Knee Surg Sports Traumatol Arthrosc (2014) 22:680–686 DOI 10.1007/s00167-013-2724-4
KNEE
No difference in in vivo polyethylene wear particles between oxidized zirconium and cobalt–chromium femoral component in total knee arthroplasty Yukihide Minoda • Kanako Hata • Hiroyoshi Iwaki Mitsuhiko Ikebuchi • Yusuke Hashimoto • Fumiaki Inori • Hiroaki Nakamura
•
Received: 11 April 2013 / Accepted: 9 October 2013 / Published online: 20 October 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Purpose Polyethylene wear particle generation is one of the most important factors affecting mid- to long-term results of total knee arthroplasties. Oxidized zirconium was introduced as a material for femoral components to reduce polyethylene wear generation. However, an in vivo advantage of oxidized zirconium on polyethylene wear particle generation is still controversial. The purpose of this study was to compare in vivo polyethylene wear particles between oxidized zirconium total knee prosthesis and conventional cobalt–chromium (Co–Cr) total knee prosthesis. Methods Synovial fluid was obtained from the knees of 6 patients with oxidized zirconium total knee prosthesis and from 6 patients with conventional cobalt–chromium (Co– Cr) total knee prosthesis 12 months after the operation. Polyethylene particles were isolated and examined using a scanning electron microscope and image analyser. Results Total number of particles in each knee was 3.3 ± 1.3 9 107 in the case of oxidized zirconium (mean ± SD) and 3.4 ± 1.2 9 107 in that of Co–Cr (n.s.). The particle size (equivalent circle diameter) was 0.8 ± 0.3 lm in the case of oxidized zirconium and 0.6 ± 0.1 lm in that of Co–Cr (n.s.). The particle shape (aspect ratio) was 1.4 ± 0.0 in the case of oxidized zirconium and 1.4 ± 0.0 in that of metal Co–Cr (n.s). Conclusions Although newly introduced oxidized zirconium femoral component did not reduce the in vivo polyethylene wear particles in early clinical stage, there was no
Y. Minoda (&) K. Hata H. Iwaki M. Ikebuchi Y. Hashimoto F. Inori H. Nakamura Department of Orthopaedic Surgery, Osaka City University Graduate School of Medicine, 1-4-3 Asahi-machi Abeno-ku, Osaka 545-8585, Japan e-mail: [email protected]
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adverse effect of newly introduced material. At this moment, there is no need to abandon oxidized zirconium femoral component. However, further follow-up of polyethylene wear particle generation should be performed to confirm the advantage of the oxidized zirconium femoral component. Level of evidence Therapeutic study, Level III. Keywords Polyethylene wear particle Total knee arthroplasty Oxidized zirconium
Introduction Polyethylene wear particles induce macrophages to release cytokines, which leads to osteolysis and aseptic loosening in total joint arthroplasties [4, 11, 12, 15]. Generation of polyethylene wear particles in total knee arthroplasties is one of the most important factors that affect mid-term and long-term clinical results [2, 5]. Therefore, to reduce polyethylene wear generation and to achieve better longterm results for patients with higher activity levels, many new designs and materials have recently been introduced for total knee prostheses. The number, size, and shape of polyethylene wear particles have been reported to be critical factors in the development of osteolysis. Greater volume, submicrometer size, and an elongated shape of polyethylene wear particles all stimulate an increased macrophage response [13, 16, 26]. The use of alumina ceramic for the material of femoral component has reported to reduce the polyethylene wear particle generation [19]. However, the alumina ceramic femoral component has potential risk of fracture. Thus, oxidized zirconium was introduced as a material for femoral component, which has less risk of fracture than
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alumina ceramic femoral component. Oxidized zirconium was produced from a wrought zirconium alloy (Zr-2.5 % Nb) that is oxidized by thermal diffusion to create a zirconium ceramic surface about 5 lm thick [8]. In vitro studies showed that wear of polyethylene against oxidized zirconium was less than that against conventional cobalt– chromium (Co–Cr) alloy (Co-28 % Cr-6 % Mo) femoral component, because oxidized zirconium was more resistant to roughing than Co–Cr [6, 23, 24]. However, there has been little information about in vivo polyethylene wear particles generated from newly introduced oxidized zirconium prostheses. It takes decades to establish the long-term performance of newly introduced total knee prostheses. Thus, it is particularly important to perform early feedback of in vivo polyethylene wear generation in such new prostheses before they come into widespread use. However, it is difficult to measure in vivo polyethylene wear by performing postoperative radiographs. For early feedback of in vivo polyethylene wear in total knee arthroplasty, we have established a method to measure in vivo polyethylene wear by isolating and analysing polyethylene wear particles in synovial fluid from well-functioning knees after total knee arthroplasty and compared the characteristics of polyethylene wear particles generated between different types of total knee prostheses [10, 17–20]. We hypothesize that oxidized zirconium femoral component reduces the in vivo polyethylene wear particle generation compared to the conventional Co–Cr femoral
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component. The aims of the current study were to compare the size, shape, and number of in vivo polyethylene wear particles isolated from the synovial fluid from patients with newly introduced oxidized zirconium total knee prostheses and conventional Co–Cr total knee prostheses that had the same articulating surface shape.
