Cavity convergence composite inlays* R. W. Wassellt,
A. W. G. Wallst
1992; 20: 294-297
angles for direct
and John F. McCabe*
tDeparrmenr of Operative Denrisrry and $ Departmenr upon Tyne, UK
of Denrai Materials
Science, The Dental School, Newcastle
ABSTRACT Direct composite inlays may sometimes be difficult to remove from the cavity following primary polymerization. The aim of this work was to find the lowest cavity convergence angle (taper) which would allow reliable inlay removal. Cavity finish and cavity size were also taken into account. Standardized mesioocclusal-distal cavities of 6” taper were cut into 10 teeth of varying mesiodistal width and finished with either lbbladed tungsten carbide or 25 pm diamond burs. After separating medium had been applied, Coltene Brilliant Dentin composite resin was packed into the cavity and light cured. The force required to remove the inlay was measured with an Instron Universal Testing Machine. The cavities were then refinished with the alternative finish and the experiment repeated. The same sequence was used for cavities of 12” and finally 18’ convergence angle which were cut consecutively in the same 10 teeth. Force levels related to cavity convergence angle, cavity finish and tooth size were tested by ANOVA and regression analysis. The forces required to remove some of the inlays from the 6”, and to a lesser extent 12” angled cavities, proved unacceptably high, while an 18 o convergence angle resulted in statistically significant lower forces which were unlikely to damage tooth or inlay. Cavity finish and tooth size did not influence inlay withdrawal force. KEY WORDS:
J. Dent. 1992; 1992)
Direct inlay, Cavity design (Received 21 August 1991;
reviewed 8 January 1992;
Correspondence should be addressed to: Mr R. W. Wassell, Department School, Framlington Place, Newcastle upon Tyne NE2 4BW, UK.
INTRODUCTION The restoration of posterior teeth with composite resin inlays is a comparatively new technique which is enjoying increasing popularity. A composite inlay is a restoration which is cemented into a dental cavity as a solid mass that has been fabricated from resin composite with a form established by either an indirect or direct procedure (Watts, 1990). Following primary polymerization, a direct inlay may sometimes prove difficult to remove from its cavity despite taking care to create rounded internal line angles and avoid undercuts. In some instances a recalcitrant inlay may be fractured or have to be cut out of the tooth. Empirically, these problems occur less frequently if the cavity has been flared out more than would be normal for a gold inlay preparation. *Presented at FDI World Dental Congress, Singapore. September 1990. @ 1992 Butterworth-Heinemann 0300-5712/92/050294-04
accepted 27 February
Dentistry, The Dental
An in vitro study was carried out prior to starting a clinical trial of a direct inlay system (Coltene DI system, Coltene AG, Altstatten, Switzerland). The aim of this study was to test the influence of cavity convergence angle, cavity finish and tooth size on the withdrawal force for mesio-occlusal-distal (MOD) inlays. Cavity convergence angle is defined as the angle subtended between opposing cavity walls and is also referred to as taper.
Ten sound third molars with mesiodistal widths varying from 7.2 to 12.8 mm were selected and mounted in polyester resin blocks. The teeth, which had been kept for no longer than 6 weeks post extraction, were examined with binocular loupes and found to be free from cracks, caries and restorations. Normal saline was used as a storage medium prior to and between experimental
Wassell et al.: Convergence
angles for direct composite
Fig. 7. Dimensions (mm) common to all inlay cavities. The depth of the proximal box was measured from the most apical part of the marginal ridge.
