Influence of implant number on the movement of …...RESEARCH AND EDUCATION Inﬂuence of implant...

RESEARCH AND EDUCATION

Supported byaGraduate stuBunkyo, TokbAssistant PrcResident, GdProfessor, G

380

Influence of implant number on the movement of mandibularimplant overdentures

Ken Oda, DDS,a Manabu Kanazawa, DDS, PhD,b Shin Takeshita, DDS, PhD,c andShunsuke Minakuchi, DDS, PhDd

ABSTRACTStatement of problem. The rotational movement of an implant overdenture (IOD) has a negativeeffect on the perceived masticatory ability of the denture wearer. However, the influence of implantnumber on the movement of IODs has not been investigated.

Purpose. The purpose of this in vitro study was to evaluate the denture movement of mandibularIODs anchored by different numbers of implants.

Material and methods. An edentulous mandibular test model with artificial mucosa and 5 experi-mental overdentures (N=5) was fabricated. The locator attachment systemwith blue nylon inserts waschosen for this study. Three implant positions were prepared: anterior midline (1-IOD), bilateral lateralincisor regions (2-IOD), and anterior midline and bilateral canine regions (3-IOD). Vertical loads of 50 Nwere applied to the mid-anterior region, the left canine region, the left premolar region, and the leftfirst molar region. The vertical and horizontal displacements at the right distal edge and the verticaldisplacements at the loading point were measured. The displacement values were statisticallyanalyzed using a 1-way analysis of variance and the post hoc Tukey honest significant differencetest with the implant number as a factor. In addition, the values of the vertical and horizontaldisplacements at the distal edge of the overdenture were statistically compared using a paired ttest, and the values of the vertical displacement at the distal edge of the overdenture werestatistically analyzed by a repeated measures analysis of variance and the post hoc Tukey honestsignificance difference test with the loading point as a factor (a=.05).

Results. Upon anterior loading, the 2-IOD showed significantly larger vertical displacements at theright distal edge than the 1-IOD or 3-IOD (P<.01). The horizontal displacements at the right distaledge were small compared with the vertical displacements at the same point, although thedisplacement of the 2-IOD was significantly larger than that of the 1-IOD upon anterior loading(P=.03). The magnitude of the vertical displacement at the loading point of the 2-IOD wassignificantly larger than that of the 3-IOD upon anterior loading (P<.01). No statisticallysignificant differences were shown under the other conditions for each loading (P>.05).

Conclusions.Within the limitationsof this in vitro study, the following conclusionsweredrawn:Duringmasticationwith the anterior teeth, the useof 2 implants for anchoring an IOD increased the rotation ofthe denture base more than the use of 1 or 3 implants. The horizontal movements of the IODs weresmall compared with the vertical movements. Denture movement under the occlusal force in themolar region was smaller than that in the anterior region. (J Prosthet Dent 2017;117:380-385)

The percentage of elderly in-dividuals in the population hasbeen increasing in a number ofcountries.1,2 The number ofpeople with edentulism isexpected to grow with the in-crease in the older population,3,4

indicating that the demand fortreatment for edentulism willalso increase.

The use of an implantoverdenture (IOD) has beenregarded as an effective treat-ment option for edentulousmandibles5,6 and has beenwidely used in clinical prac-tice.7-9 However, the rotationalmovement of the IOD has anegative effect on the perceivedmasticatory ability of denturewearers.10 Thus, rotationalmovement must be preventedto enhance the quality of life ofpatients with edentulism.

The number of implantsshould be reduced because ofthe cost of treatment and theinvasiveness of the operation.Consensus statements have

the Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science, grant 15K20429.dent, Gerodontology and Oral Rehabilitation, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Yushima,yo, Japan.ofessor, Gerodontology and Oral Rehabilitation, Tokyo Medical and Dental University, Tokyo, Japan.erodontology and Oral Rehabilitation, Tokyo Medical and Dental University, Tokyo, Japan.erodontology and Oral Rehabilitation, Tokyo Medical and Dental University, Yushima, Bunkyo, Tokyo, Japan.

THE JOURNAL OF PROSTHETIC DENTISTRY

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Figure 1. Edentulous mandibular model with silicone artificial mucosa.Five implant analogs placed in mid-anterior region, bilateral lateralincisor regions, and bilateral canine regions.

Figure 2. Experimental overdenture and 5 metal caps with blue nyloninserts (without framework).

Clinical ImplicationsMandibular 2-implant overdentures move easilyand rotate around the fulcrum line formed by 2implants during mastication with the anterior teeth.The use of 1 or 3 implants may prevent thismovement, improving denture stability and themasticatory ability of patients with edentulousmandibles.

