J, Dent. 1991; 19: 245-248
245
Effect of the curing cycle on acrylic denture base glass transition temperatures V. Jerolimov, R. G. Jagger* and P. J. Milward* Department of Removable Prosthetics, University of Zagreb, Faculty of Stomatology, Zagreb, Yugoslavia and *Department of Prosthetic Dentistry, University of Wales College of Medicine, Dental School, Cardiff, South Wales, UK
ABSTRACT Glass transition temperature of a polymer is an important physical property which may have a major effect on the dimensional stability of denture base materials. Thermomechanical analysis has been used to determine the glass transition temperature of specimens of denture base materials which had been produced by various selected curing cycles. It was found that different curing cycles produced variations in glass transition temperature of up to 20°C. KEY WORDS: Acrylic resins, Denture base materials, Properties J. Dent. 1991; 1991)
19: 245-248
(Received 14 January 199 1; reviewed 5 February 199 1; accepted 18 February
Correspondence should be addressed to: Mr R. G. Jagger, Department of Prosthetic Dentistry, University of Wales College of Medicine, Dental School, Heath Park, Cardiff CF4 4XY, South Wales, UK.
INTRODUCTION Acrylic
resins
produced
by the dough-moulded
technique
are the most commonly used denture base materials. The nature and duration of the conditions to which the mould is subjected during processing are described as the curing cycle. The effect of the curing cycle on the mechanical properties of the resultant material is well documented (Jagger, 1978; Huggett et al., 1984,1985). It has been shown that the residual monomer concentration is the most important parameter in determining variations in mechanical properties of a denture base material and that high levels of residual monomer may produce mucosal irritation (Austin and Basker, 1980). All polymers show a large variation of physical and mechanical properties with temperature. At room temperature acrylic resins are hard and glass like. When the temperature is increased a critical temperature is reached at which a transition occurs to a softer, more flexible material. Such a glass transition occurs over a finite temperature interval, but is still realized abruptly enough to merit the term ‘glass transition temperature’
(T,). @1991 Buttenvorth-Heinemann 0300-5712/91/040245-04
Ltd.
During the normal lifetime of a denture it would be expected that the denture base material would not experience temperatures in excess of its Tg. However, there are instances, for example, during polishing or cleaning in hot water, when the materials may be subjected to temperatures which approach or are in excess of this critical value (Braden and Stafford, 1968). In such cases the material may undergo irreversible distortion since very small forces are then capable of deforming the materials. TB is related to the degree of cure of a denture base material, since the glass transition temperature is directly related to molecular weight (Fox and Loshaek, 1955) and should be decreased by the plasticizing influence of residual monomers (Barsby and Braden, 1981). A variety of techniques are available for Tg determination and the value for Tg will depend on the method of determination. Huggett et al. (1990) compared several analytical techniques for measurement of the Tg of denture base acrylic resins and concluded that thermowhich measures dimensional mechanical analysis, change as a function of temperature and time under load or zero load, is a convenient and reliable technique.
246
J. Dent. 199 1; 19: No. 4
3. Long cure. The flasks were placed in cold water and the temperature raised to 70°C and maintained for 14 h. The total curing time was approximately 15 h.
It was considered of interest therefore to use thermomechanical analysis to determine the effect of the curing cycle on the Ts of acrylic resin denture base materials.
Specimens were cured and trimmed to a dimension of 7 mm length and 5 mm diameter. All specimens were conditioned in water at 37°C for 7 days prior to testing. Measurements of Ts were determined using a Stanton Redcroft Thermomechanical Analyser, model 691 (PL Thermal Sciences Ltd, Epsom, UK) linked to a conventional x-y recorder. The measuring procedure included a probe of 1 mm surface area at rest on a cylindrical specimen of 5 mm diameter and 7 mm length. The probe was connected to a transducer which allowed vertical movement of the probe to be monitored on the y-axis of the recorder. A removable heating cuff and thermos flask surrounded the probe and specimen. A thermocouple was located close to the specimen and the temperature variation was displayed on the x-axis of the recorder. The system was regulated from a control unit. A nominal load of 5 g was used throughout the study. This produced sufficient probe sensitivity. T’s values are dependent on rate of heating (McCabe and Wilson, 1980). A rate of lO”C/min was selected for this study as this allowed a reasonable time for evaluation of Ts without excessive heating which occurs at higher rates yielding elevated values of Ts. The glass transition temperature is a range of temperature and not a precise point. A Tg value was obtained for each of the materials by calculating from the trace as shown in Fig. 1. The thermocouple response was measured in millivolts. In order to convert this value to degrees centigrade, a calibration graph of temperature in “C against millivolts is supplied by the manufacturer. Ten specimens were tested for each material produced by each curing cycle.
