Acrylic resins reinforced with woven highly drawn linear polyethylene fibres. 2. Water sorption and clinical trials T. W. Chow* N. H. Ladizeskyt D. A. Clarke$

Key words: Acrylic resins, dental materials, polyethylene fibres. Abstract The results reported in this paper are part of a continuing study of the reinforcement of acrylic denture base resins with highly drawn linear polyethylene fibres. Water sorption is significantly reduced by incorporation of these fibres, even though the water diffusion processes as such remain broadly unaffected. Clinical trials are encouraging and the reinforced dentures have been well accepted by all the patients.

Studies have shown that HDLPE fibres substantially improve the mechanical properties of acrylic resins, namely flexural modulus, flexural strength and impact strength.' A thorough investigation of water sorption is now underway, and the available preliminary results are reported here because of their clinical importance. In addition, trials were carried out to understand the behaviour of dentures reinforced with woven fibres in service, to recognize possible difficulties and to ascertain the value of this technique for further development.

Clinical and experimental details Materials

(Received for publication March 1991. Accepted August 1991.)

Introduction Polymethyl methacrylate (PMMA) is the most commonly used resin for complete dentures. Various attempts have been made to improve its mechanical properties, including reinforcement with carbon fibres.'.' However, the system has not been widely accepted due to somewhat limited improvements and poor aesthetics. Recently, highly drawn linear polyethylene (HDLPE) fibres in woven f0rmY3.'have been incorporated into complete denture bases using a modified conventional production t e c h n i q ~ e . ~ HDLPE has been shown to be a fully biocompatible material.6

*Department of Prosthetic Dentistry, Faculty of Dentistry, University of Hong Kong. tDental Materials Science Unit, Faculty of Dentistry, University of Hong Kong. SDental Technology Unit, Faculty of Dentistry, University of Hong Kong. Australian Dental Journal 1992;37(6):433-8.

The HDLPE fibres were used in two forms: (a) continuous parallel and (b) the same material woven into a square texture with fibres at 90 O to each other. They were either untreated or plasma treated for improved interface strength. The characterization of the fibres and details of the plasma treatment have been reported in a previous publication.' Three heat curing resins were used, namely: (a) A clear acrylic, Trevalon C,g (b) PMMA syrup, a non-proprietary type made with one part by mass of Rapid Repair powder8 with four parts by mass of natural Coe-Lor liquid,)[ (c) RM-3,I a pour type resin diluted with 10 per cent by mass of methyl methacrylate. Details of resins (b) and (c) have been published previo~sly.'.~

BAD International, De Trey Division, UK. RCoe Laboratories, Chicago, USA. (Ivoclar AG, Liechtenstein. 433

Fabrication of dentures and bars Denture bases reinforced with one layer of woven fibres were constructed using a modified compression moulding technique described in an earlier paper.5 A similar method was used to produce bases reinforced with three woven inserts, but in this case these were presoaked in resin (b) or (c) for at least one hour to ensure good penetration into the array of fibres. The layers were placed separately on top of the packed Trevalon C, allowing a few minutes for drying before placing the next layer on top. This procedure prevented sliding of the inserts when closing the denture flask and ensured the correct positioning of the fibres which were hlly contained within the base, and running laterally and anteroposteriorly. Rectangular bars reinforced with three layers of woven fibres were fabricated with similar compression moulding technique and materials as seen above. The samples had nominal dimensions of 2 0 m m x l l mmx2.5 mm(1xwxt)andtheweave remained fully enclosed by the resin (to establish a correspondence with the denture bases). The bars with longitudinally oriented fibres were made in 'leaky' moulds7using bundles of 290 mm length and 3.5 g mass, giving final samples of 210 mm x 11 mm x 2.5 mm (1 x w x t) nominal dimensions. The length was then cut to 20 mm with a diamond disc in order to provide the predetermined dimensions for water sorption testing. The fibre content was calculated with the following formula: F v , fv, f w




Vr: V, : Fv: W,:

Volume of fibres Volume of composite Fibre content Mass of fibres ef : Density of fibre3 ( = 0.97 X 10-3g/mm3). Most samples were polished according to I S 0 rec~mmendations.~ Nevertheless, some bars with longitudinally oriented reinforcement were left unpolished to study the effect of exposed fibres on water sorption. Methods (a) Water sorption The thorough drying schedule recommended by both the International Standards Organization' and the American Dental Association'O does not represent a practical situation because acrylic dentures are never dried to such an extent, if at all. This may be of little significance when making


brand comparisons, but for fibre-reinforced resins it could affect the nature of the interface. For this research drying was carried out to a lesser extent, namely four days at 37 "C using vacuum (0.5 mm of mercury) in a desiccator containing phosphorus pentoxide. The conditioned samples were then weighed and placed in distilled water at 37 "C, and the mass changes monitored with time. The weighing procedure followed I S 0 9 recommendat ions. It should be emphasized that the drying schedule used for the present work was aimed at the reduction of history effects, that is, samples made at different times prior to immersion in water. (b) Assessment of dentures Ten patients with histories of fractured complete dentures had replacement dentures made. All single upper complete dentures were combination cases with opposing natural lower dentitions. Most of the previous upper dentures showed midline cracks, which had been reported as the most common site of fracture." Some of the bases had failed in less than twelve months in service, one of them after six months. One upper and the only lower prosthesis in the study had complete fractures resulting in separate fragments. In most cases the repaired dentures cracked again after a relatively short period of time in service. This could be due to reduced strength of repair material and unavoidable dimensional changes.12 Table 1 shows the relevant data for the new reinforced dentures which were provided. The patients were treated by Junior Hospital Dental Officers in the Prosthetic Department of Prince Philip Dental Hospital. No alterations were made to occlusal planes of the natural lower dentitions. The use of clear bases allowed checking of the placement of the fibres as well as a visual evaluation of the denture adaptation to the underlying mucosa, particularly for dentures with only a single layer of woven fibres. The construction of complete dentures followed the procedure used by dental students of the University of Hong Kong. This involved the construction of a heat-processed acrylic base plate which was used for subsequent registration and trial dentures and finally incorporated as a base in the processed denture. The patients were instructed to return immediately if they noticed cracks or experienced difficulties with the dentures, and were given appointments for regular follow-up visits. The stability of each denture was tested by pressing the' prosthesis gently against its supporting tissues. Two fingers were placed in the middle of Australian Dental Journal 1992;37:6.

Table 1. Reinforced dentures undergoing clinical trials Patients 1. CSH LH CHY WWY TSM AYK NS WLK LSY 10. NCS 2. 3. 4. 5. 6. 7. 8. 9.

Date of insertion of denture 5.5.88 26.7.88 3.5.89 10.3.89 13.6.89 26.6.89 17.8.89 28.6.89 16.8.89 22.8.89

Woven reinforcement Treatment

Syrup used for pre-soaking


Untreated Untreated Plasma treated Plasma treated Plasma treated Plasma treated Untreated Plasma treated Untreated Untreated


1 1

3 3 3 3 3 3 3

the maxillary arch base, or in the premolar areas of a mandibular arch. The examiner then tried to tip, rotate, and/or displace the denture horizontally. Stability was regarded as acceptable if little or no such movement occurred, otherwise, the stability was classified as unsati~factory.'~

Results Water sorption The water sorption at saturation of all samples is shown in Table 2. A value of 2.66 x 10-*mg/mm3 for the pure resin is reduced to less than half for samples reinforced with longitudinally oriented fibres. The water uptake of one example of each type of sample is shown in Fig. 1. Weighing of some of the samples after one year of immersion confirmed that saturation was reached at around 30 days. Clinical evaluation (a) Fractures T o date (December 1990), the longest period of service of the reinforced dentures has been 31 months, the shortest 16 months. All were intact. None of the dentures exhibited any cracks.

