Prosthododntics

Glass fiber reinforcement in repaired acrylic resin removable dentures:Preliminary results of a clinical studyPekka K. Vallitiu*

Abstract The clinical useftdness ofcotitinuous E-glass partial fiber reinforcement of acrylicresin removable dentures was evaluated an average 13 months after the insertion ofthe fibers. Twelve removable cotnplete dentures and ten removable partial dentureswith a history of recurrent fracture were selected for this study The partial fiberreinforcement was incorporated into the denture at the time of repair One completedenture and one removable partial denture fractured in the region of reinforcementduring the examination period. These fractures were most likely caused by faultyplacement of the fiber reinforcement in the denture in the dental laboratory. In sixdentures, new fractures occurred in regions without partial fiber reinforeement. Theresults revealed the importance of both the correct positioning of the partial fiberreinforcement in the denture and the use of accurate laboratory techniques.(Quintessence Int ¡997:28.39-44.)

Clinical relevance

When continuous, unidirectional glass partial fiberreinforcement is used in dentures, the fibers shouldbe oriented at a 90-degree angle to the potentialfracture line and placed as near as possible to thedenture margin, which is prone to fracture. Rein-forcement of other weak regions of the denture isrecommended to prevent the occurrence of newfractures.

Introdnction

Fiber-reinforced plastics, ie, fiber composites, arecommonly used in many fields of industry because oftheir good mechanical properties, which can betailored to specific needs.' Fiber composites for

* Guesl Researcher, NIOM. Scandinavian Instiime of Dental Malcriáis,Hasium, Nürivay, Docent. Inslitute of DeiHislry, University of TurkJ.Turku, Finland,

Reprint requesls: Dr Pekka ValliCtu. Institme of Dentislry. University ofTurku. Lemminkäisenkatu 2, nN-30520 Turku, Finland,

reinforcement of removable dentures bave been underdevelopment since the 1960s, but fiber reinforcementsare not commonly used at tbe present time. The fibersthat have been tested include glass, polyethylene,carbon, and aramid fibers incorporated in the denturepoly( methyl me t h aery I ate ) (PMMA).-'""^

The denture base can be reinforced in two ways: theentire denture base can be reinforced with a fiberweave, or a fiber reinforcement can be accuratelyplaced at the weak region of the denture. Thesereinforcements can be defined as total fiber reinforce-ment (TFR) and partial fiber reinforcement (PFR),respectively (Fig 1). If the fiber reinforcement isincorporated in the denture during repair of thedenture. PFR is the reinforcement of choice because itis easier to handle than TFR.

Continuous, unidirectional electrical-glass (E-glass)PFR has been shown to considerably Improve themechanical properties of removable complete andpartial dentures in vitro.'''^^ However, the efiect ofE-glass PFR on dentures used in the clinical situationhas not been studied so far

The aim of this study was to analyze the preliminaryresults from a clinical study of E-glass PFR of acrylicresin removable denture bases.

Quintessence International Volume 28, Number 1/1997 39

FIBER WEAVE UNIDIRECTIONALFIBERS

Fig 1 /tefi.)Totaliiberreinforcemen!and íngfiíj partial fiber reinforcementof a maxiiiaty complete denture.(arrow) Direction ot propagation ofthe midline fracture.

Table J Ages of tbe dentures and number of fractureslocated in the same region before incorporation ofthepartial fiber reinforcement

Mean Median SD Min Max No.

Complete denturesAge(y) 5.1No. offracmres 2.0

Removable partial dentaresAge (y) 4.2No. of fractures 3-0

Table 2 Type of teeth opposing the repaired maxil-lary complete dentures

4,71.0

3.22.0

2,1.

3.

21

52

1.91.0

1.31.0

8.24.0

12.06.0

1212

1010

Code Opposing

1 ! Complete denture28 MRFD (Kennedy (I mod. 1)29 Complete denture33 MRPD (Kennedy II mod. 1)40 MRPD (Kennedy I mod, 1)42 Compíete denture43 Natural dentition without teeth 36, 46, 4744 ARPD (Kennedy 1 mod. 1 )46 MRPD (Kennedy 1 mod. 1)50 MRPD (Kennedy II mod. 1 )S3 MRPD (Kennedy I)59 MRPD (Kennedy 1 mod. 1)

MRPD - RPD with s metal frame. ARPD = RPD with dr aciylic resLn base.

Method and materials

The PFR used in the present study was made fromcontinuous, unidirectional E-glass (Ahlstrom) fiberbundles (composition: SiO,. 55%; CaO, 22%; Al^Oj,15%; B,O,. 6%). The diameter of a single glass fiber inthe bundle was 10 |im. The fibers were silanized withY -methacryloxypropyltrimetboxysilane ( A174, UnionCarbide) to improve tbe adhesion between the fibersand FMMA. Four continuous, unidirectional glassfiber bundles were treated with auto polymerizingPMMA resin (Palapress, Heraeus Kulzer) before useas experimental PFR.

