only the polyethylene bearing had been revised, the standard thickness of the metal baseplate was determined from laboratory analysis of other specimens and was added to the final minimum polyethylene thickness measured with calipers.) Note that loss of polyethylene thickness in a unicondylar component, as we defined it above, sometimes represented a combination of polyethylene loss and metal loss (that is, in twenty-eight knees, the femoral component had penetrated the polyethylene bearing to contact the capture rim of the underlying metal baseplate).

To improve the validity of comparisons of data from bearings that were retrieved after varying service times, the loss of polyethylene thickness was divided by the in vivo duration.

Statistical Methods

For statistical comparisons of continuous variable datasets, the independent samples t test (if both had a normal distribution) or the Mann-Whitney U test was used. The Pearson chisquare test was used in comparisons of nominal variable datasets. In each of the analyses, probability values of <0.05 were considered significant.

Multiple linear regression was used to assess the association between the loss of polyethylene thickness and eight risk variables (Table I): age, weight, gender, postoperative hip-knee-ankle angle (measured from a 35 by 129-cm full-length bilateral weight-bearing anteroposterior radiograph made at a mean of four months after the arthroplasty), varus angle of the tibial component (measured from the full-length radiograph), initial minimum thickness of the polyethylene, shelf age of the polyethylene (i.e., the interval from the sterilization date to the implantation date), and either the type of the total knee polyethylene or the manufacturer of the unicondylar knee replacement (the polyethylene type was not available for forty-four unicondylar implants, and thus we studied the unicondylar manufacturer as an eighth variable that encompassed tibiofemoral articular geometry). For nominal variables (gender, polyethylene type, and manufacturer), one option was set as the reference standard (coded as 1) and was compared with all other variations (coded as 0) (each option was tested as the reference standard). Linear equations that expressed each of the three dependent variables (loss of polyethylene thickness in the medial unicompartmental knee replacements, in the medial compartment of total knee replacements, and in the lateral compartment of total knee replacements) in terms of constants and the independent variables that were significantly associated (p < 0.05) with the dependent variable were obtained (with use of stepwise backward elimination [version 8.0, SPSS for Windows; SPSS, Chicago, Illinois]). The coefficient of determination (R²) expressed the proportion of variation (from 0.00 to 1.00) in the dependent variable that the regression equation could account for across the study cohort. Ideally, the equation would account for 1.00, or 100%, of the variation; in other words, we could simply input the data for every variable that significantly influenced the dependent variable into the regression equation and predict the dependent variable data of each case with negligible error. A unique coefficient precedes each independent variable of each of the multiple regression equations. This coefficient allows one to determine how an arbitrary change in the corresponding independent variable would influence the dependent variable. As an example, the effect that operating on a patient who was ten years younger had on polyethylene loss can be estimated by multiplying the coefficient that preceded patient age in the regression equation by —10. This type of analysis is particularly useful when studying a result (such as polyethylene loss) that is known to be influenced by many factors in a group of many subjects.

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TABLE I Summary of Study Data

Variable	Total Knee Replacements (N = 89)	Unicondylar Knee Replacements (N = 81)	Probability Value†
Years in vivo^*	9 ± 4 (1 to 17)	8 ± 4 (1 to 18)	0.38
Loss of polyethylene thickness^*(mm/yr)
Medial compartment	0.33 ± 0.28 (0.03 to 1.31)	0.49 ± 0.40 (0.03 to 2.08)	<0.01‡
Lateral compartment§	0.19 ± 0.18 (0.02 to 1.04)	—	NA
Patient age at time of arthroplasty^*(yr)	66 ± 9 (36 to 89)	63 ± 8 (45 to 78)	0.03
Patient weight^*(kg)	86 ± 16 (59 to 132)	84 ± 14 (51 to 123)	0.43
Patient gender (F/M)	41/48	29/52	0.21#
Postoperative varus alignment of hip-knee-ankle angle^*(deg)	1 ± 3 (—6 to +7)	1 ± 3 (—5 to +10)	0.23
Varus alignment of tibial component^*(deg)	1 ± 2 (—4 to +5)	7 ± 5 (—3 to +22)	<0.01
Initial thickness of polyethylene^*(mm)	7.6 ± 1.9 (4.0 to 14.0)	5.7 ± 1.3 (4.0 to 9.7)	<0.01‡
Shelf age of polyethylene^*(yr)	1.3 ± 1.5 (0.0 to 6.3)	1.6 ± 1.3 (0.1 to 5.3)	0.01‡

