Journal of Oral Rehabilitation, 1975, Volume 2, pages 97-104
Polymethylmethacrylate as a denture base material in service
A. S. H A R G R E A V E S
School of Dental Surgery, University of Liverpool
Summary Assessments of the practical deficiencies of polymethylmethacrylate in service have been carried out by several workers. Some of these studies and one further one have been compared so as to seek any common factors, especially those concerned with the age of the appliance at failure. Times of vulnerability for different types of dentures have been demonstrated, and the scope for time-based studies in the laboratory and in service has been discussed. Introduction Mechanical testing of materials has long been realized to have the most value when carried out as nearly as possible under service conditions. This is as true of restorative dental materials as for the newly developed alloys and fabrics in aerospace aviation. Service conditions for denture base materials are still imperfectly understood and stress analysis of dentures in function has not yet been exhausted. An alternative approach has been that of examining the practical deficiencies in service, one of the commonest being fracture failure. There have now been several studies undertaken in fracture prevalence, in different areas of the country, from dental hospitals and practices, using varied samples. It was considered of value to discover any common factor from these differing studies and then see whether they might have relevance to future laboratory or clinical investigations. Method Data were drawn from (1) previous research by Smith (1957), Grant (1960), Hargreaves (1969) and Munns (1971); (2) the present investigation, using material from a dental hospital and several NHS practices treating patients from all social classes. This was on similar but simplified lines to the 1969 study, and was aimed primarily at determining the age of the appliance at time of fracture and the cause of failure. All repeated fractures were discarded so that only the primary failure of the denture base was considered. Where an alternative polymer to polymethylmethacrylate (PMMA) was present, e.g. Impact, this was examined separately. Correspondence: Miss A. S. Hargreaves, School of Dental Surgery, University of Liverpool, Pembroke Place, P.O. Box 147, Liverpool L69 3BX.
7
97
A. S. Hargreaves
98
Results Manchester (Smith, 1957) Smith only examined midline fractures of complete upper dentures that had failed in function, i.e. intra-orally, but felt that most of the considerations applied to lower dentures also. The sample drawn from hospital cases was expressed as a percentage of the total number made over the same period. If the high incidence in the first 8 months from mechanically deficient dentures or special environmental conditions, together with repeated fractures, were disregarded, the histogram showed slight maxima in the regions of 1-5, 3 and 5 years, and a sharp decline after this. Fig. 1 shows an extrapolation of his figures for comparison with later histograms. These failures were considered to be of fiexural fatigue origin. The chances of a fracture in the first 5-years of life were calculated as 1 in 650, but after this only 1 in 2000.
15 o c
cr (V
10
JL
0
_L
_L
5
10
Aqe(years) Fig. 1. Manchester sample—age of full upper denture at time of fracture.
A detailed questionnaire to twenty-five general practitioners in the area showed that those with the highest incidence of broken upper dentures tended to have the highest in lower denture failures (i.e. there was a personal factor, probably manipulative). There was no relation between the incidence of fracture and any of the five brands of material used, but there was a correlation with the use of plastic posterior teeth. Detroit {Grant, 1960) Grant compared 695 complete upper dentures with bases and teeth made of vinyl resin (Luxene) with an equal number of acrylic dentures with porcelain teeth. The length of service for both groups was 5 years. The number of upper and lower dentures that were fractured and repaired were approximately the same, but cause and type of fracture were not considered. 14-4% of the acrylic dentures had broken within this period, but only 1-6% of the vinyl resin group. If tooth fracture and base material were dissociated 8-7% of the acrylic bases had failed in comparison with 0 7% of the vinyl bases. Dundee (Hargreaves, 1969) This was a small hospital sample examining all types of denture repairs, and could only therefore suggest trends of behaviour. There was a peak incidence between 2 and
Polymethylmethacrylate denture base in service
99
Base acrylic failure
Age (years) Fig. 2. Dundee sample—age of appliance at failure, showing total repairs and base acrylic fracture.
