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The effect of an elevated level of residual monomer on the whitening of a denture base and its physical properties
J. Dent. 1989;
17: 189-l
189
94
The effect of an elevated level of residual monomer on the whitening of a denture base and its physical properties J. Arab, J. P. Newton and C. H. Lloyd Depattment KEY WORDS: J. Dent
1989;
of Dental Prosthetics and Gerontology, The Dental School, University of Dundee, UK Residual monomers, Dentures, Cleaning, Physical properties 17: 189-l
94 (Received 27 June 1988;
reviewed 17 September
1988;
accepted 20 April 1989)
ABSTRACT It has been suggested that incomplete or faulty processing leading to a high level of residual monomer is a factor in denture whitening. Laboratory tests which simulate the cleaning procedures used by patients presenting with whitened dentures have been carried out on specimen strips of acrylic denture base material with high or low levels of residual monomer. The results show that these samples exhibit similar changes and that the high level of residual monomer had no effect on the whitening process, this being attributable to the cleaning method. The major factor responsible for whitening is confirmed as a high water temperature irrespective of the presence of the denture cleaning agent. However, some physical properties were adversely affected by a high residual monomer content (due to the processing method) which has serious consequences for denture-wearing patients. The use of short/low temperature curing cycles now common in many dental laboratories
should be avoided.
INTRODUCTION Historically, the ‘bleaching’ of acrylic resin dentures has been linked to the use of household bleach or similar domestic products by patients in their cleaning procedures (Osborne, 1945). However, it was not until patients with such whitened dentures were carefUlly questioned about the materials used ‘and the procedures adopted that evidence of the role of boiling water in cleaning procedures was established (Crawford et al., 1985,1987). This was confirmed in laboratory simulation of patients’ cleaning procedures on fully cured acrylic denture base material (Crawford et al., 1986; Arab et al., 1987, 1988). It has been suggested that incomplete or faulty processing, leading to a high level of residual monomer, is a factor in denture whitening (Sexson and Phillips, 195 1; Anthony and Gibbons, 1958; Robinson et aZ., 1985), as is the in wivo exposure to solvent vapour (Robinson et al., 1987). This is of increasing importance since short/low temperature curing cycles are now commonly used in
0 1989 Butterworth & Co Publishers Ltd. 0300~5712/89/0401894I6 $0300
many dental laboratories. The temperature and curing times are selected empirically by the technicians (Austin and Basker, 1982) and, in some cases, could result in the production of acrylic dentures containing higher levels of residual monomer. The purpose of this study was to establish the role of an elevated level of residual monomer in the whitening of an acrylic resin and to investigate the consequences upon some of its physical properties.
METHODS Specimen
preparation
Specimen strips of an acrylic denture base material (Croform X10, Davis, Schottlander and Davis Ltd, London, UK) 65 X 15 X 1.7 mm were produced from dental plaster moulds. A dough formed from 2.5 : 1 powder : liquid mix was packed in flasks under pressure. These were then subjected to two different curing cycles:
190
J. Dent. 1989;
17: No. 4
one set of specimens using a processing cycle of 16 h at 70°C followed by 3 h at lOO”C, a second set using a modified cycle of 8 h at 7O”C, known to produce an elevated level of residual monomer (Austin and Basker, 1980; Huggett et al., 1985). These specimens were then finished and polished using sandpaper, pumice and whiting. Immersion
procedures
a known quantity of the internal standard was injected into the column. Instrument settings were selected to produce well separated and defined peaks, and to remove the overwhelming effect of the massive alcohol peak during integration. The calibration of the instrument was checked against standard solutions containing known amounts of monomer before and after each set of monomer determinations.
Each set of acrylic strips was divided amongst four groups to be soaked in the following solutions:
Assessment
1. Proprietary denture cleaning agent (Steradent Denture Cleaning Tablets, Reckitt Dental Care Products, Hull, UK) dissolved in water at the temperature recommended by the manufacturer ( 1 tablet to 150 ml water, at 50°C). 2. Water at recommended temperature (150 ml, at 50°C). 3. Boiling water (150 ml, at 100°C) with proprietary denture cleaning agent (concentration as treatment 1). 4. Boiling water (150 ml, at 100°C).
Visual appearance
These temperatures refer to the temperature on immersion and the solutions were allowed to cool naturally through the soak period as would occur during normal cleaning procedures used by patients. Following a soak of 30 min (Crawford et al., 1986) the samples were stored in water at room temperature for 24 h. This daily procedure was carried out for 100 days with frequent inspection for degradation. Control acrylic strips were stored in water at room temperature (18°C) for 100 days without being subject to any cleaning procedure.