Mosl ofthe dentures selected for this investigationhad a history of recurrent fracture in the PMMAdenture base. The mean ages were 5.1 years for the 12maxillary complete dentures (CDs) and 4.2 years forthe 10 removable partial dentures (RPDs) (Table 1).All fractures in the CDs were midline fractures ofthepalatal plate of the denture. Fractures of the RPDswere located at the margins of the extension bases ofthe dentures. Two CDs and three RPDs selected forthis study were previously reinforced with metal wireor a mesh. The types of teeth opposing the dentures areshown in Tables 2 and 3.

Fractured dentures were repaired in a commercialdental laboratory at the dental clinic ofthe Universityof Kuopio in Finland. Before the denture was sent tothe laboratory, the need for rebasing tbe denture wasevaltiated by a clinical instructor at the dental clinic.The dentures were repaired with normal dental labor-atory techniques, witb the exception ofthe insertion ofthe PFR.

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Table 3 Types of repaired acrylic resin removable partial dentures and the type of opposing teeth

Code Repaired denture Opposing teeth

2 Max ARPD (Kennedy II mod, 2)8 Mand ARPD (Kennedy I)9 Max ARPD (Kennedy I mod. 1 )

23 Max ARPD (Ketmedy I mod, I )31 Max ARPD ( Kennedy I mod, 1 )35 Max MRPD (Kennedy 1)41 Max ARPD (Kennedy I mod, 1}45 Max ARPD ( Kennedy II )47 Max ARPD (Kennedy III mod, 1)56 Max ARPD (Kennedy IV)

Natural dentition without teeth 36, 37, 46, 47Complete dentureMRPD(Kenndy Imod, 1)MRPD (Kennedy IV)MRPD (Kennedy II mod. 1)Natural dentition without teeth 35, 36MRPD (Kennedy IMRPD (Kennedy II mod, 1 )Natural dentition without tooth 36MRPD (Kennedy III mod. 1)

Ma\ = maxillary; Mand = mandibular.MRPD ' RPD with a metal ftame: ARPD = RPD with an acrylic resin base.

The PFR was applied to the fractured region ofthedenture on a dental cast after the PFR was wetted wilhthe methyl methacrylate (MMA) liquid of the re-pairing PMMA resin. Tbe fibers of the PFR wereoriented at a 90-iiegree angle to the fracture line in thedenture. The PFR was then embedded with PMMAresin and polymerized, together witb tbe autopoly-merizing repair PMMA, in a water bath at 55°C for15 minutes. All repairs were made by the same dentaltechnician. Special care was taken to retain the samedimensions ofthe dentures before and after the repairprocedure.

When the dentures were inserted in the mouth ofthepatient, the occlusion was carefully checked, but theocclusion was not adjusted by grinding ofthe opposingteeth. If minor occlusal adjustment was needed, thedenture base was ground.

The patients were asked to contact the dental clinicimmediately if they noticed a crack or a fracture in theirdentures. No restrictions were made on the use ofthedentures. The patients were asked to come for the firstfollow-up examination in February 1996,

In the follow-up examination by the clinical instruc-tor, the occlusion was checked and the dentures wereinspected visually in translucent light. The oral mucosaunder the denture was also inspected. If the denturehad fractured, photographs were taken for documenta-tion. In addition, the patients were asked to give theircomments on the PFR ofthe dentures.

Results

At the time ofthe first follow-up, the CDs had beenworn for a mean 1,1 years (median = 1,0; SD = 0.2;min= 0,6; max = i,3 years) and the RPDs for a mean of 1,3years (median = 1,1; SD = 0,5; min = 0.8; max = 2,7years) after insertion ofthe PKR (Figs 2a and 2b),Recurrent fracture occurred in CD 33 and in RPD 2(random identification numbers). Indentures 2, 9, 23,43, 45, and 56, fracture occurred hi a new location.The fractures of dentures 2, 23, and 43 are shown inFigs 3a to 3c.

During the clinical examination, no signs of irrita-tion ofthe oral mucosa were found under the region ofthe PFR. The patients expressed satisfaction with theirdentures with the new type of reinforcement.

Discussion

The preliminary results of a clinical study of PFR inremovable complete and partial dentures have beenreported. Although the length ofthe follow-up periodaveraged only 13 months, some clinically importantfindings were observed,

Ladizesky et al"'-''-* have tried to solve theproblem of denture base reinforcement by using TFRmade from polyethylene fibers. They tested botbcontinuous fiber weave and chopped strand mat tbathad been wetted with a mixture of PMMA powder andMMA, The use ofthe TFR has been reported to befeasible in the fabrication of dentures. However, theuse of TFR in denture repairs has not been mentioned.

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Figs 2a and 2b Wear spans (in years) of Ihe proslheses before and after inserfion oí Ihe partial fiber reinforcement Jhe coderefers lo the idenfification number ot the denlure. ¡X) Fracture; (RB¡ Rebasing; (WIRE) Metai wire reintorcement; (MESHI Meiaimesh reintorcemenf; (arrow) Insertion of Ihe PFR. 'Fraclure at the region of the PFR "New fracfure location.