3 years both in total repairs and acrylic failures (Fig. 2). It was suggested that PMMA as a denture base material might have a life of only 3 years when used as a connector for partial dentures or for complete lower dentures, the latter predominantly failing after being dropped. Tooth loss was considered fortuitous. Worcester (Munns, 1971) Munns' primary concern was broken removable orthodontic appliances but he reported 12% of his sample to have been acrylic failures. Many of these were due to definite carelessness of the patient, such as dropping and treading on it, and rarely occurred while the appliance was being worn in the mouth. Present investigation A sample of 300 dentures constructed at Liverpool Dental Hospital showed a high initial incidence of failure. However, as the mechanical faults were gradually eliminated this decreased with the age of the denture, except for a sudden sharp rise in the 2^-3 year period (Fig. 3). The overall pattern of results agreed very well with the Dundee sample when applying the Kolmogorov-Smirnov two-sample two-tailed test, giving D = 0-166 ( D ^ 0-214 for these sample sizes for the null hypothesis to be rejected). This test is more sensitive than the x^ test for quality of fit as it takes into account differences in location, dispersion and skewness. The peak at 3 years could be demonstrated both in base acrylic failure (Fig. 4a) and to a lesser degree in tooth loss (Fig. 4b). Complete lower dentures broke more often after being dropped than in function, though not all patients would readily admit to implied carelessness. Complete lower dentures with a Flexibase lining showed two patterns of failure—firstly, a shear type failure usually associated with an inadequate area of cross-section of base acrylic, and secondly a fatigue type failure where there was adequate thickness of acrylic but also a thick resilient lining. Some dentures with resilient linings had been made from Impact, and all of these failed in function within 15 months. The incidence of the classic midline fracture of F / - was surprisingly low at only 6-7 %. The practice sample of 235 repairs recorded from twelve practices showed two distinct prevalence patterns. Those practitioners who admitted providing dentures with a locked occlusion all returned a quota dominated by midline fractures of ¥/-, with a peak incidence of failure in the 18-24 month period; but those who ensured
100
A. S. Hargreaves
50-f
Age (years)
Fig. 3. Liverpool (hospital) sample—age of type of appliance at time of failure.
30
20
10 c Oi 3
cr
(a)
10
0
15
10
J
(b)
L 10
0
Age(years) Fig. 4. Liverpool sample—age of appliance where (a) failure of acrylic base; (b) tooth loss only.
that the patient had the freedom for lateral excursions (whether this was utilized or not) all had a prevalence pattern more closely resembling both the Dundee and Liverpool Hospitals' sources with an equal distribution between upper and lower dentures and a peak incidence of failure between 2\ and 3 years.
Polymethylmethacrylate denture base in service
101
Combined samples If the figures from the Manchester, Dundee and Liverpool sources are combined to provide 303F/-, 118-/F, 98P/- and 27-/P, and the acrylic failure pattern for each type of denture examined, it can be seen that the peak incidence at 2^-3 years is observed in all groups (Fig. 5). Complete upper dentures show two main peaks at 12-18 months and 30-36 months and a lesser one at about 5 years. Complete lower dentures conform more closely to their distribution by age (when intact) in the general population, as observed by Hobdell, Sheiham & Cowell (1970), except for the 3-year maximum. Acrylic partial upper dentures seem particularly prone to early mechanical faults, but the overall 40 30 20 10 (a) 10
15
10
15
20 10
Age(years)
Fig. 5. Combined sample—age at time of fracture of (a) full upper denture; (b) full lower denture; (c) partial upper denture; (d) partial lower denture.
pattern is otherwise similar to - / F . Partial lower dentures are less widely made now with acrylic rather than metal connectors so that the sample here is small, but nevertheless it shows a trend towards the same maximum observed in the other groups. Although many dentures fail after being dropped, when the ages of all dropped appliances were examined the distribution appeared random and no peak incidence for impact failure could be demonstrated. Tooth loss also showed a high initial incidence and then demonstrated a skewpositive distribution around the 3-year mark. It was not possible, however, to prove a correlation between patterns of tooth loss and acrylic failure. Discussion and conclusions
It has long been realized that acrylic is a brittle resin, prone to fatigue and impact failure when used as a denture base material, but the increased use of copolymers, reinforcement, or alternative resins has not become widespread. This is often due to either increased technical difficulties in manufacture or a deterioration in other properties.
