Process for the preparation of a sterilized ceramic body comprising or essentially consisting of stabilized zirconia of a defined colour

Abstract

A process for the preparation of a sterilized ceramic body including or essentially consisting of stabilized zirconia of a defined colour, including the steps of: providing a ceramic primary body including or essentially consisting of stabilized zirconia of a first colour A, and sterilizing the primary body using radiation sterilization whereby the primary body undergoes a colour change to a colour B. The process includes the further step of irradiating the sterilized primary body with electromagnetic radiation of at least one wavelength lying in the wavelength band ranging from 150 nm to 700 nm to induce an at least partial reversal of the colour change to obtain a colour C of the sterilized ceramic body, the colour C complying with the following requirements in the CIELAB colour space: L* being from 54 to 95, a* being from −15 to 15 and b* being from −15 to 15.

Claims

1. A process for the preparation of a sterilized ceramic body comprising stabilized zirconia of a defined colour, the process comprising a) providing a ceramic primary body comprising stabilized zirconia of a first colour A, b) sterilizing the ceramic primary body using radiation sterilization to form a sterilized ceramic primary body whereby the ceramic primary body undergoes a colour change to a colour B, and c) irradiating the sterilized ceramic primary body with electromagnetic radiation of at least one wavelength lying in a wavelength band ranging from 150 nm to 700 nm to induce an at least partial reversal of the colour change of step b) to obtain a colour C of the sterilized ceramic primary body, the colour C complying with the following requirements in the International Commission on Illumination L*a*b* (CIELAB) colour space: L* being from 54 to 95, a* being from −15 to 15 and b* being from −15 to 15, wherein step c) is carried out at a temperature below 60° C.

2. The process according to claim 1, wherein in step c) the sterilized ceramic primary body is irradiated with electromagnetic radiation of at least one wavelength lying in the wavelength band ranging from 150 nm to 600 nm.

3. The process according to claim 1, wherein the stabilized zirconia comprises vacancies in the oxygen-sublattice of its crystal structure.

4. The process according to claim 1, wherein the stabilized zirconia is yttria-stabilized zirconia.

5. The process according to claim 1, wherein the stabilized zirconia comprises extrinsic defects in the lattice of its crystal structure.

6. The process according to claim 1, wherein in step b) the ceramic primary body is sterilized using ionizing radiation sterilization.

7. The process according to claim 1, wherein in step b) an activation of colour centers in the stabilized zirconia occurs, and in step c) a transition of electrons of the colour centers from an excited state into a relaxed state is induced.

8. The process according to claim 1, wherein the wavelength band according to step c) ranges from 200 nm to 650 nm.

9. The process according to claim 1, wherein in step c) the sterilized ceramic precursor body is irradiated with electromagnetic radiation of two or more different wavelengths both lying in the wavelength band ranging from 150 to 700 nm.

10. The process according to claim 1, wherein the ceramic primary body provided in step a) is a dental article in pre-assembled and packaged form.

11. The process according to claim 1, wherein after step c) any heat treatment step is omitted.

12. The process according to claim 1, wherein the colour C complies with the requirement that L* is from 63 to 90.

13. The process according to claim 1, wherein: in step a), the ceramic primary body comprises yttria-stabilized zirconia of the first colour A, the ceramic primary body being in the form of a dental article containing two or more dental article components that are pre-assembled with one another and contained in an at least partially translucent packaging, in step b), the ceramic primary body is sterilized using gamma-sterilization whereby the ceramic primary body undergoes the colour change to the colour B, and in step c), the sterilized primary body is irradiated with electromagnetic radiation of at least one wavelength lying in the wavelength band ranging from 320 nm to 500 nm to induce the at least partial reversal of the colour change of step b) to the colour C, the colour C complying with the following requirements in the CIELAB colour space: L* being from 68 to 85, a* being from −10 to 10 and b* being from −5 to 15.