Measurement
of level of residual
monomer
Gas liquid chromatographic (GLC) analysis gives reliable values for the level of residual monomer in acrylic samples and has been used for analysing a number of denture base resins cured under a variety of conditions (McCabe and Basker, 1976; Douglas and Bates, 1978; Austin and Basker, 1980; Huggett et al., 1985). In this study, GLC analysis has been carried out using an HRGC 5300 Mega series GLC (Carla Erba Strumentazione, Milan, Italy) fitted with a Porapak Q column (80100 mesh, 1.2 m long, internal diameter 4 mm), operated with oven and injection temperatures of 200°C and a flame ionization detection temperature of 250°C. Nitrogen was used as the carrier gas at a rate of 45 ml min-I. The detector output was linked to a Shimadzu CR3A chromatopac integrator. Standard solvent extraction procedures were used to remove the residual monomer from the acrylic specimens (Huggett et al., 1985). Toluene was selected as an internal standard for chromatography since under these conditions the peak it produces is sharp and well separated from both the methanol and methylmethacrylate peaks. Of the methanolic extract, 2.5 ~1containing
of treated
specimens
Periodic visual inspection of the samples was carried out independently by three investigators. Any changes, particularly in colour (when compared with a control denture base specimen), were recorded. Surface characteristics Samples were cut from treated and control acrylic strips and the surfaces examined by scanning electron microscopy (JSM/35, Joel, Tokyo, Japan). A photographic record was obtained for the surface detail as a function of the number of immersions. Light transmission As previously reported (Crawford et al., 1985, 1986, 1987; Robinson et al., 1987), there is a change in the translucency of the acrylic denture base material during ‘bleaching’. To quantify any change, the light transmission through the treated and control samples was measured using a spectrophotometer (Unicam S.P.1800, Pye, Cambridge, UK) at a wavelength of 690 nm, this wavelength being the most sensitive for the material under test after completion of a wavelength scan analysis (Crawford et al., 1986; Arab et al., 1988). Transverse bend strength It is widely accepted that exposure to water at a high temperature is detrimental to the mechanical properties of acrylic materials. Therefore, samples of treated and control material were fractured by a three-point bend test using an Instron TTCM universal testing machine to obtain the transverse bend strength. Fracture took place at a cross-head speed of 10-4ms-‘, in air at room temperature. Ten specimens were broken (for each treatment) to obtain a mean value with a standard deviation. Surface hardness Surface hardness of the samples was investigated using a Vickers microhardness instrument (E. Leitz GmbH., Wetzlar, FRG). Measurements were taken at room temperature and under an indentation force of 1 N. Ten indentations were made for each sample. On completion of these measurements, the magnitude of the changes on the surface of the acrylic was compared
Arab et al.: Residual
Table L Percentage residual monomer content of acrylic denture base strips cured using two different processing cycles following treatment with different soak solutions (100 cycles) Soak soln water temp. f”C) 18 (control) 50* 50* + proprietary denture cleaner 100 100 + proprietary denture cleaner
% Residual monomer (f s.d) 16h at 70°C +3h at 100°C 8h at 70°C 0.27 f 0.02 0.33 f 0.01
2.62 f O-04 2.52 f 0.09
0.28 + O-02
2-48 kO.10
O-27 f 0.03
1.49 f 0.02
0.34 + 0.03
1.79 + 0.08
n = 10 for each treatment. *Manufacturer’s recommended temperature.
with the condition of the interior of the specimen. O-5 mm of the material was removed from the surface of those samples subjected to water at a temperature of lOO”C, others were treated in accordance with the manufacturer’s instructions and the untreated controls. (This removal was made by grinding under light pressures with copious water for lubrication and cooling. The surface was finished with a light polish, again with copious water for cooling and lubrication.) The procedure for measuring the microhardness was then repeated. RESULTS Using the processing cycles described, residual monomer levels of 0.27 + 0.03 per cent (cured for 16 h at 70°C + 3 h at 100°C) and 2.73 + 0.06 per cent (8 h at 70°C) were obtained from the two sets of acrylic denture base samples. The extraction and analysis methods for measuring levels of residual monomer proved to be reliable, with these levels being consistently achieved throughout this study. In view of the prolonged immersion procedures involved in this study, and the differences of opinion in the literature regarding the elution of monomer by water (Douglas and Bates, 1978; Austin and Basker, 1980), control specimen strips with high and low residual monomer content were stored in water at room temperature (18°C) for 100 days. From these, the levels of residual monomer did not change significantly, even after immersion for 100 days (2.62 f 0*05/2*50 f 0.09 and 0.27 + 0.03/0*32 f 0.05 respectively) (t = 1.824, d.f. = 8, P> O-5). It should also be noted that these specimens had not been immersed in water prior to these procedures. In specimens containing a low level of residual monomer, subjected to water at 50°C and at lOO”C, with and without the proprietary denture cleaner, no changes in residual monomer content were found after 100 soak cycles (Table I). Similarly, no change took place with higher residual monomer specimens when immersed at 50°C. However, for these, immersion in water at 100°C
monomer
and denture
bleaching
191
Table IL Light transmission through acrylic denture base samples with different levels of residual monomer after treatment with different soak solutions (100 cycles) expressed as a percentage of the transmission of control (untreated) samples (controls taken as 100 per cent light transmission)
Soak soln water temp. (“C) 50* 50* + proprietan/ denture cleaner 100 100 + proprietary denture cleaner
Light transmission (%) (* s.d) High residual Low residual monomer monomer 99*2
100
100
98*2
23rt2 23*2
23~1 23*2
n = 10 for each treatment. *Manufacturer’s recommended temperature.
Table l/1 Transverse bend strengths of acrylic denture base samples with different levels of residual monomer after treatment with different soak solutions (100 cycles)
Soak soln water temp. f”C) 18 (control) 50* 50* + proprietary denture cleaner 100 100 + proprietary denture cleaner
Transverse bend strength (k s.d) (MNm-l) Low residual High residual monomer monomer 79.8 f 3.9 7 7.4 + 2.4
74.6 f 2-9 73.1 f 1.9
76.1 f 2.2
72.0 f 1.9
66-O f 5.5
64.3 f 4.5
68.0 f 2.0
60.8 f 4-o
n = 10 for each treatment. *Manufacturer’s recommended temperature.
