_Eroziune compon polietilena

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BY

THE JOURNAL

OF

BONE

AND JOINT

SURGERY, INCORPORATED

Backside Wear of Polyethylene Tibial Inserts: Mechanism and Magnitude of Material Loss BY MICHAEL A. CONDITT, PHD, MATTHEW T. THOMPSON, MS, MOLLY M. USREY, BS, SABIR K. ISMAILY, BS, AND PHILIP C. NOBLE, PHD Investigation performed at the Institute of Orthopedic Research and Education, Houston, Texas

Background: Wear of the underside of modular tibial inserts (backside wear) in total knee replacements has been reported by several authors. Although, for some implant designs, this phenomenon seems to contribute to osteolysis, the actual volume of material lost through wear of the backside surface has not been quantified. This study describes the results of computerized measurements of tibial inserts of one design known to be associated with a high prevalence of backside wear in situ. Methods: A series of retrieved total knee components of one design were examined. The duration of implantation of the retrieved components ranged from thirty-six to 146 months. Laser surface profilometry and computer-aided design software were used to develop individual three-dimensional models of each worn, retrieved tibial insert to compare with scanned unused inserts. Volumetric subtraction of both models revealed the material lost because of backside wear. Results: Worn and unworn areas on the backside surface were easily identified by stereomicroscopy and laser profilometry. The computer reconstructions showed that, in all retrievals, all unworn surfaces on the nonarticulating surface lay in one plane. The average volume (and standard deviation) of the material lost because of backside wear was 925 ± 637 mm3 (range, 197 to 2720 mm3). On the basis of the time in situ for each implant, the average volumetric wear rate was 138 ± 95 mm3/yr. Conclusions: The predicted volume of material removed because of backside wear is substantial and may be sufficient to induce osteolysis. Our results suggest that peg-like protrusions are not generated by the extrusion of polyethylene into screw-holes within the base-plate but by abrasion of the underside of the bearing insert, leaving the protruding pegs as the only remnants of the original surface. Clinical Relevance: This study provides quantitative predictive data supporting previous qualitative studies showing that backside wear is an important and relevant damage mechanism in contemporary designs of knee replacements and may produce substantial volumes of wear debris.

I

n total knee arthroplasty, modular tibial components, consisting of a metal base-plate and a polyethylene insert, offer several practical advantages. A modular construct allows the surgeon to match the bearing surface with the femoral component, independent of the tibial baseplate 1. A modular design also reduces the complexity of revision by allowing exchange of worn polyethylene components without disruption of the bone-prosthesis or cementprosthesis interfaces. Moreover, finite element analysis has shown that metal-backed components improve fixation by A commentary is available with the electronic versions of this article, on our web site (www.jbjs.org) and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM).

reducing the stress at the bone-implant interface2-5. However, the use of modular tibial components introduces an additional site of relative motion between the baseplate and the insert. Recent studies have shown that retrieved polyethylene tibial inserts can exhibit substantial wear on both the articular and the nonarticulating surfaces6-17. An additional observation has been the occurrence of cylindrical protrusions of polyethylene extending from the undersurface of the insert into screw-holes within the tibial base-plate6,8-10,12,13,15-17. To date, this surface feature has been attributed to creep of the polyethylene under the action of compressive forces applied to the upper surface of the insert during weight-bearing6,8-10,12,13,15,16. In previous studies, backside wear has been recognized as a source of clinically important polyethylene debris, which

 THE JOUR NAL OF BONE & JOINT SURGER Y · JBJS.ORG VO L U M E 87-A · N U M B E R 2 · F E B R U A R Y 2005

Fig. 1

An Anatomic Modular Knee tibial insert with two dovetail grooves (arrows) that run from anterior to posterior. These grooves capture the polyethylene in combination with the locking pin inserted through the insert, extending into the stem of the tray.