14. The process according to claim 1, wherein the colour C complies with the requirement that L* is from 70 to 75.

15. A sterilized ceramic body obtained by the process according to claim 1.

16. A dental implant system comprising a dental article composed of the sterilized ceramic body according to claim 15.

Description

(1) The present invention is further illustrated by way of the figures, of which:

(2) FIG. 1 shows a photograph of samples 1e and 2 to 8 discussed above;

(3) FIG. 2 shows a graphical representation of the L-value in the CIELAB colour space for different irradiation wavelengths depending on the time of irradiation;

(4) FIG. 3 shows a graphical representation of the spectrum of the lamp “Nikon Intensilight C-HGFI Precentered Fiber Illuminator” used for the light treatment according to the specific working examples (experimental set I and II) of the present invention;

(5) FIG. 4 shows a graphical representation of the spectrum of the lamp “Thorlabs Inc OSL1-EC Fibre Illuminator” used for the light treatment according to the specific working examples (experimental set II) of the present invention; and

(6) FIG. 5 shows a photograph of samples 9a to 9e discussed above.

(7) FIG. 1 gives illustrative evidence of the reversal in discolouration obtainable by the process of the present invention. Whereas samples 1e and 2 to 6 exhibit a white, an off-white or an ivory colour closely resembling the colour of a natural tooth, comparative sample 7 exhibits a light brown colour and comparative sample 8 exhibits a purple-brownish colour.

(8) Ultimately, the present invention allows for preparing a sterilized sample with a natural tooth colour, which makes it particular suitable for the use as a dental article.

(9) According to FIG. 2, different irradiation wavelengths in the UV spectrum have been assessed for their effect in reversing the discolouration phenomena induced by gamma-sterilization. Specifically, the L*-values of samples irradiated with electromagnetic radiation at 312 nm (diamonds), 365 nm (squares) as well as a combined irradiation at 254 nm, 312 nm and 365 nm (circles) have been assessed punctually over time. For irradiation at 254 nm and 365 nm, a lamp comprising a 4 W tube for the respective wavelength has been used (Herolab GmbH Laborgerate, Type: NU-4, Cat. No. H 466.1, Ser.-No. 13 37 003 H466.1; WEEE-Reg.-No. DE 66734561). For irradiation at 321 nm, a lamp comprising six tubes of 8 W each has been used (VILBER LOURNMAT; TFP-M/WL, Serial No. 963718).

(10) FIG. 2 reveals that within the first hour of irradiation, the fastest increase in the L*-value was achieved for the samples irradiated at 312 nm. For the samples that were subject to a combined irradiation at the three wavelengths specified above, also a relatively fast increase in the L*-value was obtained, said increase being even faster than the one at 312 nm after the first hour of irradiation. After 1400 minutes of irradiation, the L*-value obtained by the combined irradiation was even higher than the one obtained by irradiating at 312 nm.

(11) FIG. 3 shows the spectrum of the specific lamp used with its relative intensities at the respective wavelength (in nm). The spectrum shows intensity peaks at about 370 nm, 410 nm, 440 nm and 550 nm, with the highest peak at 440 nm, i.e. in the blue range.

Experimental Set II

(12) For a further experimental set-up, zirconia samples (CeramTec MZ111) grinded to a thickness of less than 0.1 mm were provided as ceramic primary body.

(13) The samples were gamma-sterilized using a Cobalt 60 source and a dose in the range of 25-42 kGy, and then directly subjected to different treatments, namely to light treatment by exposure to the fibre optics of a “Thorlabs Inc OSL1-EC Fibre Illuminator” (in the following referred to as “Thorlabs lamp”), with a first sub-set being packed in a COC vial and a second sub-set being unpacked; or light treatment by exposure to an Intensilight lamp, with a first sub-set being packed in a COC vial and a second sub-set being unpacked.

(14) For the treatment using the Intensilight lamp, both a mode using an adapter of the type C-HGFIB HG100 W Adapter R as well as a mode without an adapter was performed. The samples were placed orthogonally to the light source with a defined distance measured from the top of the sample to the light emitting point of the fibre optics or to the additional lens adapters.