procedures, thus ensuring that the teeth did not dry out. Mesiodistal and buccolingual orientation lines were drawn perpendicular to each other on the sides and base of the block. These lines were used to position the tooth on the table of a parallelometer (Cendres & Metaux SA, Bienne, Switzerland), which was initially set at a 0” tilt. An air rotor handpiece (Supertorque, Kavo Dental Ltd, Amersham, Bucks, UK) was attached to the moveable arm of the parallelometer. A cavity of standardized dimensions (Fig. 1) was cut into each tooth under copious water spray. The dimensions were marked onto the tooth surface with a fine indelible pen and draughtsman’s dividers. An 80 urn grit diamond bur (Intensiv 8117, Coltene AG) was used to prepare the cavities. Five teeth were finished with a 25 urn grit diamond bur (Intensiv 3 117, Coltene AG) while the other five were finished with a Jet 12 bladed tungsten carbide bur (Jet lfg 7206, Cottrell & Co., Edinburgh, UK). The finishing burs were discarded after use on five cavities. No cement base was placed. The convergence angles of the diamond and tungsten carbide finishing burs were measured with a toolmaker’s microscope (Mitutoyo [UK], Andover, Hampshire, UK) and found to be 6.0” and 5.6” respectively. The 0” tilt on the parallelometer table ensured that the cavity convergence angle, nominally 6”, between all opposing cavity walls, closely approximated the bur convergence angle. A matrix band (Hawe Transparent Molar Band 774, Coltene AG) was then applied to the prepared tooth, followed by two coats of separating agent (Separator, 180988-02, Coltene, AG). A single drop of Separator was dispensed onto a brush and after painting on, excess was
H fig. 2. Measurement of inlay withdrawal force: A, inlay with metal loop attached; B, tooth in polyester block; C, block retaining jig; D, universal joint; E, wire loop; F, ball race; G, tension load cell; H, cross head.
blown off with a low pressure air hose. The composite (Brilliant Dentin, 170490-30[D3], Coltene AG) was placed as a single increment taking care to avoid voids. The inlay was cured with a Coltolux light curing unit (Coltene, AG) from the mesial, distal and occlusal aspects. A 60 s exposure was used for each aspect. Any extruded ‘flash was removed with a scalpel after the band had been taken off. A wire lug was attached to the occlusal surface of each inlay with a small increment of composite. The mounted tooth was placed in a bond strength jig (Fig. 2) (Walls et al., 1985) mounted in a Universal Testing Machine (Model 1195, Instron Ltd, High Wycombe, Bucks, UK) and the force needed to remove the inlay measured at a cross-head speed of 10 mm min-1. The sample size was effectively doubled by repeating the experiment after the teeth had been refinished: teeth previously finished with a tungsten carbide bur were refinished with a diamond bur and vice versa. The experiment was repeated in the same way using 12” and finally 18” convergence angles. The same teeth were used and the cavities flared out taking care not to increase the apical width of the cavity. Reangulation was achieved simply by altering the tilt of the parallelometer table to 6” and 12” respectively. An inclinometer was used to set the degree of tilt. The orientating lines on the tooth mounting blocks were used to ensure that all opposing cavity walls, including the axial walls, were prepared to the correct taper.
Table I. Means and standard deviations of the inlay withdrawal forces (MPa) Cavity convergence angle 12”
1 1.5 (3.2)
Statistical analysis of the inlay withdrawal forces was by ANOVA and regression analysis. A log transformation of the force values was used.
RESULTS The inlay withdrawal forces for the 6”, 12” and 18” angled cavities, subdivided into diamond and tungsten carbide cavity finish are presented in Table I. The type of cavity finish made little difference and this was confirmed by ANOVA. A clear trend was seen towards easier inlay withdrawal with increasing convergence angle. Regression analysis on cavity angulation showed that this inverse relationship was statistically highly significant (t = -5.06, d.f. = 58, P < 0.001). On the other hand, regression analysis for inlay withdrawal force with mesiodistal width showed no statistically significant relationship.