Materials and methods Between 2008 and 2009, consecutive patients awaiting TKA at our affiliated hospital were eligible to participate in the study. Patients with osteoarthritis and varus deformity of the knee were included. Exclusion criteria included inflammatory arthritis, osteonecrosis of the knee, and valgus deformity of the knee. A total of 10 patients (12 knees) consented to participate and were randomly assigned to receive either the oxidized zirconium total knee prostheses (n = 6) or a conventional Co–Cr total knee prostheses (n = 6) (Fig. 1). Two patients had bilateral procedures. Both groups had the same posterior-stabilized total knee prostheses except for the materials of femoral component (Genesis II PS; Smith & Nephew, Inc, Memphis, TN, USA). Under complete sterile conditions in an operating room 12 months post-operation, synovial fluid was obtained from 12 knees. The materials used for the femoral component were oxidized zirconium (Oxinium; Smith & Nephew, Inc. Memphis, TN, USA) or Co–Cr alloy. Roughness (Ra) of
Fig. 1 Photographs of total knee prostheses using oxidized zirconium (a) and cobalt– chromium alloy (b). Design and material of the prostheses (Genesis II PS; Smith & Nephew, Inc, Memphis, TN, USA) were the same between these 2 prostheses, except for the material of the femoral component
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the femoral component surface was 0.1 lm for both the oxidized zirconium femoral component and the Co–Cr femoral component. The manufacturing method [compression moulded sheet (GUR 1020)] and the sterilization method (ethylene oxide gas) of polyethylene insert were the same for both the groups. Tibial tray (Ti-6Al-4 V alloy and Ra of upper surface was 0.3 lm), of which the upper surface was polished, was the same for both the groups. Patella was resurfaced with polyethylene patellar component in all cases. All the components were fixed using bone cement. The preoperative diagnosis of all the patients was osteoarthritis. Preoperative and postoperative activity levels were evaluated using the University of California Los Angeles (UCLA) activity-level rating [1]. Polyethylene wear particles were isolated using a previously described technique [10, 17–20]. All solutions were filtered through a 0.2-lm pore nylon filter (150-0020, Nalge Company, Rochester, NY) to avoid contamination of extraneous particles. Synovial fluid was in each case digested with the same amount of 10 mol/l sodium hydroxide at 65 °C for 12 h, applied to a sucrose density gradient (5, 10, 20 %) in a 14-ml tube (14PA tube, Hitachi Koki Co., Ltd., Tokyo, Japan), and then ultracentrifuged at 28,000 rpm (103,700 G) at 4 °C for 3 h (CP100a, P28S1014 rotor, Hitachi Koki Co., Ltd.). The top layer was collected and applied to an isopropanol–water density gradient (0.90, 0.96 g/ml) in a 40-ml tube (40PA tube, Hitachi Koki Co., Ltd.) and ultracentrifuged again at 28,000 rpm (103,200 G) for 1 h (CP100a, P28S1004 rotor, Hitachi Koki Co., Ltd.). Polyethylene particles were collected from the interface between two layers and filtered through 0.1 lm polycarbonate filters (VCTP 013-00, Millipore Corporation, Bedford, MA). Filters were dried, attached to an aluminium specimen mount (M4, Nisshin EM Co., Ltd., Tokyo, Japan), and coated with platinum (E-1030 ion sputter, Hitachi Science Systems Ltd., Tokyo, Japan) for scanning electron microscopic examination (S4700SI, Hitachi Ltd., Tokyo, Japan) (Fig. 2). The retrieval ratio with this extraction method was determined with the use of normal synovial fluid from patients with knee osteoarthritis and commercially available high-density polyethylene powder (mean particle size, 3.5 lm) (S-395, Shamrock, Petaluma, CA). The retrieval ratio of polyethylene particles of five experimental specimens was 65.2 ± 16.7 % (mean ± SD) [17]. It is practically impossible to completely aspirate all synovial fluid from a knee joint with one puncture. Therefore, the synovial fluid collection ratio was calculated to estimate numbers of polyethylene wear particles in the residual synovial fluid in joint capsules, as previously reported. After aspiration, 20 ml of saline was injected into the joint capsule and then reaspirated with residual synovial fluid. The number of
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Fig. 2 Scanning electron microscopy photographs of polyethylene wear particles in oxidized zirconium total knee prosthesis (magnification, 1,00009)
polyethylene wear particles in aspirated fluid and that in the reaspirated fluid was analysed in seven knees. The synovial fluid collection ratio was 92.3 ± 9.3 % (mean ± SD) [17]. The total number of polyethylene wear particles in the synovial fluid was calculated from (a) the number of particles on the filter, (b) the dilution ratio during the extraction process, (c) the retrieval ratio, (d) the synovial fluid collection ratio, (e) the amount of synovial fluid used for the extraction process, and (f) the total amount of synovial fluid. Particle size was expressed with the use of the equivalent circle diameter, which is the diameter of a circle having the same area as the particle. Particle shape was determined from the aspect ratio (length to breadth) and roundness [perimeter2/(4parea)] [10, 13, 17–20]. Particle size and shape were analysed with a computerized image analyser (Mac Scope, Minami Co., Tokyo, Japan). This study was approved by institutional committee on human research in Osaka City University Graduate School of Medicine (No. 1281). Informed consent was obtained from all the patients at the time of enrolment. Statistical analysis Statistical analyses of differences between the types of prostheses were done using Mann–Whitney U test and Fisher’s exact test with a commercially available software package (StatView 4.5; Abacus Concepts Inc, Berkeley, CA, USA). Single regression analysis was performed between the number of polyethylene wear particles and factors (BMI, age, thickness of polyethylene insert, knee alignment, and postoperative knee society score [9]). The level of significance considered was 5 %. Sample size calculation showed that 6 knees in each group would allow
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detection of a difference of 2.1 9 107 counts (power = 0.8, a = 0.05) with a standard deviation of 1.2 9 107 counts in polyethylene wear particle number between 2 groups.