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cited strong evidence for the use of 2 implants as anadequate number to retain a mandibular IOD.5,6 Recently,an overdenture retained by a single implant placed in themid-anterior region (1-IOD) has been used in a clinicalapplication, and high implant survival rates and highpatient satisfaction have been reported.11-14 Furthermore,Misch15 proposed that an overdenture retained by 3implants (3-IOD) produced less force and less prosthesismovement than that of an overdenture retained by 2implants (2-IOD). Therefore, the 3-IOD has also beenused as a treatment option for edentulous mandibles.16,17

IOD movement under an occlusal load has beeninvestigated using an in vitro model that simulates theintraoral environment. Studies by Manju and Sreelal18 andTokuhisa et al19 compared the movements of 2-IODs withball, magnet, and bar attachments and reported that the2-IOD with magnetic attachments showed the mostmovement. Haruta et al20 measured the movement of2-IODs with ball and magnet attachments and reportedthat the movement was larger with the ball attachmentthan with the magnetic attachment. Maeda et al21

measured the movements of 1-IODs and 2-IODs withball and magnet attachments and reported that themovements of the 1-IODs were larger than those of the2-IODswith themagnetic attachment. Liu et al22measuredthe denture base movement and lateral force on the im-plants with IODs retained by 1 to 4 implants, using a finiteelement model. They concluded that the 2-IOD showedthe largest denture movement and the largest lateral force.

These in vitro studies have focused on the differencesamong the attachment systems.18-21 Only the studies byMaeda et al21 and Liu et al22 compared denture movementamong IODs with different numbers of implants. Thesestudies reported inconsistent findings. We are unaware ofany study that measures the denture movement of IODsretained by 1, 2, and 3 implants by using an edentulousmandibular model. Therefore, whether the number ofimplants influences the movement of IODs is not clear.

The purpose of this in vitro study was to evaluate themovement of mandibular IODs anchored by differentnumbers of implants. The null hypothesis was that theimplant number would have no influence on the denturebase movement of mandibular IODs.

Oda et al

MATERIAL AND METHODS

An edentulous mandibular test model was fabricated froman acrylic resin (Acron Clear; GC Corp). A 3-mm-thick layer of the model surface was replaced with siliconeimpression material (Exahiflex regular; GC Corp) tosimulate the mucosa. The elastic modulus of the siliconematerials was reported to be in the same range as that ofthe oral mucosa.23 Five implant replicas (Implant ReplicaInternal tri-channel connection RP; Nobel Biocare) wereplaced parallel to each other in the anterior midline,bilateral lateral incisor regions, and bilateral canineregions (Fig. 1). All tests were performed on this model.

Five experimental overdentures without a metal frame-work were fabricated from a multipurpose pourable resin(Procast DSP clear; GC Corp). Locator attachment systemswere chosen for this study. Fivemetal caps (metal cap;NobelBiocare) and blue nylon inserts (Extra Light RetentionReplacementMale [blue]; Nobel Biocare) were incorporatedinto the intaglio surface in the anterior midline, bilateral


Figure 3. Implant positions. A, Single implant in mid-anterior region(1-IOD). B, Two implants in bilateral lateral incisor regions (2-IOD). C, Threeimplants in mid-anterior region and bilateral canine regions (3-IOD).

Figure 4. Loading points and denture base movement. Loading pointsin mid-anterior region (Anterior), left canine region (Canine), leftpremolar region (Premolar), and left molar region (Molar). Two types ofdenture base movement were measured: horizontal (X) and vertical (Y)displacement of right distal edge.

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lateral incisor regions, and bilateral canine regions of eachoverdenture (Fig. 2). Three implant positions were preparedat the anteriormidline (1-IOD), the lateral incisor regions (2-IOD), and the midline and canine regions (3-IOD) (Fig. 3).These implant positionswere reproduced on themandibularmodel by changing the position of the locator abutments.


Loading tests were performed using a universal testingmachine (InstronModel 5544; Instron Corp). Vertical loadsof 50 N, reported to be the average occlusal force in pa-tients with edentulism,24 were applied on the occlusalsurface of the experimental overdentures. The loadingpoints were the mid-anterior region; the mid-anteriorpoint of the denture base, in the left canine region; 14mm apart from the mid-anterior point, in the left premolarregion; 10 mm apart from the canine point and left firstmolar region; and 10 mm apart from the premolar point.The load was applied with a 5-mm-diameter acrylic ball.The vertical and horizontal displacements at the right distaledge of the overdenture and the vertical displacement atthe loading point were measured under each loadingcondition (Fig. 4). The vertical and horizontal displace-ments at the right distal edge were measured with move-ment sensors that apply strain gauges (DTH-A-5; KyowaElectronic Instruments Co Ltd) (Fig. 5). The movementsensors were placed at the right distal edge and connectedto a computer with sensor interface boards (PCD300A;Kyowa Electronic Instruments Co Ltd). The vertical dis-placements at the loading point were recorded by theuniversal testing machine at the same time as loading.

