Reinforcement of complete denture bases with high performance polyethylene fibers N. H. Ladizesky, PhD, M Inst T. W. Chow, BDS, MSc, DRD, Faculty
of Dentistry,
University
P,a C. F. Ho, LBIST,b FRACDSe
of Hong
Kong,
Hong
continuous
and
Kong
Clinical literature details the causes for deformation and failure of acrylic resin complete denture bases. These findings are used to select patterns of high performance polyethylene fiber reinforcement that would best use the properties of the material and improve the mechanical behavior of the prostheses. A technique is described for making reinforced maxillary and mandibular bases with the fibers placed as suggested by the analysis. Microscopic observations of cross sections of reinforced bases revealed good fiber/resin integration and polish. (J PROSTHET DENT
1992;68:934-9.)
R ecent
articles presented a study of the mechanical properties of denture base resin bars reinforced with highly drawn linear polyethylene (HDLPE) fibers.l, 2 These were oriented longitudinally to increase the fiber content and the support given to the resin. The resultant composites reflected the best capabilities of the system. Previous work reported a method for the construction of acrylic resin denture bases reinforced with woven HDLPE fibers, complemented with an assessment of their clinical performance.3> * This article discusses the best placement and orientation of the fibers within the bases and the method of making maxillary and mandibular complete denture bases reinforced with parallel continuous HDLPE fibers. A future article will report on the clinical trials of dentures reinforced with HDLPE fibers in three forms, namely continuous parallel, woven, and chopped.
Properties
of fibers
and composite
resins
The HDLPE fibers have high tensile stiffness and strength.5 When longitudinally oriented they produce composite resins with high tensile and compressive moduli. In three-point bending with loads perpendicular to the fiber direction, the composite resins have excellent flexural modulus and impact strength, together with a significantly increased flexural strength, when compared with the unreinforced resin.2,6 Furthermore, HDLPE fiber-reinforced composite resins have shown somewhat unusual but highly desirable behavior during bending and impact, namely:l> 2,6-8 they have notch insensitivity, cracks do not propagate through the array of fibers, and coherence is mainPresented Study Supported
at the Annual Conference of the British Society for the of Prosthetic Dentistry, Durham, UK. by University of Hong Kong research grants, Nos. 335.263.0003, and 335.255.0004. aLecturer, Dental Materials Science Unit. bDental Technologist, Dental Technology Unit. Senior Lecturer, Department of Prosthetic Dentistry.
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tained even after a large number of testing cycles. On the other hand, there are no data on the fatigue performance of these composites.
DEFORMATION DENTURES This clinical
AND
FAIEURE
analysis is based on information literature.
Maxillary
OF available
in the
dentures
Midline fracture is the predominant mode of breakage and has been identified as fatigue failure.g Several regions have been proposed as the most likely location for crack initiation and these have been summarized, together with the most suitable reinforcement design to take advantage of the special characteristics of the fiber and resin system, namely notch insensitivity2z 6 and the inability of cracks to penetrate the array of fibers.8 Matthews and Wain’O found that the highest stresses during function occurred on the polished surface of the palatal aspect, in the region immediately behind the anterior teeth. They suggested that the palatal side of the base between the central incisors was one of the prime contributors to midline fracture. Lambrecht and Kyddll ascertained that the midline in the anteroposterior direction remains undeformed during function. Therefore, fibers in this direction will not be stress-bearing, but crack production and the resultant fatigue failure should be significantly reduced by reinforcing the palate with fibers oriented in the lateral direction. The frenal notch has been acknowledged as another major contributor to crack initiation, which then continues toward the palatal aspect.ll> l2 This can be resisted by horizontally positioned fibers in the anterior part of the labial flange. Lambrecht and Kyddll found that mastication and swallowing produce either an increase or a decrease of the curvature of the base at the mid line (Fig. 1, A), accompanied by a small extension and compression of the flanges.
DECEMBER
1992
VOLUME
68
NUMBER
6
REINFORCEMENT
OF DENTURE
BASES
Fig. 1. A, Schematic diagram Schematic diagram of possible
of possible deformation of complete maxillary denture. deformation of complete mandibular denture.
Therefore the entire denture base should be reinforced with fibers oriented in the lateral direction. Further improvement may be achieved by reinforcing the flanges with parallel fibers in the horizontal plane. Maxillary bases are thus reinforced with four layers of fibers, two in the lateral direction sandwiched between two other layers at 45 degrees from the middle ones. Previous work has indicated that oblique fibers contribute to the reinforcement in the lateral direction as well as anteroposteriorly, thus partially fulfilling the complementary role of the horizontal fibers along the buccal flanges.13
Although fatigue failure does not appear to be a serious problem, 80% of fractures are produced by impact when the dentures fall onto hard surfaces.14 The advantages of the high-impact performance of dental resins reinforced with HDLPE fiber& 2, 7 cannot be overestimated.