102
A. S. Hargreaves
Smith (1961) has already discussed the roles played in denture strength by the shape, the residual stresses, the conditions of loading and the mechanical properties of the material. Deficiencies in design and construction tend to show themselves early on, so that there appears a high initial incidence of failure; this gradually tapers away (apart from the sudden upsurge between 2^ and 3 years) in a prevalence pattern resembling that of the age of dentures in either a random population (Hobdell et al, 1970) or a population drawn from dental practice which has been provided with appliances (Gillings et al, 1967). The similarity of the prevalence patterns between Dundee and Liverpool is worth attention. One hospital has a strict recall system, so that there might have crept in an element of bias by the reduction in numbers of appliances with more than 3-years service if rebasing or remaking had been indicated; the other hospital merely encourages patients to return for a check-up unless particular circumstances dictate strict maintenance procedures. There are also differences in the teaching of design of dentures and in the processing techniques used. Nevertheless, both surveys show this 3-year peak incidence for failure of the acrylic base, and it is also seen in many of the practice samples. With a large mixed sample, individual peculiarities in manipulation or design should have little effect, and the following conclusions may be drawn about dentures in service: Once the early mechanical deficiencies such as inadequate area of cross-section of the material or premature contacts are eliminated, the complete upper denture becomes vulnerable after approximately 18 months service if it is opposed by a complete or nearly complete standing lower arch, or if it is in locked occlusion with a lower appliance in a patient who attempts to use lateral movements. Since the denture is likely to be still well adapted and retained, being of relatively recent construction, it will be 'stretched' by the lateral forces exerted by the lower cusps locking with their opponents. All dentures show vulnerability between 24 and 3 years, and this is probably of fatigue origin. It would be surprising if the carelessness of the patient with respect to the age of the denture were other than unrelated, so that although lower dentures show this maximum and also tend to fail after being dropped, the suggestion in an earlier study (Hargreaves, 1969) that acryHc had an impact life of 3 years is not really tenable. What is not yet fully known is the effect of flexure over a period of time on the structure and properties of PMMA thus affecting the development and growth of microcracks and hence modifying the mode or moment in time of impact failure. It would be interesting to postulate an associated reduction in the bond strength between acrylic teeth and base after this same period of 3-years, but it is more likely that tooth loss is simply fortuitous. After 5 or 6 years service, abrasion of the posterior teeth, if these are acrylic, may have been enough to eliminate cuspal contour and predispose to the wedge-action patterns of failure as described by Berry (1957). Recent studies of abrasion by Harrison (1974) show a wide range of values for loss of tooth surface in individuals, but a critical loss conducive to fracture could not be defined. After 6 years, the incidence of failure decreases markedly, and the reason for attendance becomes increasingly likely to be replacement of a chipped tooth, filling in a hole in the tuberosity region after wear from a lone-standing lower molar, or the regretful conclusion that the 40-year-oId appliance is at last breaking up.
Polymethylmethacrylate denture base in service
103
The early failure of bases made from Impact in function is not entirely unexpected. Stafford (1968) has shown this material to have a degree of flexibility outside the limits of the FDI Specification No. 3, and although he has also shown it to have a good fatigue life when tested under laboratory conditions (1970), the reduction in crosssectional area together with the greater flexibility of both base material and resilient lining appear to put the denture at greater risk than if the base had been constructed from unmodified PMMA. Such dentures as the latter averaged a life of 2 years 9 months, which is in the region of vulnerability for dentures without a resilient lining. Clearly, not all denture bases fail after 3-years service, but the need for some form of 'built-in obsolescence' so that patients (particularly the more elderly) may be examined periodically will not be without its advocates. It would seem that polymethylmethacrylate as a denture base material has some vulnerability with time. In service it is subjected to (I) an environment, stringent in its demands, created by heat, water, foodstuffs, drink, cleaning materials, etc.; (2) stress, which may be inherent in the complexity of its shape, or from mastication, abrasion of posterior teeth, or patient handling; (3) biodegradation. There is little that can be done about the environment except counselling the patient to avoid extremes, such as care with the choice of cleaning media and avoidance of sweets containing weak PMMA solvents. Stress analysis has been and still is under investigation in several ways, ranging from brittle lacquer techniques, the use of strain gauges and telemetry to chemical analysis and molecular weight determination after a period of service. Biodegradation is of importance in the implant field, but little is known of dentures in service. Ultraviolet and radiation degradation are probably negligible but the separate or combined effects of heat, fluid and stress with time are still largely unexplored. Fatigue studies in the laboratory do not always bear much relevance to service conditions, although the situation is improving. Despite the length of time that acrylic has been in service as a denture base material, it seems that there is still scope for investigation, using time-based studies, before our knowledge of its behaviour is complete. Acknowledgments
The author wishes to thank all those practitioners and their auxiliary staff who took part in the survey for their interest and co-operation. References BERRY, D .
