A novel method to eliminate the influence of absorbed lipids on the characterization of ultra-high molecular weight polyethylene using Fourier-transform infrared spectroscopy

Abstract

BACKGROUND:

Fourier-transform infrared spectroscopy (FTIR) is one of the standard methods to analyze ultra-high molecular weight polyethylene (UHMWPE) in orthopedic implants. For retrieved components, lipid extraction using an organic solvent prior to the measurement is necessary to eliminate the influence of lipids absorbed in vivo. However, its influence on the measurement has not been substantially investigated.

OBJECTIVE:

To investigate the influence of lipid extraction on the FTIR analysis of UHMWPE and to develop a novel method to obtain reliable results without inconvenient lipid extraction.

METHODS:

FTIR analysis was repeatedly performed on UHMWPE specimens from retrieved components before and after lipid extraction under various conditions. A method to calculate the extent of influence of the absorbed lipids from the FTIR spectra was developed using a peak separation technique.

RESULTS:

An elevated temperature was necessary for lipid extraction; however, it had the potential to influence the results if the conditions were not properly controlled. The results obtained using the peak separation technique coincided with those obtained after lipid extraction.

CONCLUSION:

The use of the peak separation technique enables the efficient acquisition of reliable results without the need for lipid extraction.

Keywords

Oxidation index lipid index retrieval study oxidative degradation lipid index subtracted oxidation index (s-OI)

1. Introduction

Joint arthroplasty has been one of the most successful surgical procedures for more than half a century [1]. Since joint arthroplasty was first successfully executed, ultra-high molecular weight polyethylene (UHMWPE) has been the material of choice for use at the articulating surface of joint replacements [2–4]. Although UHMWPE is a chemically stable and bioinert material, its oxidative degradation has been a major cause of failure. For instance, gamma irradiation, used either for sterilization or crosslinking, is known to create free radicals that result in the long-term oxidative degradation of UHMWPE [4]. The oxidative degradation of UHMWPE leads to accelerated generation of wear particles and delamination [5–7], resulting in failure of the implant. Thus, UHMWPEs containing antioxidants such as vitamin E have also been introduced [8–10].

Fourier-transform infrared spectroscopy (FTIR) has been widely used to evaluate the extent of oxidation of UHMWPE. The oxidation extent of UHMWPE has been evaluated using oxidation index (OI), which is calculated using the area of carbonyl infrared (IR) absorption peaks [11]. However, this area includes that of esters originating from absorbed lipids [11,12]. In 2013, the ASTM standard guide for the evaluation of the oxidation extent in UHMWPE using FTIR was revised to include the lipid extraction process that uses hexane or heptane to eliminate the influence of absorbed lipids on the OI calculation [11]. However, the effect of lipid extraction on OI and other parameters obtained by FTIR measurements has not been investigated thoroughly. In addition, the IR absorption peak caused by absorbed lipids has been gaining attention since the possibility of degradation induced by lipids absorbed in vivo was reported recently [13,14].

The objectives of the present study were to investigate the effect of lipid extraction on the parameters obtained from FTIR measurements as well as to introduce and validate a novel method to eliminate the influence of absorbed lipids by the peak separation technique, rather than the laborious solvent extraction method. In the present study, we assumed that the IR absorption peak between 1650 cm⁻¹ and 1850 cm⁻¹, which is used for the calculation of OI [11], consists of that of ketones originating from the oxidation of UHMWPE at 1718 cm⁻¹ and that of esters originating from absorbed lipids [12]; furthermore, we assumed that the absorption peaks have Cauchy distributions.

2. Materials and methods

2.1. Materials

A total of eight UHMWPE components, including six bipolar hip replacements, one total hip replacement, and one total ankle replacement, were randomly selected from retrieved components collected through our ongoing IRB-approved implant retrieval program (Table1). Hexane (Wako Pure Chemical Industries, Osaka, Japan) and cyclohexane (Sigma Aldrich Japan, Tokyo, Japan) were used for lipid extraction.