THE
JOURNAL
OF PROSTHETIC
DIENTISTRY
B,
When mandibular dentures fall onto hard surfaces, maximum stresses appear in the labial and lingual second premolar region.15 This is independent of the orientation of the denture at the moment of impact and substantiates the finding that impact fracture often occurs in the middle region.14s r5 It follows that parallel fibers oriented in the horizontal plane along the labial and lingual regions should provide maximum reinforcement against impact failure. Regli et a1.161r7 found three predominant types of deformation of complete mandibular dentures (Fig. 1, B): (1) movement of the molar regions away from each other; (2) movement of the molar regions toward each other; and (3) a compound movement in which the posterior teeth rotate buccally and the lingual flanges rotate lingually. Reports suggest that the strains are sma1116-18 and that the modulus, rather than the strength of the material, is the important factor. Deformations 1 and 2 are of the bending type in the horizontal plane and will be resisted by parallel fi-
935
LADIZESKY,
NO, AND
CHOW
Pig. 2. A, Set of “‘pre-pregs” ready for adaptation on duplicate cast; B, adaptation of pre-pregs to the maxillary mold after cut to the correct shape; C, maxillary denture base reinforced with four layers of parallel fibers. Fiber content 26% by volume.
bers following the contour of the denture. This same arrangement should also oppose deformation (3) because it involves elongation and compression of the anterior lingual flangeI x7 and overall tensile strains in the horizontal plane (a consequence of the “torsion” character of the deformation) . Parallel fibers following the arch, in conjunction with the complex shape of the denture and the various forces applied during function, may give rise to unexpected modes of deformations. One such possibility is the spreading apart or coming together of the flanges, for which the horizontal reinforcement design offers no increased resistance. To overcome this problem, mandibular bases were fabricated with fibers oriented at right angles to the ridge, located close to the polished and fitting surfaces. These are the regions where maximum strains occur if flange movements take place. Between the two outer layers of fiber lies the main component of the reinforcement, namely the fibers in the horizontal plane along the dental arch.
Construction
of reinforced
bases
The HDLPE yarn was supplied by Celanese Research Co. (Summit, NJ.) and had 180 filaments of 15 grn nominal diameter. Further details may be seen in a previous article.6 936
Two resins were used, (1) PMMA syrup,l a nonproprietary low-viscosity acrylic resin made with one part of polymethyl methacrylate powder (De Tray RR, AD International Ltd., Weybridge, Surrey, U.K.) mixed with four parts of methyl methacrylate liquid of a heat-curing system (Coe Lor, Coe Laboratories Inc., Chicago, Ill.) and (2) Trevalon C resin (AD International Ltd.).
Maxillary Maxillary
bases bases were reinforced
with four layers of par-
allel fibers, with a modification of the preimpregnated “pre-preg” technique used by Wylegalarg to reinforce maxillary dentures with carbon fiber mat and later by Braden et al7 to prepare acrylic bars reinforced with HDLPE fibers for testing.
TECHNIQUE Maxillary bases 1. Wet a bundle of fibers 140 mm long and 1.7 gm mass with Trevalon C monomer liquid, spread the material flat on a large sheet of polyethylene film, and sprinkle it lightly with polymer powder. 2. Wet a second bundle, spread and place the bundle at 45 degrees on top of the first layer, and follow with a light sprinkling of polymer powder. Add two further layers, DECEMBER
1992
VOLUME
66
NUMBER
6
REINFORCEMENT
OF DENTURE
BASES
3. A, Pre-preg with fibers at right angles to the mandibular arch; B, first and second pre-pregs adapted to duplicate cast, ready to receive the third pre-preg; C, adaptation of pre-preg to the mold after cutting to the correct shape; D, mandibular base reinforced with pre-preg parallel fibers. Fiber content 39% by volume.