The quantity, size, and shape of the polyethylene wear particles are listed in Table 2. The difference in the total number of polyethylene wear particles between the groups was not statistically significant. The size distribution of the polyethylene wear particles in both the groups is shown in Fig. 3. The range with greatest frequency was 0.2–0.4 lm in both groups. Although the size of polyethylene wear particles from oxidized zirconium prostheses was larger than those from Co–Cr prostheses, the difference did not reach to statistical significance. The difference in the shape (aspect ratio and roundness) between the groups was not statistically significant. There was no statistically significant correlation between the number of polyethylene wear particles and patients’ factors (BMI, age, thickness of
Results Pertinent data on patients in both the groups are shown in Table 1. The differences in these parameters were not significant. The difference in synovial fluid volume between oxidized zirconium prostheses [13.3 ± 6.9 mL (mean ± SD)] and Co–Cr prostheses (8.1 ± 1.6 mL) was not statistically significant.
Table 1 Patients’ demographic data
Parameter
Mean (and SD) Oxidized Zirconium TKA (n = 6)
p value Co–Cr TKA (n = 6)
Age at operation (years old)
59.8 ± 7.6
60.7 ± 3.6
n.s.
Gender (male/female)
6/0
6/0
n.s.
Height (cm)
165.3 ± 7.4
162.3 ± 9.5
n.s.
Body weight (kg)
75.0 ± 6.0
69.8 ± 18.4
n.s.
BMI (kg/m2)
27.5 ± 2.5
26.2 ± 4.5
n.s.
Component size femur
Size 4: 2, size 5: 2, size 6: l, size 7: l
Size 5: 3, size 6: l, size 7: l, size 8: l
n.s.
Size 3: l, size 4: 2, size 5: l, size 6: 2
Size 3: l, size 4: 2, size 5: l, size 6: 2
n.s.
Patella
Size 32: 3, size 35: 3
Size 32: 3, size 35: 3
n.s.
Thickness of PE insert
9 mm: 3; ll mm: 2 13 mm: l
9 mm: 4, ll mm: l 13 mm: l
n.s.
4.7 ± 2.4
4.2 ± 1.6
n.s.
Tibia
post-op knee alignment (degrees valgus)
TKA total knee arthroplasty, KSS knee society score, UCLA university of California Los Angeles, n.s. not statistically significant
Chi-square test
Table 2 Quantity, size, and shape of polyethylene wear particles in oxidized zirconium and Co–Cr total knee prostheses ECD equivalent circle diameter, TKA total knee arthroplasty, n.s. not statistically significant a
Common logarithm was used for statistical analysis
pre-op KSS knee
66 ± 10
70 ± 8
n.s
pre-op KSS function
67 ± 5
72 ± 10
n.s.
post-op KSS knee pre-op KSS function
94 ± 9 85 ± 15
96 ± 8 88 ± 10
n.s. n.s.
post-op extension (degree)
0±0
-1 ± 2
n.s.
post-op flexion (degree)
130 ± 4
120 ± 13
n.s.
pre-op UCLA activity-level score
4.2 ± 0.4
3.8 ± 0.4
n.s.
post-op UCLA activity-level score
5.0 ± 0.0
5.0 ± 0.0
n.s.
Characteristics
Parameter
Mean (and SD)
p value
Oxidized Zirconium TKA (n = 6)
Co–Cr TKA (n = 6)
(3.3 ± 1.3) 9 107
(3.4 ± 1.2) 9 107
n.s.a
Quantity
Total number
Size
ECD (lm)
0.8 ±0.3
0.6 ± 0.1
n.s.
Shape
Aspect ratio
1.4 ±0.1
1.4 ±0.1
n.s.
Roundness
1.4 ±0.0
1.4 ±0.0
n.s.
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Fig. 3 The size distributions of polyethylene wear particles isolated from synovial fluid from patients with the oxidized zirconium and cobalt–chromium (Co–Cr) prostheses. The range with greatest frequency was 0.2–0.4 lm in both groups
polyethylene insert, knee alignment, and postoperative knee society score).
Discussion The difference in in vivo wear particle characteristics between the newly introduced oxidized zirconium femoral component and the conventional Co–Cr femoral component was not statistically significant in early clinical stage. Factors that affect macrophage response to polyethylene wear particles have been reported; greater volume, submicron size, and an elongated shape of polyethylene wear particles all stimulate an increased macrophage response [13, 16, 26]. Our previous reports showed that modification of total knee arthroplasty materials and designs resulted in different polyethylene wear particle characteristics, and thus, these differences may be important factors in the long-term development of osteolysis [10, 17–20]. The oxidized zirconium femoral component has recently been introduced for the reduction in polyethylene wear particle generation. Higher resistance to roughing of oxidized zirconium compared to Co–Cr is expected to reduce polyethylene wear. Laskin et al. reported excellent clinical results and no clinical adverse effect of oxidized zirconium femoral component [14, 15]. Kaplan–Meir survivorship was 98 % at 5 years. Innocent et al. [8] also reported 98.7 % survivorship and no clinical adverse event of oxidized zirconium femoral component. However, an in vivo advantage of oxidized zirconium on polyethylene wear particle generation was still controversial. Our results confirmed that there was no adverse effect in polyethylene wear of newly introduced material for total knee prosthesis.