(1957) Dental fractures resulting from occlusal wear of acrylic teeth. Dental Practitioner,
7, JIO. GiLLiNGS, B.R.D., DODD, C, GRAHAM, C.H. & BARNARD, P . D . (1967) Full and partial denture survey. Australian Dental Journal, 12, 574. GRANT, W . F . D . (1960) Clinical observations of number of fractures of acrylic and modified copolymer Vinyl dentures. Journal ofthe American Dental Association, 61, 578. HARGREAVES, A.S. (1969) The prevalence of fractured dentures. British Dental Journal, 126, 451. HARRISON, A. (1974) Personal communication. HoBDELL, M.H., SHEIHAM, A. & CowELL, C.R. (1970) The prevalence of full and partial dentures in British populations. British Dental Journal, 128, 437. MUNNS, D . (1971) An analysis of broken removable appliances. Dental Practitioner, 21, 449. SMITH, D.C. (1957) Studies in denture base materials with special reference to polymethyl methacrylate. Ph.D. Thesis, University of Manchester.
104
A. S. Hargreaves
SMITH, D . C . (1961) The STAFFORD, G . D . (1968)
Acrylic Denture. British Dental Journal, 110, 257. Some studies of the properties of denture base polymers. British Dental
Journal, 125, 337. STAFFORD, G.D. (1970) Flexural fatigue tests of some denture base polymers. British Dental Journal, 128, 442.
Manuscript accepted 9 July 1974
Polymethylmethacrylate as a denture base material in service
A. S. H A R G R E A V E S
School of Dental Surgery, University of Liverpool
Summary Assessments of the practical deficiencies of polymethylmethacrylate in service have been carried out by several workers. Some of these studies and one further one have been compared so as to seek any common factors, especially those concerned with the age of the appliance at failure. Times of vulnerability for different types of dentures have been demonstrated, and the scope for time-based studies in the laboratory and in service has been discussed. Introduction Mechanical testing of materials has long been realized to have the most value when carried out as nearly as possible under service conditions. This is as true of restorative dental materials as for the newly developed alloys and fabrics in aerospace aviation. Service conditions for denture base materials are still imperfectly understood and stress analysis of dentures in function has not yet been exhausted. An alternative approach has been that of examining the practical deficiencies in service, one of the commonest being fracture failure. There have now been several studies undertaken in fracture prevalence, in different areas of the country, from dental hospitals and practices, using varied samples. It was considered of value to discover any common factor from these differing studies and then see whether they might have relevance to future laboratory or clinical investigations. Method Data were drawn from (1) previous research by Smith (1957), Grant (1960), Hargreaves (1969) and Munns (1971); (2) the present investigation, using material from a dental hospital and several NHS practices treating patients from all social classes. This was on similar but simplified lines to the 1969 study, and was aimed primarily at determining the age of the appliance at time of fracture and the cause of failure. All repeated fractures were discarded so that only the primary failure of the denture base was considered. Where an alternative polymer to polymethylmethacrylate (PMMA) was present, e.g. Impact, this was examined separately. Correspondence: Miss A. S. Hargreaves, School of Dental Surgery, University of Liverpool, Pembroke Place, P.O. Box 147, Liverpool L69 3BX.