Fig.
one with the same orientation as the second layer and the last at right angle to the first (Fig. 2, A). Cover the impregnated fibers with a sheet of polyethylene film and leave for 2 hours to allow good resin penetration. Adapt them to a duplicate cast so that the direction of the fibers of the two middle layers are oriented at 90 degrees to the mid-line of the palate. Place a sheet of polyethylene film over the cast with adapted “pre-preg” and seal it in a polyethylene bag. Leave in a refrigerator overnight at 4OC to increase the resin penetration within the array of fibers and prolong the dough stage. On the following day, allow the prepreg to reach room temperature and readapt to the cast. After 2 hours of drying time, cut the fibers to the correct shape with scissors specially designed for high performance fibers (Arston, ARS Edge Co. Ltd., Osaka, Japan) and transfer them to the mold (Fig. 2, B). Make a trial closure. Pour Trevalon C fluid (made with 2 parts of powder to 1 part of liquid by volume) onto the fitting surface of the mold. Readapt the pre-preg and close the flask under pressure for 3 hours. Prepare a new mix of Trevalon C fluid and pour over the pre-preg. Close the flask for 1 hour, then open it, remove the excessresin, make the final closure, and heat cure. A resultant base is shown in Fig. 2, C. THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Mandibular
bases
Prepare two bundles of fibers 290 mm long and 3.5 g weight. Cut into five bundles, one to the length of the arch (about 140 mm long) and four of 70 mm length each. Place a polyethylene sheet over a paper template cut in a horseshoe shape following the arch of the mandibular denture. Prepare a pre-preg by using two short bundles evenly distributed over the paper, with the fibers at right angles to the template (Fig. 3, A). Prepare an additional pre-preg with the remaining two short bundles. Use the long bundle to prepare a third pre-preg, with the fibers following the horseshoe shape of the paper. The three sets of pre-pregs can be kept in a refrigerator for several days if desired, but must be left at room temperature an hour before use. Prepare the mold as usual and adapt one pre-preg over the fitting surface, with the fibers across the ridge. Allow 1 hour for drying, by which time the adapted form will be reasonably rigid. 5. Adapt the pre-preg made with the long bundle over the first, with the fibers following the ridge, and let it dry for 1 hour (Fig. 3, B). Adapt the third pre-preg on top. Remove all of the fibers in one unit, dry it for an additional hour and cut to the correct size (Fig. 3, C). 937
LAQIZESKY,
HO, AND
CHOW
Fig. 4. Cross section of a maxillary base reinforced with four layers of parallel fibers: A, Shows the three orientation directions (original magnification x70); B fibers reach the fitting surface (FS) (original magnification x500).
6. Prepare Trevalon C resin with 2 parts of powder and 1 part of liquid. Pour a thin layer onto the cast and readapt the pre-pregs onto the mold. Pour the rest of the mix over the fibers and cover with a sheet of polyethylene before trial closure. Keep the pressure as low as possible while waiting for the mix to reach the rubber stage. Open the flask and remove the excessresin before final closure under pressure and curing. A resultant base is shown in Fig. 3, D.
ISCUSSIQN The denture bases seen in Figs. 2, C and 3, D have respectively 26 % volume and 39 % volume of fiber loading. These values were determined by the calculation method previously reported.8 The thickness of the dentures may be controlled at the wax-up stage and, if desired, thinner bases can be produced because of the high strength of the reinforced resin. Fig. 4 shows a cross section of a maxillary base reinforced with four pre-pregs cut in the palate region, approximately at right angles to the fiber direction of one of the outer layers. Three different fiber orientations are seen. Fig. 5, A shows a cross section of a m.andibular base with 938
Fig. 5. Cross section of a mandibular base reinforced with three layers of parallel fibers: A, View shows the three layers of fibers (original magnification x35); B, boundary between two layers of fibers (original magnification x400); C, cross section of mandibular base at a region where fibers reach polished surface (PS) (original magnification x800).
Fig. 6. Complete dentures with fiber reinforced bases. DECEMBER
1992
VOLUME
68
NUMBER
6
REINFORCEMENT
OF DENTURE
BASES
fibers following the arch, sandwiched between fibers at right angles to the ridge. The three layers can be seen. Fig. 5, B indicates excellent resin penetration between the different layers and between the fibers. The mechanical performance of the reinforced denture bases should be significantly superior to that of conventional prostheses. It was found that the incorporation of approximately 48% volume of parallel fibers in acrylic resin bars produced increases of approximately 70% for flexural strength, 600 % for stiffness, and 1000 % for impact strength.l These improvements were maintained in a water environment at 37’ C, as reported.2 Other workers have reported that reinforcement of denture base resins with carbon or Kevlar (E.I. DuPont de Nemours Co., Wilmington, Del.) fibers reaching the surfaces of the prostheses resulted in protruding ends when polished with conventional techniques.20-22 With reasonable care the technique presented in this article produces prostheses with no fibers exposed. However, should this occur they can be readily polished with pumice and tripoli (Fig. 5, C). Fibers reaching the Gtting side do not protrude (Fig. 4, B). Fig. 6 shows one of the complete sets of dentures presently undergoing clinical trials.