MATERIALS AND METHODS Three heat-cured acrylic resin materials were selected: Trevalon, a conventional denture base material (AD. International Ltd, Weybridge, UK); Lucitone 199, a high impact acrylic resin material (A.D. International Ltd, Weybridge, UK); a poly(methylmethacrylate) homopolymer TSI 195 (Cole Polymer Ltd, Croydon, UK). The monomer liquid used for the homopolymer was pure methylmethacrylate with 0.01 per cent quinol (BDH Ltd, Poole, UK). Specimens were produced following standardized methods of preparation. The moulds were prepared by investing master blanks in gypsum according to conventional dental flasking procedures. After the master blanks were removed, holes were made using a 5 mm drill bit. The mixing ratio was 3.5 : 1 v/v. The polymerization was effected in thermostatically controlled water-baths at the curing cycles given below. 1. Short cure. The flasks were immersed in boiling water with the heat source removed for 20 min. The water was then brought to the boil and maintained at 100“C for 10 min. The total time was approximately 40 min. 2. Medium cure. The flasks were placed in cold water and the temperature raised to 70°C and maintained at that temperature for 7 h. The temperature was raised to 100°C and maintained at that temperature for 3 h.
Room Temperature
Glass Transition Temperature Expansion
t
Penetration
Temperature
Fig. 1. Determination of the r, from the recorder trace.
Jerolimov et al.: Effect of the curing cycle on denture base T,
247
Table 1. Effect of the curing cycle on r, (“C) Material Trevalon TSI 195 Lucitone 199
x (i)
Short s.d. (+)
C.V.(%)
111.6 112.7 106.5
1.12 0.79 1.63
1.0 0.7 1.53
Curing cycles Medium x (i) s.d. (-+) C.V.(%) 120.0 1 17.9 1 10.5
RESULTS The results of the Ts determinations for the three materials are given in Table I. There was a highly significant difference in Tg due to the different curing cycles for all three materials investigated (P < 0.001). The range of the T, produced by different curing cycles was dependent on the material used. The largest range (19.5”C) was for Trevalon. When the long curing cycle with terminal boil was used, no significant difference between specimens was obtained. With medium and short cycles no difference was observed between Trevalon and TS1195 values (P > 0.05) but a significant difference was observed between Trevalon and Lucitone (P < 0.001) and between TS1195 and Lucitone (P < 0.001).
DISCUSSION Manufacturers of denture base materials may suggest several recommended curing cycles for their products. Most dental technicians have preferred curing cycles and their selection will be influenced by the working time available. The curing cycles selected for the present study are thought to represent a range of curing cycles that may be commonly used in dental production laboratories. Medium or long cure cycles of more than 7 h at 70°C and which include a terminal boil are to be preferred. Such cycles produce bases with low residual monomer levels and ensure that mechanical properties are optimized without reducing the dimensional accuracy of the bases (Huggett et al., 1985). Thermomechanical analysis has been shown to be a convenient and reliable method to be employed as an evaluation technique for Tg of denture base materials (Huggett et al., 1990). Tg values should be regarded as approximate since the value for a given specimen will depend on method of determination and on heating rate (McCabe and Wilson, 1980). The heating rate of lO”C/ min was selected for the present study as it provided sufficient probe sensitivity but avoided excessive heating which yields elevated values of Tg The effect of the curing cycles used in the present study on the Tg of the selected materials was considerable and highly significant in all cases. The curing cycle which did not include a period at 100°C produced specimens with
1.19 1.01 1.04
0.99 0.86 0.94
x (i)
Long s.d. (5)
C.V.(%)
100.5 103.1 101.7
1.0 1.99 1.97
0.99 1.94 1.94
the lowest Tg. The curing cycle which includes 3 h at 100°C produced specimens with the highest Tg These results reflect the levels of residual monomer in those specimens. The effect of the curing cycle on the high impact material, Lucitone, was less than that on the other two materials. Lucitone, the high impact material, contains a butadiene-styrene rubber which influences the plasticizing effect of the monomer, thus reducing Tg. In cycles without the terminal boil, the plasticizing effect of the residual monomer disguised the modifying effect of the butadiene-styrene. Although a high Tg provides resistance to the adverse effects of high temperatures, a high Tg can produce internal stresses during curing of a heat-cured material which could result in dimensional accuracy (McCabe and Wilson, 1980). It is considered unlikely that the Tg range obtained here could produce dimensional differences that would be of clinical significance. The curing cycle of 7 h at 70°C plus 3 h at 100°C can be recommended to produce specimens with a high Tg as well as optimum mechanical properties. It is concluded that: 1. The curing cycle has considerable influence on the Tg of the resultant denture base specimen.
