JBJS | Article

The Journal of Bone & Joint Surgery, Volume 84, Issue 3

Scientific Article | March 01, 2002

Analysis of Polyethylene Thickness of Tibial Components in Total Knee Replacement

S. A. Edwards, FRCS; H. G. Pandit, FRCS(Orth); J. L. Ramos, MD, PhD, FRCS(Orth); M. L. Grover, FRCS

Investigation performed at the Department of Orthopaedics, Queen Alexandra Hospital, Portsmouth, England

S.A. Edwards, FRCS
29 Valerian Avenue, Titchfield, Fareham, Hants P015 5TF, United Kingdom. E-mail address: stuart@eedwards.freeserve.co.uk

H.G. Pandit, FRCS(Orth)
J.L. Ramos, MD, PhD, FRCS(Orth)
M.L. Grover, FRCS
Department of Orthopaedics, Queen Alexandra Hospital, Cosham, Portsmouth PO6 3LY, England

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

The Journal of Bone & Joint Surgery. 2002; 84:369-371

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Abstract

Background: Excessive wear of the polyethylene bearing surfaces of tibial components has become an important factor in early failure of total knee arthroplasty. Inadequate thickness of the polyethylene insert is one cause of excessive wear, and various minimum thicknesses have been recommended in order to reduce contact stresses within the polyethylene. However, the true thicknesses of modular polyethylene tibial inserts typically are not stated accurately by the manufacturers in their product information. The purpose of this study was to determine whether the information about the thickness of tibial inserts supplied by the manufacturers is adequate.

Methods: Five of the thinnest available polyethylene tibial inserts from five different manufacturers were selected. The minimum thickness of each was measured with use of a Sigma electronic micrometer comparator to an accuracy of ±0.005 mm.

Results: The stated thicknesses of the polyethylene tibial inserts were 8, 9, and 10 mm, values that differed markedly from the actual minimum thicknesses, which ranged from 5.5722 to 6.2048 mm (mean values).

Conclusion: The thickness of polyethylene tibial inserts has been implicated as a potential cause of excessive wear and early failure of total knee replacements. This paper highlights the fact that the information supplied by the manufacturers is inaccurate and potentially misleading; in one case, the true thickness was much less than the recommended minimum thickness. We recommend that the minimum thickness of the tibial components as well as the combined thickness of the polyethylene insert and the metal tibial tray be specified in the product information and on the packaged insert.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Ultra-high molecular weight polyethylene is commonly used in joint replacements. Its success is due to its favorable properties, including abrasion resistance, impact strength, low coefficient of friction, chemical inertness, and resistance to stress cracking. Although the survivorship of total knee replacements with polyethylene has been reported to be as high as 98% at eleven years¹, there is growing evidence that debris from the polyethylene may limit its longer-term use^2,3 because of osteolysis and subsequent aseptic loosening^4-7.

Early failure of the bearing surface of tibial components has been attributed to excessive wear of the ultra-high molecular weight polyethylene. Several factors that control stresses associated with wear damage include the thickness of the tibial insert, the conformity between the metallic and polyethylene articulating surfaces, the elastic modulus of the polyethylene material, and the positioning and alignment of the metal tibial tray^8,9.

On the basis of retrieval analyses and biomechanical studies of total knee components, a minimum thickness of between 6 and 12 mm has been recommended in order to minimize stresses in the polyethylene^9-11. The thickness of ultra-high molecular weight polyethylene tibial inserts of knee replacements has become more important in recent years^12-14, principally since the introduction of metal-backed tibial components, which reduce the true thickness of the polyethylene insert. However, specific data regarding the actual minimum thickness of the polyethylene insert and the combined thickness with the metal tibial tray could not be found routinely in the manufacturers’ surgical technique/instrumentation booklets. Details were provided in only one manufacturer’s product information booklet.

The purpose of this study was to determine whether the information about the thickness of tibial inserts supplied by the manufacturers was adequate and accurate.

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TABLE I: Minimum Polyethylene Thickness in Millimeters

*The value in parentheses after each product represents the polyethylene thickness stated by the manufacturer.