CONCLUSION Analysis based on reports in the clinical literature suggest that patterns of continuous parallel fibers should provide maximum reinforcement to both maxillary and mandibular bases..The bases can be made with standard dental techniques, including some extra steps that do not require specialized equipment or expertise. A high fiber content was incorporated without increasing the thickness of the prostheses. Comparison with previous work indicates that these reinforced bases should have significantly improved clinical performance. We are grateful to Professor I. M. Ward and Dr. D. of the Physics Department, University of Leeds, UK, ing the HDLPE fibers. Our sincere and special thanks sor R. k. F. Clark, Head of Department of Prosthetic University of Hong Kong, for his encouragement and
W. Woods for supplyto ProfesDentistry, support.
REFERENCES 1. Lad&sky NH, Chow TW, Ward IM. The effect of highly drawn polyethylene fibers on the mechanical properties of denture base resins. Clin Mater 1990:6:209-25.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
2. Ladizesky NH, Chow TW. The effect of interface adhesion, water immersion and anatomical notches on the mechanical properties of denture base resins reinforced with continuous high performance polyethylene fibers. Aust Dent J (In press). 3. Clarke DA, Ladizesky NH, Chow TW. Acrylic resins reinforced with woven highly drawn linear polyethylene fibers: l-construction of upper denture bases. Aust Dent J (In press). 4. Chow TW, Ladizesky NH, Clarke DA. Acrylic resins reinforced with woven highly drawn linear polyethylene fibers: Z-water absorption and clinical trials. Aust Dent J (In press). 5. Ward IM. The preparation, structure and properties of ultra-highmodulus flexible polymers. Adv Polym Sci 1985;70:1-70. 6. Ladizesky NH, Ward IM. Ultra-high-modulus polyethylene fiber composites: I-the preparation and properties of conventional epoxy resin composite. Comp Sci Tech 1986;26:129-64. 7. Braden M, Davy KWM, Parker S, Ladizesky NH, Ward IM. Denture base poly(methylmethacrylate) reinforced with ultra-high modulus polyethylene fibers. Br Dent J 1988;164:109-13. 8. Ladizesky NH, Pang MKM, Chow TW, Ward IM. Acrylic resins reinforced with woven highly drawn linear polyethylene fibers: 3-mechanical properties and further aspects of denture construction. Aust Dent J (In press). 9. Smith DC. The acrylic denture. Mechanical evaluation mid-line fracture. Br Dent J 1961;110:257-67. 10. Matthews E, WainEF. Stresses in denture bases. Br Dent J 1956;100:16771. 11. Lambrecht JR, Kydd WL. A functional stress analysis of the maxillary complete denture base. J PROSTHET DENT 1962;12:865-72. 12. Kelly E. Fatigue failure in denture base polymers. J PROSTHET DENT 1969;21:257-66. 13. Ladisesky NH, Ward IM. Ultra-high-modulus polyethylene composites: III-an exploratory study of hybrid composites. Comp Sci Tech 1986;26:199-224. 14. Hargreaves AS. The prevalence of fractured dentures. A survey. Br Dent J 1969;126:451-5. 15. Ahmad R, Bates JF, Lewis TT. Measurement of strain rate behaviour in complete mandibular dentures. Biomaterials 1982;3:87-92. 16. Regli CP, Kydd WL. A preliminary study of the lateral deformation of metal base dentures in relation to plastic base dentures. J PROSTHET DENT 1953;3:326-330. 17. Regli CP, Gaskill HL. Denture base deformation during function. J PROSTHET DENT 1954;4:548-54. 18. Swoope CC Jr, Kydd WL. The effect of cusp form and occlusal surface area on denture base deformation. J PROSTHET DENT 1966;16:34-43. 19. Wylegala RT. Reinforcing denture base material with carbon fibers. Dent Techn 1973;26:97-100. 20. Gutteridge DL. The effect of including ultra-high-modulus polyethylene fiber on the impact strength of acrylic resin. Br Dent J 1988;164:17780. 21. Christensen G, ed. Reinforcement fibers. Clin Res Associates Newsletter 1987;2-3. 22. Friskopp J, Blomlof L. Intermediate fiber glass splints. J PROSTHET DENT 1984;51:334-7.