The data are given as the mean and the standard deviation, with the range in parenthesesProbability values were determined with use of the independent samples test, Mann-Whitney U test, or Pearson chi-square test. NA = not applicableMann-Whitney U testAll unicondylar knee arthroplasties involved the medial compartmentPearson chi-square test

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TABLE I Summary of Study Data

Variable	Total Knee Replacements (N = 89)	Unicondylar Knee Replacements (N = 81)	Probability Value†
Years in vivo^*	9 ± 4 (1 to 17)	8 ± 4 (1 to 18)	0.38
Loss of polyethylene thickness^*(mm/yr)
Medial compartment	0.33 ± 0.28 (0.03 to 1.31)	0.49 ± 0.40 (0.03 to 2.08)	<0.01‡
Lateral compartment§	0.19 ± 0.18 (0.02 to 1.04)	—	NA
Patient age at time of arthroplasty^*(yr)	66 ± 9 (36 to 89)	63 ± 8 (45 to 78)	0.03
Patient weight^*(kg)	86 ± 16 (59 to 132)	84 ± 14 (51 to 123)	0.43
Patient gender (F/M)	41/48	29/52	0.21#
Postoperative varus alignment of hip-knee-ankle angle^*(deg)	1 ± 3 (—6 to +7)	1 ± 3 (—5 to +10)	0.23
Varus alignment of tibial component^*(deg)	1 ± 2 (—4 to +5)	7 ± 5 (—3 to +22)	<0.01
Initial thickness of polyethylene^*(mm)	7.6 ± 1.9 (4.0 to 14.0)	5.7 ± 1.3 (4.0 to 9.7)	<0.01‡
Shelf age of polyethylene^*(yr)	1.3 ± 1.5 (0.0 to 6.3)	1.6 ± 1.3 (0.1 to 5.3)	0.01‡

Results

Materials and Methods | Results | Discussion | Appendix | References

Eighty-nine percent (seventy-two) of the eighty-one retrieved unicondylar bearings had some evidence of fatigue surface damage, with 56% (forty-five bearings) showing stage-III damage (Fig. 1). Among the eighty-nine total knee inserts, fatigue damage was seen in 84% (seventy-five) of the medial compartments and 80% (seventy-one) of the lateral compartments; stage-III damage was seen in 43% (thirty-eight) of the medial compartments and 39% (thirty-five) of the lateral compartments (see Appendix).

The loss of polyethylene thickness in the medial unicompartmental knee inserts was an average (and standard deviation) of 0.49 ± 0.40 mm/yr (range, 0.03 to 2.08 mm/yr). In the medial and lateral compartments of the total knee implants, loss of polyethylene thickness averaged 0.33 ± 0.28 mm/yr (range, 0.03 to 1.31 mm/yr) and 0.19 ± 0.18 mm/yr (range, 0.02 to 1.04 mm/yr), respectively (medial loss exceeded lateral loss in seventy of eighty-nine knees).

With use of the regression equation featuring the eight studied independent variables, we were able to account for 66% (R² = 0.66) of the variation in polyethylene loss of the retrieved medial unicompartmental implants. Three factors were significantly associated with the loss of polyethylene thickness in the medial unicompartmental knee replacements: patient age, postoperative hip-knee-ankle angle, and shelf age of the polyethylene (p < 0.01 for each). Age, postoperative hip-knee-ankle angle, and shelf age together were responsible for 63% (R² = 0.63) of the specimen-to-specimen variation in thickness loss (note that adding gender, weight, varus angle of the tibial component, initial thickness of the polyethylene, and manufacturer to the regression allowed us to account for only another 3% of the variation). A six-month increase in shelf age had an effect similar to that of operating on a patient who was ten years younger or aligning a knee in 5° more varus (less valgus); each factor was associated with an increase of 0.120 to 0.140 mm/yr in polyethylene loss per regression equation coefficients (Table II; a 0.5-year increase in shelf age × 0.241-mm/yr loss of thickness per year of shelf age = 0.120-mm/yr loss of thickness; a —10-year increase in patient age × —0.014-mm/yr loss of thickness per year of patient age = 0.140-mm/yr loss of thickness; and a 5° increase in varus alignment of the hip-knee-ankle angle × 0.026-mm/yr loss of thickness per degree of varus alignment of the hip-knee-ankle angle = 0.130-mm/yr loss of thickness). Gender, weight, varus angle of the tibial component, initial polyethylene thickness, and manufacturer were not significantly associated with polyethylene loss on the basis of the numbers studied.