DISCUSSION It is extremely important that after primary polymerization the inlay be removed from the cavity easily and without damage. Clinical experience suggests that the most reliable method of removing a direct inlay is by attaching a small knob of composite in the centre of the occlusal surface so that the inlay can be lifted out with a pair of mosquito forceps. The experiment was designed to mimic this technique. The MOD configuration was chosen as difficult inlay removal has sometimes been associated with this type of cavity. In analysing the results it is important to have some concept of the force levels encountered. A preliminary study, in which inlays were withdrawn manually from the inverted jig, showed that forces below 20 N were considered acceptable while 2035 N offered increasing degrees of difficulty. Forces above 35 N were associated with difficult removal, occasionally resulting in a fractured inlay. The manufacturers have recommended a 5-7 ’ minimum convergence angle but problems may result if the whole cavity was actually cut to this tolerance. This study showed that many of the cavities with a 6” taper had an unacceptably high inlay withdrawal force. A small proportion of the 12” tapered cavities were also associated with unacceptably high forces. Only the 18” taper allowed consistently easy removal but in terms of gold inlay preparations 18’ might be considered too great a taper. What therefore should the clinical recommendations be? Clinically, it would appear that although a dentist might aim to cut a cavity with low convergence angles, the
angles obtained are usually significantly higher. This phenomenon is well illustrated by the work of Mack (1980), who reported the isthmus convergence angle of 50 gold inlay cavities to be 22 o + 13 ‘. It is generally acknowledged that gold inlays should have a low convergence angle of around 6” (Shillingburgetal., 1981). Hence, ifone aims to cut a cavity of 12” it is likely that most of the cavities would have a larger convergence angle and the inlays would be withdrawn easily. If one aimed to cut an 18” cavity the inevitable overcutting would result in excessive tooth reduction. An inlay cavity with an 18” convergence angle needs little more tooth reduction than a cavity with a 6” convergence angle. Where a composite inlay is used to replace an existing restoration this small amount of extra tooth reduction is inconsequential providing the inlay does not require frequent replacement in the future. Nevertheless, inlays are best avoided for restoring minimal Class II cavities. Significantly smaller cavities, which outline the carious lesion, can be prepared for direct placement restorations of amalgam, composite or glass ionomer. It is unlikely that cavity convergence angle is as critical for composite inlay retention as it is for gold inlays. Gold inlays rely on having preparations with minimal convergence angles to give them adequate retention and resistance. The cements used to lute gold inlays, such as zinc phosphate and glass ionomer, are strong in compression and weak in tension. A low cavity convergence angle helps to prevent masticatory forces loading the cement in tension. Composites have tensile strengths almost ten times greater than zinc phosphate and the composite luting materials are likely to be similarly strong. An excellent bond can be created between etched enamel and composite. In addition, a strong bond in excess of 20MPa shear strength is reported to form between the inlay and the composite luting cement (Coltene AG, unpublished data). Nevertheless, it would be useful to know if the 18’ taper had a significant effect on the strength of the restored tooth. It is promising to note however that Burke et al. (1990) found the strength of premolars restored with composite inlays to be unaffected by tapers of 4,6 and 12’. If the strength of a cusp is judged to be significantly reduced, the occlusal surface should be onlayed by at least 2 mm of composite. Burke et al. (1991) reported that the strength of teeth restored in this way was similar to that of an unrestored tooth. The finding that inlay withdrawal forces were not related to tooth size was surprising. Posterior composites have polymerization contractions of approximately 2 per cent by volume (Walls et al., 1988). Forces have been measured in association with the contraction (Feilzer el al.,, 1987). We had initially hypothesized that wide teeth would need larger forces than narrow teeth due to the larger amount of composite undergoing polymerization contraction between the mesial and distal boxes. This increased amount of material might be associated with higher contraction forces which in turn would tend to pull
et a/.: Convergence
the boxes together. As this chain of events did not occur it is likely that the composite within the occlusal isthmus underwent stress relaxation (Davidson et al., 1984), which means that the polymerizing material would have flowed to some extent rather than develop progressively increasing contraction forces. Presumably, the higher convergence angle of 18 o served to reduce the frictional resistance of the inlay against the walls of the cavity so that it could be disengaged more easily from microscopic undercuts. It is likely that diamond and tungsten carbide finishing burs would have imparted different degrees of cavity wall roughness. Nevertheless, the separating medium seemed to prevent any such difference manifesting itself as a difference in inlay withdrawal force. The findings of this study are probably of as much relevance to indirect inlay preparations as they are to direct ones. The technician will need to remove and replace the inlay onto the working die. If large forces need to be used the die may be damaged making the technique unreliable.
