journal of prosthodontic research 64 (2020) 145–151

Original article

Oral mucosa pressure caused by mandibular implant overdenture withdifferent types of attachments

Hiroaki Sato, Takuya Kobayashi, Taro Nomura, Norimasa Tanabe, Kyoko Takafuji,Hidemichi Kihara, Hisatomo Kondo*Department of Prosthodontics and Oral Implantology, School of Dentistry, Iwate Medical University, 19-1 Uchimaru, Morioka, Iwate 020-8505, Japan

A R T I C L E I N F O

Article history:Received 26 February 2019Received in revised form 19 May 2019Accepted 17 June 2019Available online 29 June 2019

Keywords:OverdentureImplantAttachmentOral mucosaPressure

A B S T R A C T

Purpose: To determine the appropriate attachment and design of a denture base for mandibular implantoverdenture (IOD), the oral mucosa pressure caused by mandibular implant overdentures was measuredusing edentulous jaw models with various attachments.Methods: An experimental edentulous mandibular model with a 1.5-mm thick artificial oral mucosa wasused. Two implants were placed at the area equivalent to the bilateral canines of an experimental jawmodel. Locator attachments (LA), ball attachments (BA), magnetic attachments (MA), and round-barattachments (R-BA) were fabricated. Six miniature pressure sensors were placed at the bilateral buccalpremolar regions, bilateral buccal shelves, and bilateral lingual molar regions. A precision universaltesting machine was used to apply dynamic repetitive loads of 50 N. The load points were the center ofthe model, which should represent bilateral mastication on both sides (bilateral load), and wereequivalent to the left first molar, which should represent unilateral mastication (unilateral load).Statistical analysis was performed using one-way analysis of variance. Multiple comparisons were thenperformed using the Bonferroni post hoc test. P < 0.05 was considered statistically significant.Results: Under the bilateral load condition, the lower oral mucosa pressure value with BA was measured,compared to other attachments at all measurement sites. Under the unilateral load condition, the oralmucosa pressure value of BA was smaller than the other attachments at the measurement site on theloading side.Conclusions: BA has exerted the greatest effects on support and bracing, suggesting that, BA is suitable forreducing oral mucosa pressure during mastication.

© 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.

Journal of Prosthodontic Research

1. Introduction

Edentulous people generally demand a higher level ofmasticatory function of their prosthesis, because they oftencomplain about their complete dentures (CD). On the other hand,they expect their CD to improve their quality of life (QOL) inregards to meals. However, periodontal therapy has now becomemore commonplace, and dentists usually try to maintain as manyteeth as possible. Moreover, even patients with advancedperiodontitis do want to preserve their teeth. Thus, the numberof patients who have highly resorbed alveolar ridge has increased,resulting in a difficulty to satisfy their demands of support,retention, and bracing of their CD.

* Corresponding author at: Iwate Medical University, 19-1 Uchimaru, Morioka,Iwate 020-8505, Japan.

E-mail address: hkondo@iwate-med.ac.jp (H. Kondo).

https://doi.org/10.1016/j.jpor.2019.06.0031883-1958/© 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights re

The recent development of implant prosthesis has givenedentulous patients not only fixed bone-anchored bridges, but alsoremovable implant overdentures (IOD). Compared with conven-tional CD, IOD for edentulous mandibles anchored to two-implants(2-IOD) have been shown to provide higher levels of patientsatisfaction, comfort, and QOL, including improved chewing abilityand better masticatory force [1–6]. 2-IOD is also economicallyreasonable and less surgically invasive than fixed implant therapy[7]. According to the 2002 McGill consensus [8] and 2009 Yorkconsensus [9,10], 2-IOD was recommended as first-line therapy foredentulous mandible, indicating the usefulness of IOD.

Although IOD therapy is now accepted in many countries [11],guidelines for the design of prosthesis, including the morphologyof the denture base, number of artificial teeth, mode of occlusion,and type of attachment, have not been established yet. Most of theprevious research on IOD involved simulation of the forces onimplants and the surrounding tissue employing finite elementanalysis and investigations of harmful effects on living tissue. Honget al. found that when two unconnected implants between the

served.