2. Curing cycles which do not include a period at 100°C produce denture base resins with the lowest Tg. 3. The curing cycle of 7 h at 70°C and 3 h at 100°C can be recommended to produce specimens with high Tg as well as optimum mechanical properties.
References
Austin A T. and Basker R. M. (1980) Residual monomer in acrylic denture base materials with particular reference to a modified method of analysis. Br. Dent. J. 149,281-286. Barsby M. J. and Braden M. (1981) Visco-elastic properties of pour (fluid) denture base resins. J. Dent. Res. 60, 146-148. Braden M. and Stafford G. D. (1968) Visco-elastic properties of some denture base materials. J. Dent. Res. 47, 519-523. Fox T. G. and Loshaek S. (1955) Influence of molecular weight and degree of crosslinking on the specific volume and glass temperature of polymers. .I Polymer Sci. 371-390. Huggett R., Brooks S. C. and Bates J. F. (1984) The effect of different curing cycles on levels of residual monomer in acrylic resin denture base materials. Quintessence Dent. Technol. 8, 365-371.
248
J. Dent. 1991; 19: No. 4
Huggett R., Brooks S. C. and Bates J. F. (1985) Which curing cycle is best. Dent Technician Tech. SuppI. 38, 11-16. Huggett R., Brooks S. C., Campbell A. M. et al. (1990) Evaluation of analytical techniques for measurement of denture base acrylic resin glass transition temperature. Dent Mater. 6, 17-19.
Jagger R. G. (1978) Effect of the curing cycle on some properties of a polymethylmethacrylate denture base material. J. Oral Rehabil. 5, 151-157. McCabe J. F. and Wilson H. J. (1980) The use of differential scanning calorimetry for the evaluation of dental materials. Part II Denture base materials. J. Oral Rehabil. 7, 235-243.
Book Review Introductory Medical Statistics, 2nd edition. FL F. Mould. 1989. Pp. 192. Bristol, Adam Hilger. Softback, E9.95. Statistics in Dentistry. J. S. Bulman and J. F. Osborn. 1989. Pp. 118. London, British Dental Journal. Softback, f 6.95. Statistics with Confidence-Confidence Intervals and Statistical Guidelines. M. J. Gardner and D. G. Altman. 1989. Pp.1 40. London, British Medical Association. Softback, f 7.95. Statistical methods have become thoroughly assimilated into the practice of dental research in the last two decades and indeed the teaching of statistics is now accepted as an important part of the undergraduate dental curriculum. It is thus timely to be able to review three statistical books that have recently been published. The first of the three, by R. F. Mould, provides a general introduction to the subject of medical statistics and follows a fairly conventional pattern for guides of this sort. There is a sound but predictable exposition of basic material in the first half of the book (data presentation, descriptive statistics, the normal distribution, probability theory, etc.), followed by a series of chapters that present in turn the major classes of statistical test: chi-squared tests, t tests, analysis of variance, non-parametric procedures, regression and correlation. The book is clearly written, diagrams are clear and there are many apt working examples; the treatment is rigorous but not too mathematical. There is a rather egregious attempt to leaven the presentation by incorporating cartoons and aphorisms, but these intrusions do not detract from a sound, workmanlike guide that could form a useful course textbook or serve as an introduction to statistical techniques for the neophyte (or not so neophyte) researcher. Whilst Mould’s book is addressed at a general medical audience, Bulman and Osborn’s slim volume is aimed specifically at the dental practitioner. The book conveniently collects together the useful series of articles that first appeared in the British Dental Journal, supplemented by some further chapters and a set of worked problems. The book very much brings to mind that old friend Statistics at Square One published some years ago by the British Medical Journal. The authors’ main purpose is to introduce statistical concepts to the statistically naive dental practitioner in order to assist him/her in reading scientific reports. There are many useful examples and the book succeeds to qualified degree. However, the treatment is necessarily superficial and the book lacks the systematic development of a