(15) The following treatment distances were used:

(16) TABLE-US-00003 TABLE 3 Mode Without adapter With adapter Distance to Direct: 2.0 ± 0.1 cm Direct: 7.0 ± 0.1 cm sample In vial: 3.5 ± 0.1 cm In vial: 9.4 ± 0.1 cm Illumination 90° 90° angle

(17) The spectrum of the Intensilight lamp with its relative intensities at the respective wavelength (in nm) is shown in FIG. 3, as mentioned above, whereas the spectrum of the Thorlabs lamp is given in FIG. 4. While the spectrum of the Intensilight lamp shows intensity peaks at about 370 nm, 410 nm, 440 nm and 550 nm, the spectrum of the Thorlabs lamp shows intensity peaks at about 545 nm and 660 nm.

(18) For comparative reasons, gamma-sterilized samples were subjected to the following treatment in the unpackaged state: heat treatment at 121° C. for 30 minutes; microwave treatment 800 W for a duration of 1 minute, followed by a duration of 4 minutes; treatment in a demagnetizer for 190 seconds; and treatment using an O.sub.2 plasma cleaner, 2 times for 2 minutes at 35 W.

(19) The samples as well as their respective treatment are summarized in Table 4.

(20) TABLE-US-00004 TABLE 4 Sample Treatment after sterilization Ia Illumination with Thorlabs lamp for 3 minutes Ib Packed in COC vial; illumination with Thorlabs lamp for 61 minutes IIa Illumination with Intensilight lamp with adapter for 317 minutes IIb Packed in COC vial; illumination with Intensilight lamp with adapter for 437 minutes IIc Packed in COC vial; illumination with Intensilight lamp with adapter + 365 nm UV light source for 437 minutes IId Packed in COC vial; illumination with Intensilight lamp with fibre optics for 55 minutes IIe Packed in COC vial filled with H.sub.2O; illumination with Intensilight lamp with fibre optics for 60 minutes IIf Packed in COC vial filled with 0.9% NaCl solution; illumination with Intensilight lamp with fibre optics for 60 minutes III UV source 365 nm (4 W) for 1196 minutes IV UV source 312 nm (6*8 W) for 1196 minutes V Filter 510-560 nm (535/50) for 1680 minutes VI Filter 465-495 nm (480/30) for 900 minutes VII (comparative) demagnetizer treatment VIII (comparative) heat treatment IX (comparative) microwave treatment X (comparative) O.sub.2 plasma cleaner treatment

(21) For the respective lamps and filters used, light measurement was performed by means of a UV enhanced silicon photodiode of the type SD100-13-23-222 (from advanced Photonix Inc.).

(22) For calculating the flux density, the voltage was measured over the resistance (762±10.4Ω) with a precision of 0.025%. With a treatment distance of 6.5 cm and 2.5 cm, a flux density of 3′595±55 W/m.sup.2 and of 20′165±284 W/m.sup.2 was calculated for the Intensilight lamp, respectively. For the 510-560 nm filter, the determined flux density was 2′528 W/m.sup.2 for a treatment distance of 2.5 cm and 426 W/m.sup.2 for a treatment distance of 6.5 cm.

(23) Using the 465-495 nm filter, a flux density of 255±8 W/m.sup.2 was determined, the intensity being therefore only about one fifth of the intensity of the sun (with the solar constant E.sub.0 being 1′367 W/m.sup.2).

(24) After the treatments summarized in Table 4, the L*, a* and b* values of the respective samples were measured as mentioned above, the results of these measurements being summarized in Table 5.

(25) TABLE-US-00005 TABLE 5 Colour Colour before treatment after treatment Sample L* a* b* L* a* b* Ia 49.47 4.45 2.64 66.83 1.93 12.02 Ib 50.00 4.95 3.00 56.87 2.98 4.99 IIa 49.98 3.98 4.88 72.04 −2.15 3.13 IIb 48.37 4.15 3.89 71.58 −1.91 4.01 IIc 47.70 4.07 3.01 70.24 −1.56 4.85 IId 48.04 4.35 3.94 73.23 −1.8 1.34 IIe 48.16 4.35 3.77 71.76 −1.68 2.02 IIf 39.74 4.65 0.88 72.99 −1.64 1.36 III 49.47 5.25 3.42 65.85 1.49 3.89 IV 49.87 4.94 3.32 73.35 −0.2 3.06 V 41.50 4.40 1.40 68.01 −0.91 10.18 VI 43.16 4.47 1.26 71.03 −2.30 8.05 VII (comparative) 44.45 5.54 4.13 52.92 3.73 2.07 VIII (comparative) 51.70 4.33 3.70 64.70 2.80 12.06 IX (comparative) 49.67 4.48 2.46 51.54 4.32 2.88 X (comparative) 49.60 4.68 3.14 52.01 4.33 3.22