Fig. 1. Mandibular edentulous models.(a) Experimental mandibular edentulous model. (b) Two dental implants were placed at the position of the bilateral canines of the experimental jaw model.

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mandibular mental foramen were used to retain dentures, thefarther apart the implants, the more stress was concentrated onthe marginal bone [12]. Vafei et al. investigated the distribution ofstress on the marginal bone with the implant during lateral andprotrusive movement of a 2-IOD with ball and bar attachments,and they reported increased stress on the marginal bone withimplants in which ball attachments were used [13]. Assunção et al.reported that for 2-IOD with a ball attachment, there was anincrease in maximum stress values on the implants and super-structures when in the case of thicker and more resilient oralmucosa [14]. Liu et al. indicated that in single-, two-, three- andfour-implant retained/supported overdenture models, maximumequivalent strains in peri-implant bone under load condition of100 N were lower than the physiological tolerance threshold ofbone [15]. However, few studies have measured the force that isapplied to the residual ridge or alveolar bone via the implantsupported overdentures or have examined the harmful effectscaused by excessive occlusal force. Yoda et al. reported that theirmodel experiment using piezoelectric 3D force transducers and atactile sheet sensor enabled to clarify the effects of the ball, locatorand bar attachments used in an IOD on loading to implants and theunderlying residual ridge [16,17]. Goto et al. reported that the effectof the attachment installation conditions on the load transfer anddenture movements for implant overdentures [18]. While IOD canuse a variety of attachments, including locators, balls, magnets, andbars, most research on IOD attachments has been focused on themechanical forces on the implants and surrounding tissue [12–18].So far, no study has considered the morphology of a denture baseconfigured with a superstructure in consideration of oral mucosapressure. Therefore, we performed experiments using edentulousmodels to examine the effects of IOD attachments on the reductionof the oral mucosa pressure value, and to try and determine theappropriate morphology of such a denture base.

2. Materials and methods

2.1. Fabrication of experimental dentures

An experimental mandibular edentulous jaw model with a 1.5-mm thick artificial oral mucosa (P9-EP.30-L, Nissin, Tokyo, Japan)was purchased. Two dental implants (3.75 �11.5 mm, BränemarkSystem1 Mk III, Groovy RP, Nobel Biocare, Kloten, Switzerland)were placed at the position of the bilateral canines, perpendicularto a tentative occlusal plane (Fig.1). In this study, one experimentaldenture was fabricated and used to compare the effects of differenttypes of IOD attachments on oral mucosa pressure values. Wesupposed that neither the morphology of the denture base nor theposition of the miniature pressure sensors must be changed toevaluate the effects of attachments precisely, and only attachments

were replaced in each experiment. Experimental 2-IODs, mainlymade of acrylic resin denture base material, were fabricated. Eachattachment was inserted and connected to the experimental 2-IODwith chemical cure resin material. The shape of 2-IODs weresimilar to the record base with occlusion rim morphology that wasmade according to the standard protocol.

Locator attachments with retention discs performing retentiveforces 0.7 kg (LA, locator abutment, Bmk RP 2.0 mm, NobelBiocare), ball attachments (BA, ball abutment, Bmk RP 1 mm,Nobel Biocare), magnetic attachments (MA, MAGFIT IP-DXFL, AichiSteel, Aichi, Japan), and resilient round-bar attachments (R-BA,NobelProcera Impl Bar Overdenture Ti 2 Impl, Nobel Biocare) wereemployed (Fig. 2). Generally, retention discs of lower retentiveforces were applied when new IOD was installed. Thus, we chose0.7 kg LA to compare with other attachments in this study.

Six miniature pressure sensors (PS-10KD, Kyowa ElectronicInstruments, Tokyo, Japan) were placed in the bilateral buccalpremolar regions, the bilateral buccal shelves, and the bilaterallingual molar regions to measure pressure values on the oralmucosa when the experimental dentures were loaded (Fig. 3). Anexperimental CD without attachment was prepared as a control.