textbook; it would be of only limited value to the serious researcher or as a course-book. The final book in the clutch is a much more interesting production. Over the last several years, Sheila Gore, Douglas Altman, Martin Gardner and others have contributed to a highly distinguished, often provocative, series of articles in the British Medical Journal dealing with statistical methods in its ‘Statistics in Medicine’ column. In 1986, Gardner and Altman wrote a rather iconoclastic article which, whilst welcoming the growing adoption of statistical procedures in medical research, inveighed against the obsessive use of significance tests at the expense of more appropriate and informative techniques. Gardner and Altman’s argument was simple but cogent. Consider a piece of research concerned with blood pressure in diabetics. Mean blood pressure is determined for a sample of diabetics and a sample of normals. Significance testing allows refutation (or otherwise) of the null hypothesis that the observed differences could have arisen by sampling variation. Testing thus answers one rather narrow question; are these two samples equivalent or not? Gardner and Altman’s case in their 1986 paper was for a less restrictive approach to statistical analysis. Surely, to paraphrase their argument, the reason for examining diabetics is not just to determine whether they have qualitatively higher blood pressure than normals. Do we not wish to quantify these differences? Are we not interested in what the blood pressure of diabetics actually is? In essence, Gardner and Altman were arguing that statistical estimation is a more fundamental activity than significance testing. Constructing confidence intervals is at the core of the estimation process, allowing quantitative inferences to be made from samples. Moreover, significance testing may be done, if required, within a general confidence interval approach. Following their original article, a set of further pieces appeared describing procedures for calculating confidence limits for the more common statistical indices. These articles and the original have now been gathered together by Gardner and Altman, revised and published in book form. I believe there is much to be said for their thesis that estimation has been neglected and significance testing given undue prominence; Mould’s book as a case in point makes only two cursory references to confidence intervals. A disincentive to the confidence interval approach is that the formulae are often rather tortuous; lest the unsophisticated researcher is put off, a computer program has been made available by Gardner to provide assistance. I wholeheartedly welcome and commend this provocative and useful book. D. G. Wastell
245
Effect of the curing cycle on acrylic denture base glass transition temperatures V. Jerolimov, R. G. Jagger* and P. J. Milward* Department of Removable Prosthetics, University of Zagreb, Faculty of Stomatology, Zagreb, Yugoslavia and *Department of Prosthetic Dentistry, University of Wales College of Medicine, Dental School, Cardiff, South Wales, UK
ABSTRACT Glass transition temperature of a polymer is an important physical property which may have a major effect on the dimensional stability of denture base materials. Thermomechanical analysis has been used to determine the glass transition temperature of specimens of denture base materials which had been produced by various selected curing cycles. It was found that different curing cycles produced variations in glass transition temperature of up to 20°C. KEY WORDS: Acrylic resins, Denture base materials, Properties J. Dent. 1991; 1991)
19: 245-248
(Received 14 January 199 1; reviewed 5 February 199 1; accepted 18 February
Correspondence should be addressed to: Mr R. G. Jagger, Department of Prosthetic Dentistry, University of Wales College of Medicine, Dental School, Heath Park, Cardiff CF4 4XY, South Wales, UK.
INTRODUCTION Acrylic
resins
produced
by the dough-moulded
technique
are the most commonly used denture base materials. The nature and duration of the conditions to which the mould is subjected during processing are described as the curing cycle. The effect of the curing cycle on the mechanical properties of the resultant material is well documented (Jagger, 1978; Huggett et al., 1984,1985). It has been shown that the residual monomer concentration is the most important parameter in determining variations in mechanical properties of a denture base material and that high levels of residual monomer may produce mucosal irritation (Austin and Basker, 1980). All polymers show a large variation of physical and mechanical properties with temperature. At room temperature acrylic resins are hard and glass like. When the temperature is increased a critical temperature is reached at which a transition occurs to a softer, more flexible material. Such a glass transition occurs over a finite temperature interval, but is still realized abruptly enough to merit the term ‘glass transition temperature’
(T,). @1991 Buttenvorth-Heinemann 0300-5712/91/040245-04
Ltd.