has been linked to cases of osteolysis and implant failure8,14,16,18. In particular, the Anatomic Modular Knee (DePuy, Warsaw, Indiana) has been associated with a high prevalence of backside wear in situ and an overall prevalence of osteolysis as high as 20%19. However, the actual volume of material lost through wear of the backside surface has not been quantified. The present study describes a quantitative analysis of the backside surface with use of computerized measurement of retrieved tibial inserts of the Anatomic Modular Knee design. Materials and Methods ifteen retrieved polyethylene tibial inserts of the same design (Anatomic Modular Knee; DePuy) (Fig. 1) were selected from a large collection of inserts that had been retrieved thirty-six to 146 months (average, ninety-one months) after implantation. The thicknesses, as labeled by the manufacturer, were 8 mm (one insert), 10 mm (nine), 12 mm (four), and 14 mm (one). Fourteen inserts were from a posterior cruciate ligament-retaining insert, and one was from a posterior stabilized insert. The components were retrieved from eight women and seven men, with an average age of sixty-six years (range, fifty-four to seventy-two years) at the time of the revision arthroplasty. The average weight and height of the patients was

F

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925 N (range, 690 to 1308 N) and 1.5 m (range, 1.4 to 1.9 m), respectively, corresponding to an average body mass index of 32.4 kg/m2 (range, 23.8 to 42.5 kg/m2). The reason for the revision, reported preoperatively, was loosening (six knees), wear (six), or osteolysis (three). The capture mechanism of the Anatomic Modular Knee design consists of two dovetail slides running from anterior to posterior with a locking pin (Fig. 1). The backside of each insert was inspected visually with use of stereomicroscopy (thirty-two times magnification) to evaluate surface damage. The backside was divided into four quadrants: anteromedial, anterolateral, posterolateral, and posteromedial. Within each quadrant, the severity of burnishing, pitting, embedded acrylic debris, scratching, abrasion, delamination, and surface deformation (protrusions into screw-holes) was assessed. All damage modes were given a score of 0 to 10, in each quadrant, on the basis of either the area of the surface affected by burnishing, pitting, embedded debris, and abrasion or the severity of that particular wear pattern (delamination, scratching, and surface deformation). The damage was categorized as mild or negligible (a score of 0, 1, or 2), moderate (a score of 3, 4, or 5), or severe (a score of 6 to 10). The widths of the pegs in the anterior-posterior and medial-lateral directions were also measured. To quantify the material loss due to backside wear, each retrieved insert was scanned with use of a laser surface profilometer (Cyberware, Monterey, California). Multiple linear scans of each insert, made as the insert was rotated 360° in 10° increments, were aligned and merged, with each individual scan having a resolution of 0.1 mm in the plane of the scan and a resolution of 0.008 mm in depth. This process resulted in approximately 300,000 three-dimensional points defining each insert. An unused insert of each size was also scanned. With use of computer-aided design software (UniGraphics; EDS, Plano, Texas), individual three-dimensional surface models of each retrieved insert were developed. For each insert, the threedimensional surface was geometrically matched to its corresponding unused implant with use of unworn, undeformed patches of the retrieved insert. These unworn fiducial surfaces most commonly existed around the periphery of the implant

Fig. 2

The backside surfaces of two retrieved specimens. a: This insert (eighty-four months in situ) has four distinct polyethylene protrusions (arrows) with wear-through on the medial edge. b: This insert (eighty-two months in situ) shows wear predominantly on the medial side with polyethylene also protruding over the peripheral medial edge.

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Fig. 3

A retrieved insert (posterior cruciate ligament-sacrificing type after 108 months in situ) exhibiting unworn surfaces through the screwholes, over the medial edge, and into the notch of the tibial base-plate. Shading indicates all surfaces that lie in a plane. The cross-sectional view shows laser scans of the retrieved insert compared with an unworn insert.