2.2. Measurement system

The load on the experimental dentures was set at 50 N byreferring to the masticatory force of CD wearer [19,20]. The loadpoints were either an area equivalent to the center of the modelwhich should represent equivalent mastication on both sides(bilateral load), or an area equivalent to the left first molar whichshould represent unilateral mastication (unilateral load). A metalplate was placed on the experimental dentures and dynamicrepetitive loads of 1 Hz were applied perpendicularly to the tentativeocclusal plane by precision universal testing machine (Instron 8874,Instron, Norwood, MA, USA). Pressure distributions during theloading were measured with 6-channel frequency sensors at a200 Hz sampling rate, and recorded via a sensor interface (PCD-400A, Kyowa Electronic Instruments, Tokyo, Japan) (Fig. 4). The oralmucosa pressure values were defined as average values of the loadwith the precision universal testing machine during 5 continuouscycles.The experiments were repeated5 timeseach in the CDandtheexperimental 2-IOD using LA, BA, MA, or R-BA.

2.3. Statistical analysis

The statistical analysis was performed using IBM SPSS Statisticsversion 24 software (IBM Corp., Armonk, NY, USA). Normaldistribution was determined by the Kolmogorov–Smirnoff testand Shapiro–Wilk test. Statistical differences between the testgroups were analyzed by one-way analysis of variance with the

Fig. 3. Experimental denture.Six miniature pressure sensors were placed at bilateral buccal premolar regions, bilateral buccal shelves, and bilateral lingual molar regions. (a) Right buccal premolar region.(b) Left buccal premolar region. (c) Right buccal shelf. (d) Left buccal shelf. (e) Right lingual molar region. (f) Left lingual molar region.

Fig. 2. Attachments for 2-IOD.(a) Locator attachment (LA). (b) Ball attachment (BA). (c) Magnetic attachment (MA). (d) Round-bar attachment (R-BA).

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Bonferroni’s post hoc test. A P-value of 5% was considered asstatistically significant.

3. Results

3.1. Differences in oral mucosa pressure values between CD and 2-IOD

The pressure on the residual ridge exerted by CD was comparedto that of 2-IOD with various attachments.

Under the low, the oral mucosa pressure value exerted by 2-IODwas significantly lower in all sites than that of CD (Figs. 5–7).

Under the unilateral load condition, the oral mucosa pressurevalue exerted by 2-IOD was significantly lower in the left buccalpremolar region and the left buccal shelf than that of CD (Figs. 8and 9). 2-IOD reduced the oral mucosa pressure value of 37–190 kPa at the supportive site. The oral mucosa pressure valueexerted by 2-IOD in the left lingual molar region was not alwayslower than that of CD. The oral mucosa pressure value wasincreased significantly by MA and LA, but decreased significantlyby BA (Fig. 10). On the non-loading side, the oral mucosa pressurevalues exerted by both CD and 2-IOD were extremely low and didnot exceed 20 kPa in the right buccal premolar region, the rightbuccal shelf, or the right lingual molar region (Figs. 8–10).

3.2. Changes in oral mucosa pressure values with differentattachments

Pressures on the residual ridge exerted by 2-IOD via variousattachments were compared.

Under the bilateral load condition, the oral mucosa pressurevalues transmitted by BA and R-BA were significantly lower in thebilateral buccal premolar regions and the bilateral buccal shelvesthan those of other attachments (Figs. 5 and 6). The oral mucosapressure values exerted by BA were significantly lower in thebilateral lingual molar regions than those of other attachments. Inaddition, the oral mucosa pressure values exerted by LA were lowerthan those of MA in the right lingual molar region (Fig. 7).

Under the unilateral load condition, the oral mucosa pressurevalues exerted by BA and R-BA were significantly lower in the leftbuccal premolar region and the left buccal shelf than those of otherattachments (Figs. 8 and 9). The oral mucosa pressure valueexerted by BA was significantly lower in the left lingual molarregion than those of other attachments (Fig. 10). On the non-loading side, oral mucosa pressure values exerted by all attach-ments were extremely low and did not exceed 8.0 kPa in the rightbuccal premolar region and the right buccal shelf (Figs. 8 and 9).The oral mucosa pressure value exerted by R-BA was significantly

Fig. 4. Measurement of loads via the implant overdenture.(a) Universal testing machine. (b) Bilateral loading. (c) Unilateral loading. (d) Example of a waveform during the dynamic repeated load.