During the normal lifetime of a denture it would be expected that the denture base material would not experience temperatures in excess of its Tg. However, there are instances, for example, during polishing or cleaning in hot water, when the materials may be subjected to temperatures which approach or are in excess of this critical value (Braden and Stafford, 1968). In such cases the material may undergo irreversible distortion since very small forces are then capable of deforming the materials. TB is related to the degree of cure of a denture base material, since the glass transition temperature is directly related to molecular weight (Fox and Loshaek, 1955) and should be decreased by the plasticizing influence of residual monomers (Barsby and Braden, 1981). A variety of techniques are available for Tg determination and the value for Tg will depend on the method of determination. Huggett et al. (1990) compared several analytical techniques for measurement of the Tg of denture base acrylic resins and concluded that thermowhich measures dimensional mechanical analysis, change as a function of temperature and time under load or zero load, is a convenient and reliable technique.
246
J. Dent. 199 1; 19: No. 4
3. Long cure. The flasks were placed in cold water and the temperature raised to 70°C and maintained for 14 h. The total curing time was approximately 15 h.
It was considered of interest therefore to use thermomechanical analysis to determine the effect of the curing cycle on the Ts of acrylic resin denture base materials.
Specimens were cured and trimmed to a dimension of 7 mm length and 5 mm diameter. All specimens were conditioned in water at 37°C for 7 days prior to testing. Measurements of Ts were determined using a Stanton Redcroft Thermomechanical Analyser, model 691 (PL Thermal Sciences Ltd, Epsom, UK) linked to a conventional x-y recorder. The measuring procedure included a probe of 1 mm surface area at rest on a cylindrical specimen of 5 mm diameter and 7 mm length. The probe was connected to a transducer which allowed vertical movement of the probe to be monitored on the y-axis of the recorder. A removable heating cuff and thermos flask surrounded the probe and specimen. A thermocouple was located close to the specimen and the temperature variation was displayed on the x-axis of the recorder. The system was regulated from a control unit. A nominal load of 5 g was used throughout the study. This produced sufficient probe sensitivity. T’s values are dependent on rate of heating (McCabe and Wilson, 1980). A rate of lO”C/min was selected for this study as this allowed a reasonable time for evaluation of Ts without excessive heating which occurs at higher rates yielding elevated values of Ts. The glass transition temperature is a range of temperature and not a precise point. A Tg value was obtained for each of the materials by calculating from the trace as shown in Fig. 1. The thermocouple response was measured in millivolts. In order to convert this value to degrees centigrade, a calibration graph of temperature in “C against millivolts is supplied by the manufacturer. Ten specimens were tested for each material produced by each curing cycle.
MATERIALS AND METHODS Three heat-cured acrylic resin materials were selected: Trevalon, a conventional denture base material (AD. International Ltd, Weybridge, UK); Lucitone 199, a high impact acrylic resin material (A.D. International Ltd, Weybridge, UK); a poly(methylmethacrylate) homopolymer TSI 195 (Cole Polymer Ltd, Croydon, UK). The monomer liquid used for the homopolymer was pure methylmethacrylate with 0.01 per cent quinol (BDH Ltd, Poole, UK). Specimens were produced following standardized methods of preparation. The moulds were prepared by investing master blanks in gypsum according to conventional dental flasking procedures. After the master blanks were removed, holes were made using a 5 mm drill bit. The mixing ratio was 3.5 : 1 v/v. The polymerization was effected in thermostatically controlled water-baths at the curing cycles given below. 1. Short cure. The flasks were immersed in boiling water with the heat source removed for 20 min. The water was then brought to the boil and maintained at 100“C for 10 min. The total time was approximately 40 min. 2. Medium cure. The flasks were placed in cold water and the temperature raised to 70°C and maintained at that temperature for 7 h. The temperature was raised to 100°C and maintained at that temperature for 3 h.
Room Temperature
Glass Transition Temperature Expansion
t
Penetration
Temperature
Fig. 1. Determination of the r, from the recorder trace.
Jerolimov et al.: Effect of the curing cycle on denture base T,
247
Table 1. Effect of the curing cycle on r, (“C) Material Trevalon TSI 195 Lucitone 199
x (i)
Short s.d. (+)
C.V.(%)
111.6 112.7 106.5
1.12 0.79 1.63
1.0 0.7 1.53
Curing cycles Medium x (i) s.d. (-+) C.V.(%) 120.0 1 17.9 1 10.5
RESULTS The results of the Ts determinations for the three materials are given in Table I. There was a highly significant difference in Tg due to the different curing cycles for all three materials investigated (P < 0.001). The range of the T, produced by different curing cycles was dependent on the material used. The largest range (19.5”C) was for Trevalon. When the long curing cycle with terminal boil was used, no significant difference between specimens was obtained. With medium and short cycles no difference was observed between Trevalon and TS1195 values (P > 0.05) but a significant difference was observed between Trevalon and Lucitone (P < 0.001) and between TS1195 and Lucitone (P < 0.001).