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Previous retrieval study showed that damage score of polyethylene insert and femoral component was significantly lower with oxidized zirconium femoral component than with Co–Cr femoral component [7] at mean of 18–19 months after operation. However, these cases included early failure cases such as instability, loosening, and malposition. In this study, we investigated only wellfunctioning cases. The difference of patients’ condition was one of the reasons for different results. An in vitro simulation studies showed that the oxidized zirconium femoral component reduced polyethylene wear by 42 % during 5 million cycles [6] and by 85 % during 6 million cycles [23] comparing to Co–Cr femoral component. An increase in surface roughness during 6 million cycles was significantly less in the oxidized zirconium femoral component than in the Co–Cr femoral component, and this difference results in the reduction in polyethylene wear in the oxidized zirconium femoral component [23]. Therefore, the advantage of oxidized zirconium will be observed in longer follow-up periods. This is one of the reasons why our in vivo wear particle characteristics were not different between the oxidized zirconium femoral component and the conventional Co–Cr femoral component. Longer follow-up of in vivo wear particle analysis is required to confirm such a potential advantage in progressive surface roughing of oxidized zirconium. There were advantages in the current study. First, we used the same type of prostheses between the 2 groups except for the material of the femoral components. Design of the articulating surface also influenced in vivo polyethylene wear particle generation [17, 20]. Medial-pivot design reduced the number of polyethylene wear particle by 51 % comparing to posterior-stabilized design. Polyethylene wear particles from medial-pivot design were slightly smaller and rounder than those form posterior-stabilized design. This study eliminated the influence of the articulating surface design and thus can properly evaluate the influence of femoral component material on in vivo polyethylene wear particle characteristics. Second, we used the previous validated method for in vivo polyethylene wear particle analysis [10, 17–20]. Therefore, we can compare the results of polyethylene wear particle analysis in this study to those in previous studies using other types of total knee prostheses. The number of polyethylene wear particles in both of our groups using the Genesis II posterior-stabilized prosthesis was about 30 % of that in previous reports (11.6 ± 5.7 9 107 counts) in which other types of posterior-stabilized prostheses (I-BII, Zimmer, Warsaw, IN, USA and Osteonics Scorpio PS, Stryker Osteonics Howmedica, Rutherford, NJ, USA) had been used [17]. Polyethylene wear particles are generated not only from the upper articulating surface but also from the lower nonarticulating surface, which is the so-called backside wear.
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Polished tibial baseplates reduce back side polyethylene wear, comparing to nonpolished rough tibial baseplates [3]. The upper surface of the tibial baseplates was polished in Genesis II, but not in IB-II and Osteonics Scorpio PS. Polished tibial baseplate of Genesis II might reduce in vivo polyethylene wear particle generation between insert and tibial baseplate, and this was one of the reasons for less wear particles in Genesis II posterior-stabilized prosthesis comparing to other types of posterior-stabilized prostheses. In the present study, the UCLA activity score was used as a means of measuring patient activity. In vivo wear assessments of total joint prostheses should be based on a measure of use. The UCLA activity score correlated strongly with the average number of steps per day, as recorded by a modern pedometer, which can be a satisfactory means of quantifying the use of lower extremity joints [22]. In the present study, the difference between the 2 prostheses in UCLA scores was not statistically significant. We thus believe that the difference in patient activity level did not influence the results of the current study. Besides UCLA score, postoperative knee alignment, BMI, and age might also influence in vivo polyethylene wear. Postoperative mal-alignment affected the polyethylene wear [21]. In this study, the number of polyethylene wear particles did not correlated with postoperative knee alignment, BMI, and age. No alignment outliers and uniformity in patients’ BMI and age in this study might be possible reasons for no correlation between the number of polyethylene wear particles and such factors. There was a limitation in this study. Although our sample size of each group satisfied power analysis and was larger than that of in vitro studies [6, 23], the sample size might to be small. However, this study required invasive test. Although it might to be better to enlarge sample size, it was not easy to enlarge sample size from the point of view of infection risk due to the synovial fluid collection. Although newly introduced oxidized zirconium femoral component did not reduce the in vivo polyethylene wear particles comparing to Co–Cr femoral component in early clinical stage, there was no adverse effect of newly introduced oxidized zirconium femoral component. At this moment, there is no need to abandon oxidized zirconium femoral component. However, further follow-up of polyethylene wear particle generation should be performed to confirm the advantage of the oxidized zirconium femoral component.
Conclusion The current in vivo study showed that the difference in the characteristics of in vivo polyethylene wear particles between the newly introduced oxidized zirconium femoral
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component and conventional Co–Cr femoral component was not statistically significant in early clinical stage. Acknowledgments Nephew.