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and between the two articulating surfaces on the superior aspect of the implant. The volume of material lost because of backside wear during the time in situ was then calculated from the volume bounded by the final (worn) backside surface and the initial (unworn) backside surface. The sources of backside wear debris were separated into three sections, naturally defined by the capture mechanism grooves: under the medial plateau, under the lateral plateau, and in the center anterior-toposterior strip between the two dovetail grooves. For comparison with previous studies, the volume of the material lost was divided by the time in situ of each implant to estimate the wear rate. The planarity of a surface patch was determined by measuring the root-mean-square deviation of all of the points in the patch from a plane fit to those points. The backside surfaces of two retrieved inserts (which had been in situ for thirty-six and eighty-six months) were also examined with scanning electron microscopy (Amray 1830; Amray, Bedford, Massachusetts) operating under secondary electron mode. These two inserts were chosen as one represents typical or average wear, whereas the other exhibits severe wear. The images were used to document the microscopic appearance of the worn backside surface, particularly the transition between the worn surface and the raised protrusion. Different statistical tests were performed to elucidate the complex relationship between all of the measured variables. A logistic regression model was used to evaluate the relationship between all of the continuous variables (e.g., height, weight, Fig. 4

Retrieved inserts. A, Polyethylene protrusion on the backside of a retrieved insert (eighty-two months in situ) with visible machining marks. B, Scanning electron microscopic image from a retrieved insert (eightysix months in situ) exhibiting the shredded, eroded edge of a protrusion. C, Scanning electron microscopic image from the same insert shown in B showing a center protrusion with the same hexagonal shape of the inside of the head of the fixation screw. D, The surface of this protrusion with machining marks on the surface represents the original backside surface. All of the material around the hexagonal protrusion has been removed by contact with the top of the obliquely placed screw-head, and all of the material around the larger circular protrusion has been removed by the tibial tray.

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B A C K S I D E WE A R O F P O L YE T HY L E N E T I B I A L I N S E R T S : ME C H A N I S M A N D MA G N I T U D E O F MA TE R I A L L O S S

TABLE I Data on the Backside Surface Damage Measurement Occurrence (% of Inserts)

Type of Damage

Average

Standard Deviation

Minimum

Maximum

Medial backside burnishing*

100

7.9

2.5

1

10

Lateral backside burnishing*

100

7.8

2.5

1

10

Medial backside pitting*

87

2.3

2.0

0

6.5

Lateral backside pitting*

87

2.1

1.8

0

6

Anteromedial protrusion (mm)

100

1.08

0.92

0.12

2.68

Posteromedial protrusion (mm)

100

0.54

0.39

0.04

1.30

Anterolateral protrusion (mm)

100

0.31

0.39

0.07

1.56

Posterolateral protrusion (mm)

100

0.35

0.40

0.03

1.27

3

Medial volume loss (mm )

100

0.51

0.32

0.11

1.18

Central volume loss (mm3)

100

0.16

0.09

0.03

0.37

Lateral volume loss (mm3)

100

0.25

0.30

0.02

1.34

*The values are given as the score on a scale of 0 to 10.

body mass index, time in situ, and polyethylene thickness) and the damage measurements. Both simple logistic regression models for each damage mechanism as well as a multiple logistic regression model fit to the entire data set were analyzed. Because this technique works best with dichotomous dependent variables, the results of the logistic likelihood ratios were used to determine which independent variables significantly affected the dependent variables. For robustness in the predictive capabilities of those significant independent variables, Pearson correlation coefficients and their corresponding z transformations and p values were then calculated for the variables showing potential correlations. This analysis was

Fig. 5

Worn medial backside surface showing the wear plane with the depth of the material removed increasing from posterior to anterior.