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TABLE II Multiple Linear Regression Equations Obtained in the Study

Equation^*	Coefficient of Determination (R²)
Medial unicompartmental knee polyethylene loss = 0.959 + 0.241 × shelf age + 0.026 × postop. hip-knee-ankle varus — 0.014 × patient age	0.63
Total knee medial compartment polyethylene loss = 0.903 + 0.125 × shelf age + 0.021 × postop. hip-knee-ankle varus — 0.011 × patient age	0.57
Total knee lateral compartment polyethylene loss = 0.112 + 0.053 × shelf age — 0.012 × postop. hip-knee-ankle varus	0.31

Polyethylene loss is in millimeters per year, shelf age and patient age are in years, and hip-knee-ankle varus alignment is in degrees

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TABLE II Multiple Linear Regression Equations Obtained in the Study

Equation^*	Coefficient of Determination (R²)
Medial unicompartmental knee polyethylene loss = 0.959 + 0.241 × shelf age + 0.026 × postop. hip-knee-ankle varus — 0.014 × patient age	0.63
Total knee medial compartment polyethylene loss = 0.903 + 0.125 × shelf age + 0.021 × postop. hip-knee-ankle varus — 0.011 × patient age	0.57
Total knee lateral compartment polyethylene loss = 0.112 + 0.053 × shelf age — 0.012 × postop. hip-knee-ankle varus	0.31

Polyethylene loss is in millimeters per year, shelf age and patient age are in years, and hip-knee-ankle varus alignment is in degrees

For the retrieved total knee inserts, the corresponding regression that expressed loss of polyethylene thickness in the medial compartment in terms of the eight independent variables could account for 59% (R² = 0.59) of the variation in the loss of polyethylene thickness. Three factors were significantly associated with the loss of the polyethylene thickness in the medial compartment of total knee replacements: the shelf age of the polyethylene (p < 0.01), patient age (p < 0.01), and postoperative varus alignment of the hip-knee-ankle angle (p = 0.02). The corresponding regression equation that featured only these three variables (shelf age of the polyethylene, patient age, and postoperative hip-knee-ankle angle) could account for 57% (R² = 0.57) of the variation in the loss of polyethylene thickness in the medial compartment of total knee replacements. A one-year increase in the shelf age of the polyethylene insert had an effect similar to that of operating on a patient who was ten years younger or aligning a knee in 5° more varus (less valgus); each change would be associated with a 0.105 to 0.125-mm/yr increase in the loss of polyethylene thickness in the medial compartment if all other factors were held constant (Table II; a one-year increase in shelf age × 0.125-mm/yr loss of thickness per year of shelf age = 0.125-mm/yr loss of thickness; a —10-year increase in patient age × —0.011-mm/yr loss of thickness per year of patient age = 0.110-mm/yr loss of thickness; and a 5° increase in the varus alignment of the hip-knee-ankle angle × 0.021-mm/yr loss of thickness per degree of varus alignment of the hip-knee-ankle angle = 0.105-mm/yr loss of thickness). Entry of the same eight independent variables into the regression equation that described loss of polyethylene thickness in the lateral compartment allowed us to account for 34% (R² = 0.34) of the variation in this parameter. Shelf age (p < 0.01) and postoperative knee alignment (p = 0.04) were the only variables that were significantly associated with loss of thickness in the lateral compartment; with these two variables, we could account for 31% (R² = 0.31) of the specimen-to-specimen variation in the loss of polyethylene thickness in the lateral compartment. The loss of polyethylene thickness in the lateral compartment was rather limited (mean, 0.12 ± 0.06 mm/yr for the forty-seven inserts with a shelf age of less than nine months) when the shelf age was short. As varus alignment of the hip-knee-ankle angle decreased (or as valgus alignment of the hip-knee-ankle angle increased), loss of polyethylene thickness in the medial compartment decreased at nearly twice the rate that loss of polyethylene thickness in the lateral compartment increased (a 0.021-mm/yr decrease medially compared with a 0.012-mm/yr increase laterally per each degree of decrease in varus alignment or each degree of increase in valgus alignment) (Table II). Gender, weight, varus angle of the tibial component, initial polyethylene thickness, and polyethylene type were not significantly associated with loss of thickness of either compartment.