The displacement values were statistically analyzedusing multiple comparison 1-way analysis of variance(ANOVA) and post hoc Tukey honest significant differ-ence (HSD) test with the number of implants as a factor.In addition, the vertical and horizontal displacementvalues at the distal edge of the overdenture werestatistically compared using the paired t test, and thevalues of the vertical displacement at the distal edge of theoverdenture were statistically analyzed by repeated mea-sures ANOVA and post hoc Tukey HSD test, using theloading point as a factor (SPSS Statistics v17.0; SPSS Inc)(a=.05).

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Figure 5. Positions of movement sensors. A, Horizontal direction. B, Vertical direction.

Table 1.Mean ±SD of vertical displacement at right distal edge ofoverdenture (mm)

LoadingPoint 1-IOD 2-IOD 3-IOD P

Anterior 0.65 ±0.09 1.06 ±0.08 0.64 ±0.08 1-IOD versus 2-IOD: <.01*

2-IOD versus 3-IOD: <.01*

1-IOD versus 3-IOD: .99

Canine 0.87 ±0.25 0.97 ±0.34 0.75 ±0.11 1-IOD versus 2-IOD: .78



Premolar 0.22 ±0.08 0.27 ±0.15 0.15 ±0.07 1-IOD versus 2-IOD: .68



Molar 0.05 ±0.02 0.03 ±0.02 0.03 ±0.03 1-IOD versus 2-IOD: .26



IOD, implant overdenture. *P<.05 Tukey honest significant difference test.

Table 2.Mean ±SD of horizontal displacement at right distal edge ofoverdenture (mm)

Loadingpoint 1-IOD 2-IOD 3-IOD P

Anterior -0.01 ±0.04 0.07 ±0.05 0.04 ±0.04 1-IOD versus 2-IOD: .03*



Canine -0.07 ±0.02 -0.04 ±0.03 -0.05 ±0.03 1-IOD versus 2-IOD: .17



Premolar -0.06 ±0.0 -0.06 ±0.04 -0.06 ±0.02 1-IOD versus 2-IOD: .96

2-IOD versus 3-IOD: >.99


Molar -0.05 ±0.04 -0.09 ±0.03 -0.07 ±0.02 1-IOD versus 2-IOD: .13



IOD, implant overdenture. *P<.05 Tukey honest significant difference test.

Table 3. Comparison of mean ±SD vertical versus horizontal displace-ment on all loadings (mm)

Type of IOD

Displacement Direction

PVertical Horizontal

1-IOD 0.44 ±0.36 -0.05 ±0.04 <.01*

2-IOD 0.58 ±0.49 -0.03 ±0.07 <.01*

3-IOD 0.39 ±0.33 -0.04 ±0.05 <.01*

IOD, implant overdenture. *P<.05 paired t test.

Table 4.Mean ±SD of vertical displacement at loading point ofoverdenture (mm)

Loadingpoint 1-IOD 2-IOD 3-IOD P

Anterior -0.80 ±0.09 -0.99 ±0.13 -0.66 ±0.13 1-IOD versus 2-IOD: .05

2-IOD versus 3-IOD: <.01a


Canine -0.77 ±0.09 -0.65 ±0.14 -0.64 ±0.10 1-IOD versus 2-IOD: .23



Premolar -0.52 ±0.13 -0.49 ±0.08 -0.39 ±0.03 1-IOD versus 2-IOD: .83



Molar -0.61 ±0.10 -0.54 ±0.13 -0.48 ±0.07 1-IOD versus 2-IOD: .53



IOD, implant overdenture. aP<.05 Tukey honest significant difference test.

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RESULTS

Table 1 shows the average vertical displacement at the rightdistal edge of the overdenture. The 2-IOD showedsignificantly larger displacements than the 1-IOD or the 3-IOD on anterior loading (P<.01). The displacementresulting from anterior and canine loading was slightlylarger than that resulting frompremolar andmolar loading,but the differences were not statistically significant (P>.05).

Table 2 shows the average horizontal displacement atthe right distal edge of the overdenture. The horizontaldisplacement of the 2-IOD was significantly larger than

Oda et al

that of the 1-IOD upon anterior loading (P=.03).Furthermore, statistically significant differences were notdetected under the other conditions. However, regardlessof the loading point, the horizontal displacements were


Table 5.Mean ±SD of vertical displacement at right distal edge ofoverdenture upon each loading point (mm)

Loading Point P

Anterior 0.78 ±0.22 A versus C: .59

Canine 0.86 ±0.25 A versus P: <.01*

Premolar 0.21 ±0.11 A versus M: <.01*

Molar 0.03 ±0.02 C versus P: <.01*

C versus M: <.01*

P versus M: <.04*

A, anterior loading; C, canine; IOD, implant overdenture; M, molar; P, premolar. *P<.05Tukey honest significant difference test.

