Flexural strength of heat-polymerized polymethyl methacrylate dentureresin reinforced with glass, aramid, or nylon fibers

Jacob John, MDS,a Shivaputrappa A. Gangadhar, MDS,b and Ila Shah, MDSc

Bapuji Dental College and Hospital, Kuvempu University, Davangere, Karnatata, India

Statement of problem. Despite the favorable properties of conventional PMMA used as a denturebase material, its fracture resistance could be improved.Purpose. This in vitro study was performed to determine whether the flexural strength of a commercial-ly available, heat-polymerized acrylic denture base material could be improved through reinforcementwith 3 types of fibers.Material and methods. Ten specimens of similar dimensions were prepared for each of the 4 experi-mental groups: conventional acrylic resin and the same resin reinforced with glass, aramid, or nylon fibers.Flexural strength was evaluated with a 3-point bending test. The results were analyzed with a 1-wayanalysis of variance.Results. All reinforced specimens showed better flexural strength than the conventional acrylic resin.Specimens reinforced with glass fibers showed the highest flexural strength, followed by aramid andnylon.Conclusion. Within the limitations of this study, the flexural strength of heat-polymerized PMMA den-ture resin was improved after reinforcement with glass or aramid fibers. It may be possible to apply theseresults to distal extension partial denture bases and provisional fixed partial dentures. (J Prosthet Dent2001;86:424-7.)

424 THE JOURNAL OF PROSTHETIC DENTISTRY VOLUME 86 NUMBER 4

PMMA1 currently is the material of choice for den-ture base fabrication. Introduced in 1937 by Dr WalterWright,2 PMMA continues to be used because of itsfavorable working characteristics, processing ease,accurate fit, stability in the oral environment, superioresthetics, and use with inexpensive equipment. Despitethese excellent properties, there is a need for improve-ment in the fracture resistance of PMMA.

Most fractures of the denture occur inside themouth during function, primarily because of resinfatigue.3,4 The denture base resin is subjected to vari-ous stresses during function; these include compressive,tensile, and sheer stresses. Some of the factors respon-sible for denture fracture6 include stress intensification,increased ridge resorption leading to an unsupporteddenture base, deep incisal notching at the labial frena,sharp changes at the contours of the denture base, deepscratches, and induced processing stresses.

To overcome these shortcomings, experimentswith alternate polymers were conducted, but thepolymers failed to produce dentures of greater accu-racy or better performance.7 Various modificationsof PMMA also have been tested to improve theexisting material; these modifications include chem-ical modification to produce graft copolymerhigh-impact resins7 and mechanical reinforcementthrough the inclusion of glass fibers,8,9 sapphirewhiskers,10,11 aramid fibers,10,12 carbon fibers,13,14

stainless steel mesh,15 nylon,16 or (more recently)ultra-high-modulus polyethylene fibers.17-19

Numerous studies have evaluated the use of indi-vidual reinforcing fibers to improve the strength of thedenture base.7,8,10 In the absence of any single studycomparing the properties of PMMA resin reinforcedwith different types of fibers, this in vitro study wasperformed to compare the flexural strength of com-monly used PMMA resin and that of PMMA resinreinforced with glass, aramid, and nylon fibers.

MATERIAL AND METHODS

Forty dental stone molds were prepared using stain-less steel dies of specific dimensions. The 4 experimental

aFormer postgraduate student in prosthodontics. Lecturer, Departmentof Prosthodontics, Ragas Dental College and Hospital, Chennai,India.

bProfessor and Head, Department of Prosthodontics.cFormer Professor and Head, Department of Prosthodontics.

CLINICAL IMPLICATIONS

This in vitro study demonstrated a significant improvement in the flexural strength ofconventional acrylic resin when it was reinforced with glass or aramid fibers.

groups consisted of conventional acrylic resin and thesame resin reinforced with glass, aramid, or nylon fibers.Ten specimens were fabricated in a standardized fashionfor each of the experimental groups. Flexural strengthwas tested with a 3-point universal testing machine.

Forty dental stone molds were prepared in dentalflasks with preformed stainless steel metal dies (each65 × 10 × 3 min).20 Each die was coated with a thinlayer of petroleum jelly before being invested in dentalstone. After the final set of the dental stone, the flaskwas opened, and the die was gently removed from theinvesting material. The master die had a threaded holein the center to facilitate easy removal from the stonemold. The prepared molds then were immersed in hotwater to remove any trace of impurities and to facili-tate the application of separating medium. The moldcavities obtained were used for the preparation ofacrylic resin test specimens.

