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Micro-Raman spectroscopy for the crystallinity characterization of UHMWPE hip cups run on joint simulators☆
MOLSTR 11220
Journal of Molecular Structure 521 (2000) 89–95 www.elsevier.nl/locate/molstruc
Micro-Raman spectroscopy for the crystallinity characterization of UHMWPE hip cups run on joint simulators 夽 A. Bertoluzza a, C. Fagnano a, M. Rossi a, A. Tinti a,*, G.L. Cacciari b a
Sezione di Chimica e Propedeutica Biochimica, Dipartimento di Biochimica “G. Moruzzi”, Centro di Studio sulla Spettroscopia Raman, Universita` di Bologna, 40126 Bologna, Italy b Officine Ortopediche Rizzoli S.p.A., Bologna, Italy Received 5 April 1999; accepted 21 June 1999
Abstract In this work Raman microspectrometry was used to evaluate the crystallinity changes of hip cups made of ultra-high molecular weight polyethylene (UHMWPE). In vitro experiments were carried out using hip joint movement-wear simulators, run for five million cycles in water or bovine calf serum. The hip cups were subjected to mechanical friction with ceramic femoral heads (alumina–zirconia blend). The crystallinity of the polymer hip cups was studied as a function of inner surface position and thickness, from the stressed surface to the unstressed outer one. The Partial Least Square (PLS-l) calibration was used to correlate the Raman spectra with the crystallinity of the polymer measured with DSC calorimetry. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: UHMWPE hip cups; Polyethylene crystallinity; Hip joint simulator; Raman spectroscopy; Multivariate PLS calibration
1. Introduction UHMWPE has been used as hip cup material for about 30 years, giving satisfactory clinical results which improved as the fixation and prosthesis design methods were refined. As a matter of fact, this polymeric material, being rather inert, has passed a lot of tests regarding chemical biocompatibility. Today, unfortunately, the major cause of failure for loaded prostheses, like those of the hip joint, is due to the cellular events induced by the polyethylene debris coming from the bearing surfaces [1,2]. This is, indeed, why the future use of this material is inexor夽 In honour of Professor Giuseppe Zerbi on the occasion of his 65th birthday. * Corresponding author. Tel.: ⫹39-051-354280/1; fax: ⫹39-051243119.
ably linked to the study and discussion of its wear properties [3]. The polymer crystallinity, especially in the superficial regions, is related to the production of debris; in fact the brittleness and the fracture toughness mostly depend on the morphological situation of the material, particularly when the crystallinity is modified by the mechanical action, and the polymer chains are induced to reorganize in crystalline domains which are more extended and flaky [4,5]. From this point of view, it is important to have a tool to monitor the crystallinity changes during the in vitro experiments, in order to evaluate the degradation and ageing of polymer prosthesis. The “ideal tool” should be non-destructive and able to evaluate in situ with sufficient accuracy, the degree of crystallinity during the wear tests, as for example between the periodic lubricant changes. The prince technique for estimating crystallinity of polymeric materials is
0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00427-5
90
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
Fig. 1. UHMWPE hip cup and its subdivision in sectors.
the differential scanning calorimetry (DSC) [6]. Unfortunately, this method does not permit the subsequent analysis of the same sample during the test run. The vibrational spectroscopy is a technique that does not compromise the sample but allows obtaining the morphology of the polyethylene only with long and complex spectral analysis [7,8]. It has been clear for a long time that the Raman spectra are related to the crystallinity [7–9]. Moreover, it has been shown that it is possible to obtain this kind of information from the multivariate regression. For
Fig. 2. PRESS as a function of the number of factors.
example the density of polyethylene has been correlated to Raman spectra using the Partial Least Square regression, obtaining very good results [10]. In this work, we are attempting to suggest a non-destructive Raman method to evaluate the morphology of this material and then to permit the design of in vitro wear experiments that have the control of the surface crystallinity, during the mechanical tests, without sacrificing the samples.