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Fig. 1: Representative examples of stage-I, II, and III fatigue damage are shown from left to right for the total knee (top row) and unicondylar knee (bottom row) polyethylene tibial bearings.

To gain further insight into the influence of knee alignment in the coronal plane, we studied a subset of total knee replacements performed in younger patients with polyethylene tibial bearings with a shorter shelf age. Twenty-eight retrieved total knee inserts had a shelf age of less than nine months (mean, four months) and had been implanted into patients who were sixty-eight years old or less (mean, fifty-nine years). Losses of polyethylene thickness of >0.2 mm/yr were largely isolated to the medial compartment and were especially common in knees with varus alignment of the postoperative hip-knee-ankle angle (Fig. 2). Polyethylene loss in the medial compartment decreased at three times the rate that polyethylene loss in the lateral compartment increased with decreasing varus alignment (or increasing valgus alignment) of the knee.

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Fig. 2: The loss of polyethylene thickness in relation to the hip-knee-ankle angle (as measured during the first postoperative year) is plotted for the twenty-eight total knee inserts that had a shelf age of less than nine months and had been implanted in patients who were sixty-eight years old or less. The loss of polyethylene thickness was greatest in the medial compartment of knees that had postoperative varus alignment of the hip-knee-ankle angle. On the basis of the regression trend lines, with the solid line indicating loss of polyethylene thickness in the medial compartment (+0.030 mm/yr x postoperative degree of varus alignment of the hip-knee-ankle angle + 0.255; R = 0.35) and the dashed line indicating loss of polyethylene thickness in the lateral compartment (—0.011 mm/yr x postoperative degree of varus alignment of the hip-knee-ankle angle + 0.124; R = 0.22), the loss of polyethylene in the medial compartment decreased at nearly three times the rate that the loss of polyethylene in the lateral compartment increased with less varus or more valgus angulation (a 0.030-mm/yr/deg decrease medially compared with a 0.011-mm/yr/deg increase laterally).

Notable reductions in the loss of polyethylene thickness were observed when the surgeon had implanted polyethylene with a shorter shelf age and avoided varus alignment of the postoperative hip-knee-ankle angle (Table III).

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TABLE III Data on Loss of Polyethylene Thickness According to Shelf Age of the Polyethylene and Postoperative Hip-Knee-Ankle Angle

				Total Knee Replacement
		Medial Unicompartmental Knee Replacement				Loss of Polyethylene Thickness^*(mm/yr)
Shelf Age of Polyethylene (mo)	Varus Alignment of Postop. Hip-Knee-Ankle Angle	No. of Knees	Loss of Polyethylene Thickness^*(mm/yr)	No. of Knees	Medial Compartment	Lateral Compartment
<9	<1°	11	0.23 ± 0.11 (0.09 to 0.38)	26	0.17 ± 0.09 (0.05 to 0.45)	0.12 ± 0.07 (0.04 to 0.31)
<9	=1°	14	0.27 ± 0.19 (0.03 to 0.60)	19	0.33 ± 0.17 (0.06 to 0.59)	0.11 ± 0.05 (0.04 to 0.21)
>9	<1°	16	0.52 ± 0.51 (0.13 to 2.08)	18	0.47 ± 0.39 (0.03 to 1.31)	0.39 ± 0.30 (0.05 to 1.04)
>9	=1°	35	0.65 ± 0.41 (0.12 to 2.07)	23	0.44 ± 0.33 (0.06 to 1.09)	0.18 ± 0.10 (0.02 to 0.42)