produced a significant reduction of 43 per cent after 100 soak cycles (t = 14.437, d.f. = 18, P < O*OOl) and it might be reasonable to assume that most, if not all of this reduction could be due to further polymerization. The proprietary denture cleaner may affect the reduction in level of residual monomer at 100°C and this is the subject of further investigation. After completion of 100 soak cycles, following the procedures recommended by the manufacturer of the proprietary denture cleaner, no apparent differences were observed between acrylic specimens containing high and low levels of residual monomer for: (a) visual appearance, (b) surface morphology (Fig. I), (c) light transmission (Table II) and (d) transverse bend strength (Table III). However, the level of residual monomer did affect the surface hardness with a significant reduction for those containing a high residual monomer content (P < 040 1) (Table IV). When the denture base acrylic (regardless of the level of residual monomer) was subjected to water at an initial temperature of 100°C (whether with or without the cleaner), (a) a marked whitening, (b) loss of translucency of the acrylic resin, (c) a loss of surface integrity (Fig. I), (cl) a reduction in light transmission of
192
J. Dent. 1989; 17: No. 4
a
b
d Fig. 1. Scanning electron microscope evidence of loss of surface integrity in whitened acrylic strips (a d) (water temperature 100°C) compared with relatively intact and smooth surface of unaffected strips (a, c) (water temperature 50°C) on completion of 100 soak cycles. Low residual monomer specimens: a, 50°C water temperature; 6, 100°C. High residual monomer specimens: c, 50°C; d, 100°C. Scale bar = 1Op.m.
per cent (t = 89.6, d.f. = 78, P < 0401) (Table II), (e) a reduction in the transverse bend strength (t = 9.925, d.f. = 78, P < O-001 ) (Table III) and df) an increase in the surface hardness (t = 10585, d.f. = 78, P < 0401) (Table IV) was observed. When 0.5 mm of the surface material was removed from those specimens subjected to water at 50°C no changes were found (Table v) when compared with the original measurement (Table IV). Exposure to water at 77
100°C during cleaning increased the hardness in the interior of the material containing the higher level of residual monomer towards that measured in material with a lower level of residual monomer. This is not surprising since the terminal boil was omitted during the curing of the former. In both cases, there is a significant difference between the hardness of the surface and interior. This supports the contention that the phenomena of bleaching is surface related.
Arab et ab: Residual monomer and denture bleaching
Tab/e /K Hardness number of acrylic denture base samples with different levels of residual monomer after treatment with different soak solutions (100 cycles)
Soak soln water temp. f”C) 18 (control) 50* 50* + proprietary denture cleaner 100 100 + proprietary denture cleaner
Hardness number (& s.d) (VHW Low residual High residual monomer monomer 17.8 f 0.9 18-l kO.7
13.6 f 0.2 14-2 f 0.6
17.8 f 0.5
13.8 f O-9
22.7 f 1.1
19.3 f 0.8
22.8 f 1 .O
19.4 + 2.0
n = 10 for each treatment. *Manufacturer’s recommended temperature.
DISCUSSION It has been suggested previously that the whitening or bleaching of acrylic resin dentures may be due to faulty processing. However, this suggestion had not been tested in the laboratory following cleaning procedures used by patients. From this study, the increased opacity, loss of surface integrity, reduction in transverse bend strength and increase in surface hardness after exposure to a cleaning procedure using boiling water, confirm results previously reported in ‘correctly’ cured acrylic specimens (Crawford et al., 1986; Arab et al., 1988). In addition, further support is given to the surface nature of the whitening effect by the changes observed in hardness values when the surface layer is removed from the affected strips. However, in view of similar changes being observed in both high and low residual monomer samples, when subject to patients’ cleaning procedures, it must be concluded that an elevated level of residual monomer does not play a principal role in the bleaching or whitening of the acrylic denture base material. Nevertheless, this does not rule out the proposition that higher levels of residual monomer give dentures a greater susceptibility to bleaching in a minority of patients who expel acetone in their breath due to their diabetic condition (Robinson et al., 1987). This may be described as an in vivo effect, distinct from the in vitro cleaning effect investigated here. However, it is worth noting that important physical properties such as transverse bend strength and surface hardness are affected by high levels of residual monomer, confirming observations made by other workers (Dougles and Bates, 1978; Jagger, 1978; Austin and Basker, 1982; Huggett et aZ.. 1985). It is probable that residual monomer adversely affects these properties by a plasticizing effect which effectively reduces interchain forces so that deformation could occur more easily under load.
193
Tab/e V: Hardness number of acrylic denture base samples with different levels of residual monomer after treatment with different solutions (100 cycles) when O-5 mm of surface layer removed
Soak soln water temp. PC) 18 (control) 50* 50* + proprietary denture cleaner 100 100 + proprietary denture cleaner
Hardness number (& s.d) (VHW Low residual High residual monomer monomer 17-8 f 0.4 17.7 f 0.4
13*7*0*1 13.7 it-5
17-4 f 0.4
13.3 f 0.4
18.4 f: 0.8
15.8 + 0.4
18.3 kO.8
15.8 f 0.3
n = 10 for each treatment. *Manufacturer’s recommended temperature.
This has important consequences for commercial dental laboratories using short/low temperature processing procedures and more seriously for denture-wearing patients, particularly those who exhibit a sensitivity to elevated levels of residual monomer (Ali et al., 1986). Acknowledgements Thanks are due to Mr D. D. Chalmers, Mrs H. Bennett and Mr J. McTear for technical assistance.