2. Experimental Micro-Raman spectra were obtained using a Jasco NRS-2000C microscope at 20 × of magnitude. All the spectra were recorded in backscattering conditions with 5 cm ⫺1 spectral resolution and ca 20 mW power of the noble gas laser line at 488 nm (Innova Coherent 70). The detector was a 160 K frozen CCD from Pinceton Instruments Inc. Polyethylene samples used for the calibration were 12 nascent powders with different synthesis conditions and with degree of crystallinity spanning from 63 to 77%. The DSC analyses of the powders were carried out with a Mettler Toledo TA-STAR Model 821 e, using 5⬚C/ min as the scanning rate. The crystallinity was obtained giving the value of 69.2 cal/g to the fusion enthalpy for a totally crystalline sample. The hip cups were sterilized in ethylene oxide and worn against ceramic femoral heads. Two hip-joint movement simulators were used. In the first simulator the prostheses were subjected to the “Paul load curve by walking” (1967) with a load peak of 4000 N. The interstitial lubricant was distilled water. The second simulator was a sinusoidal one (maximum load 2000 N), and the liquid used was bovine calf serum. For both simulators the lubricant was changed every 5 × 10 5 cycles [11]. Even though Raman spectroscopy permits non-destructive analysis, the cups were cut in order to evaluate the differences of crystallinity between the surface and the depth. The 90⬚ angle of a quarter of the cup was ideally divided into nine parts, each 10⬚ wide, and indexed from 1 to 9 starting from the vertex (see Fig. 1). The Raman spectra were collected from the first to the eighth sector, either on the inner surface or on the cut surface for different depth values. The cuts were performed without warming the samples and considering the original
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
91
PRESS (Predicted Residual Error Sum of Squares), as a function of the number of factors [12]. 3. Results and discussion
Fig. 3. Cross-validated calibration plot for crystallinity. A onefactor model gave this calibration.
clamping of the cups into the holders, to assure the analysis of the same region for every cup. The ninth sector was not examined because it was very distant from the maximum load position. The software used for the PLS-Type 1 analysis was the commercial Grams 386 with the Add-On PLSplus/IQ V.3.0, from Galactic Ind. All the spectra were baseline corrected and subtracted, normalized to unit area and mean centered as pretreatment to the calibration. The calibration was fully cross-validated using the F-test significative minimization of the Table 1 Average crystallinity of the surface for reference cups Cup A Cup B Cup C
65 ^ 1% 65 ^ 2% 63 ^ 1%
In Fig. 2 the PRESS as a function of the number of factors is reported. Fig. 3 shows the regression of the predicted versus the actual crystallinity. The R 2 of the calibration curve is 0.925. The Standard Error of Prediction (SEP) for one factor is 1.1%. The minimum PRESS is 15.02 and is reached with only one factor. The model is therefore based on the first PLS eigenvector. A test has been carried out to verify the robustness of the calibration: various “blind” samples were used to check the predictive powers of the model, the crystallinity of which has been measured with Raman spectroscopy and with DSC for comparison. We verified that the error in predicting the “unknown” samples was ca twice the SEP. To control the degree of morphological homogeneity of the virgin cups, two of them were examined in ten different random points on the internal hollow surface and once on the external convex surface (cups A and B). In the same way we also tested another cup sterilized with ethylene oxide (cup C). For every cup the crystallinity measures are reported in Table 1. The degree of debris production of the worn cups was measured considering the weight loss of the cups themselves [11], according to standard ISO 9326. 3.1. Specimens immersed in distilled water Different experimental conditions were performed in order to create changes in friction forces and wear debris. Lack of lubricant for short times was used to obtain cups with major wear damage (W1 and W2). The cup W3 was instead normally worn. In Figs. 4 and 5 the Raman spectra of the W3 cup, which had the smaller weight loss, and of the W1 cup, which had the larger weight loss, are reported. The figures show the comparison of the spectra of the surface and the depth for each cup. For the W3 cup the spectra are almost identical, differing only for small variations of some band intensities. On the contrary, the surface spectrum of the W1 cup, compared to the inner one, shows a strong decrease in the 1080 cm ⫺1 band and in the 1303 cm ⫺1 shoulder, both characteristic of the gauche
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Fig. 4. Raman spectra of PE W3 cup: (–-) surface spectrum and (–) spectrum at 3 mm of depth. The spectra are those taken at the fifth sector.
conformation of the polymer (associated with the amorphous phase). Moreover, the two spectra of W1 have different band intensities, particularly for the bands at 1416 and 1131 cm ⫺1, which are related to the crystallinity of the sample [7]. Even from a qualitative spectral analysis, based on these marker bands
of crystallinity, it is possible to see that a difference in morphology between the surface and the depth of W1 with respect to W3 has arisen. Fig. 6a–c shows the crystallinity values obtained by PLS-1 calibration for W1, W2 and W3 cups, which were worn in water: the figure shows the values
Fig. 5. Raman spectra of PE W1 cup: (–-) surface spectrum and (–) spectrum at 3 mm of depth. The spectra are those taken at the fifth sector.