CODE -91 -92 '93 -94 -95 -96 -97

sa ^|„g

33

4446 " ' ^ E ^

505359

RB 9

/

Q

n

a

. .V

n0

Fig 2a Maxillary complete dentures.

VEARCODE -91 .92 -93 -94 -95 -96 -97

e9

aa31

35

41

45

56

u

WIRE

3H

O

0

•?

Fjg 2b Removable partiai dentures.

No results from clinicai studies ofpoiyethylene TFR ofdenture bases have been reported so far,

A ciinical study of PFR has been published byBowman and Manley,* They used unidirectional,continuous carbon fibers to reinforce new maxillaryCDs with PFR (see Fig 1 ), The 28 new dentures withPFR replaced existing dentures that had a history ofrecurrent fracture. No fractures occurred in the carboniiber-reinforced dentures during the 7-year follow-upperiod.^ This suggests that there might not be a needfor TFR of maxillary CDs. \\Vlegala'' described the use

of carbon fibers in the fabrication of dentures withTFR. Unfortunately, the black color of the carbonfibers causes a certain amount of difficulty from theperspective of cosmetics,"

The results of the present study emphasize theimportance of following accurate laboratory proce-dures when PFR is used. This is especially importantwhen RPDs are reinforced. The PFR should be placedon the side of the denture that is afîected by tensilestress during mastication- Placement ofthe fibers onthe tension side ofthe denture may prevent fractures

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Fig 3a Fracture in denture 2. Fig 3b Fracture in denture 23,

Fig 3c Fracture in denture 43.

such as those found in denture 23 (see Fig 3b), Thefracture propagated from the side under tension until itreached the fibers of the PFR, which stopped thepropagation ofthe fracture.

Another example of poor placement is CD 33.Initialiy. the PFR was placed ciose to the orai mucosain the denture base. However, the region ofthe highesttensiie stress located between the incisors, ie, fromwhere the midline fracture begins. The PFR initiallyplaced had only a minor reinforcing effect, andrecurrent fracture was unavoidable. Since the positionofthe PFR in CD 33 was changed to near the dentalarch ofthe denmre. no fractures have occurred. Thus,placement of the fibers as near as possible to thelocation ofthe highest tensile stress in dentures mayprevent the initiation of fracture.

Another problem is related to the length ofthe PFR,In CD 43, for example, PFR eliminated the fracture

that initiated from the region between the centraiincisors. However, a new fracture was found betweenthe canine and the laterai incisor (see Fig 3c), Thiscould have been eliminated by the use ofa PFR thatwas long enough. In the region ofthe premolars andthe molars, there is no high tensile stress that couldlead to fracture initiation; ie. the PFR should extend atleast to the secotid premolar or preferably to the firstmolar.

The fracture of RFD 2 was most likely caused by acombination of the aforementioned conditions (seeFig 3a), The fracture was initiated because there wereno fibers on the tension side ofthat particular region ofthe denture. The fracture propagated until it reachedthe fibers ofthe PFR, The fibers ofthe PFR changedthe direction ofthe fracture from vertical to anterior,and. as soon as the fracture reached the end of thePFR, the denture fractured into two pieces. LongerPFR under tooth 22 ofthe RPD might have prolongedthe time before total fracture occurred, or it might evenhave eliminated the fracture entirely.

The new fractures in the dentures reinforced withPFR might lead one to believe that TFR is thereinforcement of choice because it reinforces the entiredenture base. However. In TFR the fibers are used inthe form of weaves or mats. The direction ofthe fibersIn the weave, or especially in the mat. is not optimal toreinforce a denture base in which the possible fractureline can easily be predicted. It is well known that thehighest strength for fiber composite can be obtained byfibers that are oriented in one direction,'" For thisreason, weaves or mats, which are formed with fibersrunning in various directions, do not contribute to theoptimal strength ofthe tlber composite. In addition,the application of TFR in denture repairs is difficult

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Vallittu

and may require rebasing ofthe denfure. Furthermore,when TFR is applied during the rebasing of thedenture, the regions ofthe highest tensile stress remainunreinforced. These facts support the use of PFR indenture repairs. However, the positioning ofthe fibersof the PFR should be carefully planned before therepair is made.

Early experiments with glass fiber reinforcement ofdentures resuHed in failure because of the irritatingnature of the fibers that protruded from ihe finishedsurfaces.-̂ ' This is a problem with TFR. especiallywhen a chopped fiber mat with various fiber directionsis used. This problem can be avoided by using PFR.Partial fiber reinforcement, which is placed accuratelyin the desired region of the denture, can be easilycovered with a thin layer of unreinforced PMMA. Inaddition, the continuous fibers ofthe PFR. which runin the direction ofthe surface ofthe denture, do notprotrude from the finished surface like fibers runningin various directions do. The clinical examination ofthe mucosa supporting the denture and the commentsofthe patients support the aforementioned aspects ofthe feasibility of PFR for removable dentures.

The results of this study were promising, despite tworecurrent fractures in the region ofthe PFR. A longerfollow-up period is essential to show the clinicalusefulness of E-glass PFR. In addition, the use ofK-glass PFR in the fabrication of new denmres shouldbe clinically evaluated.

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