This study was supported in part by Smith and
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686 18. Minoda Y, Kobayashi A, Iwaki H et al (2004) Characteristics of polyethylene wear particles isolated from synovial fluid after mobile-bearing and posterior-stabilized total knee arthroplasties. J Biomed Mater Res 71B:1–6 19. Minoda Y, Kobayashi A, Iwaki H et al (2005) Polyethylene wear particle generation in vivo in an alumina medial pivot total knee prosthesis. Biomaterials 26:6034–6040 20. Minoda Y, Kobayashi A, Iwaki H et al (2009) In vivo analysis of polyethylene wear particles after total knee arthroplasty: the influence of improved materials and designs. J Bone Joint Surg 91A(Suppl 6):67–73 21. Nishikawa K, Okazaki K, Matsuda S et al (2013) Improved design decreases wear in total knee arthroplasty with varus malalignment. Knee Surg Sports Traumatol Arthrosc. doi:10. 1007/s00167-013-2506-z 22. Schmalzried TP, Szuszczewicz ES, Northfield MR et al (1998) Quantitative assessment of walking activity after total hip or knee replacement. J Bone Joint Surg 80A:54–59
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Knee Surg Sports Traumatol Arthrosc (2014) 22:680–686 23. Spector BM, Ries MD, Bourne RB et al (2001) Wear performance of ultra-high molecular weight polyethylene on oxidized zirconium total knee femoral components. J Bone Joint Surg 83A(Suppl 2 Pt 2):80–86 24. White SE, Whiteside LA, McCarthy DS et al (1944) Simulated knee wear with cobalt chromium and oxidized zirconium knee femoral components. Clin Orthop Relat Res 309:176–184 25. Willert HG (1977) Reactions of the articular capsule to wear products of artificial joint prostheses. J Biomed Mater Res 11:157–164 26. Yang SY, Ren W, Park Y et al (2002) Diverse cellular and apoptotic responses to variant shapes of UHMWPE particles in a murine model of inflammation. Biomaterials 23:3535–3543
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No difference in in vivo polyethylene wear particles between oxidized zirconium and cobalt–chromium femoral component in total knee arthroplasty Yukihide Minoda • Kanako Hata • Hiroyoshi Iwaki Mitsuhiko Ikebuchi • Yusuke Hashimoto • Fumiaki Inori • Hiroaki Nakamura
•
Received: 11 April 2013 / Accepted: 9 October 2013 / Published online: 20 October 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Purpose Polyethylene wear particle generation is one of the most important factors affecting mid- to long-term results of total knee arthroplasties. Oxidized zirconium was introduced as a material for femoral components to reduce polyethylene wear generation. However, an in vivo advantage of oxidized zirconium on polyethylene wear particle generation is still controversial. The purpose of this study was to compare in vivo polyethylene wear particles between oxidized zirconium total knee prosthesis and conventional cobalt–chromium (Co–Cr) total knee prosthesis. Methods Synovial fluid was obtained from the knees of 6 patients with oxidized zirconium total knee prosthesis and from 6 patients with conventional cobalt–chromium (Co– Cr) total knee prosthesis 12 months after the operation. Polyethylene particles were isolated and examined using a scanning electron microscope and image analyser. Results Total number of particles in each knee was 3.3 ± 1.3 9 107 in the case of oxidized zirconium (mean ± SD) and 3.4 ± 1.2 9 107 in that of Co–Cr (n.s.). The particle size (equivalent circle diameter) was 0.8 ± 0.3 lm in the case of oxidized zirconium and 0.6 ± 0.1 lm in that of Co–Cr (n.s.). The particle shape (aspect ratio) was 1.4 ± 0.0 in the case of oxidized zirconium and 1.4 ± 0.0 in that of metal Co–Cr (n.s). Conclusions Although newly introduced oxidized zirconium femoral component did not reduce the in vivo polyethylene wear particles in early clinical stage, there was no
Y. Minoda (&) K. Hata H. Iwaki M. Ikebuchi Y. Hashimoto F. Inori H. Nakamura Department of Orthopaedic Surgery, Osaka City University Graduate School of Medicine, 1-4-3 Asahi-machi Abeno-ku, Osaka 545-8585, Japan e-mail: [email protected]
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adverse effect of newly introduced material. At this moment, there is no need to abandon oxidized zirconium femoral component. However, further follow-up of polyethylene wear particle generation should be performed to confirm the advantage of the oxidized zirconium femoral component. Level of evidence Therapeutic study, Level III. Keywords Polyethylene wear particle Total knee arthroplasty Oxidized zirconium
Introduction Polyethylene wear particles induce macrophages to release cytokines, which leads to osteolysis and aseptic loosening in total joint arthroplasties [4, 11, 12, 15]. Generation of polyethylene wear particles in total knee arthroplasties is one of the most important factors that affect mid-term and long-term clinical results [2, 5]. Therefore, to reduce polyethylene wear generation and to achieve better longterm results for patients with higher activity levels, many new designs and materials have recently been introduced for total knee prostheses. The number, size, and shape of polyethylene wear particles have been reported to be critical factors in the development of osteolysis. Greater volume, submicrometer size, and an elongated shape of polyethylene wear particles all stimulate an increased macrophage response [13, 16, 26]. The use of alumina ceramic for the material of femoral component has reported to reduce the polyethylene wear particle generation [19]. However, the alumina ceramic femoral component has potential risk of fracture. Thus, oxidized zirconium was introduced as a material for femoral component, which has less risk of fracture than
Knee Surg Sports Traumatol Arthrosc (2014) 22:680–686
alumina ceramic femoral component. Oxidized zirconium was produced from a wrought zirconium alloy (Zr-2.5 % Nb) that is oxidized by thermal diffusion to create a zirconium ceramic surface about 5 lm thick [8]. In vitro studies showed that wear of polyethylene against oxidized zirconium was less than that against conventional cobalt– chromium (Co–Cr) alloy (Co-28 % Cr-6 % Mo) femoral component, because oxidized zirconium was more resistant to roughing than Co–Cr [6, 23, 24]. However, there has been little information about in vivo polyethylene wear particles generated from newly introduced oxidized zirconium prostheses. It takes decades to establish the long-term performance of newly introduced total knee prostheses. Thus, it is particularly important to perform early feedback of in vivo polyethylene wear generation in such new prostheses before they come into widespread use. However, it is difficult to measure in vivo polyethylene wear by performing postoperative radiographs. For early feedback of in vivo polyethylene wear in total knee arthroplasty, we have established a method to measure in vivo polyethylene wear by isolating and analysing polyethylene wear particles in synovial fluid from well-functioning knees after total knee arthroplasty and compared the characteristics of polyethylene wear particles generated between different types of total knee prostheses [10, 17–20]. We hypothesize that oxidized zirconium femoral component reduces the in vivo polyethylene wear particle generation compared to the conventional Co–Cr femoral
681
component. The aims of the current study were to compare the size, shape, and number of in vivo polyethylene wear particles isolated from the synovial fluid from patients with newly introduced oxidized zirconium total knee prostheses and conventional Co–Cr total knee prostheses that had the same articulating surface shape.