The values are given as the mean and the standard deviation, with the range in parentheses. Five unicondylar and three total knee bearings could not be grouped according to the criteria because of uncertainty with regard to the shelf age or the hip-knee-ankle angle

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TABLE III Data on Loss of Polyethylene Thickness According to Shelf Age of the Polyethylene and Postoperative Hip-Knee-Ankle Angle

				Total Knee Replacement
		Medial Unicompartmental Knee Replacement				Loss of Polyethylene Thickness^*(mm/yr)
Shelf Age of Polyethylene (mo)	Varus Alignment of Postop. Hip-Knee-Ankle Angle	No. of Knees	Loss of Polyethylene Thickness^*(mm/yr)	No. of Knees	Medial Compartment	Lateral Compartment
<9	<1°	11	0.23 ± 0.11 (0.09 to 0.38)	26	0.17 ± 0.09 (0.05 to 0.45)	0.12 ± 0.07 (0.04 to 0.31)
<9	=1°	14	0.27 ± 0.19 (0.03 to 0.60)	19	0.33 ± 0.17 (0.06 to 0.59)	0.11 ± 0.05 (0.04 to 0.21)
>9	<1°	16	0.52 ± 0.51 (0.13 to 2.08)	18	0.47 ± 0.39 (0.03 to 1.31)	0.39 ± 0.30 (0.05 to 1.04)
>9	=1°	35	0.65 ± 0.41 (0.12 to 2.07)	23	0.44 ± 0.33 (0.06 to 1.09)	0.18 ± 0.10 (0.02 to 0.42)

Discussion

Materials and Methods | Results | Discussion | Appendix | References

Tibial polyethylene wear has traditionally been assessed by grading articular surface damage of retrieved implants^{18,19,24,25,28,30,32-34,36,45-47}. In the last decade, researchers began to quantify in vivo changes in tibial polyethylene thickness from analysis of retrievals^{14,22,31,39,48-50} or radiographs^{7,13,20,21,23,26,51}. Measuring changes in tibial polyethylene thickness is challenging because radiographic analysis requires carefully obtained images to maximize accuracy^{13,20,21,23,26,51-54}, manufacturing tolerances allow for some variation in implant dimensions, wear may progress obliquely, and thickness may be lost because of a variety of damage modes^30,45,46.

The loss in thickness in our retrieved implants was related to some combination of burnishing, deformation, and mechanical failure (half of the total knee inserts and three-fourths of the unicondylar bearings exhibited stage-II or III fatigue damage). Rarely is it possible to examine a meaningful number of retrieved implants that were in vivo for the same duration. We divided the change in the minimum thickness of each tibial component by the in vivo duration to improve the validity of comparisons of implants that had differing service times rather than to suggest that the loss of thickness progressed linearly. Components with a minimum thickness that had changed less per years in vivo were considered to have performed superiorly to those with a minimum thickness that had changed more per years in vivo. Mean losses in polyethylene thickness from the medial compartment (0.3 mm/yr) and lateral compartment (0.2 mm/yr) of our retrieved total knee inserts are within the mean values of 0.1 to 0.4 mm/yr reported with contemporaneous total knee designs that employed gamma-irradiated-in-air modular polyethylene inserts^{13,14,22,23,49,51}. The mean loss of polyethylene thickness in our retrieved unicondylar implants (0.5 mm/yr) is within the range of mean values (0.1 to 1.7 mm/yr) reported with fixed-bearing tibial components^7,26,50.