DISCUSSION Manufacturers of denture base materials may suggest several recommended curing cycles for their products. Most dental technicians have preferred curing cycles and their selection will be influenced by the working time available. The curing cycles selected for the present study are thought to represent a range of curing cycles that may be commonly used in dental production laboratories. Medium or long cure cycles of more than 7 h at 70°C and which include a terminal boil are to be preferred. Such cycles produce bases with low residual monomer levels and ensure that mechanical properties are optimized without reducing the dimensional accuracy of the bases (Huggett et al., 1985). Thermomechanical analysis has been shown to be a convenient and reliable method to be employed as an evaluation technique for Tg of denture base materials (Huggett et al., 1990). Tg values should be regarded as approximate since the value for a given specimen will depend on method of determination and on heating rate (McCabe and Wilson, 1980). The heating rate of lO”C/ min was selected for the present study as it provided sufficient probe sensitivity but avoided excessive heating which yields elevated values of Tg The effect of the curing cycles used in the present study on the Tg of the selected materials was considerable and highly significant in all cases. The curing cycle which did not include a period at 100°C produced specimens with
1.19 1.01 1.04
0.99 0.86 0.94
x (i)
Long s.d. (5)
C.V.(%)
100.5 103.1 101.7
1.0 1.99 1.97
0.99 1.94 1.94
the lowest Tg. The curing cycle which includes 3 h at 100°C produced specimens with the highest Tg These results reflect the levels of residual monomer in those specimens. The effect of the curing cycle on the high impact material, Lucitone, was less than that on the other two materials. Lucitone, the high impact material, contains a butadiene-styrene rubber which influences the plasticizing effect of the monomer, thus reducing Tg. In cycles without the terminal boil, the plasticizing effect of the residual monomer disguised the modifying effect of the butadiene-styrene. Although a high Tg provides resistance to the adverse effects of high temperatures, a high Tg can produce internal stresses during curing of a heat-cured material which could result in dimensional accuracy (McCabe and Wilson, 1980). It is considered unlikely that the Tg range obtained here could produce dimensional differences that would be of clinical significance. The curing cycle of 7 h at 70°C plus 3 h at 100°C can be recommended to produce specimens with a high Tg as well as optimum mechanical properties. It is concluded that: 1. The curing cycle has considerable influence on the Tg of the resultant denture base specimen.
2. Curing cycles which do not include a period at 100°C produce denture base resins with the lowest Tg. 3. The curing cycle of 7 h at 70°C and 3 h at 100°C can be recommended to produce specimens with high Tg as well as optimum mechanical properties.
References
Austin A T. and Basker R. M. (1980) Residual monomer in acrylic denture base materials with particular reference to a modified method of analysis. Br. Dent. J. 149,281-286. Barsby M. J. and Braden M. (1981) Visco-elastic properties of pour (fluid) denture base resins. J. Dent. Res. 60, 146-148. Braden M. and Stafford G. D. (1968) Visco-elastic properties of some denture base materials. J. Dent. Res. 47, 519-523. Fox T. G. and Loshaek S. (1955) Influence of molecular weight and degree of crosslinking on the specific volume and glass temperature of polymers. .I Polymer Sci. 371-390. Huggett R., Brooks S. C. and Bates J. F. (1984) The effect of different curing cycles on levels of residual monomer in acrylic resin denture base materials. Quintessence Dent. Technol. 8, 365-371.
248
J. Dent. 1991; 19: No. 4
Huggett R., Brooks S. C. and Bates J. F. (1985) Which curing cycle is best. Dent Technician Tech. SuppI. 38, 11-16. Huggett R., Brooks S. C., Campbell A. M. et al. (1990) Evaluation of analytical techniques for measurement of denture base acrylic resin glass transition temperature. Dent Mater. 6, 17-19.
