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Abstract

Introduction:

Sequentially annealed highly cross-linked polyethylene (HXLPE) was recently introduced to reduce the wear in total knee arthroplasty (TKA). However, an in vivo advantage of sequentially annealed HXLPE on wear particle generation is still controversial. The purpose of this study is to compare the characteristics of in vivo wear particles between sequentially annealed HXLPE and conventional polyethylene after TKA.

Materials and Methods:

Synovial fluid was obtained from the eight knees with sequentially annealed HXLPE and from eight knees with conventional polyethylene 12 months after the operation. Polyethylene particles were isolated and examined using a scanning electron microscope and image analyzer.

Results:

Total number of wear particles in each knee was 2.1 ± 1.0 × 10⁷ with sequentially annealed HXLPE (mean ± standard deviation) and 4.9 ± 3.6 × 10⁷ with conventional polyethylene (p = 0.036). Particle size (equivalent circle diameter) was 1.01 ± 0.26 μm with sequentially annealed HXLPE and 1.02 ± 0.20 μm with conventional polyethylene (p = 0.674). Aspect ratio was 1.33 ± 0.04 with sequentially annealed HXLPE and 1.39 ± 0.10 with conventional polyethylene (p = 0.462).

Conclusions:

The sequentially annealed HXLPE reduced the in vivo polyethylene wear particles by 58% compared with conventional polyethylene without the significant change of particle size and shape.

Keywords

polyethylene wear particle sequentially annealed highly cross-linked polyethylene total knee arthroplasty

Introduction

Polyethylene wear particles from the articulating surface induce macrophages to release cytokines, which leads to osteolysis and loosening of the prosthesis.^1

–4 Greater volume, sub-micrometer size, and an elongated shape of polyethylene wear particles all stimulate an increased macrophage response.^5
–7 Aseptic loosening is one of the main factors that negatively affect the longevity for total knee arthroplasty (TKA).^8,9 To reduce the polyethylene wear particles and aseptic loosening, new materials have been developed. A new process of sequential irradiation and annealing highly cross-linked polyethylene (HXLPE) has recently been introduced in TKA.¹⁰ In vitro studies have demonstrated that sequentially annealed HXLPE can reduce the volumetric wear of a polyethylene insert by 64–91% comparing to the conventional polyethylene.^11,12 However, the wear particles were smaller than those of conventional polyethylene.¹³ Smaller polyethylene wear particles would have higher biological activity.⁶ Therefore, the smaller wear particles of sequentially annealed HXLPE might offset the benefit of reduced volumetric wear.

Previous reports showed that newly introduced materials, which reduced the polyethylene wear particles in in vitro simulator study, do not always reduce in vivo polyethylene wear particles and improve clinical results such as Hylamer,¹⁴ heat-pressed polyethylene,¹⁵ and carbon fiber-reinforced polyethylene.¹⁶ However, it takes decades to determine whether newly introduced materials reduce the polyethylene wear particles and improve clinical results or not. Thus, it is particularly important to provide early feedback of in vivo polyethylene wear particles before newly introduced materials come into widespread use. As one of the methods to address these problems, we have established a method to measure in vivo polyethylene wear particles by isolating polyethylene wear particles in synovial fluid from well-functioning knees after TKA in early clinical stage.^17

–22 There is still little information of in vivo data on the wear particles of sequentially annealed HXLPE.

We hypothesized that newly introduced sequentially annealed HXLPE reduces the in vivo polyethylene wear particles in TKA compared to conventional polyethylene. 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 total knee prosthesis using newly introduced sequentially annealed HXLPE and conventional polyethylene that had the same articulating surface shape.

Material and methods

Consecutive patients awaiting TKA were eligible to participate in the study. Exclusion criteria included inflammatory arthritis, osteonecrosis of the knee, and valgus deformity of the knee. Osteoarthritis and varus deformity of the knee were permitted. A total of 12 patients (16 knees) consented to participate and four patients had bilateral procedures. All patients had the same design posterior-stabilized total knee prosthesis (Scorpio NRG PS; Stryker Orthopaedics, Mahwah, NJ, USA), which only differed in the type of polyethylene used in the tibial insert and patellar component. This material specification was randomly assigned. Simple randomization was performed by a computer-generated random-number list. Some patients received sequentially annealed HXLPE (X3, Stryker Orthopaedics, Mahwah, NJ, USA; n = 8) and others conventional polyethylene (N₂Vac, Stryker Orthopaedics, Mahwah, NJ, USA; n = 8).

All the operations were performed by a single surgeon. Synovial fluid was obtained from 16 knees in a sterile operating room 12 months postoperation. This study complied with the Declaration of Helsinki (2013) and was approved by the Institutional Review Board for Clinical Research of Osaka City University Graduate School of Medicine (ref. 1281). Informed consent was obtained from all the patients at the time of enrolment.