7
97
A. S. Hargreaves
98
Results Manchester (Smith, 1957) Smith only examined midline fractures of complete upper dentures that had failed in function, i.e. intra-orally, but felt that most of the considerations applied to lower dentures also. The sample drawn from hospital cases was expressed as a percentage of the total number made over the same period. If the high incidence in the first 8 months from mechanically deficient dentures or special environmental conditions, together with repeated fractures, were disregarded, the histogram showed slight maxima in the regions of 1-5, 3 and 5 years, and a sharp decline after this. Fig. 1 shows an extrapolation of his figures for comparison with later histograms. These failures were considered to be of fiexural fatigue origin. The chances of a fracture in the first 5-years of life were calculated as 1 in 650, but after this only 1 in 2000.
15 o c
cr (V
10
JL
0
_L
_L
5
10
Aqe(years) Fig. 1. Manchester sample—age of full upper denture at time of fracture.
A detailed questionnaire to twenty-five general practitioners in the area showed that those with the highest incidence of broken upper dentures tended to have the highest in lower denture failures (i.e. there was a personal factor, probably manipulative). There was no relation between the incidence of fracture and any of the five brands of material used, but there was a correlation with the use of plastic posterior teeth. Detroit {Grant, 1960) Grant compared 695 complete upper dentures with bases and teeth made of vinyl resin (Luxene) with an equal number of acrylic dentures with porcelain teeth. The length of service for both groups was 5 years. The number of upper and lower dentures that were fractured and repaired were approximately the same, but cause and type of fracture were not considered. 14-4% of the acrylic dentures had broken within this period, but only 1-6% of the vinyl resin group. If tooth fracture and base material were dissociated 8-7% of the acrylic bases had failed in comparison with 0 7% of the vinyl bases. Dundee (Hargreaves, 1969) This was a small hospital sample examining all types of denture repairs, and could only therefore suggest trends of behaviour. There was a peak incidence between 2 and
Polymethylmethacrylate denture base in service
99
Base acrylic failure
Age (years) Fig. 2. Dundee sample—age of appliance at failure, showing total repairs and base acrylic fracture.
3 years both in total repairs and acrylic failures (Fig. 2). It was suggested that PMMA as a denture base material might have a life of only 3 years when used as a connector for partial dentures or for complete lower dentures, the latter predominantly failing after being dropped. Tooth loss was considered fortuitous. Worcester (Munns, 1971) Munns' primary concern was broken removable orthodontic appliances but he reported 12% of his sample to have been acrylic failures. Many of these were due to definite carelessness of the patient, such as dropping and treading on it, and rarely occurred while the appliance was being worn in the mouth. Present investigation A sample of 300 dentures constructed at Liverpool Dental Hospital showed a high initial incidence of failure. However, as the mechanical faults were gradually eliminated this decreased with the age of the denture, except for a sudden sharp rise in the 2^-3 year period (Fig. 3). The overall pattern of results agreed very well with the Dundee sample when applying the Kolmogorov-Smirnov two-sample two-tailed test, giving D = 0-166 ( D ^ 0-214 for these sample sizes for the null hypothesis to be rejected). This test is more sensitive than the x^ test for quality of fit as it takes into account differences in location, dispersion and skewness. The peak at 3 years could be demonstrated both in base acrylic failure (Fig. 4a) and to a lesser degree in tooth loss (Fig. 4b). Complete lower dentures broke more often after being dropped than in function, though not all patients would readily admit to implied carelessness. Complete lower dentures with a Flexibase lining showed two patterns of failure—firstly, a shear type failure usually associated with an inadequate area of cross-section of base acrylic, and secondly a fatigue type failure where there was adequate thickness of acrylic but also a thick resilient lining. Some dentures with resilient linings had been made from Impact, and all of these failed in function within 15 months. The incidence of the classic midline fracture of F / - was surprisingly low at only 6-7 %. The practice sample of 235 repairs recorded from twelve practices showed two distinct prevalence patterns. Those practitioners who admitted providing dentures with a locked occlusion all returned a quota dominated by midline fractures of ¥/-, with a peak incidence of failure in the 18-24 month period; but those who ensured
100
A. S. Hargreaves
50-f
Age (years)
Fig. 3. Liverpool (hospital) sample—age of type of appliance at time of failure.