Materials and methods Between 2008 and 2009, consecutive patients awaiting TKA at our affiliated hospital were eligible to participate in the study. Patients with osteoarthritis and varus deformity of the knee were included. Exclusion criteria included inflammatory arthritis, osteonecrosis of the knee, and valgus deformity of the knee. A total of 10 patients (12 knees) consented to participate and were randomly assigned to receive either the oxidized zirconium total knee prostheses (n = 6) or a conventional Co–Cr total knee prostheses (n = 6) (Fig. 1). Two patients had bilateral procedures. Both groups had the same posterior-stabilized total knee prostheses except for the materials of femoral component (Genesis II PS; Smith & Nephew, Inc, Memphis, TN, USA). Under complete sterile conditions in an operating room 12 months post-operation, synovial fluid was obtained from 12 knees. The materials used for the femoral component were oxidized zirconium (Oxinium; Smith & Nephew, Inc. Memphis, TN, USA) or Co–Cr alloy. Roughness (Ra) of
Fig. 1 Photographs of total knee prostheses using oxidized zirconium (a) and cobalt– chromium alloy (b). Design and material of the prostheses (Genesis II PS; Smith & Nephew, Inc, Memphis, TN, USA) were the same between these 2 prostheses, except for the material of the femoral component
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the femoral component surface was 0.1 lm for both the oxidized zirconium femoral component and the Co–Cr femoral component. The manufacturing method [compression moulded sheet (GUR 1020)] and the sterilization method (ethylene oxide gas) of polyethylene insert were the same for both the groups. Tibial tray (Ti-6Al-4 V alloy and Ra of upper surface was 0.3 lm), of which the upper surface was polished, was the same for both the groups. Patella was resurfaced with polyethylene patellar component in all cases. All the components were fixed using bone cement. The preoperative diagnosis of all the patients was osteoarthritis. Preoperative and postoperative activity levels were evaluated using the University of California Los Angeles (UCLA) activity-level rating [1]. Polyethylene wear particles were isolated using a previously described technique [10, 17–20]. All solutions were filtered through a 0.2-lm pore nylon filter (150-0020, Nalge Company, Rochester, NY) to avoid contamination of extraneous particles. Synovial fluid was in each case digested with the same amount of 10 mol/l sodium hydroxide at 65 °C for 12 h, applied to a sucrose density gradient (5, 10, 20 %) in a 14-ml tube (14PA tube, Hitachi Koki Co., Ltd., Tokyo, Japan), and then ultracentrifuged at 28,000 rpm (103,700 G) at 4 °C for 3 h (CP100a, P28S1014 rotor, Hitachi Koki Co., Ltd.). The top layer was collected and applied to an isopropanol–water density gradient (0.90, 0.96 g/ml) in a 40-ml tube (40PA tube, Hitachi Koki Co., Ltd.) and ultracentrifuged again at 28,000 rpm (103,200 G) for 1 h (CP100a, P28S1004 rotor, Hitachi Koki Co., Ltd.). Polyethylene particles were collected from the interface between two layers and filtered through 0.1 lm polycarbonate filters (VCTP 013-00, Millipore Corporation, Bedford, MA). Filters were dried, attached to an aluminium specimen mount (M4, Nisshin EM Co., Ltd., Tokyo, Japan), and coated with platinum (E-1030 ion sputter, Hitachi Science Systems Ltd., Tokyo, Japan) for scanning electron microscopic examination (S4700SI, Hitachi Ltd., Tokyo, Japan) (Fig. 2). The retrieval ratio with this extraction method was determined with the use of normal synovial fluid from patients with knee osteoarthritis and commercially available high-density polyethylene powder (mean particle size, 3.5 lm) (S-395, Shamrock, Petaluma, CA). The retrieval ratio of polyethylene particles of five experimental specimens was 65.2 ± 16.7 % (mean ± SD) [17]. It is practically impossible to completely aspirate all synovial fluid from a knee joint with one puncture. Therefore, the synovial fluid collection ratio was calculated to estimate numbers of polyethylene wear particles in the residual synovial fluid in joint capsules, as previously reported. After aspiration, 20 ml of saline was injected into the joint capsule and then reaspirated with residual synovial fluid. The number of
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Fig. 2 Scanning electron microscopy photographs of polyethylene wear particles in oxidized zirconium total knee prosthesis (magnification, 1,00009)
polyethylene wear particles in aspirated fluid and that in the reaspirated fluid was analysed in seven knees. The synovial fluid collection ratio was 92.3 ± 9.3 % (mean ± SD) [17]. The total number of polyethylene wear particles in the synovial fluid was calculated from (a) the number of particles on the filter, (b) the dilution ratio during the extraction process, (c) the retrieval ratio, (d) the synovial fluid collection ratio, (e) the amount of synovial fluid used for the extraction process, and (f) the total amount of synovial fluid. Particle size was expressed with the use of the equivalent circle diameter, which is the diameter of a circle having the same area as the particle. Particle shape was determined from the aspect ratio (length to breadth) and roundness [perimeter2/(4parea)] [10, 13, 17–20]. Particle size and shape were analysed with a computerized image analyser (Mac Scope, Minami Co., Tokyo, Japan). This study was approved by institutional committee on human research in Osaka City University Graduate School of Medicine (No. 1281). Informed consent was obtained from all the patients at the time of enrolment. Statistical analysis Statistical analyses of differences between the types of prostheses were done using Mann–Whitney U test and Fisher’s exact test with a commercially available software package (StatView 4.5; Abacus Concepts Inc, Berkeley, CA, USA). Single regression analysis was performed between the number of polyethylene wear particles and factors (BMI, age, thickness of polyethylene insert, knee alignment, and postoperative knee society score [9]). The level of significance considered was 5 %. Sample size calculation showed that 6 knees in each group would allow
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detection of a difference of 2.1 9 107 counts (power = 0.8, a = 0.05) with a standard deviation of 1.2 9 107 counts in polyethylene wear particle number between 2 groups.