Jagger R. G. (1978) Effect of the curing cycle on some properties of a polymethylmethacrylate denture base material. J. Oral Rehabil. 5, 151-157. McCabe J. F. and Wilson H. J. (1980) The use of differential scanning calorimetry for the evaluation of dental materials. Part II Denture base materials. J. Oral Rehabil. 7, 235-243.
Book Review Introductory Medical Statistics, 2nd edition. FL F. Mould. 1989. Pp. 192. Bristol, Adam Hilger. Softback, E9.95. Statistics in Dentistry. J. S. Bulman and J. F. Osborn. 1989. Pp. 118. London, British Dental Journal. Softback, f 6.95. Statistics with Confidence-Confidence Intervals and Statistical Guidelines. M. J. Gardner and D. G. Altman. 1989. Pp.1 40. London, British Medical Association. Softback, f 7.95. Statistical methods have become thoroughly assimilated into the practice of dental research in the last two decades and indeed the teaching of statistics is now accepted as an important part of the undergraduate dental curriculum. It is thus timely to be able to review three statistical books that have recently been published. The first of the three, by R. F. Mould, provides a general introduction to the subject of medical statistics and follows a fairly conventional pattern for guides of this sort. There is a sound but predictable exposition of basic material in the first half of the book (data presentation, descriptive statistics, the normal distribution, probability theory, etc.), followed by a series of chapters that present in turn the major classes of statistical test: chi-squared tests, t tests, analysis of variance, non-parametric procedures, regression and correlation. The book is clearly written, diagrams are clear and there are many apt working examples; the treatment is rigorous but not too mathematical. There is a rather egregious attempt to leaven the presentation by incorporating cartoons and aphorisms, but these intrusions do not detract from a sound, workmanlike guide that could form a useful course textbook or serve as an introduction to statistical techniques for the neophyte (or not so neophyte) researcher. Whilst Mould’s book is addressed at a general medical audience, Bulman and Osborn’s slim volume is aimed specifically at the dental practitioner. The book conveniently collects together the useful series of articles that first appeared in the British Dental Journal, supplemented by some further chapters and a set of worked problems. The book very much brings to mind that old friend Statistics at Square One published some years ago by the British Medical Journal. The authors’ main purpose is to introduce statistical concepts to the statistically naive dental practitioner in order to assist him/her in reading scientific reports. There are many useful examples and the book succeeds to qualified degree. However, the treatment is necessarily superficial and the book lacks the systematic development of a
textbook; it would be of only limited value to the serious researcher or as a course-book. The final book in the clutch is a much more interesting production. Over the last several years, Sheila Gore, Douglas Altman, Martin Gardner and others have contributed to a highly distinguished, often provocative, series of articles in the British Medical Journal dealing with statistical methods in its ‘Statistics in Medicine’ column. In 1986, Gardner and Altman wrote a rather iconoclastic article which, whilst welcoming the growing adoption of statistical procedures in medical research, inveighed against the obsessive use of significance tests at the expense of more appropriate and informative techniques. Gardner and Altman’s argument was simple but cogent. Consider a piece of research concerned with blood pressure in diabetics. Mean blood pressure is determined for a sample of diabetics and a sample of normals. Significance testing allows refutation (or otherwise) of the null hypothesis that the observed differences could have arisen by sampling variation. Testing thus answers one rather narrow question; are these two samples equivalent or not? Gardner and Altman’s case in their 1986 paper was for a less restrictive approach to statistical analysis. Surely, to paraphrase their argument, the reason for examining diabetics is not just to determine whether they have qualitatively higher blood pressure than normals. Do we not wish to quantify these differences? Are we not interested in what the blood pressure of diabetics actually is? In essence, Gardner and Altman were arguing that statistical estimation is a more fundamental activity than significance testing. Constructing confidence intervals is at the core of the estimation process, allowing quantitative inferences to be made from samples. Moreover, significance testing may be done, if required, within a general confidence interval approach. Following their original article, a set of further pieces appeared describing procedures for calculating confidence limits for the more common statistical indices. These articles and the original have now been gathered together by Gardner and Altman, revised and published in book form. I believe there is much to be said for their thesis that estimation has been neglected and significance testing given undue prominence; Mould’s book as a case in point makes only two cursory references to confidence intervals. A disincentive to the confidence interval approach is that the formulae are often rather tortuous; lest the unsophisticated researcher is put off, a computer program has been made available by Gardner to provide assistance. I wholeheartedly welcome and commend this provocative and useful book. D. G. Wastell