(26) As shown, discolouration reversal is achievable by irradiation with electromagnetic radiation emitted from the Thorlabs lamp, the Intensilight lamp and the UV sources (365 nm and 312 nm) as well as using the filters (510-560 nm and 465-495 nm). Further, discolouration reversal was achieved for both the samples packed in a COC vial as well as the unpacked samples, as can for example be seen from samples Ia and Ib as well as from IIa and IIb. This shows that light of a wavelength lying in the wavelength band according to step c) can at least partially pass the COC vial.

(27) As shown by samples IIe and IIf, discolouration reversal is even achieved in the case where the COC vial-packed samples are stored in water or in a 0.9% NaCl solution.

(28) Thus, the initial lightness of the gamma-sterilized zirconia can be reached by the treatment of the present invention, independent of whether the samples are packed in a COC packaging and whether the packed samples are held in a storing liquid during treatment. However, the more light is absorbed by the packaging and, as the case may be, the liquid, the longer irradiation duration is required to obtain an appearance closely matching the initial colour before sterilization.

(29) Some samples exhibited an even higher lightness (L*) after treatment than the raw samples before gamma-sterilization. After prolonged storage for several months, samples with higher L* values than initially present (before gamma-sterilization) showed a slight decrease in the L* value, but still exhibited a lightness similar or higher than the raw samples.

(30) Within the framework of the above experimental set-up, the best results are obtained with the Intensilight lamp. Specifically, for a zirconia sample in a COC vial (without liquid) an L* value of 70 can be reached after 14 minutes of irradiation using the Intensilight lamp.

(31) Further testing showed that the irradiation duration to reach an L* value of 70 is 4.5 times lower when using radiation at 465 to 495 nm compared to illuminating the samples using radiation at 510 to 560 nm of the same (normalized) intensity. Nevertheless, a discolouration reversal is also achieved by using electromagnetic radiation at 510 to 560 nm and thus clearly outside the UV band. Within the framework of the experimental set-up, the best effects regarding the desired discolouration reversal are achieved by using light in a wavelength band from 312 nm to 495 nm, specifically in a wavelength band from 350 nm to 495 nm.

(32) It was further found that for some samples the development of the L*, the a* value and the b* value do not coincide perfectly. This effect can be used to obtain a desired colour or shade by adapting the time of treatment correspondingly.

Experimental Set III

(33) For a further experimental set, polyethylene blister-packed zirconia caps (MZ111) of a first sample I (without a bore) and of a second sample II (with a 1.8 mm diameter bore) were gamma-sterilized (using a Cobalt 60 source and a dose in the range of 25-42 kGy) and then irradiated using a device of the type Polylux-P.

(34) After Polylux-P irradiation of samples I for 17 hours and hours and of samples II for 30 hours and 45 hours, respectively, the L*, a* and b* values were measured as explained above. For comparative reasons, the L*, a* and b* values before and immediately after gamma-sterilization (i.e. before irradiation) were also measured. The results of these measurements are given in Table 6.

(35) TABLE-US-00006 TABLE 6 treatment L* a* b* I.sub.0 non-gamma sterilized; 73.71 −0.52 3.23 non-irradiated Ia gamma sterilized; 67.93 0.83 9.66 irradiated 17 h Ib gamma sterilized; 70.06 −0.28 8.06 irradiated 24 h IIa gamma sterilized; 68.08 −0.88 9.26 irradiated 30 h IIb gamma sterilized; 70.81 −1.52 6.87 irradiated 45 h

(36) The results show that a sufficient colour change of the packaged samples can also be achieved by using a relatively simple irradiation device, if a sufficiently long irradiation duration is chosen. Thus, the process of the present invention can be performed by using a relatively simple and small equipment and allows to obtain the desired discolouration reversal even when the ceramic body is in its packaged state.