The quantity, size, and shape of the polyethylene wear particles are listed in Table 2. The difference in the total number of polyethylene wear particles between the groups was not statistically significant. The size distribution of the polyethylene wear particles in both the groups is shown in Fig. 3. The range with greatest frequency was 0.2–0.4 lm in both groups. Although the size of polyethylene wear particles from oxidized zirconium prostheses was larger than those from Co–Cr prostheses, the difference did not reach to statistical significance. The difference in the shape (aspect ratio and roundness) between the groups was not statistically significant. There was no statistically significant correlation between the number of polyethylene wear particles and patients’ factors (BMI, age, thickness of
Results Pertinent data on patients in both the groups are shown in Table 1. The differences in these parameters were not significant. The difference in synovial fluid volume between oxidized zirconium prostheses [13.3 ± 6.9 mL (mean ± SD)] and Co–Cr prostheses (8.1 ± 1.6 mL) was not statistically significant.
Table 1 Patients’ demographic data
Parameter
Mean (and SD) Oxidized Zirconium TKA (n = 6)
p value Co–Cr TKA (n = 6)
Age at operation (years old)
59.8 ± 7.6
60.7 ± 3.6
n.s.
Gender (male/female)
6/0
6/0
n.s.
Height (cm)
165.3 ± 7.4
162.3 ± 9.5
n.s.
Body weight (kg)
75.0 ± 6.0
69.8 ± 18.4
n.s.
BMI (kg/m2)
27.5 ± 2.5
26.2 ± 4.5
n.s.
Component size femur
Size 4: 2, size 5: 2, size 6: l, size 7: l
Size 5: 3, size 6: l, size 7: l, size 8: l
n.s.
Size 3: l, size 4: 2, size 5: l, size 6: 2
Size 3: l, size 4: 2, size 5: l, size 6: 2
n.s.
Patella
Size 32: 3, size 35: 3
Size 32: 3, size 35: 3
n.s.
Thickness of PE insert
9 mm: 3; ll mm: 2 13 mm: l
9 mm: 4, ll mm: l 13 mm: l
n.s.
4.7 ± 2.4
4.2 ± 1.6
n.s.
Tibia
post-op knee alignment (degrees valgus)
TKA total knee arthroplasty, KSS knee society score, UCLA university of California Los Angeles, n.s. not statistically significant
Chi-square test
Table 2 Quantity, size, and shape of polyethylene wear particles in oxidized zirconium and Co–Cr total knee prostheses ECD equivalent circle diameter, TKA total knee arthroplasty, n.s. not statistically significant a
Common logarithm was used for statistical analysis
pre-op KSS knee
66 ± 10
70 ± 8
n.s
pre-op KSS function
67 ± 5
72 ± 10
n.s.
post-op KSS knee pre-op KSS function
94 ± 9 85 ± 15
96 ± 8 88 ± 10
n.s. n.s.
post-op extension (degree)
0±0
-1 ± 2
n.s.
post-op flexion (degree)
130 ± 4
120 ± 13
n.s.
pre-op UCLA activity-level score
4.2 ± 0.4
3.8 ± 0.4
n.s.
post-op UCLA activity-level score
5.0 ± 0.0
5.0 ± 0.0
n.s.
Characteristics
Parameter
Mean (and SD)
p value
Oxidized Zirconium TKA (n = 6)
Co–Cr TKA (n = 6)
(3.3 ± 1.3) 9 107
(3.4 ± 1.2) 9 107
n.s.a
Quantity
Total number
Size
ECD (lm)
0.8 ±0.3
0.6 ± 0.1
n.s.
Shape
Aspect ratio
1.4 ±0.1
1.4 ±0.1
n.s.
Roundness
1.4 ±0.0
1.4 ±0.0
n.s.
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Fig. 3 The size distributions of polyethylene wear particles isolated from synovial fluid from patients with the oxidized zirconium and cobalt–chromium (Co–Cr) prostheses. The range with greatest frequency was 0.2–0.4 lm in both groups
polyethylene insert, knee alignment, and postoperative knee society score).