References Ali A., Bates J. F., Reynolds A. J. et al., (1986) The burning mouth sensation related to the wearing of acrylic dentures: an investigation. Br. Dent. J. 161, 444-447. Anthony D. H. and Gibbons P. ( 195 8) The nature and behaviour of denture cleaners. J. Prosthet. Dent. 8, 796-810. Arab J., Newton J. P. and Lloyd C. H. (1987) The effect of water temperature on denture ‘bleaching’. .I Dent. Res. 66, 885. Arab J., Newton J. P. and Lloyd C. H. (1988) The importance of water temperature in denture cleaning procedures. J. Dent. 16,277-281. Austin A. T. and Basker R M. (1980) The level of residual monomer in acrylic resin denture base materials. Br. Dent. J. 149,281-286. Austin A. T. and Basker R M. (1982) Residual monomer levels in denture bases. Br. Dent. J. 153, 424-426. Crawford C. A., Newton J. P. and Yemm R. (1985) Misuse of a denture cleaning agent. J. Dent. Res. 64, 673. Crawford C. A., Lloyd C. H., Newton J. P. et al. (1986) Denture bleaching: a laboratory simulation of patients’ cleaning procedures. J. Dent. 14, 258-261. Crawford C. A., Newton J. P. and Yemm R (1987) Bleached dentures: evidence of misuse of a denture cleaning agent. Dent. Update 14, 29-32. Douglas W. H. and Bates J. F. (1978) The determination of residual monomer in polymethylmethacrylate denture-base resins. J. Mater. Sci. 13, 2600-2604.
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J. Dent.
1989; 17: No. 4
Huggett R, Brooks S. and Bates J. F. (1985) Which curing cycle is best? Dent. Technician Techn. Suppl. 38, 11-16. Jagger R G. (1978) Effect of the curing cycle on some properties of a polymethacrylate denture base material. J. Oral Rehabil. 5, 151-157. McCabe J. F. and Basker R M. (1976) Tissue sensitivity to acrylic resin. Br. Dent. J. 140, 347-350. Osborne J. (1945) Clinical survey of acrylic resin dentures. Br. Dent. J. 78, 324-329.
Robinson J. G., McCabe J. F. and Storer R (1985) The whitening of acrylic dentures: the role of denture cleaners. Br. Dent. J. 159,247-250. Robinson J. G., McCabe J. F. and Storer R (1987) Denture bases: the effects of various treatments on clarity, strength and structure. J. Dent. 15, 159-165. Sexson J. C. and Phillips R W. (1951) Studies on the effects of abrasures on acrylic resins. J. Prosthet. Dent. 1,454-471.
Correspondence should be addressed to: Dr J. P. Newton, Department of Dental Prosthetics and Gerontology, The Dental School, University of Dundee DDl 4HN, UK.
Book
Reviews
Colour Atlas of a New Concept of Enamel Caries. L. K. Bandlish. Pp. 105. 1987. London, L. K. Bandlish. Softback, f 35.00.
The Social Psychology of Facial Appearance. Edited by R. Bull and N. Rumsey. Pp. 355. 1988. London, Springer Verlag. Hardback, DM 125.00.
In the nineteenth century it was quite common for scientists to publish their own books so as to propound their own theories. In recent years this has become a rare event, no doubt because of the cost of publishing. This book is one of the exceptions. It puts forward the author’s view on the site of approximal caries attack on teeth, and Mr Bandlish argues that this occurs at the contact, or attrition, area rather than below it, as has been the accepted wisdom for many years. The book consists of two parts. Part I, of 15 pages, is the text where his argument is developed. Part II, of the remaining 88 pages, consists entirely of 126 colour photographs of whole teeth or sections of teeth. These plates are accompanied by legends pointing out the caries in many teeth and all showing, or claiming to show, lesions at the contact areas. This is really the sum total of the book. The author obviously feels strongly that the teaching of the histology of enamel caries is wrong as to its site of initiation, but this thesis is, however, based solely on his clinical findings. There does not seem to have been a properly designed study, preferably in a blind manner, of a large number of teeth coupled with enamel histology to prove his theory. It so happens that this reviewer, for other reasons, at one time looked at the approximal surfaces of 3500 first or second premolars extracted for orthodontic purposes. Under a dissecting microscope the presence or absence of caries was noted in relation to the contact area, and in all cases caries was below this area. The author of this book does not seem to have carried out any similar research to prove his theory. The purpose of this book is to have dental scientists reconsider their views on the site of caries initiation. While many may read this book out of curiosity, the evidence presented is unconvincing. Besides dental scientists it is not clear who else should read this book. It would not be suitable for undergraduate students, without training in critical evaluation and the scientific method. Nor is it likely that general dental practitioners will be interested. Mr Bandlish is to be admired for making the effort to pursue his theory with the singlemindedness to publish a book on it, but his lack of scientific evidence detracts from its value. M. E. J. Cutzon
As the editors frankly admit at the outset, despite the inherently fascinating aspects of behaviour in response to facial appearance and the huge expenditure of the cosmetic industries, research in this area is characterized by modest quality and the lack of any comprehensive theoretical models that merit serious investigation. What they have succeeded in doing is to collect together an impressively wide range of published studies under various headings. Such an endeavour might easily have got out of hand but they have exercised a degree of selectivity and in doing so may well have omitted some readers’ pet notions. For instance, this reviewer is particularly interested in the contribution of cosmetic surgery and orthodontics to facial appearance, and its personal and social consequences, yet this topic hardly merits a mention, occupying just eight pages. On the other hand, the editors have classed the available research into six major areas in which facial appearance has been demonstrated to have an effect; personnal liking, persuasion and politics, criminal justice, education, children’s effect on adults and vice versa, and facial disfigurement. The penultimate chapter provides an outline of various psychological approaches to helping individuals suffering from the latter and the book closes with an attempt to define in more effective terms the basic issues which lie behind much of the research. They are no more successful in describing facial attractiveness than an educated lay man but they do imply that closeness to the social norm of appearance is helpful and certain facial attributes, such as dental appearance, eye colour, facial hair, are identified as being significant. In conclusion, five theoretical stances are outlined with the authors calling for more and better research, if possible in a real life environment where the interaction between appearance and behaviour might be better observed. This text is not one for bedtime reading; it is much too packed with references to published studies for that, but it does merit a place on any researcher’s shelves. The 800 quoted references are a testament to the industry of the authors and they do succeed in providing a structure for the better understanding of such an extensive and confusing area of study. A. R. Dabbs
17: 189-l
189
94
The effect of an elevated level of residual monomer on the whitening of a denture base and its physical properties J. Arab, J. P. Newton and C. H. Lloyd Depattment KEY WORDS: J. Dent
1989;
of Dental Prosthetics and Gerontology, The Dental School, University of Dundee, UK Residual monomers, Dentures, Cleaning, Physical properties 17: 189-l
94 (Received 27 June 1988;
reviewed 17 September
1988;
accepted 20 April 1989)
ABSTRACT It has been suggested that incomplete or faulty processing leading to a high level of residual monomer is a factor in denture whitening. Laboratory tests which simulate the cleaning procedures used by patients presenting with whitened dentures have been carried out on specimen strips of acrylic denture base material with high or low levels of residual monomer. The results show that these samples exhibit similar changes and that the high level of residual monomer had no effect on the whitening process, this being attributable to the cleaning method. The major factor responsible for whitening is confirmed as a high water temperature irrespective of the presence of the denture cleaning agent. However, some physical properties were adversely affected by a high residual monomer content (due to the processing method) which has serious consequences for denture-wearing patients. The use of short/low temperature curing cycles now common in many dental laboratories
should be avoided.