Reprint
requests
to:
DR. T. W. CHOW DEPARTMENT OF PROSTHETIC DENTISTRY PRINCE PHILLIP DENTAL HOSPITAL 34 HOSPITAL ROAD HONG KONG
939
of Dentistry,
University
P,a C. F. Ho, LBIST,b FRACDSe
of Hong
Kong,
Hong
continuous
and
Kong
Clinical literature details the causes for deformation and failure of acrylic resin complete denture bases. These findings are used to select patterns of high performance polyethylene fiber reinforcement that would best use the properties of the material and improve the mechanical behavior of the prostheses. A technique is described for making reinforced maxillary and mandibular bases with the fibers placed as suggested by the analysis. Microscopic observations of cross sections of reinforced bases revealed good fiber/resin integration and polish. (J PROSTHET DENT
1992;68:934-9.)
R ecent
articles presented a study of the mechanical properties of denture base resin bars reinforced with highly drawn linear polyethylene (HDLPE) fibers.l, 2 These were oriented longitudinally to increase the fiber content and the support given to the resin. The resultant composites reflected the best capabilities of the system. Previous work reported a method for the construction of acrylic resin denture bases reinforced with woven HDLPE fibers, complemented with an assessment of their clinical performance.3> * This article discusses the best placement and orientation of the fibers within the bases and the method of making maxillary and mandibular complete denture bases reinforced with parallel continuous HDLPE fibers. A future article will report on the clinical trials of dentures reinforced with HDLPE fibers in three forms, namely continuous parallel, woven, and chopped.
Properties
of fibers
and composite
resins
The HDLPE fibers have high tensile stiffness and strength.5 When longitudinally oriented they produce composite resins with high tensile and compressive moduli. In three-point bending with loads perpendicular to the fiber direction, the composite resins have excellent flexural modulus and impact strength, together with a significantly increased flexural strength, when compared with the unreinforced resin.2,6 Furthermore, HDLPE fiber-reinforced composite resins have shown somewhat unusual but highly desirable behavior during bending and impact, namely:l> 2,6-8 they have notch insensitivity, cracks do not propagate through the array of fibers, and coherence is mainPresented Study Supported
at the Annual Conference of the British Society for the of Prosthetic Dentistry, Durham, UK. by University of Hong Kong research grants, Nos. 335.263.0003, and 335.255.0004. aLecturer, Dental Materials Science Unit. bDental Technologist, Dental Technology Unit. Senior Lecturer, Department of Prosthetic Dentistry.
10/1/39426
934
tained even after a large number of testing cycles. On the other hand, there are no data on the fatigue performance of these composites.
DEFORMATION DENTURES This clinical
AND
FAIEURE
analysis is based on information literature.
Maxillary
OF available
in the
dentures
Midline fracture is the predominant mode of breakage and has been identified as fatigue failure.g Several regions have been proposed as the most likely location for crack initiation and these have been summarized, together with the most suitable reinforcement design to take advantage of the special characteristics of the fiber and resin system, namely notch insensitivity2z 6 and the inability of cracks to penetrate the array of fibers.8 Matthews and Wain’O found that the highest stresses during function occurred on the polished surface of the palatal aspect, in the region immediately behind the anterior teeth. They suggested that the palatal side of the base between the central incisors was one of the prime contributors to midline fracture. Lambrecht and Kyddll ascertained that the midline in the anteroposterior direction remains undeformed during function. Therefore, fibers in this direction will not be stress-bearing, but crack production and the resultant fatigue failure should be significantly reduced by reinforcing the palate with fibers oriented in the lateral direction. The frenal notch has been acknowledged as another major contributor to crack initiation, which then continues toward the palatal aspect.ll> l2 This can be resisted by horizontally positioned fibers in the anterior part of the labial flange. Lambrecht and Kyddll found that mastication and swallowing produce either an increase or a decrease of the curvature of the base at the mid line (Fig. 1, A), accompanied by a small extension and compression of the flanges.
DECEMBER
1992
VOLUME
68
NUMBER
6
REINFORCEMENT
OF DENTURE
BASES
Fig. 1. A, Schematic diagram Schematic diagram of possible
of possible deformation of complete maxillary denture. deformation of complete mandibular denture.