	Product*
NexGen (9 mm)	PFC (8 mm)	Kinemax (8 mm)	LCS (10 mm)	Scorpio (8 mm)	All
No. of implants	5	5	5	5	5	25
Mean	5.5722	5.9843	6.0524	6.0709	6.2048	5.9769
Standard deviation	0.0567	0.0271	0.0227	0.0859	0.0200	0.2236
Range	5.454-5.666	5.925-6.011	5.994-6.096	5.975-6.237	6.176-6.265

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TABLE I: Minimum Polyethylene Thickness in Millimeters

*The value in parentheses after each product represents the polyethylene thickness stated by the manufacturer.

	Product*
NexGen (9 mm)	PFC (8 mm)	Kinemax (8 mm)	LCS (10 mm)	Scorpio (8 mm)	All
No. of implants	5	5	5	5	5	25
Mean	5.5722	5.9843	6.0524	6.0709	6.2048	5.9769
Standard deviation	0.0567	0.0271	0.0227	0.0859	0.0200	0.2236
Range	5.454-5.666	5.925-6.011	5.994-6.096	5.975-6.237	6.176-6.265

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TABLE II: Analysis-of-Variance Table

*The Fisher statistic or ratio compares the variability attributable to each source or component under investigation with that expected as a result of chance. †P values of £0.05 indicated significance.

Source	Degrees of Freedom	Sum of Squares	Mean Square	Fisher Statistic or Ratio*	P Value†
Product	4	9.21135	2.30284	117.54	0.000
Side	1	0.00249	0.00249	??4.77	0.030
Product.side	4	0.03428	0.00857	?16.40	0.000
Sample (product)	20	0.39185	0.01959	?37.49	0.000
Observer	1	0.00038	0.00038	??0.73	0.394
Error	169	0.08832	0.00052
Total	199	9.72867

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TABLE II: Analysis-of-Variance Table

Source	Degrees of Freedom	Sum of Squares	Mean Square	Fisher Statistic or Ratio*	P Value†
Product	4	9.21135	2.30284	117.54	0.000
Side	1	0.00249	0.00249	??4.77	0.030
Product.side	4	0.03428	0.00857	?16.40	0.000
Sample (product)	20	0.39185	0.01959	?37.49	0.000
Observer	1	0.00038	0.00038	??0.73	0.394
Error	169	0.08832	0.00052
Total	199	9.72867

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

The thinnest available polyethylene tibial inserts from five different manufacturers most frequently used in our District General Hospital were obtained. Packaged sterile prostheses, ready for use in patients, were randomly selected. Five implants of each design were measured at an ambient temperature of 20Â° ± 3Â°C with 55% ± 20% relative humidity.

The inserts were given unique reference numbers, and the right and left sides of each concave surface were marked. The minimum thickness of each was measured with use of a Sigma electronic micrometer comparator (202/11SIG; Herberts Controls and Instruments, Hertfordshire, England), engineering parallels, and metric block gauges, to an accuracy of 0.005 mm. Observations were made by two independent calibration engineers who were blinded with respect to the type of prosthesis and its stated thickness. The minimum thickness was measured twice on each of the concave surfaces (left and right) of the polyethylene insert. The response variable was the average of eight repeated measurements (four by each of the two observers) made on each sample.

Individual measurements were analyzed with use of a hierarchical analysis of variance for repeated measures. Observers and samples (nested within product type) were treated as random factors, and their components of variance were calculated. Product type and side were treated as fixed effects in the model. An analysis-of-variance table was then determined.

Results

Introduction | Materials and Methods | Results | Discussion | References

The stated thicknesses of the tibial inserts were 8, 9, and 10 mm, values that differed substantially from the actual minimum thicknesses, which ranged from a mean of 5.5722 mm to a mean of 6.2048 mm (Table I).