(b) Aesthetics All patients accepted the reinforced bases. Plasma treated fibres gave a slight yellow tint to the clear bases but did not change the colour of the pink acrylic. (c) Exposure of fibres There were two dentures with exposed fibres. The first case had a small number of ends showing on the polished surface of the flange near the sulcus. Most of them were subsequently covered by pink acrylic and the remaining fibre ends smoothed out with fine diamond burs and polished in the convenAustralian Dental Journal 1992;37:6

Complete denture

No. of layers

Upper Upper Upper Upper Upper Upper Upper Upper Lower Upper

tional manner. The labial tissue in contact with the flange remained comfortable and healthy. The second case had fibres exposed on the fitting surface after adjustment for a denture sore spot at a followup visit. The rough area was first smoothed out with a hot ball-ended burnisher as polyethylene fibres melt at about 135 "C.This was followed by polishing with a silicone rubber point. The mucosal tissue healed and remained healthy with no signs of inflammatory reaction.

(d) Stability Stability of all the base plates at the jaw record stage was satisfactory and no change in stability was noticed following the reprocessing of bases necessitated for the attachment of the artificial teeth and supporting acrylic superstructure. All dentures remained satisfactory and their functions were assessed as very good by the patients themselves.

Discussion Water sorption This initial study was carried out on a small number of samples to ascertain that the incorporation of fibres did not have a deleterious effect on water sorption. Information on this parameter is important for clinical application of reinforced resin as water uptake may affect dimensional stability and other physical properties of denture bases. From Table 2 it can be seen that lower water sorption is associated with higher fibre content. This is because polyethylene fibres are hydrophobic with negligible water uptake. The reduction in water uptake of all the samples is broadly proportional to their fibre contents, indicating that the fibrehesin interface does not provide sites for water diffusion. Polishing of composite bars with longitudinally oriented reinforcement exposed numerous fibres, giving a furry appearance (Fig. 2a,b). However, 435







Days Fig. 1.-Mass change in acrylic resin during immersion in water at 37 "C: unreinforced resin; A reinforced with three layers of untreated woven fibres; 0 reinforced with longitudinally oriented plasma treated fibres.

Table 2. Water sorption at saturation of reinforced acrylic resins Polishing

Reinforcement Form

Continuous woven (3 layers)

Fibre treatment


Untreated Plasma treated

Continuous parallel

Fibre content (Vol. Yo)


7.9 (SD: 0.6) 48 (SD: 2)

Yes Yes

No Yes Plasma treated

exposed fibres did not increase the water sorption of the composite (Table 2). There was no difference between untreated and plasma treated fibres samples. Notwithstanding the large differences in water sorption (Table 2), all the samples studied in this work showed a similar pattern of mass gain (Fig. 1). Figure 3 shows the mass changes during drying and early water uptake. It is interesting to note that, in broad terms, the water lost during the drying period was regained between one and two days after immersion. It therefore appeared that the mechanism for water uptake was controlled by the resin, with no specific effect associated with the fibres. 436

48 (SD: 2)

Water sorption at saturation (mg x 10-'/mm3)


2.38 2.36 2.47 2.47 1.07 1.09 1,14 1.19 1.25 1.22 1.31 1.07


2.42 (SD: k0.06) 1.12 (SD: k 0.05)

1.21 (SD:k0.10)

BradenI4 showed that sorption and desorption of water by acrylic resin obeyed the laws of diffusion and, in particular, the time to attain equilibrium was a hnction of the diffusion coefficient only. This is of considerable clinical importance because it is also the time during which dimensional changes may occur. It can be calculated that a sheet of acrylic resin 2.5 mm thick reached 98.5 per cent saturation after 17 days immersion in water at 37.4 O C . I 4 The results presented in Fig. 1 show no drastic departure from this behaviour and suggest that the presence of the fibres may not significantly affect this particular characteristic of the resin. Australian Dental Journal 1992;37:6.

Fig. 2. -Surface of composite bars reinforced with longitudinally oriented plasma treated fibres. a. Unpolished; b. polished according to IS0 recommendations. x 8.