The control group test specimens were made withconventional heat-polymerized acrylic resin (Travalon;Denstply International, York, Pa.). A mixture ofmonomer and polymer in the ratio of 1:2.4 by weightwas allowed to reach dough stage, then kneaded andplaced in the mold. Trial closure was performed with ahydropress at 40,000 N (KaVo EWL, Leutkirch,Germany). The flask was clamped, and low pressurewas maintained for 30 minutes to allow proper pene-tration of monomer into the polymer, even flow of thematerial, and outward flow of excess material. Theflask was immersed in water in an acrylizer (KaVoEWL) at room temperature. The temperature wasraised slowly to 73°C and maintained for a half hour.After completion of the polymerization cycle, the flaskwas allowed to cool in the water bath to room tem-perature before deflasking. The acrylic specimens thenwere retrieved, finished, and polished.

The remaining 3 experimental groups consisted ofPMMA resin specimens of the same dimensions rein-forced with glass (Ahlstrom Corp, Karhula, Finland),

JOHN, GANGADHAR, AND SHAH THE JOURNAL OF PROSTHETIC DENTISTRY

OCTOBER 2001 425

aramid (Kevlar; DuPont, Wilmington, Del.), or nylon(MRF Ltd, Chennai, India) fibers. These fibers had athickness of 10 to 15 µm and were cut to 5 mmlength. The cut fibers were soaked in monomer for 10minutes2l for better bonding with the acrylic resin;after the fibers were removed from the monomer,excess liquid was allowed to dry. The resin and fibers(2% by weight12) were mixed thoroughly to dispersethe fibers. On reaching dough stage, the mixture waskneaded and packed into the prepared mold. Thespecimens were polymerized and recovered in thesame manner as the control group. After deflasking, ifthe specimens revealed exposed fibers at the peripher-al border, trimming was performed with diamond bursto avoid delamination of the reinforcement.

All specimens were stored in water at room tem-perature for 1 week before testing. Specimens werelabeled on each end before testing so that fracturedpieces could be reunited and examined subsequentto testing.

All samples were tested for flexural strength with a3-point bending test with a universal testing machine22

(WPM Leipzig, Leipzig, Germany) at a crossheadspeed of 2 mm/min. A load was applied by a centrallylocated rod until fracture occurred. The flexuralstrength was calculated with the following formula:

where FS is flexural strength, p is the peak loadapplied, l is the span length, b is the sample width, andd is the sample thickness. The results were analyzedwith a 1-way analysis of variance (ANOVA).

RESULTS

Figure 1 shows the mean and standard deviationvalues of flexural strength for each of the experimental

FS = 3 pl

×2 bd2

Fig. 1. Flexural strengths (±2 SD) of the control and 3 experimental groups.

groups. In group A (control), the force required tofracture the specimens ranged from 14 to 18.5 kg; theflexural strength of these specimens ranged from624.6 to 825.4 MPa, with a mean of 696 MPa. Ingroup B (glass fiber reinforced), the force required tofracture the specimens ranged from 18.5 to 23.5 kg;the flexural strength of these specimens ranged from825.4 to 1048.5 MPa, with a mean of 979.2 MPa, thehighest among all groups. In group C (aramid fiberreinforced), the force required to fracture the speci-mens ranged from 16.5 to 22 kg; the flexural strengthof these specimens ranged from 736.2 to 981.5 MPa,with a mean of 849.9 MPa. Finally, in group D (nylonfiber reinforced), the force required to fracture thespecimens ranged from 15 to 19 kg; the flexuralstrength of these specimens ranged from 619.5 to847.7 MPa, with a mean of 733.4 MPa. Group B spec-imens had the highest flexural strength, followed bygroups C, D, and A (in that order). The higher theload or force required to fracture the specimens, thehigher the fracture resistance.

An analysis of the difference in flexural strengths wasperformed with a 1-way ANOVA, and the result wasfound to be significant (P<.001). The minimum signifi-cant value was 87.8 MPa. Only the control andnylon-reinforced denture resins did not differ significantly.