The size of our group of retrieved implants allowed us to determine the importance of selected patient, alignment, and implant factors with multivariate statistical methods (which previously had been used only in radiographic studies)^20,21,23. Use of these methods is advantageous in a study like this one, in which many intercorrelations exist between variables (weight with age and gender, varus-valgus alignment of the total knee tibial component with limb alignment in the coronal plane, and initial polyethylene thickness with shelf age). The three factors that were significantly associated with loss of polyethylene thickness in medial unicondylar tibial components were the same three factors that were significantly associated with loss of polyethylene thickness from the medial compartment of total knee tibial inserts: shelf age of the polyethylene, patient age, and postoperative knee alignment in the coronal plane (hypotheses accepted). Wear of a medial unicondylar bearing and of the medial compartment of a total knee bearing appear very much alike; this inference is supported by the similarity of the linear regression equations that described the loss of polyethylene thickness at each site. Fifty-seven percent to 63% of the variation in the polyethylene loss in the medial compartment of the retrieved unicondylar and total knee bearings was accounted for by patient age, postoperative alignment of the knee in the coronal plane, and the shelf age of the polyethylene. As a frame of comparison, multiple regression analyses of the wear of acetabular components (which seldom demonstrate fatigue) could account for no more than one-third of the specimen-to-specimen variation in the loss of polyethylene thickness^55-57.

The loss of polyethylene thickness in the medial compartment was affected similarly by implanting a total knee insert with a shelf age that was one year longer or a unicondylar bearing with a shelf age that was six months longer, performing either procedure in a patient who was ten years younger, or placing a total or unicondylar knee in 5° more varus alignment (hip-knee-ankle angle); each hypothetical change would increase polyethylene loss by 0.11 to 0.14 mm/yr if all other factors were held constant (with the opposite event decreasing it by the same amount). With less postoperative varus alignment (more postoperative valgus alignment), polyethylene loss in the medial compartment of a total knee insert decreased by two to three times as much as the polyethylene loss in its lateral compartment increased. The surgeon should consider the optimal alignment of each limb along with the potentially disparate consequences of erring on the varus or valgus side of the planned target^58,59.

Polyethylene loss was not associated with weight, gender, polyethylene thickness, or varus angle of the tibial component on the basis of the numbers (hypotheses rejected). Nor was it associated with polyethylene type (for studied total knee inserts) or design (for the group of retrieved unicondylar bearings). Admittedly, comprehensive analyses of polyethylene type and design would require study of multiple polyethylene types across single designs and study of single polyethylene types across multiple designs. Rather than dismiss the role of polyethylene type or design in wear of gamma-irradiated-in-air bearings, we just deduced that neither factor was important in our study.

Our findings may not apply to a limb with preoperative valgus malalignment or to mobile-bearing or all-polyethylene tibial components. A noteworthy limitation of retrieval studies is that revised bearings and specimens retrieved post mortem are often drawn from opposite extremes of the pool of arthroplasty patients (such patients tend to be younger, more active, and in better health during the years preceding revision than during the years preceding death).

Other multivariate statistical analysis studies are needed to further isolate and prioritize factors contributing to the loss of tibial polyethylene thickness and to provide reference standards against which the performance of contemporary types of ultra-high molecular weight polyethylene tibial bearings (those sterilized with gamma radiation in an oxygen-free environment or with radiation-free ethylene oxide or gas plasma methods)⁴² and future types of such bearings can be gauged. While the long-term clinical performance of contemporary polyethylene tibial bearings (which are expected to be less susceptible to oxidative degradation before, if not also after, implantation) will remain uncertain for many years, the surgeon could cautiously assume in the interim that knee alignment continues to influence bearing wear and could aim to avoid the postoperative varus alignment of the mechanical axis (the hip-knee-ankle angle). We believe that this strategy is particularly wise in younger patients, whose risk for wear-related complications remains elevated because of greater activity levels and longer life expectancies.

Appendix

Materials and Methods | Results | Discussion | Appendix | References

A table summarizing the reasons for retrieval of the studied components and a table characterizing the extent of surface fatigue damage of the components are available with the electronic versions of this article, on our web site at (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). ?

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Topics

ankle ; hip region ; knee region ; tibia ; knee replacement arthroplasty ; polyethylene

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Matthew B. Collier, C. Anderson EnghJr., James P. McAuley, Gerard A. Engh; Factors Associated with the Loss of Thickness of Polyethylene Tibial Bearings After Knee Arthroplasty. The Journal of Bone & Joint Surgery. 2007 Jun;89(6):1306-1314.

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