INTRODUCTION Historically, the ‘bleaching’ of acrylic resin dentures has been linked to the use of household bleach or similar domestic products by patients in their cleaning procedures (Osborne, 1945). However, it was not until patients with such whitened dentures were carefUlly questioned about the materials used ‘and the procedures adopted that evidence of the role of boiling water in cleaning procedures was established (Crawford et al., 1985,1987). This was confirmed in laboratory simulation of patients’ cleaning procedures on fully cured acrylic denture base material (Crawford et al., 1986; Arab et al., 1987, 1988). It has been suggested that incomplete or faulty processing, leading to a high level of residual monomer, is a factor in denture whitening (Sexson and Phillips, 195 1; Anthony and Gibbons, 1958; Robinson et aZ., 1985), as is the in wivo exposure to solvent vapour (Robinson et al., 1987). This is of increasing importance since short/low temperature curing cycles are now commonly used in
0 1989 Butterworth & Co Publishers Ltd. 0300~5712/89/0401894I6 $0300
many dental laboratories. The temperature and curing times are selected empirically by the technicians (Austin and Basker, 1982) and, in some cases, could result in the production of acrylic dentures containing higher levels of residual monomer. The purpose of this study was to establish the role of an elevated level of residual monomer in the whitening of an acrylic resin and to investigate the consequences upon some of its physical properties.
METHODS Specimen
preparation
Specimen strips of an acrylic denture base material (Croform X10, Davis, Schottlander and Davis Ltd, London, UK) 65 X 15 X 1.7 mm were produced from dental plaster moulds. A dough formed from 2.5 : 1 powder : liquid mix was packed in flasks under pressure. These were then subjected to two different curing cycles:
190
J. Dent. 1989;
17: No. 4
one set of specimens using a processing cycle of 16 h at 70°C followed by 3 h at lOO”C, a second set using a modified cycle of 8 h at 7O”C, known to produce an elevated level of residual monomer (Austin and Basker, 1980; Huggett et al., 1985). These specimens were then finished and polished using sandpaper, pumice and whiting. Immersion
procedures
a known quantity of the internal standard was injected into the column. Instrument settings were selected to produce well separated and defined peaks, and to remove the overwhelming effect of the massive alcohol peak during integration. The calibration of the instrument was checked against standard solutions containing known amounts of monomer before and after each set of monomer determinations.
Each set of acrylic strips was divided amongst four groups to be soaked in the following solutions:
Assessment
1. Proprietary denture cleaning agent (Steradent Denture Cleaning Tablets, Reckitt Dental Care Products, Hull, UK) dissolved in water at the temperature recommended by the manufacturer ( 1 tablet to 150 ml water, at 50°C). 2. Water at recommended temperature (150 ml, at 50°C). 3. Boiling water (150 ml, at 100°C) with proprietary denture cleaning agent (concentration as treatment 1). 4. Boiling water (150 ml, at 100°C).
Visual appearance
These temperatures refer to the temperature on immersion and the solutions were allowed to cool naturally through the soak period as would occur during normal cleaning procedures used by patients. Following a soak of 30 min (Crawford et al., 1986) the samples were stored in water at room temperature for 24 h. This daily procedure was carried out for 100 days with frequent inspection for degradation. Control acrylic strips were stored in water at room temperature (18°C) for 100 days without being subject to any cleaning procedure.