Analysis of variance revealed a significant difference among the products with regard to mean minimum thickness (p < 0.001) (Table II). The mean thickness was found to depend on the side of the insert that had been measured (p = 0.030). The difference between the mean thicknesses on the left and right sides, however, was not consistent among manufacturers (p < 0.001). There was no significant variation between observers. The corresponding component of variance was estimated to be zero. Variation among samples within product type was also found to be significant (p < 0.001), and the component of variance due to this source was estimated to be 0.00238. The within-sample component of variance reflecting variability between repeated (intraobserver) observations on each sample was estimated to be 0.00052. The within-sample (error) component of variance was used to provide an estimate of repeatability. Repeatability was defined as the maximum expected difference between two observations made by the same observer 95% of the time and was estimated to be 0.061 mm in this study. This value is small, indicating good repeatability, and the lack of significant differences between the observers indicates good reproducibility.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Improved knowledge of the behavior of polyethylene within total knee replacements and improved surgical techniques to ensure accurate alignment are the most important factors in reducing the rate of early aseptic loosening9,12.

The introduction of modular components resulted from concerns about the deformation of all-polyethylene components¹⁵. The metal-backed tibial tray and ultra-high molecular weight polyethylene inserts of varying thicknesses has allowed relatively simple adjustment of soft-tissue tension and has provided a potential for simplifying revision surgery when necessary¹⁶.

Modularity has disadvantages, with accumulating evidence of cold flow and wear between surfaces and the production of polyethylene debris, which leads to localized osteolysis^10,14. Also, the presence of the metal tray decreases the available space for the polyethylene.

A compromise is required between bone resection and soft-tissue balance, and adequate polyethylene thickness. Excessive bone resection may compromise revision surgery; it also may damage ligaments and hence compromise stability. The use of thinner polyethylene inserts may still be indicated, and they should therefore still be available. Work by Bartel and Wright, who analyzed contact stresses as a function of the thickness of the insert, led to the recommendation of a desirable minimum thickness of between 6 and 8 mm for the polyethylene insert^9,11.

The present study documented a significant difference between the stated thicknesses of the tibial inserts and their actual thicknesses in implants supplied by five manufacturers. The actual minimum thickness of one of the inserts was well below that recommended by Bartel and Wright. The analysis also revealed that there was significant variation among samples within each product type. Despite previous calls for a change in the way tibial insert thicknesses are described^12,14, product information supplied by the manufacturers typically fails to specify the actual minimum thickness. We strongly recommend that the minimum thickness of the polyethylene tibial insert and the metal tibial tray be clearly stated.

Note: The authors acknowledge Mr. D. Abbott of Absolute Calibration Limited, for his assistance in measuring the thicknesses of the tibial polyethylene inserts, and B. Higgins for his assistance in the statistical analysis.

References

Introduction | Materials and Methods | Results | Discussion | References

Colizza WA, Insall JN,Scuderi GR. The posterior stabilized total knee prosthesis. Assessment of polyethylene damage and osteolysis after a ten-year-minimum follow-up. J Bone Joint Surg Am,1995;77: 1713-20. 771713 1995 [PubMed]

Dannemaier WC, Haynes DW,Nelson CL. Granulomatous reaction and cystic bony destruction associated with higher wear rate in a total knee prosthesis. Clin Orthop,1985;198: 224-30. 198224 1985 [PubMed]

Peters PC Jr, Engh GA, Dwyer KA,Vinh TN. Osteolysis after total knee replacement without cement. J Bone Joint Surg Am,1992;74: 864-76. 74864 1992 [PubMed]

Goodman SB, Fonasier VL, Lee J,Kei J. The histological effects of the implantation of different sizes of polyethylene particles in the rabbit tibia. J Biomed Mater Res,1990;24: 517-24. 24517 1990 [PubMed]

Harris WH. The problem is osteolysis. Clin Orthop,1995;311: 46-53. 31146 1995 [PubMed]

Howie DW, Vernon-Roberts B, Oakeshott R,Manthey B. A rat model of resorption of bone at the cement-bone interface in the presence of polyethylene wear particles. J Bone Joint Surg Am,1988;70: 257-63. 70257 1988 [PubMed]

McKellop HA, Campbell P, Park SH, Schmalzried TP, Grigoris P, Amstutz HC,Sarmiento A. The origin of the submicron polyethylene wear debris in total hip arthroplasty. Clin Orthop,1995;311: 3-20. 3113 1995 [PubMed]