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
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Both W1 and W2 cups show a marked increase in the surface crystallinity in comparison with the internal ones. Both cups have an average surface crystallinity higher than 75%, about 10% higher than the internal values, and however undoubtedly higher than the measurement error. It can be seen that the values of internal crystallinity is not far from those obtained for the reference cups. Two measurements of crystallinity were taken at the eighth sector of the cup W2: the first on the external edge and the second in a rather evident crack. The crystallinity in the crack was higher than that on the corresponding edge. As Fig. 6c shows, the cup W3 shows different behavior from the other two. These results are in agreement with the wear undergone by the cups: in fact W3 has irrelevant weight loss, while the other two cups underwent considerable wear. The results show that high weight loss, and therefore considerable wear, corresponds to a noticeable increase in surface crystallinity.
3.2. Specimens immersed in filter-sterilized bovine calf serum
Fig. 6. Crystallinity values obtained by PLS-1 analysis for: (a) W1; (b) W2 and (c) W3 cups, worn in water, at the surface and at various depths.
determined for the eight examined sectors, both at the prostheses surface and at various depths. The cups are ordered from the most worn (Fig. 6a) to the least worn (Fig. 6c), in accordance with weight loss.
In this instance there was no lack of lubricant. It is important to note that the friction coefficient is correlated to the lubricant used in the in vitro test. The bovine serum, which is the best sinovial fluid simulator, sensibly reduces the friction in comparison to water [13,14]. Fig. 7a and b shows the crystallinity values obtained for the two cups (BS1 and BS2), which were worn in bovine serum, at the prostheses surface and at various depths, as for the eight examined sectors. In this case, the crystallinity changes, between surface and depth, are moderate. Moreover, the loss of weight caused by wear, was also of low intensity. In addition it seems that in very low wear conditions the surface is sligthly less crystalline than the subsurface regions for the first sectors. On the contrary, for the last sectors where the load is minimum, the crystallinity of the surface is quite similar to the crystallinity of the depth. This should be seen both for the cup W3 (in water, normal condition) and for the two cups run in the bovine serum hip joint simulator. It is therefore
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A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
shown by the amount of debris which increases as the surface crystallinity of the cup increases. The debris remaining in the surrounding tissues can be phagocytized and consequently give rise to the formation of granulomatous lesions, osteolysis and bone resorption [15]. Therefore, the research of new UHMWPE acetabular cups with better wear resistance, is an important goal in the prosthetic field. In order to check the morphology changes while the in vitro wear experiments are carried out, a non-destructive and non-invasive structural analysis is needed. Raman microspectroscopy, as used in this work, is able to evaluate the surface changes of the acetabular cups undergoing in vitro mechanical strain. For this experiment to be really non-destructive, it is possible to use an optical fiber probe to automatically detect the difference in crystallinity along the inner surface of the cup, at different times, during the breaks of the mechanical tests. The probe could be mounted on a curve line, to lay in the cup and could be driven by the computer. In this case the mapping of the inner surface crystallinity should be made automatically and this information could be useful for the further study and design of the prosthesis. We are currently carrying out studies of this kind.
Acknowledgements Fig. 7. Crystallinity values obtained by PLS-1 analysis for the: (a) BS1 and (b) BS2 cups, worn in bovine serum, at the surface and at various depths.
possible to say that least worn cups show the least crystallinity changes.
