Journal of Oral Rehabilitation, 1992, Volume 19, pages 281-287
Non-cast titanium restorations in fixed prosthodontics K.W. B O E N I N G , M . H . W A L T E R , anJ P . - D . KEVV^EL Department of Prosthetic Dentistry, Dental School, Free tJniversity of Berlin, Berlin, Germany
The problems encountered in casting titanium in dentistry have not been completely resolved. The Procera system forms crowns by means of a combination of sparkerosion and milling. The accuracy of fit was examined before and after ceramic veneering both in vitro and in vivo. Before veneering, on conical surfaces space widths were approximately 53j.im in vitro and 69|.im in vivo. At shoulders and occlusal surfaces, spaces of about 430 (_im were measured in vitro and of about 500|_tm were measured in vivo. After ceramic veneering, slight increases in space widths could be observed. The metal-ceramic compound was tested by the 3-point bending test (DIN) and the bending test (ISO). The DIN test was satisfactory, but not the ISO test. It is concluded that titanium crowns processed by the Procera System are suitable for clinical usage, if the space widths at shoulders and the occlusal surface and the special requirements of tooth preparation are taken into account. Introduction
In recent years titanium has become a material of great interest in prosthetic dentistry. Having been used most successfully for dental implants, it was found to be a highly biocompatible material, with excellent corrosion resistance. In addition, it has a low thermal conductivity, low density and a considerable translucency to X-rays, enabling the production of diagnostic radiographs through titanium restorations. Finally, the global supply of titanium is enormous, estimated to be 300 million tons (Suckert, 1987). Due to gas absorption and the high chemical reactivity of the melt, titanium is extremely difficult to process by the conventional lost-wax casting technique. Recently, the Procera system*, a precision mechanical technique in combination with spark erosion, has been developed by Andersson et al. (1989) for the fabrication of unalloyed titanium crowns and bridges. In a first step, the stone die is scanned with low force while graphite electrodes are milled with a duplication milling machine. In a similar way, the wax model of a crown or metal substructure is duplicated from a solid piece of titanium. After milling the outer form, the inner cavity of the crown is processed by spark erosion. There is a certain wear of the graphite electrode, so three electrodes are normally required to give the crown the accurate shape of the cavity. After removing the ridges at the Correspondenee: Dr K.W. Boening. The Dental School. Department of Prosthetic Dentistry. Eree University of Berlin. Assmannshauser Str. 4 - 6 . 1000 Berlin 33. Germany. ' Nobelpharma, Goteborg. Sweden.
K.W. Boening et al.
crown margin and a final check, the metal surface is either veneered or polished, using the same materials and methods as for polishing gold castings. Only single crowns can be produced by the Procera system. For larger restorations, crowns and pontics are fused by laser welding (Sjogren, Andersson & Bergmann, 1988). A laser welding unit forms part of the Procera system. Materials and methods
To test the clinical suitability of the Procera system, the following properties of titanium specimens were investigated in this study: (i) the accuracy of fit; (ii) the bonding strength of the metal-ceramic compound; (iii) the roughness of the eroded surface. The accuracy of fit was tested before and after ceramic veneering in vitro as well as in vivo. In vitro., 10 double-mix impressions were taken from a metal die (Fig. 1) under standardized conditions, using addition-curing silicone material (Provil M, Provil H)*. Stone dies were fabricated and the titanium frameworks were processed according to the manufacturer's instructions. The fit of the specimens was tested by measuring the thickness of a light body silicone layer (Xantopren blue)* between the original die and the crown under the light microscope. To avoid false test data due to an increase in the flowing pressure of the silicone material, drainage was applied to the occlusal surface of the die. In vivo, the space widths of 24 crowns (10 veneered and 14 nonveneered) were examined in the marginal area of the buccal and the oral side as well as on the occlusal surface by measuring a silicone layer between tooth and crown. The metal-ceramic bonding strength was tested by the 3-point bending test (DIN 13927, 1989) and the bending test (ISO/DIS 9693, 1989). For the DIN test, the middle of 10 titanium specimens of dimensions 0-5 x 3 x 25 mm were veneered with ceramic layers of thickness 1 mm and length 8 mm. The distance between the supporting points
7 mm 1
Fig 1. Die for testing the accuracy of fit in vitro. The numbers indicate the measuring points.
Bayer. Levorkuscn, Germany.
Non-cast titanium restorations
was 20mm. A rising load was applied to the middle of the specimen, until the metalceramic compound cracked. In the ISO test, 10 specimens were bent over a rod of diameter 10mm, until a right angle was made. After rebending the specimens to their original form, at least 50% of the metal surface in the middle third should still be covered with ceramic particles. All veneerings were made of Duceratin Ceramics*. The surface roughness of the eroded cavity was tested according to DIN 4768. Results Accuracy of fit The results are shown in Fig. 2. Before veneering, space widths in the range 25—75 \im (average 53 ^m) were measured at the conical surfaces and the bevel of the die. On the shoulder and the plane occiusal surface and the gap widths were in the range 290— 710|_im (average 430 ^m). The bar graphs show a tendency towards enhanced space widths after ceramic veneering (67 ^^im at the bevel, 500^m at the shoulder and the occiusal surface). However, no statistically significant differences could be found after ceramic veneering in vitro. The results in vivo showed, before veneering, spaces on the buccal and oral side in the range 20—150^m (average 69|.im) (Fig. 3). On the occiusal surface, the average gap width was about 400(xm (range 110-860pim). After ceramic veneering, there
I 300 CL
4 5 Measuring point
Fig 2. Space widths in vitro. The measuring points are shown in Fig. 1. (S) = before eeramie veneering. (^) = after ceramic veneering.
