Acrylic resins reinforced with highly drawn linear polyethylene woven fibres. 1. Construction of upper denture bases D. A. Clarke* N. H. Ladizeskyt T. W. Chow$
Key words: Acrylic resins, dental materials, polyethylene fibres. Abstract This paper presents a detailed description of the constructionof complete acrylic upper dentures reinforced with highly drawn linear polyethylene fibres. The work forms part of a continuing project to elucidate the properties and potential of the new system. (Received for publication February 1991. Accepted June 1991.)
Introduction During recent years the introduction of new materials together with advances in technology have ensured that dental prostheses can be produced to a high standard. One material still in constant use which has not changed significantly since it was introduced to dentistry over fifty years ago is acrylic resin or polymethyl methacrylate (PMMA), although modifications have been made to improve the original material by using co-polymers, cross-linking agents and even including a rubber phase in order to achieve better properties. Polymethyl methacrylate has many advantages, being easy to use and repair. It is well tolerated by *Dental Technology Unit, Faculty of Dentistry, University of Hong Kong. ?Dental Materials Science Unit, Faculty of Dentistry, University of Hong Kong. $Department of Prosthetic Dentistry, Faculty of Dentistry, University of Hong Kong. 394
the majority of patients and, by adding pigments and so on, can give excellent aesthetic results. However, the strength and stiffness of these resins are still a cause for concern. In complete upper dentures the most common cause of fracture during service seems to be when the prosthesis is opposed to a natural dentition in the lower jaw, particularly if the position or alignment of the teeth make it difficult to achieve an even or balanced There are other factors which may influence failure during service such as a severe fraenal a prominent torus palatinus or if the fit of the denture base is not accurate. Also, it is not uncommon for a patient to accidentally drop a denture on a hard surface resulting in an impact fract~re.~ A further deficiency of PMMA relates to its relatively low flexural stiffness and the tendency for the denture base to deform under load, influencing changes to the underlying tissue surface and bone.6 It has been said that if a denture is made correctly it will not fail in service. It is possible to agree or disagree with this statement but the amount of research carried out in this field suggests that there is a need for a material possessing better physical strength properties.
Reinforcement of denture bases with carbon fibres Several researchers have investigated the reinforcement of acrylic resins with carbon fibres to improve flexural and impact strength, as well as to prevent fatigue fracture.’.*Further work included Wylegala’s method for the production of acrylic dentures reinAustralian Dental Journal 1992;37(5):394-99.
b Fig. la, b.-Pink acrylic upper denture reinforced with continuous HDLPE fibre. a, Incident light; b, transmitted light.
forced with carbon fibre^.^ However, this material has some potential disadvantages, namely the unsightly appearance of the fibre within the denture and its obvious effect on aesthetics, as well as some doubts about their biocompatibility if fibres become exposed on the surface of the denture base.'O This may happen as a consequence of fibre displacement during processing, polishing, or repeated cleaning over long periods of time. Bowman" suggested confining the carbon fibres to the area directly behind the anterior teeth to reduce these problems. However, this method puts restraints on the placement of the reinforcement necessary to achieve dentures with optimum properties. Australian Dental Journal 1992375
The use of highly drawn linear polyethylene fibres as reinforcement The present research concentrates on a new type of reinforcement, namely highly drawn linear polyethylene (HDLPE) fibres. Earlier work by Capaccio and Ward12 has shown that polyethylene, a crystalline polymer, may be drawn at temperatures below its melting point to produce a material of enhanced modulus and strength in the axial direction. The natural colour, low density and known biocompatibility together with the fact that the fibres are chemically inert, solvent resistant and hydrophobic make them worthy of consideration as a reinforcement for denture bases. When comparing with carbon fibres, they remain practically unnoticed 395
Fig. 2.-Complete upper denture with clear base reinforced with continuous HDLPE fibres.
Fig. 3.-Draining the woven insert after soaking in PMMA syrup.
within pink acrylic (Fig. 1) and are not unsightly when used as a reinforcement for clear resins (Fig. 2). Fibre material should bond to the matrix it is intended to reinforce in order to resist stresses that may be applied. Polyethylene has a low surface energy and consequently poor wettability but, if necessary, this can be improved considerably by plasma treatment which etches the fibres so that they can bond mechanically with the matrix phase.13 The first application of this material to the reinforcement of dentures was carried out by Braden et al. l 4 More recent work by Gutteridgels investigated the effectiveness of incorporating chopped fibres 396
into denture base resin and its effect on impact strength. In this case the fibres were used in lengths of 6 mm, and amounts of 0.5 to 3 mass per cent were incorporated into the resin during mixing. Larger amounts than this made the dough unmanageable, and the authors also found that even 2 mass per cent significantly altered the handling of the resin. Nevertheless, the results obtained by the above workers indicated that the technique merited further attention. Ideally, reinforcement is most effective when fibres are arranged in a strategic direction with as many fibres parallel to the surface of the denture base Australian Dental Journal 1992;37:5.