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higher in the right lingual molar region than those of otherattachments (Fig. 10).

4. Discussion

To ensure the long term prognosis of prosthetic therapy usingIOD, it is important to control the loads on the implants and theresidual ridge oral mucosa [21–27]. Namely, it is necessary toconsider the position of implants and design of the denture of thesuperstructure. A treatment plan must be drawn up so that theimplants and residual ridge are not subjected to excessive loads.

In previous studies related to IOD, finite element analysis [12–15],strain gauges [16], and pressure sensors [17,18] were employed toelucidate the impacts of attachments on residual ridge oralmucosa. Most experiments have fabricated 2-IOD models andinvestigated designs for embedding implants based on measure-ments of strain on the bone surrounding the implants. However,few studies have measured the pressure on the residual ridge orthe jawbone of supportive tissue or examined superstructuredesigns. Yoda et al. reported that under the unilateral loadcondition, different 2-IOD attachments led to different amountsof oral mucosa pressure in the molar region [17]. Goto et al.

Fig. 5. Oral mucosa pressure in the buccal premolar regions under the bilateral loadcondition.

Fig. 6. Oral mucosa pressure in buccal shelves under the bilateral load condition. Fig. 8. Oral mucosa pressure in buccal premolar regions under the unilateral loadcondition.

Fig. 7. Oral mucosa pressure in lingual molar regions under the bilateral loadcondition.

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reported that the way of fixing attachments changed the mucosapressure values in the molar regions under the unilateralcondition [18]. However, the measurements in those studieswere limited to the molar region, and no study covered the wholeresidual ridge. To determine the denture base morphology of 2-IOD, the measurements over the entire jaw should be required.However, it might be more important to measure the change inmucosa pressure caused by implant placement in the supportiveand bracing regions of CD.

To examine the pressure phases of the whole denture region, itwould be suitable to place a sheet sensor over the entire mucosalsurface of the denture base. However, such experimental devicescannot separately measure the pressure values on multipleattachments. Thus, in this study, we conducted experiments byplacing 6 miniature pressure sensors in the bilateral supportiveregions (the buccal premolar regions and the buccal shelves) andthe bilateral bracing regions (the lingual molar regions). Moreover,

in 2-IOD, the attachments installed on the implants can change thepressures on the residual ridge and jawbone [13,28–31]. It isimportant to consider characteristics of attachments whendesigning the denture base morphology. Therefore, we examinedthe differences in oral mucosa pressure values when using LA, BA,MA, and R-BA attachments.

Under the bilateral load condition, the oral mucosa pressurevalues exerted by 2-IOD were significantly lower at all measure-ment points than those of CD. This reduction in oral mucosapressure values are supposed to be the result of the supportiveeffect provided by the implants and the stabilized effect of theattachments. Depending on the shape of attachments, variouskinds of occlusal load to implants and attachments reduced theoral mucosa pressure value at least 40 or 15% at the supportiveregions or bracing regions, respectively. This finding demonstratesthat the stability of 2-IOD is better than CD.

Fig. 9. Oral mucosa pressure in buccal shelves under the unilateral load condition.

Fig. 10. Oral mucosa pressure in lingual molar regions under the unilateral loadcondition.

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Usually, a specific side is used during mastication, and weapplied a load of 50 N to an area equivalent to the left first molar. Inthe supportive regions of the loading side, the buccal premolarregion and the buccal shelf, the oral mucosa pressure value of 2-IOD was approximately 37–190 kPa lower, in other words about15–85% lower, than that of CD. In the bracing region, which isequivalent to the lingual molar region, the oral mucosa pressurevalue depends on the type of attachment. LA increased the oralmucosa pressure value of 13 kPa and BA reduced the value of29 kPa, compared to CD. These results suggested that thesupportive effect of 2-IOD is high but the bracing effect is not sohigh on the loading side. On the non-loading side, 90% of the oralmucosa pressure value in both the supportive and bracing regionswas reduced by 2-IOD compared to CD, because unilateral loadcauses the denture to rotate and rise up from the residual ridge.