CONCLUSIONS Direct MOD inlay cavities cut with an 18” convergence angle resulted in predictable and acceptable composite inlay withdrawal forces. The withdrawal forces associated with 6”, and to a lesser extent 12” convergence angles, were sometimes sufficiently high to be potentially damaging to the inlay. The mesiodistal tooth width and the type of cavity finish did not influence inlay withdrawal force significantly.
angles for direct composite
Acknowledgements We are grateful to Dr D. Appleton, Department of Medical Statistics, University of Newcastle upon Tyne, for performing the statistical analysis.
References Burke F. J. T., Watts D. C. and Wilson N. H. F. (1990) Fracture resistance of premolar teeth restored with MOD composite inlays. J. Dent. Res. 69, 956 (abstr. 14). Burke F. J. T., Watts D. C. and Wilson N. H. F. (1991) Effect of cuspal coverage on fracture resistance of teeth restored with composite inlays. J. Dent. Res. 70, 701 (abstr. 264). Davidson C. L., de Gee A. J. and Feilzer A. (1984) The composition between the composite-dentine bond strength and the polymerization contraction stress. J. Dent. Res. 63, 1396-1399. Feilzer A. J., De Gee A J. and Davidson C. L. (1987) Setting stress in composite resin in relation to configuration of the restoration. J. Dent Res. 66, 1636-1639. Mack P. J. (1980) A theoretical and clinical investigation into the taper achieved on crown and inlay preparations. J. Oral Rehabil. 7, 255-265. Shillingburg H. T., Hobo S. and Whitsett L. D. (1981) Fundamentals of Fixed Prosthodontics, 2nd edn. Chicago, Quintessence, p. 132. Walls A. W. G., McCabe J. F. and Murray J. J. (1985) The bond strength of composite laminate veneers. J. Dent. Res. 64, 1261-1264. Walls A. W. G., McCabe J. F. and Murray J. J. (1988) The polymerization contraction of visible light-activated composite resins. J. Dent. 16, 177-181. Watts D. C. (1990) Composite inlay systems: material properties and design. J. Dent. 18, 69-70.
Book Review ODT 1990/1991. Edited by R. P. Renner. Pp. 222. 1991. Quintessence. Hardback, f 38.00.
This annual publication styles itself ‘a current reference source of dental technology’. The editorial mentions the globalization of dental research and this is reflected in the residences of the authors: 10 from the USA, four from West Germany, three from Japan and one each from Australia, Canada, Greece, Turkey and the UK. The articles range from the highly detailed to the relatively brief. The feature article is a discussion between three Japanese experts on ‘fundamentals of aesthetics: contouring techniques for metal ceramic restorations’. It is 71 pages long and lavishly illustrated. There are, however, few intraoral views; it is therefore impossible to judge the aesthetics of the completed bridges in their natural environment. This reviewer also found the discussion style of presentation difficult to read. The remaining 1 18 pages embrace ceramic restorations, fixed partial dentures, complete dentures, removable partial dentures, orthodontics, dental laboratory technology, technical tips
and new products information. Articles include the comparison of various ceramic systems, polyethylene fibre reinforcement of interim restorations, the accuracy of visible light-cured denture base resins, attaching precious metal precision attachments to non-precious alloy constructions, removable spring aligners and arch expanders, and the effect of mixing techniques on some properties of a repair resin. Reviews include a basic but useful overview of overdentures for the general practitioner, and, highly topical, recommendations for the disinfection of impressions. Technical tips consider the waxing, spruing and venting of cast restorations. New products information comprises items of relevance to dental technology that have previously been published in the International Journal of Prosthodontics. As would be expected from Quintessence, the publication is printed on good quality paper and is profusely illustrated with high quality photographs, many in colour. It is considered to be good value for money, especially for anyone contouring ceramics for bridges. J. R. Heath