Argenson JN,O’Connor JJ. Polyethylene wear in meniscal knee replacement. A one to nine-year retrieval analysis of the Oxford knee. J Bone Joint Surg Br,1992;74: 228-32. 74228 1992 [PubMed]

Bartel DL, Bicknell VL,Wright TM. The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement. J Bone Joint Surg Am,1986;68: 1041-51. 681041 1986 [PubMed]

Chang DC, Goh JC, Teoh SH,Bose K. Cold extrusion deformation of UHMWPE in total knee replacement prostheses. Biomaterials,1995;16: 219-23. 16219 1995 [PubMed]

Wright TM,Bartel DL. The problem of surface damage in polyethylene total knee components. Clin Orthop,1986;205: 67-74. 20567 1986 [PubMed]

Chillag KJ,Barth E. An analysis of polyethylene thickness in modular total knee components. Clin Orthop,1991;273: 261-3. 273261 1991 [PubMed]

Howmedica International. Polyethylene wear of the bearing surfaces of the 7mm resurfacing tibial components of the PCA primary knee prosthesis. Medical Devices Agency, adverse incident centre. MDA SN 9629 Sept 1996.

Weber AB,Morris HG. Thickness of tibial inserts in total knee arthroplasty. J Arthroplasty,1996;11: 856-8. 11856 1996 [PubMed]

Scott RD. Duopatellar total knee replacement: the Brigham experience. Orthop Clin North Am,1982;13: 89-102. 1389 1982 [PubMed]

Lewis JL, Askew MJ,Jaycox DP. A comparative evaluation of tibial component designs of total knee prosthesis. J Bone Joint Surg Am,1982;64: 129-35. 64129 1982 [PubMed]

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TABLE I: Minimum Polyethylene Thickness in Millimeters

*The value in parentheses after each product represents the polyethylene thickness stated by the manufacturer.

	Product*
NexGen (9 mm)	PFC (8 mm)	Kinemax (8 mm)	LCS (10 mm)	Scorpio (8 mm)	All
No. of implants	5	5	5	5	5	25
Mean	5.5722	5.9843	6.0524	6.0709	6.2048	5.9769
Standard deviation	0.0567	0.0271	0.0227	0.0859	0.0200	0.2236
Range	5.454-5.666	5.925-6.011	5.994-6.096	5.975-6.237	6.176-6.265

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TABLE I: Minimum Polyethylene Thickness in Millimeters

*The value in parentheses after each product represents the polyethylene thickness stated by the manufacturer.

	Product*
NexGen (9 mm)	PFC (8 mm)	Kinemax (8 mm)	LCS (10 mm)	Scorpio (8 mm)	All
No. of implants	5	5	5	5	5	25
Mean	5.5722	5.9843	6.0524	6.0709	6.2048	5.9769
Standard deviation	0.0567	0.0271	0.0227	0.0859	0.0200	0.2236
Range	5.454-5.666	5.925-6.011	5.994-6.096	5.975-6.237	6.176-6.265

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TABLE II: Analysis-of-Variance Table

Source	Degrees of Freedom	Sum of Squares	Mean Square	Fisher Statistic or Ratio*	P Value†
Product	4	9.21135	2.30284	117.54	0.000
Side	1	0.00249	0.00249	??4.77	0.030
Product.side	4	0.03428	0.00857	?16.40	0.000
Sample (product)	20	0.39185	0.01959	?37.49	0.000
Observer	1	0.00038	0.00038	??0.73	0.394
Error	169	0.08832	0.00052
Total	199	9.72867

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TABLE II: Analysis-of-Variance Table

Source	Degrees of Freedom	Sum of Squares	Mean Square	Fisher Statistic or Ratio*	P Value†
Product	4	9.21135	2.30284	117.54	0.000
Side	1	0.00249	0.00249	??4.77	0.030
Product.side	4	0.03428	0.00857	?16.40	0.000
Sample (product)	20	0.39185	0.01959	?37.49	0.000
Observer	1	0.00038	0.00038	??0.73	0.394
Error	169	0.08832	0.00052
Total	199	9.72867