4. Conclusions The PLS-1 calibration was used to correlate the Raman spectra to the crystallinity of the polymer obtained by DSC measurements. The results show that the changes in surface crystallinity of UHMWPE acetabular cups is caused by the mechanical friction during the in vitro tests. The increase in surface crystallinity is probably due to a breakdown in the polymer chains, together with an increase in the macromolecules mobility and a lower crack and wear resistance. This fact is also
We devote this article to the memory of our scientific guide, Professor Alessandro Bertoluzza, who has passed away before the publication. The authors are very grateful to Dott. Stefano Ottani (Centro Studio Fisica Macromolecole del CNR, Bologna, Italy) for providing the powder samples which made this work possible. A part of the above mentioned wear tests were performed by Dr S. Affatato in the Laboratorio di Tecnologia dei Materiali of the Istituto Ortopedico Rizzoli directed by Dr A. Toni. Research supported by financial aid of the University of Bologna (funds for selected research topics) and Progetto Eureka EU 294 Biomaterials.
References [1] K. Hirakawa, T.W. Bauer, B.N. Stulberg, A.H. Wilde, J. Biomed. Mater. Res. 31 (1996) 257.
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95 [2] G. Lewis, J. Biomed. Mater. Res. 38 (1997) 55 and references therein. [3] L. Pruitt, L. Bailey, Polymer 39 (1998) 1545. [4] P. Eyerer, M. Kurt, A.H. McKellup, T. Mittlmeier, J. Biomed. Mater. Res. 21 (1987) 275. [5] P. Eyerer, Y.C. Ke, J. Biomed. Mater. Res. 18 (1984) 1137. [6] H. Witkiewicz, M. Deng, T. Vidovszky, M.E. Bolander, M.G. Rock, B.F. Morrey, S.W. Shalaby, J. Biomed. Mater. Res. 33 (1996) 73. [7] G.R. Strobl, W. Hagedorn, J. Polym. Sci.: Polym. Phys. Ed. 16 (1978) 1181. [8] F. Rull, A.C. Prieto, J.M. Casado, F. Sobron, H.G.M. Edwards, J. Raman Spectrosc. 24 (1993) 545.
95
[9] D.V. Luu, L. Cambon, C. Lapeyre, J. Raman Spectrosc. 9 (1980) 172. [10] K.P.J. Williams, N.J. Everall, J. Raman Spectrosc. 26 (1995) 427. [11] S. Affatato, M. Testoni, G.L. Cacciari, A. Toni, Biomaterials 20 (1999) 971. [12] P. Geladi, B.R. Kowalski, Anal. Chim. Acta 185 (1986) 1. [13] H. McKellop, I.C. Clarke, K.L. Markolf, H.C. Amstutz, J. Biomed. Mater. Res. 12 (1978) 895. [14] A. Wang, A. Essner, C. Stark, J.H. Dumbleton, Biomaterials 17 (1996) 865. [15] W.J. Maloney, R.L. Smith, Instr. Course Lect. 45 (1996) 171.
Journal of Molecular Structure 521 (2000) 89–95 www.elsevier.nl/locate/molstruc
Micro-Raman spectroscopy for the crystallinity characterization of UHMWPE hip cups run on joint simulators 夽 A. Bertoluzza a, C. Fagnano a, M. Rossi a, A. Tinti a,*, G.L. Cacciari b a
Sezione di Chimica e Propedeutica Biochimica, Dipartimento di Biochimica “G. Moruzzi”, Centro di Studio sulla Spettroscopia Raman, Universita` di Bologna, 40126 Bologna, Italy b Officine Ortopediche Rizzoli S.p.A., Bologna, Italy Received 5 April 1999; accepted 21 June 1999
Abstract In this work Raman microspectrometry was used to evaluate the crystallinity changes of hip cups made of ultra-high molecular weight polyethylene (UHMWPE). In vitro experiments were carried out using hip joint movement-wear simulators, run for five million cycles in water or bovine calf serum. The hip cups were subjected to mechanical friction with ceramic femoral heads (alumina–zirconia blend). The crystallinity of the polymer hip cups was studied as a function of inner surface position and thickness, from the stressed surface to the unstressed outer one. The Partial Least Square (PLS-l) calibration was used to correlate the Raman spectra with the crystallinity of the polymer measured with DSC calorimetry. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: UHMWPE hip cups; Polyethylene crystallinity; Hip joint simulator; Raman spectroscopy; Multivariate PLS calibration
1. Introduction UHMWPE has been used as hip cup material for about 30 years, giving satisfactory clinical results which improved as the fixation and prosthesis design methods were refined. As a matter of fact, this polymeric material, being rather inert, has passed a lot of tests regarding chemical biocompatibility. Today, unfortunately, the major cause of failure for loaded prostheses, like those of the hip joint, is due to the cellular events induced by the polyethylene debris coming from the bearing surfaces [1,2]. This is, indeed, why the future use of this material is inexor夽 In honour of Professor Giuseppe Zerbi on the occasion of his 65th birthday. * Corresponding author. Tel.: ⫹39-051-354280/1; fax: ⫹39-051243119.