Measurement
of level of residual
monomer
Gas liquid chromatographic (GLC) analysis gives reliable values for the level of residual monomer in acrylic samples and has been used for analysing a number of denture base resins cured under a variety of conditions (McCabe and Basker, 1976; Douglas and Bates, 1978; Austin and Basker, 1980; Huggett et al., 1985). In this study, GLC analysis has been carried out using an HRGC 5300 Mega series GLC (Carla Erba Strumentazione, Milan, Italy) fitted with a Porapak Q column (80100 mesh, 1.2 m long, internal diameter 4 mm), operated with oven and injection temperatures of 200°C and a flame ionization detection temperature of 250°C. Nitrogen was used as the carrier gas at a rate of 45 ml min-I. The detector output was linked to a Shimadzu CR3A chromatopac integrator. Standard solvent extraction procedures were used to remove the residual monomer from the acrylic specimens (Huggett et al., 1985). Toluene was selected as an internal standard for chromatography since under these conditions the peak it produces is sharp and well separated from both the methanol and methylmethacrylate peaks. Of the methanolic extract, 2.5 ~1containing
of treated
specimens
Periodic visual inspection of the samples was carried out independently by three investigators. Any changes, particularly in colour (when compared with a control denture base specimen), were recorded. Surface characteristics Samples were cut from treated and control acrylic strips and the surfaces examined by scanning electron microscopy (JSM/35, Joel, Tokyo, Japan). A photographic record was obtained for the surface detail as a function of the number of immersions. Light transmission As previously reported (Crawford et al., 1985, 1986, 1987; Robinson et al., 1987), there is a change in the translucency of the acrylic denture base material during ‘bleaching’. To quantify any change, the light transmission through the treated and control samples was measured using a spectrophotometer (Unicam S.P.1800, Pye, Cambridge, UK) at a wavelength of 690 nm, this wavelength being the most sensitive for the material under test after completion of a wavelength scan analysis (Crawford et al., 1986; Arab et al., 1988). Transverse bend strength It is widely accepted that exposure to water at a high temperature is detrimental to the mechanical properties of acrylic materials. Therefore, samples of treated and control material were fractured by a three-point bend test using an Instron TTCM universal testing machine to obtain the transverse bend strength. Fracture took place at a cross-head speed of 10-4ms-‘, in air at room temperature. Ten specimens were broken (for each treatment) to obtain a mean value with a standard deviation. Surface hardness Surface hardness of the samples was investigated using a Vickers microhardness instrument (E. Leitz GmbH., Wetzlar, FRG). Measurements were taken at room temperature and under an indentation force of 1 N. Ten indentations were made for each sample. On completion of these measurements, the magnitude of the changes on the surface of the acrylic was compared
Arab et al.: Residual
Table L Percentage residual monomer content of acrylic denture base strips cured using two different processing cycles following treatment with different soak solutions (100 cycles) Soak soln water temp. f”C) 18 (control) 50* 50* + proprietary denture cleaner 100 100 + proprietary denture cleaner
% Residual monomer (f s.d) 16h at 70°C +3h at 100°C 8h at 70°C 0.27 f 0.02 0.33 f 0.01
2.62 f O-04 2.52 f 0.09
0.28 + O-02
2-48 kO.10
O-27 f 0.03
1.49 f 0.02
0.34 + 0.03
1.79 + 0.08
n = 10 for each treatment. *Manufacturer’s recommended temperature.
with the condition of the interior of the specimen. O-5 mm of the material was removed from the surface of those samples subjected to water at a temperature of lOO”C, others were treated in accordance with the manufacturer’s instructions and the untreated controls. (This removal was made by grinding under light pressures with copious water for lubrication and cooling. The surface was finished with a light polish, again with copious water for cooling and lubrication.) The procedure for measuring the microhardness was then repeated. RESULTS Using the processing cycles described, residual monomer levels of 0.27 + 0.03 per cent (cured for 16 h at 70°C + 3 h at 100°C) and 2.73 + 0.06 per cent (8 h at 70°C) were obtained from the two sets of acrylic denture base samples. The extraction and analysis methods for measuring levels of residual monomer proved to be reliable, with these levels being consistently achieved throughout this study. In view of the prolonged immersion procedures involved in this study, and the differences of opinion in the literature regarding the elution of monomer by water (Douglas and Bates, 1978; Austin and Basker, 1980), control specimen strips with high and low residual monomer content were stored in water at room temperature (18°C) for 100 days. From these, the levels of residual monomer did not change significantly, even after immersion for 100 days (2.62 f 0*05/2*50 f 0.09 and 0.27 + 0.03/0*32 f 0.05 respectively) (t = 1.824, d.f. = 8, P> O-5). It should also be noted that these specimens had not been immersed in water prior to these procedures. In specimens containing a low level of residual monomer, subjected to water at 50°C and at lOO”C, with and without the proprietary denture cleaner, no changes in residual monomer content were found after 100 soak cycles (Table I). Similarly, no change took place with higher residual monomer specimens when immersed at 50°C. However, for these, immersion in water at 100°C
monomer
and denture
bleaching
191
Table IL Light transmission through acrylic denture base samples with different levels of residual monomer after treatment with different soak solutions (100 cycles) expressed as a percentage of the transmission of control (untreated) samples (controls taken as 100 per cent light transmission)
Soak soln water temp. (“C) 50* 50* + proprietan/ denture cleaner 100 100 + proprietary denture cleaner
Light transmission (%) (* s.d) High residual Low residual monomer monomer 99*2
100
100
98*2
23rt2 23*2
23~1 23*2
n = 10 for each treatment. *Manufacturer’s recommended temperature.
Table l/1 Transverse bend strengths of acrylic denture base samples with different levels of residual monomer after treatment with different soak solutions (100 cycles)
Soak soln water temp. f”C) 18 (control) 50* 50* + proprietary denture cleaner 100 100 + proprietary denture cleaner
Transverse bend strength (k s.d) (MNm-l) Low residual High residual monomer monomer 79.8 f 3.9 7 7.4 + 2.4
74.6 f 2-9 73.1 f 1.9
76.1 f 2.2
72.0 f 1.9
66-O f 5.5
64.3 f 4.5
68.0 f 2.0
60.8 f 4-o
n = 10 for each treatment. *Manufacturer’s recommended temperature.