Table 1
Summary of the retrieved components used in this study

ID Implant Year in vivo Gender Diagnosis Height [m] Weight [kg] BMI

#1 Bipolar 15.2 F OA 1.55 49 20.4

#2 Bipolar 5.3 F F 1.67 68 24.4

#3 THR 16.2 F OA 1.46 44 20.6

#4 TAA 3.9 F RA 1.5 45 20.0

#5 Bipolar 2 F F 1.54 50 21.1

#6 Bipolar 10.2 F OA 1.56 56 23.0

#7 Bipolar 20.5 F ON 1.46 55 25.8

#8 Bipolar 28 F ON 1.59 58 22.9

ID	Implant	Year in vivo	Gender	Diagnosis	Height [m]	Weight [kg]	BMI
#1	Bipolar	15.2	F	OA	1.55	49	20.4
#2	Bipolar	5.3	F	F	1.67	68	24.4
#3	THR	16.2	F	OA	1.46	44	20.6
#4	TAA	3.9	F	RA	1.5	45	20.0
#5	Bipolar	2	F	F	1.54	50	21.1
#6	Bipolar	10.2	F	OA	1.56	56	23.0
#7	Bipolar	20.5	F	ON	1.46	55	25.8
#8	Bipolar	28	F	ON	1.59	58	22.9

THR: total hip arthroplasty, TAA: total ankle arthroplasty, F (gender): female, OA: osteoarthritis, F (diagnosis): fracture, ON: osteonecrosis, BMI: body mass index.

2.2. FTIR measurements

The retrieved UHMWPE components were first washed by sonication in a neutral detergent solution and immersion in an enzymatic detergent (Cidezyme, ASP Japan, Tokyo, Japan). They were then disinfected by immersing in a phtharal solution (Cidex Opa, ASP Japan, Tokyo, Japan). Following visual inspection, cross-sectional specimens with a thickness of 100–200 μm were prepared using a rotary microtome (PR-50, Yamato Kohki Industrial, Asaka, Japan). These thin specimens were analyzed using microscopic FTIR spectroscopy (SPX200 and IR-MAU110, JEOL, Tokyo, Japan). The microscopic FTIR measurements were performed in the depth direction from the original surface at intervals of 100 to 500 μm with an aperture size of 100 × 100 μm, 10 scans, and a resolution of 4 cm⁻¹.

OI was calculated by normalization of the peak area between 1650 cm⁻¹ and 1850 cm⁻¹ with respect to the peak area at 1370 cm⁻¹ [11]. The maximum OI in a specimen was denoted as OI_MAX. The ketone oxidation index (KOI) was calculated by normalization of the peak height at 1718 cm⁻¹ with respect to the peak height at 1370 cm⁻¹ [7]. The crystallinity index (CI) was calculated using the following formula: $\begin{eqnarray}\displaystyle \text{CI}=(A_{1896}/A_{1305})/(A_{1896}/A_{1305}+0.25)\times 100, & & \displaystyle \nonumber\end{eqnarray}$ where A ₁₈₉₆ and A ₁₃₀₅ are the peak areas for the crystalline band at 1896 cm⁻¹ and the amorphous band at 1305 cm⁻¹, respectively [15]. The average values from the CI profile were then used for further analyses. The trans-vinylene index (TVI) was calculated by normalization of the peak area at 965 cm⁻¹ with respect to the peak area at 1370 cm⁻¹ [16]. The average values from the TVI profile were then used for further analysis.

2.3. Lipid index calculation

The Cauchy distribution parameters (height, location, and half-width at half-maximum) for IR absorption peaks of ketones and esters at 1718 and 1740 cm⁻¹, respectively, that minimize the deviation between the sum of the calculated peaks and the measurement were calculated using FTIR spectra between 1650 cm⁻¹ and 1850 cm⁻¹ on Microsoft Excel with the generalized reduced gradient (GRG) method [17]. An example of the calculated peaks is shown in Fig. 1. The lipid index (LI) was defined by normalization of the peak area of calculated lipid peak at 1740 cm⁻¹ with respect to the peak area at 1370 cm⁻¹ in the same manner as for OI. Then, LI-subtracted OI (s-OI) was defined as follows: $\begin{eqnarray}\displaystyle \text{s}\text{-}\text{OI}=\text{OI}-\text{LI}. & & \displaystyle \nonumber\end{eqnarray}$

2.4. Lipid extraction

We performed lipid extraction by immersion in hexane at room temperature (RT) or reflux using 100 mL of hexane [11] or cyclohexane [12]. FTIR measurements were repeated after each extraction procedure. OI after lipid extraction in either hexane or cyclohexane at the boiling point (BP) was defined as lipid extracted OI (e-OI).

Fig. 1.

Example of peak separation. The sum of the calculated lipid peak and calculated oxidation peak coincides well with the measurement (specimen #4).

2.5. Statistical analysis

A paired Student’s t-test for one-tailed distributions was used to statistically analyze if a lipid extraction process induced a reduction in OI. A paired Student’s t-test for two-tailed distributions was used to statistically analyze if a lipid extraction process had affected CI and TVI. The differences were considered statistically significant when p < 0.05.