performed on all possible combinations of independent variables and damage measurements. Results hree modes of surface damage were observed on the nonarticulating backside surface of the retrieved inserts: burnishing, pitting, and surface deformation in the form of protrusions into screw-holes. Some degree of burnishing was noted on the backside surface of every component. Pitting was also very common (thirteen of the fifteen tibial inserts). The average burnishing score (and standard deviation) was 7.9 ± 2.5 (range, 1 to 10), and the average pitting score for the entire backside surface was 2.2 ± 1.8 (range, 0 to 5.5). All components exhibited polyethylene material protruding from the undersurface of the polyethylene insert into screw-holes within the tibial backing plate. The average protrusion height was 0.6 ± 0.6 mm (range, 0.03 to 2.7 mm) above the surrounding surface. Protrusions over the anteromedial screwhole of the tray (average height, 1.1 ± 0.9 mm) were significantly larger than those over the other three locations (p = 0.002) (Table I). Fourteen of the fifteen components had a flange of polyethylene that overlapped the peripheral margin of the base-plate on the medial side (Fig. 2). With the numbers available, the original thickness of the insert was found to have no effect on the height of the protrusions (p = 0.45). Patient weight was positively correlated with the average medial protrusion height (r2 = 0.79, p < 0.05). Ninety percent of the pegs were out of round with the anterior-posterior dimension, measuring smaller than the mediallateral dimension. The average difference in the two dimensions was 0.26 ± 0.21 mm (range, 0 to 0.83 mm). Three-dimensional computer reconstructions of the retrieved inserts demonstrated that, for all retrievals, the unworn surface of the remaining protrusions, the rim of material

T

 THE JOUR NAL OF BONE & JOINT SURGER Y · JBJS.ORG VO L U M E 87-A · N U M B E R 2 · F E B R U A R Y 2005

protruding over the medial edge, and the unworn surfaces on the anterior-lateral edge all lay in a single plane. The average root-mean-square deviation from a plane of the points defining the unworn surfaces was 0.07 ± 0.05 mm. An example of a computer reconstruction of the backside surface of the posterior cruciate ligament-sacrificing insert showed that the surfaces of the material protruded into the four screw-holes and that the surface of the unworn posterior aspect that protruded into the posterior cruciate ligament notch of the tibial base-plate all had well-defined machining marks and all were in one plane (root-mean-square error of points was 0.06 mm) (Fig. 3). This example demonstrates that the protrusions and their inferior surface represent the original backside surface and that the worn surface surrounding the protrusions demonstrates loss of material. Electron microscopy revealed that the typical polyethylene protrusion had eroded, shredded edges, providing evidence of wear as opposed to a deformation phenomenon (Fig. 4, A through D). On the basis of the three-dimensional computer reconstructions, the average volume of material lost during the time in situ was 925 ± 637 mm3 (range, 197 to 2720 mm3); 521 ± 354 mm3 (range, 112 to 1183 mm3) from the medial compartment, 245 ± 325 mm3 (range, 16 to 1338 mm3) from the lateral compartment, and 159 ± 96 mm3 (range, 34 to 366 mm3) from the center area between the medial and lateral grooves for the locking mechanism. The corresponding average volumetric wear rate was 138 ± 95 mm3/yr from the backside surface. Patient weight was positively correlated with increased volumetric wear from the medial backside surface (r2 = 0.52, p < 0.01). Discussion his study is the first, as far as we know, to quantitatively estimate the volume removed from the backside surface of retrieved modular tibial inserts. These results suggest that the protrusions on the underside of worn retrieved tibial components were not primarily due to creep or cold flow of the polyethylene through the screw-holes on the base-plate, as has been previously suggested6,8-10,12,13,15,16, but instead were primarily due to wear of the inserts. Our data are not consistent with a coldflow surface deformation phenomenon. We found that surfaces of all of the material descending below the worn backside surface existed in a single plane, independent of the location of each individual outcropping or overhang, an observation unlikely to occur if the protrusions were due to extrusion. Evidence in support of our conclusion that the protrusions are not the consequence of extrusion of polyethylene into the screw-holes is provided by a laboratory study quantifying the cold extrusion process. Chang et al.20 examined the surface profiles of ultra-high molecular weight polyethylene that was cold extruded through a hole in a cobalt-chromium disc with cyclic loading at a frequency of 20 Hz and a peak amplitude of 2059 N for one million cycles. It was found that a polyethylene specimen thickness of 10 mm had a maximum extrusion of <100 µm. Extrapolation of these data by Chang et al. led to the suggestion that cold extrusion is negli-