Discussion The difference in in vivo wear particle characteristics between the newly introduced oxidized zirconium femoral component and the conventional Co–Cr femoral component was not statistically significant in early clinical stage. Factors that affect macrophage response to polyethylene wear particles have been reported; greater volume, submicron size, and an elongated shape of polyethylene wear particles all stimulate an increased macrophage response [13, 16, 26]. Our previous reports showed that modification of total knee arthroplasty materials and designs resulted in different polyethylene wear particle characteristics, and thus, these differences may be important factors in the long-term development of osteolysis [10, 17–20]. The oxidized zirconium femoral component has recently been introduced for the reduction in polyethylene wear particle generation. Higher resistance to roughing of oxidized zirconium compared to Co–Cr is expected to reduce polyethylene wear. Laskin et al. reported excellent clinical results and no clinical adverse effect of oxidized zirconium femoral component [14, 15]. Kaplan–Meir survivorship was 98 % at 5 years. Innocent et al. [8] also reported 98.7 % survivorship and no clinical adverse event of oxidized zirconium femoral component. However, an in vivo advantage of oxidized zirconium on polyethylene wear particle generation was still controversial. Our results confirmed that there was no adverse effect in polyethylene wear of newly introduced material for total knee prosthesis.
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Previous retrieval study showed that damage score of polyethylene insert and femoral component was significantly lower with oxidized zirconium femoral component than with Co–Cr femoral component [7] at mean of 18–19 months after operation. However, these cases included early failure cases such as instability, loosening, and malposition. In this study, we investigated only wellfunctioning cases. The difference of patients’ condition was one of the reasons for different results. An in vitro simulation studies showed that the oxidized zirconium femoral component reduced polyethylene wear by 42 % during 5 million cycles [6] and by 85 % during 6 million cycles [23] comparing to Co–Cr femoral component. An increase in surface roughness during 6 million cycles was significantly less in the oxidized zirconium femoral component than in the Co–Cr femoral component, and this difference results in the reduction in polyethylene wear in the oxidized zirconium femoral component [23]. Therefore, the advantage of oxidized zirconium will be observed in longer follow-up periods. This is one of the reasons why our in vivo wear particle characteristics were not different between the oxidized zirconium femoral component and the conventional Co–Cr femoral component. Longer follow-up of in vivo wear particle analysis is required to confirm such a potential advantage in progressive surface roughing of oxidized zirconium. There were advantages in the current study. First, we used the same type of prostheses between the 2 groups except for the material of the femoral components. Design of the articulating surface also influenced in vivo polyethylene wear particle generation [17, 20]. Medial-pivot design reduced the number of polyethylene wear particle by 51 % comparing to posterior-stabilized design. Polyethylene wear particles from medial-pivot design were slightly smaller and rounder than those form posterior-stabilized design. This study eliminated the influence of the articulating surface design and thus can properly evaluate the influence of femoral component material on in vivo polyethylene wear particle characteristics. Second, we used the previous validated method for in vivo polyethylene wear particle analysis [10, 17–20]. Therefore, we can compare the results of polyethylene wear particle analysis in this study to those in previous studies using other types of total knee prostheses. The number of polyethylene wear particles in both of our groups using the Genesis II posterior-stabilized prosthesis was about 30 % of that in previous reports (11.6 ± 5.7 9 107 counts) in which other types of posterior-stabilized prostheses (I-BII, Zimmer, Warsaw, IN, USA and Osteonics Scorpio PS, Stryker Osteonics Howmedica, Rutherford, NJ, USA) had been used [17]. Polyethylene wear particles are generated not only from the upper articulating surface but also from the lower nonarticulating surface, which is the so-called backside wear.
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Polished tibial baseplates reduce back side polyethylene wear, comparing to nonpolished rough tibial baseplates [3]. The upper surface of the tibial baseplates was polished in Genesis II, but not in IB-II and Osteonics Scorpio PS. Polished tibial baseplate of Genesis II might reduce in vivo polyethylene wear particle generation between insert and tibial baseplate, and this was one of the reasons for less wear particles in Genesis II posterior-stabilized prosthesis comparing to other types of posterior-stabilized prostheses. In the present study, the UCLA activity score was used as a means of measuring patient activity. In vivo wear assessments of total joint prostheses should be based on a measure of use. The UCLA activity score correlated strongly with the average number of steps per day, as recorded by a modern pedometer, which can be a satisfactory means of quantifying the use of lower extremity joints [22]. In the present study, the difference between the 2 prostheses in UCLA scores was not statistically significant. We thus believe that the difference in patient activity level did not influence the results of the current study. Besides UCLA score, postoperative knee alignment, BMI, and age might also influence in vivo polyethylene wear. Postoperative mal-alignment affected the polyethylene wear [21]. In this study, the number of polyethylene wear particles did not correlated with postoperative knee alignment, BMI, and age. No alignment outliers and uniformity in patients’ BMI and age in this study might be possible reasons for no correlation between the number of polyethylene wear particles and such factors. There was a limitation in this study. Although our sample size of each group satisfied power analysis and was larger than that of in vitro studies [6, 23], the sample size might to be small. However, this study required invasive test. Although it might to be better to enlarge sample size, it was not easy to enlarge sample size from the point of view of infection risk due to the synovial fluid collection. Although newly introduced oxidized zirconium femoral component did not reduce the in vivo polyethylene wear particles comparing to Co–Cr femoral component in early clinical stage, there was no adverse effect of newly introduced oxidized zirconium femoral component. At this moment, there is no need to abandon oxidized zirconium femoral component. However, further follow-up of polyethylene wear particle generation should be performed to confirm the advantage of the oxidized zirconium femoral component.
Conclusion The current in vivo study showed that the difference in the characteristics of in vivo polyethylene wear particles between the newly introduced oxidized zirconium femoral
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component and conventional Co–Cr femoral component was not statistically significant in early clinical stage. Acknowledgments Nephew.
This study was supported in part by Smith and
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