DISCUSSION

PMMA resin is the material of choice for the fabri-cation of denture bases. However, fracture of the basemay occur during function because of the poor trans-verse, impact, and flexural strengths of PMMA. One ofthe most common causes for breakage of dentures isfatigue4 (namely, continued flexing of the base duringfunction, which leads to crack development). Midlinefracture of a denture base is a flexural fatigue failurethat results from cyclic deformation of the base duringfunction. This fracture stems from the initiation andpropagation of a crack, and it requires the presence ofa stress raiser or localized stress.

This study compared the flexural strengths of con-ventional PMMA resin and the same resin reinforcedwith glass, aramid, or nylon fibers in loose form. Glassis an inorganic substance that has been cooled to arigid condition without crystallization. Different typesof glass fibers are produced commercially; theseinclude E-glass, S-glass, R-glass, V-glass, and Cemfil.Of these, E-glass fiber, which has high alumina andlow alkali and borosilicate, is claimed to be superior inflexural strength.8

The flexural strength of a material is a combinationof compressive, tensile, and shear strengths. As thetensile and compressive strengths increase, the forcerequired to fracture the material also increases.Compared with conventional polymer materials, fiber-reinforced polymers are successful in their application

primarily because of their high specific modulus andspecific strength. Because the modulus of elasticity ofglass fibers is very high, most of the stresses arereceived by them without deformation.23 Thus, in thisstudy, glass-reinforced specimens exhibited better flex-ural strength than the other specimen groups.

Aramid is a generic term for wholly aromatic fibers.These fibers are resistant to chemicals, are thermallystable, and have a high mechanical stability, meltingpoint, and glass transitional temperature. They alsohave pleated structure (molecules are radiallyarranged in the form of sheets) that makes aramidweak as far as flexural, compression, and abrasionbehavior are concerned. This explains why aramidfiber-reinforced specimens demonstrated a lower flex-ural strength than specimens reinforced with glassfiber.

Nylon fibers are polyamide fibers and are based pri-marily on aliphatic chains. The chief advantage ofnylon lies in its resistance to shock and repeated stress-ing. However, water absorption affects the mechanicalproperties of nylon. In this study, nylon-reinforcedspecimens bases had a higher fracture resistance thanthe control PMMA specimens.

Neither thermocycling nor fatigue testing was con-ducted in this study. Fatigue would be better simulatedin intraoral conditions, which would also reveal anyeffect of water sorption on the properties of fiber-rein-forced PMMA resins.

CONCLUSIONS

Within the limitations of this study, the reinforce-ment of denture base resin with glass, aramid, or nylonfibers improved the flexural strength of the resin.Which type of fiber is preferable depends on the typeof prosthesis being fabricated. Glass and aramid fibersappear to be suitable for long-term use in completedentures and distal extension partial denture bases,which are considered prone to fracture. Glass fiberreinforcement may also help prevent fracture in provi-sional fixed partial dentures by strengthening them atthe connector sites.

REFERENCES

1. Eick JD. Biological properties of denture base resins. Dent Clin North Am1977;21:459-64.

2. Peyton FA. History of resins in dentistry. Dent Clin North Am1975;19:211-22.

3. Smith DC. Recent developments and prospects in dental polymers. JProsthet Dent 1962;12:1066-78.

4. Stafford GD, Smith DC. Flexural fatigue tests of some denture base poly-mers. Br Dent J 1970;128:442-5.

5. Lambrecht JR, Kydd WL. A functional stress analysis of the maxillarycomplete denture base. J Prosthet Dent 1962;12:865-72.

6. Beyli MS, von Fraunhofer JA. An analysis of causes of fracture of acrylicresin dentures. J Prosthet Dent 1981;46:238-41.

7. Gutteridge DL. The effect of including ultra-high modulus polyethyl-ene fibre on the impact strength of acrylic resin. Br Dent J1988;164:177-80.

THE JOURNAL OF PROSTHETIC DENTISTRY JOHN, GANGADHAR, AND SHAH

426 VOLUME 86 NUMBER 4

JOHN, GANGADHAR, AND SHAH THE JOURNAL OF PROSTHETIC DENTISTRY

OCTOBER 2001 427

8. Solanit GS. The effect of methyl methacrylate reinforcement with silane-treated and untreated glass fibers. J Prosthet Dent 1991;66:310-4.

9. Vallittu PK. Glass fiber reinforcement in repaired acrylic resin removabledentures: preliminary results of a clinical study. Quintessence Int1997;28:39-44.

10. Berrong JM, Weed RM, Young JM. Fracture resistance of Kevlar-rein-forced poly (methyl methacrylate) resin: a preliminary study. Int JProsthodont 1990;3:931-5.