3. Results

3.1. Basic characteristics of specimens

The results of initial FTIR measurements before lipid extraction are summarized in Table2. Based on a previous study [18], components with TVI values of 0.01 or higher were considered to have experienced gamma irradiation.

Table 2
Results of initial FTIR measurements before lipid extraction

ID Implant Position Year in vivo OI_MAX CI TVI Irradiation

#1a Bipolar Rim 15.2 0.55 64.0 0.000 No

#1b Articular 0.75 63.8 0.002

#2 Bipolar Articular 5.3 1.16 66.6 0.011 Yes

#3 THA Articular 16.2 1.03 62.3 -0.001 No

#4 TAA Articular 3.9 1.67 60.7 0.000 No

#5 Bipolar Rim 2 0.44 67.1 0.011 Yes

#6 Bipolar Ring 10.2 5.91 70.3 0.014 Yes

#7 Bipolar Ring 20.5 3.99 70.7 0.016 Yes

#8 Bipolar Rim 28 4.44 76.3 0.019 Yes

ID	Implant	Position	Year in vivo	OI_MAX	CI	TVI	Irradiation
#1a	Bipolar	Rim	15.2	0.55	64.0	0.000	No
#1b		Articular		0.75	63.8	0.002
#2	Bipolar	Articular	5.3	1.16	66.6	0.011	Yes
#3	THA	Articular	16.2	1.03	62.3	-0.001	No
#4	TAA	Articular	3.9	1.67	60.7	0.000	No
#5	Bipolar	Rim	2	0.44	67.1	0.011	Yes
#6	Bipolar	Ring	10.2	5.91	70.3	0.014	Yes
#7	Bipolar	Ring	20.5	3.99	70.7	0.016	Yes
#8	Bipolar	Rim	28	4.44	76.3	0.019	Yes

THR: total hip arthroplasty, TAA: total ankle arthroplasty, OI_MAX: maximum oxidation index, CI: average crystallinity index, TVI: average trans vinylene index.

3.2. Lipid extraction at RT

Lipid extraction was first performed sequentially using one specimen (#1a) for 2, 4, and 24 h in hexane at RT. The FTIR measurements were repeated after each extraction. Note that the specimen had undergone a total of 30 h of lipid extraction after the last step of lipid extraction for 24 h. The IR absorption peak of lipids at 1740 cm⁻¹ became smaller after every extraction step, as shown in Fig. 2. This trend was also confirmed by the calculated OI, KOI, and LI, as shown in Fig. 3, indicating the need for lipid extraction for 24 h in hexane at RT to minimize the effect of absorbed lipids on the FTIR measurements.

Fig. 2.

FTIR spectra before and after each lipid extraction process in hexane at room temperature. A consecutive reduction of the lipid absorption peak at 1740 cm⁻¹, as indicated by the black arrow, was observed from consecutive measurements of FTIR spectra at the same position of specimen #1a. The vertical magnification of each spectrum is normalized by the peak at 1370 cm⁻¹. INI indicates the initial condition before lipid extraction.

Fig. 3.

Effects of lipid extraction in hexane at room temperature. The oxidation index (OI, left axis), ketone oxidation index (KOI, right axis), and lipid index (LI, left axis) at the same position of specimen #1a before and after each lipid extraction process in hexane at room temperature are shown. The indices are reduced after each lipid extraction process. INI indicates the initial condition before lipid extraction.

3.3. Effect of elevated temperature and solvent selection

In a previous study, several organic solvents were compared, and cyclohexane was reported to yield the largest amount of extract [19]. Therefore, the extraction using cyclohexane at BP is considered to produce a higher yield than that using hexane at BP, probably due to its higher BP, as indicated in Table3. Five specimens were used for lipid extraction, first in hexane at RT for 24 h, followed by extraction in hexane at BP for approximately 16 h, and finally extraction in cyclohexane at BP for approximately 16 h. Hexane extraction at RT significantly decreased the OI_MAX of the specimens, as shown in Fig. 4, demonstrating its effectiveness to extract lipids at RT. However, the subsequent extraction in hexane at BP further decreased the OI_MAX value, as shown in Fig. 4, indicating insufficiency of hexane extraction at RT for 24 h. In contrast, additional extraction in cyclohexane at BP did not significantly change the OI_MAX value.