30
20
10 c Oi 3
cr
(a)
10
0
15
10
J
(b)
L 10
0
Age(years) Fig. 4. Liverpool sample—age of appliance where (a) failure of acrylic base; (b) tooth loss only.
that the patient had the freedom for lateral excursions (whether this was utilized or not) all had a prevalence pattern more closely resembling both the Dundee and Liverpool Hospitals' sources with an equal distribution between upper and lower dentures and a peak incidence of failure between 2\ and 3 years.
Polymethylmethacrylate denture base in service
101
Combined samples If the figures from the Manchester, Dundee and Liverpool sources are combined to provide 303F/-, 118-/F, 98P/- and 27-/P, and the acrylic failure pattern for each type of denture examined, it can be seen that the peak incidence at 2^-3 years is observed in all groups (Fig. 5). Complete upper dentures show two main peaks at 12-18 months and 30-36 months and a lesser one at about 5 years. Complete lower dentures conform more closely to their distribution by age (when intact) in the general population, as observed by Hobdell, Sheiham & Cowell (1970), except for the 3-year maximum. Acrylic partial upper dentures seem particularly prone to early mechanical faults, but the overall 40 30 20 10 (a) 10
15
10
15
20 10
Age(years)
Fig. 5. Combined sample—age at time of fracture of (a) full upper denture; (b) full lower denture; (c) partial upper denture; (d) partial lower denture.
pattern is otherwise similar to - / F . Partial lower dentures are less widely made now with acrylic rather than metal connectors so that the sample here is small, but nevertheless it shows a trend towards the same maximum observed in the other groups. Although many dentures fail after being dropped, when the ages of all dropped appliances were examined the distribution appeared random and no peak incidence for impact failure could be demonstrated. Tooth loss also showed a high initial incidence and then demonstrated a skewpositive distribution around the 3-year mark. It was not possible, however, to prove a correlation between patterns of tooth loss and acrylic failure. Discussion and conclusions
It has long been realized that acrylic is a brittle resin, prone to fatigue and impact failure when used as a denture base material, but the increased use of copolymers, reinforcement, or alternative resins has not become widespread. This is often due to either increased technical difficulties in manufacture or a deterioration in other properties.
102
A. S. Hargreaves
Smith (1961) has already discussed the roles played in denture strength by the shape, the residual stresses, the conditions of loading and the mechanical properties of the material. Deficiencies in design and construction tend to show themselves early on, so that there appears a high initial incidence of failure; this gradually tapers away (apart from the sudden upsurge between 2^ and 3 years) in a prevalence pattern resembling that of the age of dentures in either a random population (Hobdell et al, 1970) or a population drawn from dental practice which has been provided with appliances (Gillings et al, 1967). The similarity of the prevalence patterns between Dundee and Liverpool is worth attention. One hospital has a strict recall system, so that there might have crept in an element of bias by the reduction in numbers of appliances with more than 3-years service if rebasing or remaking had been indicated; the other hospital merely encourages patients to return for a check-up unless particular circumstances dictate strict maintenance procedures. There are also differences in the teaching of design of dentures and in the processing techniques used. Nevertheless, both surveys show this 3-year peak incidence for failure of the acrylic base, and it is also seen in many of the practice samples. With a large mixed sample, individual peculiarities in manipulation or design should have little effect, and the following conclusions may be drawn about dentures in service: Once the early mechanical deficiencies such as inadequate area of cross-section of the material or premature contacts are eliminated, the complete upper denture becomes vulnerable after approximately 18 months service if it is opposed by a complete or nearly complete standing lower arch, or if it is in locked occlusion with a lower appliance in a patient who attempts to use lateral movements. Since the denture is likely to be still well adapted and retained, being of relatively recent construction, it will be 'stretched' by the lateral forces exerted by the lower cusps locking with their opponents. All dentures show vulnerability between 24 and 3 years, and this is probably of fatigue origin. It would be surprising if the carelessness of the patient with respect to the age of the denture were other than unrelated, so that although lower dentures show this maximum and also tend to fail after being dropped, the suggestion in an earlier study (Hargreaves, 1969) that acryHc had an impact life of 3 years is not really tenable. What is not yet fully known is the effect of flexure over a period of time on the structure and properties of PMMA thus affecting the development and growth of microcracks and hence modifying the mode or moment in time of impact failure. It would be interesting to postulate an associated reduction in the bond strength between acrylic teeth and base after this same period of 3-years, but it is more likely that tooth loss is simply fortuitous. After 5 or 6 years service, abrasion of the posterior teeth, if these are acrylic, may have been enough to eliminate cuspal contour and predispose to the wedge-action patterns of failure as described by Berry (1957). Recent studies of abrasion by Harrison (1974) show a wide range of values for loss of tooth surface in individuals, but a critical loss conducive to fracture could not be defined. After 6 years, the incidence of failure decreases markedly, and the reason for attendance becomes increasingly likely to be replacement of a chipped tooth, filling in a hole in the tuberosity region after wear from a lone-standing lower molar, or the regretful conclusion that the 40-year-oId appliance is at last breaking up.