Clinical trials The present trials suggested that crack initiation in dentures under service may be prevented by only one layer of woven HDLPE reinforcement. Furthermore, previous showed that acrylic resins containing these fibres may get damaged but do not break. From the safety point of view this is a desirable feature, minimizing the risk of ingestion or inhalation of denture fragments and removing the requirement to develop radiopaque denture base resins, which so far has not been successful. Further clinical studies are currently in progress, and these will be reported in future publications. Australian Dental Journal 1992;37:6.

Conclusions 1. Fibre reinforcement significantly reduces the water sorption of acrylic denture base resin. 2. The fibrehesin interface does not provide a path for water diffusion. 3. Present clinical results are encouraging, suggesting that this technique merits further attention. Acknowledgements The authors are grateful to Professor I. M. Ward of the Department of Physics, University of Leeds, for supplying the highly drawn polyethylene fibres and for general discussions. Thanks are also due to 437

Days Fig. 3.-Mass change in acrylic resin during drying and subsequent immersion in water at 37°C. Symbols as per Fig. 1.

Mr Y. Y. Cheng of Dental Technology Unit, University of Hong Kong, for help with computer programming. This work was supported by a research grant from the Faculty of Dentistry, University of Hong Kong, No. 335.263.0003. References 1. Schreiber CK. Polymethyl methacrylate reinforced with carbon fibres. Br Dent J 1971;130:29-33. 2. Wylegala RT. Reinforcing denture base material with carbon fibres. Dent Techn 1973;26:97-100. 3. Ward IM. The preparation, structure and properties of ultrahigh-modulus flexible polymers. Advances in Polym Sci 1985;70:1-70. 4. Ladizesky NH, Ward IM.Ultra high-modulus polyethylene fibre composites: I - The preparation and properties of conventional epoxy resin composite. Comp Sci Tech 1986;26: 129-64. 5. Clarke DA, Ladizesky NH,Chow TW. Acrylic resins reinforced with highly drawn linear polyethylene woven fibres. 1. Construction of upper denture bases. Aust Dent J (in press). 6. Rubin LR. Polyethylene as a bone and cartilage substitute: a 32-year retrospective. In: Rubin LR, ed. Biomaterials in reconstructive surgery. St Louis: CV Mosby, 1983:474-93. 7. Ladizesky NH,Chow TW, Ward IM. The effect of highly drawn polyethylene fibre on the mechanical properties of denture base resins. Clin Mater 1990;6:209-25. 430

8. Ladizesky NH.The integration of dental resins with highly drawn polyethylene fibres. Clin Mater 1990;6: 181-92. 9. International Standard I S 0 1567-1978(E): Denture base resin. 10. American Dental Association Council on Dental Materials and Devices. Revised American Dental Association Specification No. 12 for denture base polymers. J Am Dent Assoc 1975;90:45 1-8. 11. Smith DC. The acrylic denture. Br Dent J 1961;110:257-67. 12. Ware AL, Docking AR. Strength of acrylic repairs. Aust J Dent 1950;54:27-32. 13. Agerbery G, Viklund L. Functional disturbance in complete denture patients. Int J Prosthodont 1964;14:307-16. 14. Braden M. The absorption of water by acrylic resins and other materials. J Prosthetic Dent 1964;14:307-16. 15. Braden M, Davy KWM, Parkers S, Ladizesky NH, Ward IM. Denture base poly(methylmethacry1ate)reinforced with ultra-high modulus polyethlene fibres. Br Dent J 1988; 164:109-13.

Address for correspondenceheprints: T. W. Chow, Department of Prosthetic Dentistry, Faculty of Dentistry, University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong. Australian Dental Journal 1992;37:6.

Acrylic resins reinforced with woven highly drawn linear polyethylene fibres. 2. Water sorption and clinical trials.

The results reported in this paper are part of a continuing study of the reinforcement of acrylic denture base resins with highly drawn linear polyeth...
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