Table 3
Boiling point of typical organic solvents used for lipid extraction

Boiling point [°C]

Hexane [11] 69

Cyclohexane [12] 81

Heptane [11] 98

	Boiling point [°C]
Hexane [11]	69
Cyclohexane [12]	81
Heptane [11]	98

Fig. 4.

Effects of lipid extraction on the maximum oxidation index (OI_MAX). INI indicates the initial condition before lipid extraction. He and CH indicate extraction using hexane and cyclohexane, respectively. n = 5 except for CH (BP), for which n = 4.

3.4. Effect of the extraction process on other indices

The effect of each lipid extraction on CI is shown in Fig. 5. The lipid extraction process in hexane at either RT or BP did not change the CI values significantly. In contrast, extraction in cyclohexane at BP significantly increased the CI values by up to 10% compared to the initial measurement before lipid extraction. The TVI in irradiated specimens tended to increase after lipid extraction in cyclohexane at BP, as shown in Fig. 6, although this increase was not statistically significant. The effect of lipid extraction on KOI is shown in Fig. 7. KOI was found to decrease after hexane or cyclohexane extraction at BP.

Fig. 5.

Effects of lipid extraction on the crystallinity index (CI). INI indicates the initial condition before lipid extraction. He and CH indicate extraction using hexane and cyclohexane, respectively. n = 5 except for INI and CH (BP), for which n = 8 and 7, respectively.

Fig. 6.

Effects of lipid extraction on trans vinylene index (TVI) of irradiated specimens. INI indicates the initial condition before lipid extraction. CH indicates extraction using cyclohexane.

Fig. 7.

Effects of lipid extraction on the maximum ketone oxidation index (KOI_MAX). The maximum ketone oxidation indices of specimens after lipid extraction in either hexane or cyclohexane at BP (e-KOI_MAX) were significantly lower than those before lipid extraction (KOI_MAX).

3.5. Relation between s-OI and OI after lipid extraction

Figure 8 shows an example for the depth profiles of OI, s-OI, and e-OI. Good agreement was observed between s-OI and e-OI. Figure 9 shows the comparison of maximum values of OI, s-OI, and e-OI of all specimens used in this study. Owing to either LI subtraction or lipid extraction, s-OI_MAX and e-OI_MAX were significantly lower than the initial OI_MAX. In addition, s-OI_MAX and e-OI_MAX coincided well with each other, as shown in Fig. 10.

Fig. 8.

Comparison of the depth profile of the oxidation index (OI). The depth profile of the OI, lipid index subtracted OI (s-OI), and lipid extracted OI (e-OI) of specimen #4 are compared.

Fig. 9.

Comparison of the maximum oxidation index (OI_MAX). The OI_MAX, maximum lipid index subtracted oxidation index (s-OI_MAX), and maximum lipid extracted oxidation index (e-OI_MAX) of the specimens used in this study are compared with n = 8.

Fig. 10.

Relation between s-OI_MAX and e-OI_MAX. The maximum lipid index subtracted oxidation index (s-OI_MAX) and maximum lipid extracted oxidation index (e-OI_MAX) coincided well. n = 8.

4. Discussion

We performed a series of lipid extractions for 2, 4, and 24 h at RT and 16 h at BP to investigate their effects on the indices obtained from FTIR measurements. It was considered that the duration of extraction for 16 h in hexane or cyclohexane at BP is sufficiently long to minimize the influence of the absorbed lipids since further reduction of OI_MAX was not observed by the additional lipid extraction using cyclohexane at BP.

The possibility that lipid extraction at an elevated temperature affects other indices was investigated. It was found that CI values were increased significantly by the extraction process when a solvent with a high BP was used. Furthermore, the elevated temperature during lipid extraction was considered to have promoted the crystallization of molecular fragments freed by chain scissions owing to prior oxidative degradation [20]. The reduction of KOI by the solvent extraction, as shown in Fig. 7, suggested the influence of absorbed lipids on the height of the IR absorption peak of ketones. The distribution of the calculated absorption peak of esters shown in Fig. 1 also supported this observation. Thus, it was concluded that KOI is not free from the influence of absorbed lipids.