Both types of polyethylene were manufactured by sheet compression molding of GUR 1020 resin powder. The sequentially annealed HXLPE was manufactured using a cycle of 3 Mrad of gamma irradiation followed by the thermal treatment below belting point (130 °C) for 8 h, repeated three times. After sequentially annealing, it was machined, sterilized using gas plasma, and stored in air. The conventional polyethylene was machined from GUR 1020 resin sheet, sterilized using 3 Mrad of gamma irradiation in N₂, stored in N₂.

The preoperative diagnosis of all patients was osteoarthritis. Preoperative and postoperative activity levels were evaluated using the University of California Los Angeles (UCLA) activity-level rating.²³ Knee Society score,²⁴ range of motion of the knee joint,²⁵ postoperative implant alignment according to Knee Society,²⁶ and mechanical axis using full-length leg radiography were also evaluated.

Polyethylene wear particles were isolated using our previously developed technique.^17

–22 All solutions were filtered through a 0.2-μm-pore nylon filter (150-0020, Nalge Company, Rochester, NY, USA) to avoid contamination of extraneous particles. In each case, synovial fluid was 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 r/min (103,700 G) at 4 °C for 3 h (CP100α, 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 28,00 r/min (103,200 G) at 4 °C for 1 h (CP100α, P28S1004 rotor, Hitachi Koki Co., Ltd.). Polyethylene particles were collected from the interface between two layers and filtered through 0.1-μm polycarbonate filters (VCTP 013-00, Millipore Corporation, Bedford, MA). Filters were dried, attached to an aluminum 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 (S-4700SI, Hitachi Ltd., Tokyo, Japan). 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 μm; S-395, Shamrock, Petaluma, CA, USA). The retrieval ratio of polyethylene particles of five experimental specimens was 65.2 ± 16.7% (mean ± standard deviation).¹⁷ It was not feasible to completely aspirate all synovial fluid from a knee joint with one puncture. Therefore, the synovial fluid collection ratio was used to estimate numbers of polyethylene wear particles in the residual synovial fluid in joint capsules, as previously reported. After aspiration, 2 ml of saline was injected into the joint capsule and then re-aspirated of residual synovial fluid. The number of polyethylene wear particles in aspirated fluid and that in the re-aspirated fluid was analyzed in seven knees. The synovial fluid collection ratio was 92.3 ± 9.3%.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 in the extraction process, and (f) the total amount of synovial fluid. Particle size was expressed as the equivalent circle diameter; that 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/(4π·area)).^{7,17

–22} Particle size and shape were analyzed with a computerized image analyzer (Mac Scope, Minami Co., Tokyo, Japan).

Statistical analyses of differences between the types of prostheses were done using the Mann–Whitney U test and Fisher’s exact test with a commercially available software package (StatView 4.5; Abacus Concepts Inc., Berkeley, CA, USA). We set the significance level at 5%.We set the significance level at 5%. Sample size calculation showed that seven knees in each group would allow detection of a difference of 3 × 10⁷ counts/knee of particles (power = 0.8, α = 0.05) with a standard deviation of 2 × 10⁷ counts/knee between two groups.

Results

Salient patient data for both groups are shown in Table 1. The differences in these parameters were not significant. The difference in synovial fluid volume between the conventional polyethylene group (8.8 ± 3.1 ml; mean ± standard deviation) and the sequentially annealed HXLPE group (7.5 ± 2.9 ml) was not statistically significant.

Table 1.

Patients’ demographic data.

Parameter	Mean (and standard deviation)		p Value
Parameter	Conventional PETKA (n = 8)	Sequentially annealed HXLPE TKA (n = 8)	p Value
Age at operation (years old)	73.5 ± 11.7	77.6 ± 5.9	0.462
Gender (male/female)	8/0	8/0	>0.999^a
BMI (kg/m²)	26.9 ± 2.3	26.0 ± 3.2	0.52
Thickness of PE insert	8 mm:6, 10 mm:2	8 mm:7, 10 mm:1	0.674
Postoperative hip-knee-ankle angle (degrees valgus)	0.0 ± 1.8	0.2 ± 1.7	0.804
Postoperative implant alignment (degree)
α	95.7 ± 1.9	96.9 ± 1.6	0.248
β	89.5 ± 0.8	89.7 ± 1.1	0.600
γ	2.7 ± 1.0	1.8 ± 2.3	0.495
δ	88.0 ± 1.8	87.9 ± 1.4	0.958
Preoperative KSS knee	30 ± 9	34 ± 13	0.462
Preoperative KSS function	31 ± 22	26 ± 21	0.753
Postoperative KSS knee	96 ± 7	96 ± 9	0.958
Postoperative KSS function	79 ± 16	78 ± 25	0.834
Postoperative extension (degree)	–3 ± 7	0 ± 0	0.401
Postoperative flexion (degree)	138 ± 10	128 ± 14	0.093
Preoperative UCLA activity-level score	2.9 ± 0.4	2.8 ± 0.5	0.674
Postoperative UCLA activity-level score	4.8 ± 0.7	4.8 ± 0.7	>0.999

PE: polyethylene; TKA: total knee arthroplasty; KSS: Knee Society Score; UCLA: University of California Los Angeles; HXLPE: highly cross-linked polyethylene.