Therefore the entire denture base should be reinforced with fibers oriented in the lateral direction. Further improvement may be achieved by reinforcing the flanges with parallel fibers in the horizontal plane. Maxillary bases are thus reinforced with four layers of fibers, two in the lateral direction sandwiched between two other layers at 45 degrees from the middle ones. Previous work has indicated that oblique fibers contribute to the reinforcement in the lateral direction as well as anteroposteriorly, thus partially fulfilling the complementary role of the horizontal fibers along the buccal flanges.13
Although fatigue failure does not appear to be a serious problem, 80% of fractures are produced by impact when the dentures fall onto hard surfaces.14 The advantages of the high-impact performance of dental resins reinforced with HDLPE fiber& 2, 7 cannot be overestimated.
THE
JOURNAL
OF PROSTHETIC
DIENTISTRY
B,
When mandibular dentures fall onto hard surfaces, maximum stresses appear in the labial and lingual second premolar region.15 This is independent of the orientation of the denture at the moment of impact and substantiates the finding that impact fracture often occurs in the middle region.14s r5 It follows that parallel fibers oriented in the horizontal plane along the labial and lingual regions should provide maximum reinforcement against impact failure. Regli et a1.161r7 found three predominant types of deformation of complete mandibular dentures (Fig. 1, B): (1) movement of the molar regions away from each other; (2) movement of the molar regions toward each other; and (3) a compound movement in which the posterior teeth rotate buccally and the lingual flanges rotate lingually. Reports suggest that the strains are sma1116-18 and that the modulus, rather than the strength of the material, is the important factor. Deformations 1 and 2 are of the bending type in the horizontal plane and will be resisted by parallel fi-
935
LADIZESKY,
NO, AND
CHOW
Pig. 2. A, Set of “‘pre-pregs” ready for adaptation on duplicate cast; B, adaptation of pre-pregs to the maxillary mold after cut to the correct shape; C, maxillary denture base reinforced with four layers of parallel fibers. Fiber content 26% by volume.
bers following the contour of the denture. This same arrangement should also oppose deformation (3) because it involves elongation and compression of the anterior lingual flangeI x7 and overall tensile strains in the horizontal plane (a consequence of the “torsion” character of the deformation) . Parallel fibers following the arch, in conjunction with the complex shape of the denture and the various forces applied during function, may give rise to unexpected modes of deformations. One such possibility is the spreading apart or coming together of the flanges, for which the horizontal reinforcement design offers no increased resistance. To overcome this problem, mandibular bases were fabricated with fibers oriented at right angles to the ridge, located close to the polished and fitting surfaces. These are the regions where maximum strains occur if flange movements take place. Between the two outer layers of fiber lies the main component of the reinforcement, namely the fibers in the horizontal plane along the dental arch.
Construction
of reinforced
bases
The HDLPE yarn was supplied by Celanese Research Co. (Summit, NJ.) and had 180 filaments of 15 grn nominal diameter. Further details may be seen in a previous article.6 936
Two resins were used, (1) PMMA syrup,l a nonproprietary low-viscosity acrylic resin made with one part of polymethyl methacrylate powder (De Tray RR, AD International Ltd., Weybridge, Surrey, U.K.) mixed with four parts of methyl methacrylate liquid of a heat-curing system (Coe Lor, Coe Laboratories Inc., Chicago, Ill.) and (2) Trevalon C resin (AD International Ltd.).
Maxillary Maxillary
bases bases were reinforced
with four layers of par-
allel fibers, with a modification of the preimpregnated “pre-preg” technique used by Wylegalarg to reinforce maxillary dentures with carbon fiber mat and later by Braden et al7 to prepare acrylic bars reinforced with HDLPE fibers for testing.
TECHNIQUE Maxillary bases 1. Wet a bundle of fibers 140 mm long and 1.7 gm mass with Trevalon C monomer liquid, spread the material flat on a large sheet of polyethylene film, and sprinkle it lightly with polymer powder. 2. Wet a second bundle, spread and place the bundle at 45 degrees on top of the first layer, and follow with a light sprinkling of polymer powder. Add two further layers, DECEMBER
1992
VOLUME
66
NUMBER
6
REINFORCEMENT
OF DENTURE
BASES
3. A, Pre-preg with fibers at right angles to the mandibular arch; B, first and second pre-pregs adapted to duplicate cast, ready to receive the third pre-preg; C, adaptation of pre-preg to the mold after cutting to the correct shape; D, mandibular base reinforced with pre-preg parallel fibers. Fiber content 39% by volume.