produced a significant reduction of 43 per cent after 100 soak cycles (t = 14.437, d.f. = 18, P < O*OOl) and it might be reasonable to assume that most, if not all of this reduction could be due to further polymerization. The proprietary denture cleaner may affect the reduction in level of residual monomer at 100°C and this is the subject of further investigation. After completion of 100 soak cycles, following the procedures recommended by the manufacturer of the proprietary denture cleaner, no apparent differences were observed between acrylic specimens containing high and low levels of residual monomer for: (a) visual appearance, (b) surface morphology (Fig. I), (c) light transmission (Table II) and (d) transverse bend strength (Table III). However, the level of residual monomer did affect the surface hardness with a significant reduction for those containing a high residual monomer content (P < 040 1) (Table IV). When the denture base acrylic (regardless of the level of residual monomer) was subjected to water at an initial temperature of 100°C (whether with or without the cleaner), (a) a marked whitening, (b) loss of translucency of the acrylic resin, (c) a loss of surface integrity (Fig. I), (cl) a reduction in light transmission of
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J. Dent. 1989; 17: No. 4
a
b
d Fig. 1. Scanning electron microscope evidence of loss of surface integrity in whitened acrylic strips (a d) (water temperature 100°C) compared with relatively intact and smooth surface of unaffected strips (a, c) (water temperature 50°C) on completion of 100 soak cycles. Low residual monomer specimens: a, 50°C water temperature; 6, 100°C. High residual monomer specimens: c, 50°C; d, 100°C. Scale bar = 1Op.m.
per cent (t = 89.6, d.f. = 78, P < 0401) (Table II), (e) a reduction in the transverse bend strength (t = 9.925, d.f. = 78, P < O-001 ) (Table III) and df) an increase in the surface hardness (t = 10585, d.f. = 78, P < 0401) (Table IV) was observed. When 0.5 mm of the surface material was removed from those specimens subjected to water at 50°C no changes were found (Table v) when compared with the original measurement (Table IV). Exposure to water at 77
100°C during cleaning increased the hardness in the interior of the material containing the higher level of residual monomer towards that measured in material with a lower level of residual monomer. This is not surprising since the terminal boil was omitted during the curing of the former. In both cases, there is a significant difference between the hardness of the surface and interior. This supports the contention that the phenomena of bleaching is surface related.
Arab et ab: Residual monomer and denture bleaching
Tab/e /K Hardness number of acrylic denture base samples with different levels of residual monomer after treatment with different soak solutions (100 cycles)
Soak soln water temp. f”C) 18 (control) 50* 50* + proprietary denture cleaner 100 100 + proprietary denture cleaner
Hardness number (& s.d) (VHW Low residual High residual monomer monomer 17.8 f 0.9 18-l kO.7
13.6 f 0.2 14-2 f 0.6
17.8 f 0.5
13.8 f O-9
22.7 f 1.1
19.3 f 0.8
22.8 f 1 .O
19.4 + 2.0
n = 10 for each treatment. *Manufacturer’s recommended temperature.
DISCUSSION It has been suggested previously that the whitening or bleaching of acrylic resin dentures may be due to faulty processing. However, this suggestion had not been tested in the laboratory following cleaning procedures used by patients. From this study, the increased opacity, loss of surface integrity, reduction in transverse bend strength and increase in surface hardness after exposure to a cleaning procedure using boiling water, confirm results previously reported in ‘correctly’ cured acrylic specimens (Crawford et al., 1986; Arab et al., 1988). In addition, further support is given to the surface nature of the whitening effect by the changes observed in hardness values when the surface layer is removed from the affected strips. However, in view of similar changes being observed in both high and low residual monomer samples, when subject to patients’ cleaning procedures, it must be concluded that an elevated level of residual monomer does not play a principal role in the bleaching or whitening of the acrylic denture base material. Nevertheless, this does not rule out the proposition that higher levels of residual monomer give dentures a greater susceptibility to bleaching in a minority of patients who expel acetone in their breath due to their diabetic condition (Robinson et al., 1987). This may be described as an in vivo effect, distinct from the in vitro cleaning effect investigated here. However, it is worth noting that important physical properties such as transverse bend strength and surface hardness are affected by high levels of residual monomer, confirming observations made by other workers (Dougles and Bates, 1978; Jagger, 1978; Austin and Basker, 1982; Huggett et aZ.. 1985). It is probable that residual monomer adversely affects these properties by a plasticizing effect which effectively reduces interchain forces so that deformation could occur more easily under load.
193
Tab/e V: Hardness number of acrylic denture base samples with different levels of residual monomer after treatment with different solutions (100 cycles) when O-5 mm of surface layer removed
Soak soln water temp. PC) 18 (control) 50* 50* + proprietary denture cleaner 100 100 + proprietary denture cleaner
Hardness number (& s.d) (VHW Low residual High residual monomer monomer 17-8 f 0.4 17.7 f 0.4
13*7*0*1 13.7 it-5
17-4 f 0.4
13.3 f 0.4
18.4 f: 0.8
15.8 + 0.4
18.3 kO.8
15.8 f 0.3
n = 10 for each treatment. *Manufacturer’s recommended temperature.
This has important consequences for commercial dental laboratories using short/low temperature processing procedures and more seriously for denture-wearing patients, particularly those who exhibit a sensitivity to elevated levels of residual monomer (Ali et al., 1986). Acknowledgements Thanks are due to Mr D. D. Chalmers, Mrs H. Bennett and Mr J. McTear for technical assistance.