Fig. 4.-The soaked woven insert in the recess made on the packed resin.
Fig. 5. -Denture base with embedded thermocouples.
as possible. Arranged at random a large proportion of the reinforcement serves little purpose. Consequently, the present work concentrated on continuous fibres in the form of a woven texture, allowing the incorporation of a high fibre content while at the same time retaining control of their placement and orientation. Details of the fibres and woven texture are the subject of another paper.16
monitoring of the final position of the fibres within the denture in the area where stress and deformation may be anticipated, namely the anterior section of the hard palate extending over the crest of the ridge and into the labial flange.3.17,18 The reinforcing fibres should also be embedded in the base so that polishing of the palatal surface is unlikely to expose them.
Laboratory procedure The resin used for the present work was a clear heat Cure acrylic, Trevalon c,§ permitting critical
SAD International Ltd, De TIC^ Division, UK.
Australian Dental Journal 1992;37:5.
Fig. 6a, b.-Clinical appearance of complete upper denture reinforced with woven HDLPE fibres.
In the technique used, the base of the complete denture was processed first to ensure accurate fit and correct extension before proceeding to jaw registration. A soft spacer shaped to the pattern of the required reinforcement was constructed on the master cast using a pressure-forming machine. After the usual flasking procedures and removal of the wax base pattern the spacer was placed in the upper half of the mould. The fitting surface (cast) was coated with sodium alginate and packed with acrylic resin, followed by a trial closure with a polyethylene separating sheet to allow removal of any excess material. The flask was then kept under pressure for about 60 minutes with the spacer and polyethylene 398
separating sheet still in place. On removing the soft spacer a recess was clearly visible on the surface of the acrylic resin, ready to receive the fibre insert. This must be cut to the required shape using a pair of scissors specially designed for high performance fibres. For maximum reinforcement the resin should penetrate the weave, ensuring no voids at the fibrehesin interface. The consistency of heat cure acrylic resin, being a dough-type material, may prevent this occurring and it is advisable to moisten the insert with a more fluid polymerizable material. Two methods were used: (a) wetting the insert with PMMA monomer or a thin PMMA slurry prior to Australian Dental Journal 1992;37:5.
the final closure or, (b) pre-soaking the insert for ten minutes in a Petri dish containing a diluted pour type resin, RM-3,I)*I9 or a PMMA syrup.2oThe woven insert was then removed from the fluid resin and any excess allowed to drain away (Fig. 3). The insert was then carefilly positioned on top of the packed resin, in the recess which had been provided by the spacer (Fig. 4). Using a polyethylene separating sheet light pressure was applied in a bench press. On re-opening the flask the position of the fibres was checked and, if satisfactory, a second mix of acrylic resin was used to complete the packing procedure. Alternatively the second stage may be trial packed prior to placement of the insert, ensuring that any excess resin will only relate to the space taken by the fibres. Over-packing of the mould at this stage must be avoided to prevent increasing the thickness of the base and possible displacement of the insert. A hrther trial closure may be needed before processing. Processing was carried out using an initial low temperature (72 "C) for 6 h 30 min followed by 1 h 30 min at 100 "C. Temperature control is important. A sudden increase in exothermic heat can damage the fibres which melt at about 130 "C. No problems were encountered in this respect and this was c o n f i e d by experiments aimed at determining the maximum polymerization temperature at different points within the denture base (Fig. 5). It was found that the temperature thus recorded followed very closely the pre-determined cycle of the water bath with a variation of not more than two degrees centrigrade. After deflasking fibres may occasionally be exposed at the peripheral border, in which case trimming should be carried out with diamond burs to avoid delamination of the reinforcement.
Conclusions Woven HDLPE fibres can be successhlly used to reinforce acrylic dentures using modified laboratory procedures. It is possible to place the fibres as desired and in intimate contact with the resin.lg A reinforced maxillary complete denture is shown in Fig. 6. This has been in service for 31 months. The aesthetics are good and the denture is readily accepted by the parent. Acknowledgements The authors are gratefil to Professor I. M. Ward of the Department of Physics, University of Leeds, United Kingdom for supplying the highly drawn polyethylene fibres. Ivoclar AG, Liechtenstein. Australian Dental Journal 199237:5.
This work was supported by research grants from the Faculty of Dentistry, University of Hong Kong, Nos. 335.263.0003 and 335.255.0004.
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Address for correspondendrqrints: T. W. Chow, Department of Prosthetic Dentistry, University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong. 399