Fig.
one with the same orientation as the second layer and the last at right angle to the first (Fig. 2, A). Cover the impregnated fibers with a sheet of polyethylene film and leave for 2 hours to allow good resin penetration. Adapt them to a duplicate cast so that the direction of the fibers of the two middle layers are oriented at 90 degrees to the mid-line of the palate. Place a sheet of polyethylene film over the cast with adapted “pre-preg” and seal it in a polyethylene bag. Leave in a refrigerator overnight at 4OC to increase the resin penetration within the array of fibers and prolong the dough stage. On the following day, allow the prepreg to reach room temperature and readapt to the cast. After 2 hours of drying time, cut the fibers to the correct shape with scissors specially designed for high performance fibers (Arston, ARS Edge Co. Ltd., Osaka, Japan) and transfer them to the mold (Fig. 2, B). Make a trial closure. Pour Trevalon C fluid (made with 2 parts of powder to 1 part of liquid by volume) onto the fitting surface of the mold. Readapt the pre-preg and close the flask under pressure for 3 hours. Prepare a new mix of Trevalon C fluid and pour over the pre-preg. Close the flask for 1 hour, then open it, remove the excessresin, make the final closure, and heat cure. A resultant base is shown in Fig. 2, C. THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Mandibular
bases
Prepare two bundles of fibers 290 mm long and 3.5 g weight. Cut into five bundles, one to the length of the arch (about 140 mm long) and four of 70 mm length each. Place a polyethylene sheet over a paper template cut in a horseshoe shape following the arch of the mandibular denture. Prepare a pre-preg by using two short bundles evenly distributed over the paper, with the fibers at right angles to the template (Fig. 3, A). Prepare an additional pre-preg with the remaining two short bundles. Use the long bundle to prepare a third pre-preg, with the fibers following the horseshoe shape of the paper. The three sets of pre-pregs can be kept in a refrigerator for several days if desired, but must be left at room temperature an hour before use. Prepare the mold as usual and adapt one pre-preg over the fitting surface, with the fibers across the ridge. Allow 1 hour for drying, by which time the adapted form will be reasonably rigid. 5. Adapt the pre-preg made with the long bundle over the first, with the fibers following the ridge, and let it dry for 1 hour (Fig. 3, B). Adapt the third pre-preg on top. Remove all of the fibers in one unit, dry it for an additional hour and cut to the correct size (Fig. 3, C). 937
LAQIZESKY,
HO, AND
CHOW
Fig. 4. Cross section of a maxillary base reinforced with four layers of parallel fibers: A, Shows the three orientation directions (original magnification x70); B fibers reach the fitting surface (FS) (original magnification x500).
6. Prepare Trevalon C resin with 2 parts of powder and 1 part of liquid. Pour a thin layer onto the cast and readapt the pre-pregs onto the mold. Pour the rest of the mix over the fibers and cover with a sheet of polyethylene before trial closure. Keep the pressure as low as possible while waiting for the mix to reach the rubber stage. Open the flask and remove the excessresin before final closure under pressure and curing. A resultant base is shown in Fig. 3, D.
ISCUSSIQN The denture bases seen in Figs. 2, C and 3, D have respectively 26 % volume and 39 % volume of fiber loading. These values were determined by the calculation method previously reported.8 The thickness of the dentures may be controlled at the wax-up stage and, if desired, thinner bases can be produced because of the high strength of the reinforced resin. Fig. 4 shows a cross section of a maxillary base reinforced with four pre-pregs cut in the palate region, approximately at right angles to the fiber direction of one of the outer layers. Three different fiber orientations are seen. Fig. 5, A shows a cross section of a m.andibular base with 938
Fig. 5. Cross section of a mandibular base reinforced with three layers of parallel fibers: A, View shows the three layers of fibers (original magnification x35); B, boundary between two layers of fibers (original magnification x400); C, cross section of mandibular base at a region where fibers reach polished surface (PS) (original magnification x800).
Fig. 6. Complete dentures with fiber reinforced bases. DECEMBER
1992
VOLUME
68
NUMBER
6
REINFORCEMENT
OF DENTURE
BASES
fibers following the arch, sandwiched between fibers at right angles to the ridge. The three layers can be seen. Fig. 5, B indicates excellent resin penetration between the different layers and between the fibers. The mechanical performance of the reinforced denture bases should be significantly superior to that of conventional prostheses. It was found that the incorporation of approximately 48% volume of parallel fibers in acrylic resin bars produced increases of approximately 70% for flexural strength, 600 % for stiffness, and 1000 % for impact strength.l These improvements were maintained in a water environment at 37’ C, as reported.2 Other workers have reported that reinforcement of denture base resins with carbon or Kevlar (E.I. DuPont de Nemours Co., Wilmington, Del.) fibers reaching the surfaces of the prostheses resulted in protruding ends when polished with conventional techniques.20-22 With reasonable care the technique presented in this article produces prostheses with no fibers exposed. However, should this occur they can be readily polished with pumice and tripoli (Fig. 5, C). Fibers reaching the Gtting side do not protrude (Fig. 4, B). Fig. 6 shows one of the complete sets of dentures presently undergoing clinical trials.