We proposed two new indices, LI and s-OI, which can be obtained by peak-separation calculation, and the use of s-OI instead of lipid extraction to eliminate the effect of absorbed lipids. In general, s-OI_MAX coincided well with e-OI_MAX, as shown in Fig. 10, but a difference between them was observed in one specimen, which had the highest initial OI_MAX (Fig. 9). In the FTIR spectra of this specimen, reductions at the top and shoulder of the IR absorption peak of ketones, in addition to the peak of esters at 1740 cm⁻¹, were observed after the solvent extraction, as shown in Fig. 11. Previously, Sakoda et al. investigated the extracts using FTIR [19] and gas chromatography/mass spectrometry (GC/MS) [21]. In these preliminary studies, they found the IR absorption peak of ketones at 1720 cm⁻¹ by FTIR measurement and carbonic acids and dicarbonic acids with various numbers of carbon chains by GC/MS analysis. Therefore, it was considered that oxidized polyethylene fragments may be extracted from highly oxidized UHMWPE by solvent extraction, leading to the underestimation of OI, although further investigation is needed to draw a strong conclusion. Taking this observation into account, an even higher correlation of R ² = 0.996 was obtained between s-OI_MAX and e-OI_MAX when such highly oxidized specimens (initial OI_MAX > 5) were excluded.

Fig. 11.

Effects of cyclohexane extraction on FTIR spectra. The white arrow indicates the difference between the FTIR spectra of specimen #6 at 400 μm beneath the original surface before lipid extraction (Initial) and after lipid extraction with cyclohexane at BP (Extracted). The black arrows indicate the difference between the spectra, which was probably due to the extraction of substances other than lipids. The vertical magnification of each spectrum is normalized by the peak at 1370 cm⁻¹.

These results strongly indicated that s-OI can be used instead of OI measurement after lipid extraction. This alternative method can avoid the time-consuming lipid extraction process and any uncertainty associated with the extraction process on the FTIR measurements.

We used eight conventional UHMWPE components in the present study. An additional investigation using modern materials such as highly crosslinked UHMWPE and UHMWPE containing an anti-oxidant will further strengthen the significance of this study.

5. Conclusions

Lipid extraction using organic solvents should be performed at an elevated temperature to maximize its effect. However, the conditions of this process must be properly controlled if OI, CI, and TVI are simultaneously measured because it has the potential to affect CI and TVI. To avoid this inconvenience, a method to minimize the influence of absorbed lipids by using a peak separation technique was proposed. This method enables efficient acquisition of reliable parameters without the need for a lipid extraction process prior to FTIR measurement.

Footnotes

Conflict of interest

None to report.

References

Charnley

and Cupic

, The nine and ten year results of the low-friction arthroplasty of the hip, Clin Orthop Relat Res 95 (1973), 9. doi:10.1097/00003086-197309000-00003.

National Joint Registry for England, Wales, Northern Ireland and the Isle of Man. National Joint Registry 13th Annual Report 2016. Available from: http://www.njrreports.org.uk/.

Australian Orthopaedic Association. National Joint Replacement Registry Annual Report 2016. Available from: https://aoanjrr.sahmri.com/annual-reports-2016.

Kurtz

S.M.

Muratoglu

O.K.

Evans

and Edidin

A.A.

, Advances in the processing, sterilization, and crosslinking of ultra-high molecular weight polyethylene for total joint arthroplasty, Biomaterials 20(18) (1999), 1659. doi:10.1016/s0142-9612(99)00053-8.

Sutula

L.C.

Collier

J.P.

Saum

K.A.

Currier

B.H.

Currier

J.H.

Sanford

W.M.

Mayor

M.B.

Wooding

R.E.

Sperling

D.K.

Williams

I.R.

Kasprzak

D.J.

and Surprenant

V.A.

, The Otto Aufranc Award. Impact of gamma sterilization on clinical performance of polyethylene in the hip, Clin Orthop Relat Res 319 (1995), 28. doi:10.1097/00003086-199510000-00004.

Fisher

Chan

K.L.

Hailey

J.L.

Shaw

and Stone

, Preliminary study of the effect of aging following irradiation on the wear of ultrahigh-molecular-weight polyethylene, J Arthroplasty 10(5) (1995), 689. doi:10.1016/s0883-5403(05)80217-7.

Currier

B.H.

Currier

J.H.

Mayor

M.B.

Lyford

K.A.

Collier

J.P.

and Van Citters

D.W.

, Evaluation of oxidation and fatigue damage of retrieved crossfire polyethylene acetabular cups, J Bone Joint Surg Am 89(9) (2007), 2023. doi:10.2106/00004623-200709000-00019.

Tomita

Kitakura

Onmori

Ikada

and Aoyama

, Prevention of fatigue cracks in ultrahigh molecular weight polyethylene joint components by the addition of vitamin E, J Biomed Mater Res. 48(4) (1999), 474. doi:10.1002/(sici)1097-4636(1999)48:4%3C474::aid-jbm11%3E3.0.co;2-t.