^aChi-square test.

The quantity, size, and shape of the polyethylene wear particles are listed in Table 2. The total number of polyethylene wear particles from the knees with sequentially annealed HXLPE was 58% less than those made with conventional polyethylene (p = 0.036). The size distribution of the polyethylene wear particles is shown in Figure 1. Particles of size 0.8–1.0 µm occur most frequently in both groups. The difference in the size and shape (aspect ratio and roundness) between the groups was not statistically significant.

Table 2.

Quantity, size, and shape of wear particles in conventional polyethylene and sequentially annealed HXLPE.

		Mean (and standard deviation)
		Conventional PE	Sequentially annealed HXLPE
Characteristics	Parameter	(n = 8)	(n = 8)	p Value
Quantity	Total number	(4.9 ± 3.6) × 10⁷	(2.1 ± 1.0) × 10⁷	0.036
Size	ECD (μm)	1.02 ± 0.20	1.01 ± 0.26	0.674
Shape	Aspect ratio	1.39 ± 0.10	1.33 ± 0.04	0.462
	Roundness	1.36 ± 0.06	1.34 ± 0.05	0.600

HXLPE: highly cross-linked polyethylene; PE: polyethylene; ECD: equivalent circle diameter.

Figure 1.

Size distributions of polyethylene wear particles isolated from the synovial fluid of patients with conventional polyethylene and sequentially annealed HXLPE. HXLPE: highly cross-linked polyethylene.

Discussion

This study showed that the sequentially annealed HXLPE significantly decreased the in vivo polyethylene wear particles compared with conventional polyethylene in the early clinical stage. This is in agreement with the previous in vitro studies.^11,12 However, in contrast to the previous in vitro studies,¹³ we found that the sequentially annealed HXLPE did not change the particle size or shape compared to the conventional polyethylene. Therefore, we expected that sequentially annealed HXLPE might reduce the likelihood of osteolysis and aseptic loosening after TKA.

There were several advantages in this study. First, we used not only polyethylene tibial insert but also polyethylene patellar component in all the patients. However, in vitro simulators could not use the polyethylene patellar component. The difference of such tribological condition between our in vivo study and the previous in vitro studies^11
–13 might result in the difference of wear particle size of sequentially annealed HXLPE between our in vivo study and the previous in vitro studies. Second, we used the same prosthesis for all the patients, except for the materials for the tibial insert and patellar component. It was reported that the design of the articulating surface and the material of femoral component also influenced in vivo polyethylene wear particle generation.^17,20 Our study design was suitable for the comparison between sequentially annealed HXLPE and conventional polyethylene. Third, we investigate age, gender, BMI, knee alignment, patients’ activity level. Previous reports showed that such factors can influence in vivo polyethylene wear.^27,28 However, in this study, the differences of age, gender, BMI, thickness of polyethylene insert, postoperative implant alignment, postoperative alignment of lower extremity, Knee Society score, the knee range of motion, and UCLA activity-level score between two groups were not statistically significant.

In contract to this study, Hinarejos et al. showed that the sequentially annealed HXLPE did not decreased the in vivo polyethylene wear particle generation compared with conventional polyethylene in TKA.²⁹ However, the prosthesis used in their study (Triathlon PS; Stryker Orthopaedics, Mahwah, NJ, USA) was different from that used in this study (Scorpio NRG PS). Although both the prostheses were made by the same company, the design concept in coronal plane between two prostheses was different. Previous reports showed that the design of the articulating surface can influence in vivo polyethylene wear particle generation.^17,20 The articulating surface design of Scorpio NRG PS might be more sensitive to the difference of polyethylene insert materials than that of Triathlon PS. This study suggested that the efficacy of sequentially annealed HXLPE also depends on the design of articulating surface of total knee prostheses.

This study has some limitations. First, pore size of the filter was 0.1 µm. Wear particles under a size of 0.1 µm might be passed through the filter. However, the most frequent size distribution was 0.8–1.0 µm in both groups. We thus believe that the pore size of the filter did not have a great influence on the results of this study. Second, the number of patients was limited. The more number of patients might be desirable. However, the collection of joint fluid potentially has a risk of infection. From the point of view of safety, we have to limit the number of patients as less as possible.

Our in vivo study of newly introduced materials showed the advantage of sequentially annealed HXLPE over conventional polyethylene, regarding the characteristics of polyethylene wear particles. Although sequentially annealed HXLPE is expected to reduce wear particles and the osteolysis, further follow-up of wear particle generation and clinical results is mandatory.

Conclusions

The current in vivo study showed that the newly introduced sequentially annealed HXLPE reduced the in vivo polyethylene wear particles in TKA by 58% compared with conventional polyethylene in the early clinical stage.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received grants in support of their research (Stryker Japan).

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Sequentially annealed highly cross-linked polyethylene reduced in vivo wear particle generation in total knee arthroplasty

Abstract

Introduction:

Materials and Methods:

Results:

Conclusions:

Keywords

Introduction

Material and methods

Results

Discussion

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References