CONCLUSION Analysis based on reports in the clinical literature suggest that patterns of continuous parallel fibers should provide maximum reinforcement to both maxillary and mandibular bases..The bases can be made with standard dental techniques, including some extra steps that do not require specialized equipment or expertise. A high fiber content was incorporated without increasing the thickness of the prostheses. Comparison with previous work indicates that these reinforced bases should have significantly improved clinical performance. We are grateful to Professor I. M. Ward and Dr. D. of the Physics Department, University of Leeds, UK, ing the HDLPE fibers. Our sincere and special thanks sor R. k. F. Clark, Head of Department of Prosthetic University of Hong Kong, for his encouragement and
W. Woods for supplyto ProfesDentistry, support.
REFERENCES 1. Lad&sky NH, Chow TW, Ward IM. The effect of highly drawn polyethylene fibers on the mechanical properties of denture base resins. Clin Mater 1990:6:209-25.
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2. Ladizesky NH, Chow TW. The effect of interface adhesion, water immersion and anatomical notches on the mechanical properties of denture base resins reinforced with continuous high performance polyethylene fibers. Aust Dent J (In press). 3. Clarke DA, Ladizesky NH, Chow TW. Acrylic resins reinforced with woven highly drawn linear polyethylene fibers: l-construction of upper denture bases. Aust Dent J (In press). 4. Chow TW, Ladizesky NH, Clarke DA. Acrylic resins reinforced with woven highly drawn linear polyethylene fibers: Z-water absorption and clinical trials. Aust Dent J (In press). 5. Ward IM. The preparation, structure and properties of ultra-highmodulus flexible polymers. Adv Polym Sci 1985;70:1-70. 6. Ladizesky NH, Ward IM. Ultra-high-modulus polyethylene fiber composites: I-the preparation and properties of conventional epoxy resin composite. Comp Sci Tech 1986;26:129-64. 7. Braden M, Davy KWM, Parker S, Ladizesky NH, Ward IM. Denture base poly(methylmethacrylate) reinforced with ultra-high modulus polyethylene fibers. Br Dent J 1988;164:109-13. 8. Ladizesky NH, Pang MKM, Chow TW, Ward IM. Acrylic resins reinforced with woven highly drawn linear polyethylene fibers: 3-mechanical properties and further aspects of denture construction. Aust Dent J (In press). 9. Smith DC. The acrylic denture. Mechanical evaluation mid-line fracture. Br Dent J 1961;110:257-67. 10. Matthews E, WainEF. Stresses in denture bases. Br Dent J 1956;100:16771. 11. Lambrecht JR, Kydd WL. A functional stress analysis of the maxillary complete denture base. J PROSTHET DENT 1962;12:865-72. 12. Kelly E. Fatigue failure in denture base polymers. J PROSTHET DENT 1969;21:257-66. 13. Ladisesky NH, Ward IM. Ultra-high-modulus polyethylene composites: III-an exploratory study of hybrid composites. Comp Sci Tech 1986;26:199-224. 14. Hargreaves AS. The prevalence of fractured dentures. A survey. Br Dent J 1969;126:451-5. 15. Ahmad R, Bates JF, Lewis TT. Measurement of strain rate behaviour in complete mandibular dentures. Biomaterials 1982;3:87-92. 16. Regli CP, Kydd WL. A preliminary study of the lateral deformation of metal base dentures in relation to plastic base dentures. J PROSTHET DENT 1953;3:326-330. 17. Regli CP, Gaskill HL. Denture base deformation during function. J PROSTHET DENT 1954;4:548-54. 18. Swoope CC Jr, Kydd WL. The effect of cusp form and occlusal surface area on denture base deformation. J PROSTHET DENT 1966;16:34-43. 19. Wylegala RT. Reinforcing denture base material with carbon fibers. Dent Techn 1973;26:97-100. 20. Gutteridge DL. The effect of including ultra-high-modulus polyethylene fiber on the impact strength of acrylic resin. Br Dent J 1988;164:17780. 21. Christensen G, ed. Reinforcement fibers. Clin Res Associates Newsletter 1987;2-3. 22. Friskopp J, Blomlof L. Intermediate fiber glass splints. J PROSTHET DENT 1984;51:334-7.
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