T

B A C K S I D E WE A R O F P O L YE T HY L E N E T I B I A L I N S E R T S : ME C H A N I S M A N D MA G N I T U D E O F MA TE R I A L L O S S

gible at a thickness of 12 mm. In the present study, the 10mm-thick inserts exhibited an average protrusion height of 0.7 mm (maximum, 2.5 mm), whereas the 12-mm inserts had an average protrusion height of 0.5-mm (maximum, 2.7 mm). Even the 14-mm insert showed an average protrusion of 0.2 mm (maximum, 0.5 mm). The maximum extrusion for the 6-mm-thick inserts in the study by Chang et al. was approximately 0.3 mm. Thus, we speculate that, as an insert was worn because of backside wear, its thickness may have decreased enough to allow some contribution of creep to the backside changes. However, we believe that the majority of the magnitude of the protrusions measured in our retrieved inserts cannot be explained on the basis of cold flow of polyethylene alone. The geometry of the protrusions also was not consistent with cold flow. Surface profiles of cold extruded ultrahigh molecular weight polyethylene, again from the study by Chang et al.20, showed a parabolic surface profile as the surface of the polyethylene was stretched when the material flowed through a hole in the cobalt-chromium disk. In comparison, polyethylene protrusions from the backside of retrieved tibial inserts in our study demonstrated unworn surfaces on each protrusion exhibiting visible machining marks (Fig. 4, A through D). Scanning electron microscopy of the protrusions revealed eroded, shredded edges in the transition between the backside surface area in contact with the tibial tray and the surface opposite the hole in the baseplate, not a parabolic stretched surface. Thus, the geometry of the observed wear does not support a cold extrusion mechanism. Several studies have identified tibial osteolysis under the screw-holes in uncemented modular components7-9,18-22. In relation to observed backside wear, Wasielewski et al.8 found that larger protrusions on retrieved inserts were correlated with a higher occurrence of tibial osteolysis. More recently, Surace et al.16 found that the distal penetration of granulomas along fixation screws increased as the depth of the polyethylene protrusions increased. A cold extrusion phenomenon does not necessarily explain this, as deformation of the polyethylene could conceivably plug the screw-hole and prevent particulate debris from reaching the cancellous bone. However, if the protrusions are instead caused by severe wear of the backside surface, then the deeper the protrusion, the more severe the wear, thus increasing the likelihood of more severe osteolysis. While volumetric wear rates for total hip replacement have been reported to range from 50 to 150 mm3/yr23-25, a recent study of the articular surface wear from tibial components retrieved postmortem showed a volumetric wear rate of 32 mm3/yr26. Few studies have measured volumetric wear from components retrieved at the time of a revision total knee arthroplasty. Our average estimated volumetric wear rate of 138 mm3/yr from the backside surface alone is comparable with severe wear of acetabular components23-25,27 and is comparable with, and perhaps even worse than, the volume of wear debris from the articular surface of tibial inserts26.

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The retrieved inserts in the present study were chosen because of the history of severe backside wear for this design, with protrusions extending as much as 3 mm from the worn backside surface (Fig. 5)17. However, polyethylene protrusions on the worn backside surface of retrieved components are not unique to this design and, in fact, occur in many designs with base-plate screw-holes6,8-10,12,13,15-17. The subset of retrieved inserts in this study suggests that polyethylene protrusions on the underside of other tibial insert designs with screw-holes on the base-plate may not primarily be due to creep but, instead, may be due to severe wear eroding large volumes of the insert, leaving the protrusions. Our results also suggest that substantial relative motion between the tray and the inserts occurred in the retrievals that we studied, as indicated by the oblong shape of the protrusions. This study provides quantitative predictive data supporting previous qualitative studies showing that backside wear is an important and relevant damage mechanism in contemporary designs of total knee replacements that may produce substantial volumes of wear debris.