ably linked to the study and discussion of its wear properties [3]. The polymer crystallinity, especially in the superficial regions, is related to the production of debris; in fact the brittleness and the fracture toughness mostly depend on the morphological situation of the material, particularly when the crystallinity is modified by the mechanical action, and the polymer chains are induced to reorganize in crystalline domains which are more extended and flaky [4,5]. From this point of view, it is important to have a tool to monitor the crystallinity changes during the in vitro experiments, in order to evaluate the degradation and ageing of polymer prosthesis. The “ideal tool” should be non-destructive and able to evaluate in situ with sufficient accuracy, the degree of crystallinity during the wear tests, as for example between the periodic lubricant changes. The prince technique for estimating crystallinity of polymeric materials is
0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00427-5
90
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
Fig. 1. UHMWPE hip cup and its subdivision in sectors.
the differential scanning calorimetry (DSC) [6]. Unfortunately, this method does not permit the subsequent analysis of the same sample during the test run. The vibrational spectroscopy is a technique that does not compromise the sample but allows obtaining the morphology of the polyethylene only with long and complex spectral analysis [7,8]. It has been clear for a long time that the Raman spectra are related to the crystallinity [7–9]. Moreover, it has been shown that it is possible to obtain this kind of information from the multivariate regression. For
Fig. 2. PRESS as a function of the number of factors.
example the density of polyethylene has been correlated to Raman spectra using the Partial Least Square regression, obtaining very good results [10]. In this work, we are attempting to suggest a non-destructive Raman method to evaluate the morphology of this material and then to permit the design of in vitro wear experiments that have the control of the surface crystallinity, during the mechanical tests, without sacrificing the samples.
2. Experimental Micro-Raman spectra were obtained using a Jasco NRS-2000C microscope at 20 × of magnitude. All the spectra were recorded in backscattering conditions with 5 cm ⫺1 spectral resolution and ca 20 mW power of the noble gas laser line at 488 nm (Innova Coherent 70). The detector was a 160 K frozen CCD from Pinceton Instruments Inc. Polyethylene samples used for the calibration were 12 nascent powders with different synthesis conditions and with degree of crystallinity spanning from 63 to 77%. The DSC analyses of the powders were carried out with a Mettler Toledo TA-STAR Model 821 e, using 5⬚C/ min as the scanning rate. The crystallinity was obtained giving the value of 69.2 cal/g to the fusion enthalpy for a totally crystalline sample. The hip cups were sterilized in ethylene oxide and worn against ceramic femoral heads. Two hip-joint movement simulators were used. In the first simulator the prostheses were subjected to the “Paul load curve by walking” (1967) with a load peak of 4000 N. The interstitial lubricant was distilled water. The second simulator was a sinusoidal one (maximum load 2000 N), and the liquid used was bovine calf serum. For both simulators the lubricant was changed every 5 × 10 5 cycles [11]. Even though Raman spectroscopy permits non-destructive analysis, the cups were cut in order to evaluate the differences of crystallinity between the surface and the depth. The 90⬚ angle of a quarter of the cup was ideally divided into nine parts, each 10⬚ wide, and indexed from 1 to 9 starting from the vertex (see Fig. 1). The Raman spectra were collected from the first to the eighth sector, either on the inner surface or on the cut surface for different depth values. The cuts were performed without warming the samples and considering the original
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
91
PRESS (Predicted Residual Error Sum of Squares), as a function of the number of factors [12]. 3. Results and discussion
Fig. 3. Cross-validated calibration plot for crystallinity. A onefactor model gave this calibration.