Turner

A.C.

Sugimoto

Uetsuki

Hyon

S.H.

and Tomita

, Mechanical and oxidative performance of high-dose electron-beam irradiated, dl-𝛼-tocopherol (vitamin E) blended UHMWPE, J Biomech Sci Eng 10(1) (2015), 14-00238. doi:10.1299/jbse.14-00238.

10.

Oral

Wannomae

K.K.

Hawkins

Harris

W.H.

and Muratoglu

O.K.

, Alpha-tocopherol-doped irradiated UHMWPE for high fatigue resistance and low wear, Biomaterials 25(24) (2004), 5515. doi:10.1016/j.biomaterials.2003.12.048.

11.

ASTM F2102-13. Standard Guide for Evaluating the Extent of Oxidation in Polyethylene Fabricated Forms Intended for Surgical Implants, ASTM International, West Conshohocken (PA), 2013.

12.

Costa

Bracco

del Prever

E.B.

Luda

M.P.

and Trossarelli

, Analysis of products diffused into UHMWPE prosthetic components in vivo, Biomaterials 22(4) (2001), 307. doi:10.1016/s0142-9612(00)00182-4.

13.

Oral

Ghali

B.W.

Neils

and Muratoglu

O.K.

, A new mechanism of oxidation in ultrahigh molecular weight polyethylene caused by squalene absorption, J Biomed Mater Res B Appl Biomater 100(3) (2012), 742. doi:10.1002/jbm.b.32507.

14.

Sakoda

and Niimi

, Impact of lipid-induced degradation on the mechanical properties of ultra-high molecular weight polyethylene for joint replacements, J Mech Behav Biomed Mater 53 (2016), 218. doi:10.1016/j.jmbbm.2015.08.025.

15.

Costa

Jacobson

Brunella

and Bracco

, Effects of microtomy on the material properties of ultra high molecular weight polyethylene, Polym Test 20(6) (2001), 649. doi:10.1016/s0142-9418(00)00088-x.

16.

ASTM F2381-10. Standard Test Method for Evaluating Trans-Vinylene Yield in Irradiated Ultra-High-Molecular-Weight Polyethylene Fabricated Forms Intended for Surgical Implants by Infrared Spectroscopy, ASTM International, West Conshohocken (PA), 2010.

17.

Lasdon

L.S.

Fox

R.L.

and Ratner

M.W.

, Nonlinear optimization using the generalized reduced gradient method, Revue Française d’Automatique, Informatique et Recherche Opérationnelle V3 (1974), 73. doi:10.1051/ro/197408v300731.

18.

Sakoda

Ishikawa

Jung

D.Y.

Wakitani

Tensho

Sato

and Tsuchiya

, Direct evaluation of fatigue property of ultra-high molecular weight polyethylene components of retrieved knee implants using small specimens, Strength, Fracture and Complexity 6 (2010), 103–114.

19.

Sakoda

Kawakami

Haishima

Tensho

Wakitani

and Matsuoka

, Effects of lipid extraction from retrieved components on the analysis of UHMWPE by FTIR, in: 56th Annual Meeting, Vol. 56, Orthopaedic Research Society, 2010, p. 2292.

20.

Slouf

Synkova

Baldrian

Marek

Kovarova

Schmidt

Dorschner

Stephan

and Gohs

, Structural changes of UHMWPE after e-beam irradiation and thermal treatment, J Biomed Mater Res B Appl Biomater 85(1) (2008), 240. doi:10.1002/jbm.b.30942.

21.

Sakoda

Kawakami

Haishima

Tensho

Wakitani

and Matsuoka

, Quantitative analysis of lipids extracted from retrieved UHMWPE knee component by organic solvents, in: 57th Annual Meeting, Vol. 57, Orthopaedic Research Society, 2011, p. 1246.

A novel method to eliminate the influence of absorbed lipids on the characterization of ultra-high molecular weight polyethylene using Fourier-transform infrared spectroscopy

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Materials and methods

2.1. Materials

2.3. Lipid index calculation

2.4. Lipid extraction

3. Results

3.1. Basic characteristics of specimens

Table 3 Boiling point of typical organic solvents used for lipid extraction Boiling point [°C] Hexane [11] 69 Cyclohexane [12] 81 Heptane [11] 98

Footnotes

Conflict of interest

References

Table 3
Boiling point of typical organic solvents used for lipid extraction

Boiling point [°C]

Hexane [11] 69

Cyclohexane [12] 81

Heptane [11] 98