B A C K S I D E WE A R O F P O L YE T HY L E N E T I B I A L I N S E R T S : ME C H A N I S M A N D MA G N I T U D E O F MA TE R I A L L O S S

Michael A. Conditt, PhD Matthew T. Thompson, MS Molly M. Usrey, BS Sabir K. Ismaily, BS Institute of Orthopedic Research and Education, 6550 Fannin, Suite 2512, Houston, TX 77030. E-mail address for M.A. Conditt: [email protected] Philip C. Noble, PhD Barnhart Department of Orthopedic Surgery, Baylor College of Medicine, 6550 Fannin, Suite 2625, Houston, TX 77030 The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

doi:10.2106/JBJS.C.01308

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15. Rodriguez JA, Baez N, Rasquinha V, Ranawat CS. Metal-backed and allpolyethylene tibial components in total knee replacement. Clin Orthop. 2001; 392:174-83. 16. Surace MF, Berzins A, Urban RM, Jacobs JJ, Berger RA, Natarajan RN, Andriacchi TP, Galante JO. Backsurface wear and deformation in polyethylene tibial inserts retrieved postmortem. Clin Orthop. 2002;404:14-23. 17. Conditt MA, Stein JA, Noble PC. Factors affecting the severity of backside wear of modular tibial inserts. J Bone Joint Surg Am. 2004;86:305-11. 18. Engh GA, Parks NL, Ammeen DJ. Tibial osteolysis in cementless total knee arthroplasty. A review of 25 cases treated with and without tibial component revision. Clin Orthop. 1994;309:33-43. 19. Ezzet KA, Garcia R, Barrack RL. Effect of component fixation method on osteolysis in total knee arthroplasty. Clin Orthop. 1995;321:86-91. 20. Chang DC, Goh JC, Teoh SH, Bose K. Cold extrusion deformation of UHMWPE in total knee replacement prostheses. Biomaterials. 1995;16:219-23. 21. Engh GA, Koralewicz LM, Pereles TR. Clinical results of modular polyethylene insert exchange with retention of total knee arthroplasty components. J Bone Joint Surg Am. 2000;82:516-23. 22. Lewis PL, Rorabeck CH, Bourne RB. Screw osteolysis after cementless total knee replacement. Clin Orthop. 1995;321:173-7. 23. Devane PA, Robinson EJ, Bourne RB, Rorabeck CH, Nayak NN, Horne JG. Measurement of polyethylene wear in acetabular components inserted with and without cement. A randomized trial. J Bone Joint Surg Am. 1997;79:682-9. 24. Hernandez JR, Keating EM, Faris PM, Meding JB, Ritter MA. Polyethylene wear in uncemented acetabular components. J Bone Joint Surg Br. 1994;76:263-6. 25. Livermore J, Ilstrup D, Morrey B. Effect of femoral head size on wear of the polyethylene acetabular component. J Bone Joint Surg Am. 1990;72:518-28. 26. Lavernia CJ, Sierra RJ, Hungerford DS, Krackow K. Activity level and wear in total knee arthroplasty: a study of autopsy retrieved specimens. J Arthroplasty. 2001;16:446-53. 27. Jasty M, Goetz DD, Bragdon CR, Lee KR, Hanson AE, Elder JR, Harris WH. Wear of polyethylene acetabular components in total hip arthroplasty. An analysis of one hundred and twenty-eight components retrieved at autopsy or revision operations. J Bone Joint Surg Am. 1997;79:349-58.