clamping of the cups into the holders, to assure the analysis of the same region for every cup. The ninth sector was not examined because it was very distant from the maximum load position. The software used for the PLS-Type 1 analysis was the commercial Grams 386 with the Add-On PLSplus/IQ V.3.0, from Galactic Ind. All the spectra were baseline corrected and subtracted, normalized to unit area and mean centered as pretreatment to the calibration. The calibration was fully cross-validated using the F-test significative minimization of the Table 1 Average crystallinity of the surface for reference cups Cup A Cup B Cup C
65 ^ 1% 65 ^ 2% 63 ^ 1%
In Fig. 2 the PRESS as a function of the number of factors is reported. Fig. 3 shows the regression of the predicted versus the actual crystallinity. The R 2 of the calibration curve is 0.925. The Standard Error of Prediction (SEP) for one factor is 1.1%. The minimum PRESS is 15.02 and is reached with only one factor. The model is therefore based on the first PLS eigenvector. A test has been carried out to verify the robustness of the calibration: various “blind” samples were used to check the predictive powers of the model, the crystallinity of which has been measured with Raman spectroscopy and with DSC for comparison. We verified that the error in predicting the “unknown” samples was ca twice the SEP. To control the degree of morphological homogeneity of the virgin cups, two of them were examined in ten different random points on the internal hollow surface and once on the external convex surface (cups A and B). In the same way we also tested another cup sterilized with ethylene oxide (cup C). For every cup the crystallinity measures are reported in Table 1. The degree of debris production of the worn cups was measured considering the weight loss of the cups themselves [11], according to standard ISO 9326. 3.1. Specimens immersed in distilled water Different experimental conditions were performed in order to create changes in friction forces and wear debris. Lack of lubricant for short times was used to obtain cups with major wear damage (W1 and W2). The cup W3 was instead normally worn. In Figs. 4 and 5 the Raman spectra of the W3 cup, which had the smaller weight loss, and of the W1 cup, which had the larger weight loss, are reported. The figures show the comparison of the spectra of the surface and the depth for each cup. For the W3 cup the spectra are almost identical, differing only for small variations of some band intensities. On the contrary, the surface spectrum of the W1 cup, compared to the inner one, shows a strong decrease in the 1080 cm ⫺1 band and in the 1303 cm ⫺1 shoulder, both characteristic of the gauche
92
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
Fig. 4. Raman spectra of PE W3 cup: (–-) surface spectrum and (–) spectrum at 3 mm of depth. The spectra are those taken at the fifth sector.
conformation of the polymer (associated with the amorphous phase). Moreover, the two spectra of W1 have different band intensities, particularly for the bands at 1416 and 1131 cm ⫺1, which are related to the crystallinity of the sample [7]. Even from a qualitative spectral analysis, based on these marker bands
of crystallinity, it is possible to see that a difference in morphology between the surface and the depth of W1 with respect to W3 has arisen. Fig. 6a–c shows the crystallinity values obtained by PLS-1 calibration for W1, W2 and W3 cups, which were worn in water: the figure shows the values
Fig. 5. Raman spectra of PE W1 cup: (–-) surface spectrum and (–) spectrum at 3 mm of depth. The spectra are those taken at the fifth sector.
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
93
Both W1 and W2 cups show a marked increase in the surface crystallinity in comparison with the internal ones. Both cups have an average surface crystallinity higher than 75%, about 10% higher than the internal values, and however undoubtedly higher than the measurement error. It can be seen that the values of internal crystallinity is not far from those obtained for the reference cups. Two measurements of crystallinity were taken at the eighth sector of the cup W2: the first on the external edge and the second in a rather evident crack. The crystallinity in the crack was higher than that on the corresponding edge. As Fig. 6c shows, the cup W3 shows different behavior from the other two. These results are in agreement with the wear undergone by the cups: in fact W3 has irrelevant weight loss, while the other two cups underwent considerable wear. The results show that high weight loss, and therefore considerable wear, corresponds to a noticeable increase in surface crystallinity.
3.2. Specimens immersed in filter-sterilized bovine calf serum
Fig. 6. Crystallinity values obtained by PLS-1 analysis for: (a) W1; (b) W2 and (c) W3 cups, worn in water, at the surface and at various depths.
determined for the eight examined sectors, both at the prostheses surface and at various depths. The cups are ordered from the most worn (Fig. 6a) to the least worn (Fig. 6c), in accordance with weight loss.