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small compared with the vertical displacements at allimplant positions (P<.01) (Table 3).

Table 4 indicates the vertical displacement of the over-denture at the loading point. The magnitude of thedisplacement of the 2-IOD at the loading pointwas significantly larger than that of the 3-IODuponanteriorloading (P< .01), while no statistically significant differenceswere observed under the other conditions (P>.05).

Table 5 shows the average of all vertical displace-ments at the distal edge of the overdenture on eachloading point. Statistically significant differences existedamong all loading points (P<.05), except between ante-rior and canine loading. The value of the displacementson molar loading was the smallest, followed by premolar,canine, and anterior loading.

DISCUSSION

The results of this study support the hypothesis that theimplant number affects the denture base movement of amandibular IOD upon anterior loading. The null hy-pothesis of the study was rejected for anterior loading butnot for canine, premolar, and molar loading.

Figure 6. Denture base movement in vertical direction. A, Overdenture rotatwith molar loading.


The 2-IOD showed significantly larger vertical dis-placements at the right distal edge than the 1-IOD uponanterior loading, which is in agreement with previousresearch using a finite element model.22 This result maybe because the 2-IOD rotated easily around the fulcrumline formed between the 2 implants. By contrast, the1-IOD rotated little because of the proper fit between thedenture base and the edentulous mandibular model usedin this study and because the 1-IOD showed smallervertical displacements. Under actual oral conditions, the1-IOD may allow complex denture movements because asingle implant cannot limit the direction of movement.The experimental overdentures had an appropriatefit with the alveolar ridge in this in vitro study, preventingdenture rotation under occlusal loading. The 1-IOD maybe an effective treatment option for the edentulousmandible, with a low cost and reduced invasiveness.

The vertical displacement of the 3-IOD at the rightdistal edge upon anterior loading was significantlysmaller than that of the 2-IOD, supporting the proposalof Mericske-Stern et al25 that the use of 3 or moreimplants prevents denture rotation. Denture baserotation has a negative effect on perceived masticatoryability and patient satisfaction10 and must be minimized.The 3-IOD treatment may be more effective than that of2-IOD for patients with edentulism to prevent denturerotation if the cost and invasiveness required for 3implants are acceptable.

Regardless of the implant number, upon anterior andcanine loading, the vertical displacements at the rightdistal edge and the loading point of the overdenturesuggest that the IODwas pressed into the mandible at theloading point and raised above it at the distal edge. Thisresulted in rotation around the implant. In contrast, upon

ed around implants on anterior loading. B, Minimal overdenture rotation

Oda et al

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premolar and molar loading, although the IOD waspressed into the mandible at the loading point, the IODhardly rotated and was slightly raised at the distal edge(Fig. 6). The denture-bearing area of the molar region islarger than that of the anterior region and resisted occlusalforcemore strongly. Therefore, the IODhardly rotated andwas slightly raised in the distal region. The horizontaldisplacements at the right distal edge measured in thisstudy were small; therefore, they hardly affected denturestability. This study suggests that the IOD moved mainlyin a vertical direction on anterior loading, simulating theincising of food with the anterior teeth. To prevent thismovement, an IOD should not be given definitive occlusalcontacts in the anterior area.

The actual denture movement in a patient’s oral cavityis not same as that observed in in vitro mandibularmodels, and denture movements should be measured inthe actual oral cavity. However, measuring denturemovement intraorally is difficult as the denture move-ment is smaller than the movement of the mandibleduring mastication. Therefore, denture movement wasmeasured using a simple model which could notcompletely simulate clinical situations such as behavior ofthe occlusal load. In addition, the success of the IODtreatment depends on complex factors such as forceapplied to the implants, retention of dentures, andprosthodontic complications. Further studies investi-gating these factors are required.

CONCLUSIONS

Within the limitations of this in vitro study, the followingconclusions were drawn:

1. The use of 2 implants for anchoring an IOD resultsin easier rotation of the denture base duringmastication with the anterior teeth than the use of1 or 3 implants.

2. The horizontal movements of IODs are smallcompared with the vertical movements.

3. Denture movement under occlusal force in themolar region is small compared with that in theanterior region.

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Corresponding author:Dr Manabu Kanazawa1-5-45, Yushima, BunkyoTokyo 113-8549JAPANEmail: [email protected]

Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.


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