References Ali A., Bates J. F., Reynolds A. J. et al., (1986) The burning mouth sensation related to the wearing of acrylic dentures: an investigation. Br. Dent. J. 161, 444-447. Anthony D. H. and Gibbons P. ( 195 8) The nature and behaviour of denture cleaners. J. Prosthet. Dent. 8, 796-810. Arab J., Newton J. P. and Lloyd C. H. (1987) The effect of water temperature on denture ‘bleaching’. .I Dent. Res. 66, 885. Arab J., Newton J. P. and Lloyd C. H. (1988) The importance of water temperature in denture cleaning procedures. J. Dent. 16,277-281. Austin A. T. and Basker R M. (1980) The level of residual monomer in acrylic resin denture base materials. Br. Dent. J. 149,281-286. Austin A. T. and Basker R M. (1982) Residual monomer levels in denture bases. Br. Dent. J. 153, 424-426. Crawford C. A., Newton J. P. and Yemm R. (1985) Misuse of a denture cleaning agent. J. Dent. Res. 64, 673. Crawford C. A., Lloyd C. H., Newton J. P. et al. (1986) Denture bleaching: a laboratory simulation of patients’ cleaning procedures. J. Dent. 14, 258-261. Crawford C. A., Newton J. P. and Yemm R (1987) Bleached dentures: evidence of misuse of a denture cleaning agent. Dent. Update 14, 29-32. Douglas W. H. and Bates J. F. (1978) The determination of residual monomer in polymethylmethacrylate denture-base resins. J. Mater. Sci. 13, 2600-2604.
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Huggett R, Brooks S. and Bates J. F. (1985) Which curing cycle is best? Dent. Technician Techn. Suppl. 38, 11-16. Jagger R G. (1978) Effect of the curing cycle on some properties of a polymethacrylate denture base material. J. Oral Rehabil. 5, 151-157. McCabe J. F. and Basker R M. (1976) Tissue sensitivity to acrylic resin. Br. Dent. J. 140, 347-350. Osborne J. (1945) Clinical survey of acrylic resin dentures. Br. Dent. J. 78, 324-329.
Robinson J. G., McCabe J. F. and Storer R (1985) The whitening of acrylic dentures: the role of denture cleaners. Br. Dent. J. 159,247-250. Robinson J. G., McCabe J. F. and Storer R (1987) Denture bases: the effects of various treatments on clarity, strength and structure. J. Dent. 15, 159-165. Sexson J. C. and Phillips R W. (1951) Studies on the effects of abrasures on acrylic resins. J. Prosthet. Dent. 1,454-471.
Correspondence should be addressed to: Dr J. P. Newton, Department of Dental Prosthetics and Gerontology, The Dental School, University of Dundee DDl 4HN, UK.
Book
Reviews
Colour Atlas of a New Concept of Enamel Caries. L. K. Bandlish. Pp. 105. 1987. London, L. K. Bandlish. Softback, f 35.00.
The Social Psychology of Facial Appearance. Edited by R. Bull and N. Rumsey. Pp. 355. 1988. London, Springer Verlag. Hardback, DM 125.00.
In the nineteenth century it was quite common for scientists to publish their own books so as to propound their own theories. In recent years this has become a rare event, no doubt because of the cost of publishing. This book is one of the exceptions. It puts forward the author’s view on the site of approximal caries attack on teeth, and Mr Bandlish argues that this occurs at the contact, or attrition, area rather than below it, as has been the accepted wisdom for many years. The book consists of two parts. Part I, of 15 pages, is the text where his argument is developed. Part II, of the remaining 88 pages, consists entirely of 126 colour photographs of whole teeth or sections of teeth. These plates are accompanied by legends pointing out the caries in many teeth and all showing, or claiming to show, lesions at the contact areas. This is really the sum total of the book. The author obviously feels strongly that the teaching of the histology of enamel caries is wrong as to its site of initiation, but this thesis is, however, based solely on his clinical findings. There does not seem to have been a properly designed study, preferably in a blind manner, of a large number of teeth coupled with enamel histology to prove his theory. It so happens that this reviewer, for other reasons, at one time looked at the approximal surfaces of 3500 first or second premolars extracted for orthodontic purposes. Under a dissecting microscope the presence or absence of caries was noted in relation to the contact area, and in all cases caries was below this area. The author of this book does not seem to have carried out any similar research to prove his theory. The purpose of this book is to have dental scientists reconsider their views on the site of caries initiation. While many may read this book out of curiosity, the evidence presented is unconvincing. Besides dental scientists it is not clear who else should read this book. It would not be suitable for undergraduate students, without training in critical evaluation and the scientific method. Nor is it likely that general dental practitioners will be interested. Mr Bandlish is to be admired for making the effort to pursue his theory with the singlemindedness to publish a book on it, but his lack of scientific evidence detracts from its value. M. E. J. Cutzon
As the editors frankly admit at the outset, despite the inherently fascinating aspects of behaviour in response to facial appearance and the huge expenditure of the cosmetic industries, research in this area is characterized by modest quality and the lack of any comprehensive theoretical models that merit serious investigation. What they have succeeded in doing is to collect together an impressively wide range of published studies under various headings. Such an endeavour might easily have got out of hand but they have exercised a degree of selectivity and in doing so may well have omitted some readers’ pet notions. For instance, this reviewer is particularly interested in the contribution of cosmetic surgery and orthodontics to facial appearance, and its personal and social consequences, yet this topic hardly merits a mention, occupying just eight pages. On the other hand, the editors have classed the available research into six major areas in which facial appearance has been demonstrated to have an effect; personnal liking, persuasion and politics, criminal justice, education, children’s effect on adults and vice versa, and facial disfigurement. The penultimate chapter provides an outline of various psychological approaches to helping individuals suffering from the latter and the book closes with an attempt to define in more effective terms the basic issues which lie behind much of the research. They are no more successful in describing facial attractiveness than an educated lay man but they do imply that closeness to the social norm of appearance is helpful and certain facial attributes, such as dental appearance, eye colour, facial hair, are identified as being significant. In conclusion, five theoretical stances are outlined with the authors calling for more and better research, if possible in a real life environment where the interaction between appearance and behaviour might be better observed. This text is not one for bedtime reading; it is much too packed with references to published studies for that, but it does merit a place on any researcher’s shelves. The 800 quoted references are a testament to the industry of the authors and they do succeed in providing a structure for the better understanding of such an extensive and confusing area of study. A. R. Dabbs