In this instance there was no lack of lubricant. It is important to note that the friction coefficient is correlated to the lubricant used in the in vitro test. The bovine serum, which is the best sinovial fluid simulator, sensibly reduces the friction in comparison to water [13,14]. Fig. 7a and b shows the crystallinity values obtained for the two cups (BS1 and BS2), which were worn in bovine serum, at the prostheses surface and at various depths, as for the eight examined sectors. In this case, the crystallinity changes, between surface and depth, are moderate. Moreover, the loss of weight caused by wear, was also of low intensity. In addition it seems that in very low wear conditions the surface is sligthly less crystalline than the subsurface regions for the first sectors. On the contrary, for the last sectors where the load is minimum, the crystallinity of the surface is quite similar to the crystallinity of the depth. This should be seen both for the cup W3 (in water, normal condition) and for the two cups run in the bovine serum hip joint simulator. It is therefore
94
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95
shown by the amount of debris which increases as the surface crystallinity of the cup increases. The debris remaining in the surrounding tissues can be phagocytized and consequently give rise to the formation of granulomatous lesions, osteolysis and bone resorption [15]. Therefore, the research of new UHMWPE acetabular cups with better wear resistance, is an important goal in the prosthetic field. In order to check the morphology changes while the in vitro wear experiments are carried out, a non-destructive and non-invasive structural analysis is needed. Raman microspectroscopy, as used in this work, is able to evaluate the surface changes of the acetabular cups undergoing in vitro mechanical strain. For this experiment to be really non-destructive, it is possible to use an optical fiber probe to automatically detect the difference in crystallinity along the inner surface of the cup, at different times, during the breaks of the mechanical tests. The probe could be mounted on a curve line, to lay in the cup and could be driven by the computer. In this case the mapping of the inner surface crystallinity should be made automatically and this information could be useful for the further study and design of the prosthesis. We are currently carrying out studies of this kind.
Acknowledgements Fig. 7. Crystallinity values obtained by PLS-1 analysis for the: (a) BS1 and (b) BS2 cups, worn in bovine serum, at the surface and at various depths.
possible to say that least worn cups show the least crystallinity changes.
4. Conclusions The PLS-1 calibration was used to correlate the Raman spectra to the crystallinity of the polymer obtained by DSC measurements. The results show that the changes in surface crystallinity of UHMWPE acetabular cups is caused by the mechanical friction during the in vitro tests. The increase in surface crystallinity is probably due to a breakdown in the polymer chains, together with an increase in the macromolecules mobility and a lower crack and wear resistance. This fact is also
We devote this article to the memory of our scientific guide, Professor Alessandro Bertoluzza, who has passed away before the publication. The authors are very grateful to Dott. Stefano Ottani (Centro Studio Fisica Macromolecole del CNR, Bologna, Italy) for providing the powder samples which made this work possible. A part of the above mentioned wear tests were performed by Dr S. Affatato in the Laboratorio di Tecnologia dei Materiali of the Istituto Ortopedico Rizzoli directed by Dr A. Toni. Research supported by financial aid of the University of Bologna (funds for selected research topics) and Progetto Eureka EU 294 Biomaterials.
References [1] K. Hirakawa, T.W. Bauer, B.N. Stulberg, A.H. Wilde, J. Biomed. Mater. Res. 31 (1996) 257.
A. Bertoluzza et al. / Journal of Molecular Structure 521 (2000) 89–95 [2] G. Lewis, J. Biomed. Mater. Res. 38 (1997) 55 and references therein. [3] L. Pruitt, L. Bailey, Polymer 39 (1998) 1545. [4] P. Eyerer, M. Kurt, A.H. McKellup, T. Mittlmeier, J. Biomed. Mater. Res. 21 (1987) 275. [5] P. Eyerer, Y.C. Ke, J. Biomed. Mater. Res. 18 (1984) 1137. [6] H. Witkiewicz, M. Deng, T. Vidovszky, M.E. Bolander, M.G. Rock, B.F. Morrey, S.W. Shalaby, J. Biomed. Mater. Res. 33 (1996) 73. [7] G.R. Strobl, W. Hagedorn, J. Polym. Sci.: Polym. Phys. Ed. 16 (1978) 1181. [8] F. Rull, A.C. Prieto, J.M. Casado, F. Sobron, H.G.M. Edwards, J. Raman Spectrosc. 24 (1993) 545.
95
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