Key Engineering Materials Vols. 309-311 (2006) pp 1257-1260 Online: 2006-05-15 (2006) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/kem.309-311.1257 Mechanical Properties of Hip Joint Heads Made of the Dispersion Ceramic - Alumina Toughened Zirconia Sabine Begand, Thomas Oberbach, Wilfried Glien, Mathys Orthopaedie GmbH, An den Trillers Büschen 2, 07646 Mörsdorf/Germany sabine.begand@mathysmedical.com, thomas.oberbach@mathysmedical.com, wilfried.glien@mathysmedical.com Keywords: dispersion ceramic, fatigue test, wear, ring-on-disc Abstract. For orthopaedic applications an Alumina Toughened Zirconia Ceramic was tested regarding its mechanical behaviour. We observed a marked higher strength compared to the requirements according to FDA guidelines and a significantly higher strength compared to alumina ceramic despite of 15 % decreased fatigue burst strength. The wear resistance after ring-on-disc method is significantly improved comparing to standard Al 2 O 3 /Al 2 O 3 pairings. It means the application of endoprosthetic components made of ATZ-ceramic offers more safety and reliability. Introduction Aluminium oxide ceramic has been proved successfully in joint endoprosthetics. In order to meet the increasing requirements of materials the use of innovative materials is essential. The expectation of the patients has changed to higher mobility for a long term and therefore new materials with adapted properties are necessary. For orthopaedic applications an Alumina Toughened Zirconia Ceramic was tested regarding its mechanical behaviour. The so-called ATZ-Ceramic consists of 80% ZrO 2 (stabilized with Y 2 O 3 ) and 20% Al 2 O 3. This paper describes the static and dynamical characteristics measured on hip joint heads and discs. Materials and Methods According to norm ISO 6474 the static strength (biaxial bending strength) was measured. Samples were made from ATZ-Ceramic and reference samples were made from Al 2 O 3 whose density has values of >99.5%TD. With hip joint heads a fatigue test based on ISO 7206-10 and FDA-guidance was carried out with following conditions: A number of 10 heads of each neck length was cyclic loaded with an upper force of 14 kn (frequency 10 Hz). The test was executed under ambient temperature in ringer solution. Upper Load: Minimal Load: Frequency: Duration: Medium: 14 kn 0.5 kn 10 Hz 10 Mio Ringer solution Fig. 1. Schematic fixture of the pulsation system The femoral head is loaded through a conical bore. After the dynamical loading the remaining static load was measured. The test was carried out with 3 different neck lengths, each group with a number of 10 samples. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 153.96.53.5, Fraunhofer-Gesellschaft - FhG, Muenchen, Germany-23/12/15,14:21:06)
1258 Bioceramics 18 Wear measurements (ring-on-disc-method) were applied for the evaluation of material fatigue. Under the standardized testing conditions the three pairings Al 2 O 3 /Al 2 O 3, Al 2 O 3 /ATZ and ATZ/ATZ were tested. The surfaces were polished like femoral heads or cups. The abrasion volume was determined gravimetrically. Rotation angle: +25 /-25 Axial Load: 1500 N Frequency: 1 Hz Test Time: 200 h Temperature: 37 C Fluid Test Medium: Serum Fig. 2. Condition and ring/disc body structure of the specimen In comparison to the ISO norm we upgraded the conditions in order to increase the force of expression: a) extension of the duration up to 200 h b) modification of sample and holder geometry to improve the resistance against rotation c) determination of the wear volume by means of weighting d) increased number of n=5 pairings per group We interrupted the test each 50 h for weighting and to inspect and analyse the surface. All tests were performed together with the company ENDOLAB GMBH in Germany. With XRD-method we measured the phase composition quantitatively by means of Rietfeldmethod. Results The static biaxial flexural strength was investigated for 30 test discs and a median flexural strength of 1160 MPa was calculated with a Weibul-modulus >10. The static strength of hip joint heads with a diameter of 28 mm was determined for three neck length S, M and L. Before testing we checked the sphericity and roughness of the samples. All heads owned optimal sphericity < 1.0 µm and roughness R a = 0.003 µm on the surface. That means the samples met the requirements of the FDA guidelines. On a titanium-alloy taper we reached for instance a static burst strength of 139 kn for 28 M. In comparison to standard material we achieved a increasing of static strength of about 70 %. Fracture Load [kn] 200 160 120 80 40 static strength Median value according FDA dynamic strength after 10 Mill. cycles 0 28 L 28 M 28 S Fig. 3. Static strength of hip joint heads made of ATZ in comparison to alumina heads