11. Goldberg AJ, Burstone CJ. The use of continuous fiber reinforcement indentistry. Dent Mater 1992;8:197-202.

12. Vallittu PK, Lassila VP, Lappalainen R. Acrylic resin-fiber composite—Part I: the effect of fiber concentration on fracture resistance. J ProsthetDent 1994;71:607-12.

13. De Boer J, Vermilyea SG, Brady RE. The effect of carbon fiber orientationon the fatigue resistance and bending properties of two denture resins. JProsthet Dent 1984;51:119-21.

14. Vallittu PK, Vojtkova H, Lassila VP. Impact strength of denture polymethylmethacrylate reinforced with continuous glass fibers or metal wire. ActaOdontol Scand 1995;53:392-6.

15. Vallittu PK, Lassila VP. Reinforcement of acrylic resin denture base mate-rial with metal or fibre strengtheners. J Oral Rehabil 1992;19:225-30.

16. Matthews E. Nylon as a denture base material. Br Dent J 1955;98:231-7.17. Gutterridge DL. Reinforcement of poly (methyl methacrylate) with ultra-

high-modulus polyethylene fibre. J Dent 1992;20:50-4.18. Ladizesky NH, Pang MK, Chow TW, Ward IM. Acrylic resin reinforced with

woven highly drawn linear polyethylene fibres. Mechanical properties andfurther aspects of denture construction. Aust Dent J 1993;38:28-38.

19. Williamson DL, Boyer DB, Aquilino AS, Leary JM. Effect of polyethylenefiber reinforcement on the strength of denture base resins polymerizedby microwave energy. J Prosthet Dent 1994;72:635-8.

20. Dixon DL, Fincher M, Breeding LC, Mueninghoff LA. Mechanical prop-erties of a light-polymerizing provisional restorative material with andwithout reinforcement fibers. J Prosthet Dent 1995;73:510-4.

21. Chow TW, Ladizesky NH, Clarke DA. Acrylic resins reinforced withwoven highly drawn linear polyethylene fibres. 2. Water sorption andclinical trials. Aust Dent J 1992;37:433-8.

22. Robinson JG, McCabe JF. Impact strength of acrylic resin denture basematerials with surface defects. Dent Mater 1993;9:355-60.

23. Clark HA, Plueddemann EP. Bonding of silane coupling agents in glass-reinforced plastics. Modern Plastics 1963;40:133-8.

Reprint requests to:DR JACOB JOHN

52, MAIN ROAD

PERAMBUR

CHENNAI 600 011INDIAFAX: (91)44-854-6674E-MAIL: jacob_john69@yahoo.com

Copyright © 2001 by The Editorial Council of The Journal of ProstheticDentistry.

0022-3913/2001/$35.00 + 0. 10/1/118564

doi:10.1067/mpr.2001.118564

Saliva in health and disease: an appraisal and updateSreebny LM. Int Dent J 2000;50:140-61.

Purpose. Saliva plays an extremely important role in oral health monitoring, as it regulates andmaintains the integrity of the oral cavity’s hard tissue as well as some of the soft tissues. This arti-cle reviews the role of saliva in health and disease and highlights the current use of salivary testsand the use of saliva as a diagnostic fluid. Discussion. This article reviews the prevalence of oral dryness (xerostomia) and its importanceto the dental practitioner. The discussion includes the following topics: (1) stimulated andunstimulated salivary flow rates and how they are affected by aging; (2) the relationship betweenxerostomia and salivary gland hypofunction; (3) the causes of oral dryness, which include druguse, irradiation to the head and neck, decreased masticatory function, and xerostomia in theSjögren’s Syndrome patient; (4) the role of saliva as a diagnostic fluid; and (5) the methods cur-rently available for testing saliva.Recommendations. The author, an acknowledged world-class scientist of salivary function,makes 12 major recommendations related to saliva education and treatment. Among other things,he suggests that reference values for saliva should be developed; that dental associations world-wide should be encouraged to promote media campaigns on the health benefits of saliva and thesignificance of dry mouth; that brochures about saliva, dry mouth, and Sjögren’s Syndromeshould be published and given to patients by dentists; and that industry should be encouraged todevelop salivary testing kits as well as develop products and drugs that do not promote drymouth. 134 References. —RP Renner

Noteworthy Abstractsof theCurrent Literature