Polymethylmethacrylate denture base in service
103
The early failure of bases made from Impact in function is not entirely unexpected. Stafford (1968) has shown this material to have a degree of flexibility outside the limits of the FDI Specification No. 3, and although he has also shown it to have a good fatigue life when tested under laboratory conditions (1970), the reduction in crosssectional area together with the greater flexibility of both base material and resilient lining appear to put the denture at greater risk than if the base had been constructed from unmodified PMMA. Such dentures as the latter averaged a life of 2 years 9 months, which is in the region of vulnerability for dentures without a resilient lining. Clearly, not all denture bases fail after 3-years service, but the need for some form of 'built-in obsolescence' so that patients (particularly the more elderly) may be examined periodically will not be without its advocates. It would seem that polymethylmethacrylate as a denture base material has some vulnerability with time. In service it is subjected to (I) an environment, stringent in its demands, created by heat, water, foodstuffs, drink, cleaning materials, etc.; (2) stress, which may be inherent in the complexity of its shape, or from mastication, abrasion of posterior teeth, or patient handling; (3) biodegradation. There is little that can be done about the environment except counselling the patient to avoid extremes, such as care with the choice of cleaning media and avoidance of sweets containing weak PMMA solvents. Stress analysis has been and still is under investigation in several ways, ranging from brittle lacquer techniques, the use of strain gauges and telemetry to chemical analysis and molecular weight determination after a period of service. Biodegradation is of importance in the implant field, but little is known of dentures in service. Ultraviolet and radiation degradation are probably negligible but the separate or combined effects of heat, fluid and stress with time are still largely unexplored. Fatigue studies in the laboratory do not always bear much relevance to service conditions, although the situation is improving. Despite the length of time that acrylic has been in service as a denture base material, it seems that there is still scope for investigation, using time-based studies, before our knowledge of its behaviour is complete. Acknowledgments
The author wishes to thank all those practitioners and their auxiliary staff who took part in the survey for their interest and co-operation. References BERRY, D .
(1957) Dental fractures resulting from occlusal wear of acrylic teeth. Dental Practitioner,
7, JIO. GiLLiNGS, B.R.D., DODD, C, GRAHAM, C.H. & BARNARD, P . D . (1967) Full and partial denture survey. Australian Dental Journal, 12, 574. GRANT, W . F . D . (1960) Clinical observations of number of fractures of acrylic and modified copolymer Vinyl dentures. Journal ofthe American Dental Association, 61, 578. HARGREAVES, A.S. (1969) The prevalence of fractured dentures. British Dental Journal, 126, 451. HARRISON, A. (1974) Personal communication. HoBDELL, M.H., SHEIHAM, A. & CowELL, C.R. (1970) The prevalence of full and partial dentures in British populations. British Dental Journal, 128, 437. MUNNS, D . (1971) An analysis of broken removable appliances. Dental Practitioner, 21, 449. SMITH, D.C. (1957) Studies in denture base materials with special reference to polymethyl methacrylate. Ph.D. Thesis, University of Manchester.
104
A. S. Hargreaves
SMITH, D . C . (1961) The STAFFORD, G . D . (1968)
Acrylic Denture. British Dental Journal, 110, 257. Some studies of the properties of denture base polymers. British Dental
Journal, 125, 337. STAFFORD, G.D. (1970) Flexural fatigue tests of some denture base polymers. British Dental Journal, 128, 442.
Manuscript accepted 9 July 1974