Key Engineering Materials Vols. 309-311 1259 After fatigue of the other samples tests we investigated the remaining static strength. For the heads 28 M we reached a duration strength of 116 kn. The decrease of the strength after 10 million cycles is 15 % in average, which is comparable to this of alumina heads. The static strength for alumina heads of the same type before fatigue test is 84 kn and after duration we determined a strength of 70 kn, which corresponds to a reduction of approximately 18 %. Next to the dynamical investigations we applied the ring-on-disc-method to appreciate the tribological behaviour. Due to changing the fixation of specimen it was possible to utilize beside the discs the behaviour of the rings too. The measured weight loss is plotted in Fig. 4. 2,00 1,50 ATZdisc ATZdisc Al2O3disc ATZ ring Al2O3ring Al2O3ring Weight loss [mg] 1,00 0,50 0,00 Fig. 4. Cumulative amount of wear after ring-on-disc experiments After 200 h in bovine serum the following average wear volumes were measured: Al 2 O 3 /ATZ ATZ/ATZ Al 2 O 3 /Al 2 O 3 0 50 100 150 200 250 Duration [h] 0.054 mm³ 0.152 mm³ 0.157 mm³ These wear tests gave the following results: Exceptional the Al 2 O 3 rings of couple Al 2 O 3 /Al 2 O 3 we observed a linear increasing of wear amount. The measured weight loss for all three groups was slightly increased for the rings compared to the discs. The wear volume for both own pairings had nearly the same value. In this case the mixed pairing Al 2 O 3 -ring against ATZ-disc showed the best wear resistance. The calculated wear rate was 0.07 *10-7 mm³/nm, which was significantly improved compared to published values of approximately 0.2 *10-7 mm³/nm for Al 2 O 3 /Al 2 O 3 pairings [1,2,3]. The extension of duration showed, that even after 200h no catastrophic failure occurred. Although there is a high local mechanical stress at the edge of contact zone no initial worsening of sliding behaviour could be realized.
1260 Bioceramics 18 Additionally we inspected the surface with XRD in order to control the phase composition. Results are presented in Figure 5. 6 Monoclinic content [wt-%] 5 4 3 2 1 0 ATZ disc (ATZ ring) ATZ disc ( Al2O3 ring) initial content 0 50 100 150 200 250 Duration [h] X-ray focus inside of the wear track Fig. 5. Relation between amount of monoclinic-zro 2 and testing time during ring-on-disc-test (Calculation of monoclinic content related to 100 % Y-TZP) The phase transformation of cubic and tetragonal ZrO 2 to monoclinic phase happened in a low scale. The Rietfeld-quantification results in 4.3 4.8 % monoclinic ZrO 2, which is conform to the ASTM F 1873-98 where the monoclinic content should be < 5 %. This fact correlates to the measured wear volume; only a small amount of wear particles was generated. We detected no evidence of starting catastrophic transformation tetragonal/monoclinic in consequence of low temperature degradation. Discussion and Conclusion The static strength of ATZ-Ceramic showed a marked dependence on technological conditions. After optimizing the sintering parameters strengths of >1100 MPa were measured for the dispersion-ceramic which thus showed a three times increased strength compared to Al 2 O 3 -ceramic and a 1.5 times increased strength compared to ZrO 2 -ceramic. It can be summarized that the dispersion-ceramic possesses a markedly higher bending strength compared to Al 2 O 3 with nearly the same wear behaviour. Despite of 15 % decreased fatigue burst strength, we observed a marked higher value compared to the required strength according to FDA guidelines and a significantly higher strength compared to alumina ceramic. In consideration of these results the application of endoprosthetic components made of ATZ-ceramic offers more safety and reliability. References: [1] Früh, H.J., Willmann, G., Biomaterials 18 (1997) 873-876 [2] Murakami, T., Sawae, Y., 7 th World Biomaterial Congress 2004 [3] Kaddick, C., Pfaff, H.G., Reliability and Long-term Results of Ceramics in Orthopaedics (1999) 96-101 [4] ISO 7206-10: Implants for surgery partial and total hip-joint prostheses [5] FDA: Guidance document for the preparation of premarket notifications for ceramic ball hip system. (1995)
Bioceramics 18 10.4028/www.scientific.net/KEM.309-311 Mechanical Properties of Hip Joint Heads Made of the Dispersion Ceramic - Alumina Toughened Zirconia 10.4028/www.scientific.net/KEM.309-311.1257