Ducera, Rosbaeh. Germany.
K.W. Boening et al.
800 r 700 600 500
Fig. 3. Space widths of titanium crowns //; vivo. veneering. (0) = alter ceramic veneering.
I) = without veneering. (S) = before ceramic
were distinct increases in gap widths at the occiusal surface (average 560|_im) in nine out of ten cases (Fig. 4). From the geometrical relationship between the occiusal gap (Socc)> the conus angle (a) and the marginal gap at the bevel (Si^,). which is given by the equation; Shev
sin ^ a it is clear that slight deformations of the metal substructure during ceramic firing cause the greatest changes in space width at the occiusal surface. Metal-ceramic bonding strength The breaking load of the specimens in the 3-point bending test is affected by the modulus of elasticity of the metal substructure. Therefore the modulus of elasticity is associated with a coefficient by which the breaking load must be multiplied. The modulus of elasticity was calculated from the load deflection diagram of non-veneered titanium specimens by the following equation: 4 b d-M
where E = modulus of elasticity, F = applied load, L = distance between points of support (20-0mm), b = width of specimen (3-0mm), d = thickness of specimen (0-5mm), and 1 = way of bending. The results are shown in Fig. 5. The modulus of elasticity of unalloyed titanium is
Non-cast titanium restorations
i o 400
1 % u
Crown number Fig. 4. Occiusal space widths in vivo before (*) and after (O) ceramic veneering. The spaces inereased in all cases exeept for no. 9, where a decrease was observed.
approximately 81,000 N mm ^, a value similar to that of gold alloys, and associated with a coefficient of 5, according to the DIN draft. The product of breaking load and coefficient is equal to 36N mm ", and exceeds the lower limit of 30N mm ". Thus the metal-ceramic compound passes the DIN test. Light microscopic examination of titanium specimens after bending according to ISO/DIS 9693 showed an estimated share of the remaining ceramic of only 20%, and therefore this test was not passed. Surface roughness The spark erosion produces a mean roughness R;, of 3-6±0-4|-im. The maximum roughness of the specimens was in the range 12-3-27-3^m. Discussion
It can be concluded from this study that crowns processed by the Procera system show a high accuracy of fit in vitro and in vivo, if feather-edge or chamfer preparation is used. Due to the construction of the duplication milling machine, it is not possible to shape perfect shoulders. Space widths between the specimens and the shoulder of the metal die were in the range 290-750|im. However, CFM restorations require a certain space. If shoulder preparation is planned, a bevel is neccessary for a good fit at the crown margin. Furthermore, the occiusal discrepancy approximated to several
K.W. Boening et al. \
3-point bending test
TJ 03 O
20 10 0 Hg. 5. Three point bending test: breaking load and modulus of elasticity.
hundred f^tm. When preparing the occiusal tooth surface, this additional space must be taken into account. Increases in space width could be observed after ceramic veneering. The differences were statistically significant in vivo, but should not be overestimated, because clinical requirements are still fulfilled. Due to the impossibility of standardizing clinical treatment, the effects of the dental impressions, different conus angles, and the stone casts must be taken into account when considering the results in vivo. The suitability of the Procera system for clinical use is confirmed by the studies of Andersson et al. (1989). No final conclusion concerning the metal-ceramic compound can be drawn at present, because the specimens passed the DIN test but not the ISO test. The present study suggests that the physical properties of titanium do meet the clinical requirements. However, long-term studies will be necessary to determine whether the titaniumceramic bond is strong enough for use in prosthetic dentistry. References P. & NILSON. H . (1989) Clinical results with titanium erowns fabricated with machine duplication and spark erosion. Acta
ANDERSSON, M . , BERGMANN. B . . BESSING. C .
ERICSON, G . . LUNDQUIST.
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Odontologica Scandinavica. 47, 279. DEUTSCHE INDUSTRIE NORM (DIN) 13927 (1989) Metall-Kcramik, Anfordcrungen, Priifung. INTERNATIONAL STANDARDS ORGANISATION ( I S O ) / D I S 9693 (1989) Dcntal ceramic fused
to metal restorative materials. Draft International Standard. SJOGREN. G . , ANDERSSON. M . & BERGMANN, M . (1988) Laser welding of titanium in dentistry. Acta Odontologica Scandinavica. 46, 247. SucKERT, S. (1987) Reines Titanium, ein Altemativ Werkstoff fur die Prothetik? Dentallabor, 35, 1544.