These findings are consistent with previous studies, whichreported that unilateral load on CD increased the oral mucosa

pressure value in the buccal regions of the loading side butdecreased pressure in the lingual regions compared with bilateralload [32–34]. Although it is necessary to consider the character-istics of attachments, this study suggests that the denture base areain both the supportive and bracing regions of 2-IOD could bereduced.

The results of this study also showed that under a bilateral loadcondition, oral mucosa pressure value was the highest when MAwas employed, followed by LA, R-BA, and then BA in thesupportive regions. Under a unilateral load condition, the oralmucosa pressure value was the highest when LA was employed,followed by MA, R-BA, and then BA in the supportive regions. Inthe bracing regions, the oral mucosa pressure values under thebilateral load condition was the highest when MA was used,followed by LA, R-BA, and then BA. Under the unilateral loadcondition, the oral mucosa pressure values were similar betweenLA, MA and R-BA groups, and these values were higher than BAwhen LA was used.

In our study, BA contributed to the lowest oral mucosa pressurevalue under all load conditions and in all regions. Because malestuds were surrounded with metal housings and no free spacebetween BA components, resulting in the smallest amounts ofdenture movement and rotation. Similar concepts have beenpreviously reported, in which the oral mucosa pressure value wasthe highest when R-BA was used under the unilateral loadcondition of 100 N, followed by LA and then BA [17]. Goto et al.reported that, under the unilateral load condition of 50 N, oralmucosa pressure value was the highest when MA was used,followed by LA and then BA [18]. Although those studies wereconducted under different load conditions, BA componentssignificantly reduced the oral mucosa pressure value. Yoda et al.claimed that LA only allowed a small amount of attachmentrotation and that the dentures were strongly fixed to the frontaltooth area, resulting in a decrease in the load on the oral mucosa.However, the load on the oral mucosa was significantly lower whenBA was used, compared to LA. Thus, those previous studiessupported our results, suggesting that BA reduced the oral mucosapressure value significantly compared to other attachments [17].Moreover, Tokuhisa et al. reported that MA allows a large amountof rotation, which increases the oral mucosa pressure value in themolar region [35]. In observations of denture behavior, Goto et al.found that while there is little settling of the dentures with MA, themagnetic structure separates from the MA keeper, which mayresult in a loss of retentive force. They also reported that thisseparation causes posterior rotation of the dentures and increasesthe oral mucosa pressure value in the molar region [18]. Regardingthe movements of 2-IOD, MA showed the greatest amount ofpitching, yawing, and rolling, and LA also showed a large amount ofpitching [17,35]. In our study, when BA and R-BA was employed, weobserved less denture pitching and a smaller amount of oralmucosa pressure value in the supportive regions compared to LAand MA. Regarding R-BA, we used the Hader bar system, which isequipped with cantilever attachments. Compared with the otherattachments, this bar is thought to suppress pitching, yawing, androlling of the denture. While the oral mucosa pressure value withR-BA was lower than that with MA and LA, it was greater than withBA. This difference between BA and R-BA might be caused by thevariance of materials. The clips used in this study were made ofpolypropylene, which should not fix 2-IOD as tightly as metalhousings of BA. The application of regular round-type R-BAsplinting two implants without cantilevers, resulted in greaterpitching and oral mucosa pressure value than BA [17,35]. RegardingR-BA, the oral mucosa pressure value of each point may be affectedby the shape of the bar and materials used for the attachment.Thus, it is important to consider all the characteristics of R-BAduring the design process for 2-IOD.

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Generally, the oral mucosa pressure value should not exceed200 kPa with any type of attachment. A previous study thatexamined jawbone resorption with 2-IOD demonstrated that therewere no harmful effects on the jawbone if the oral mucosa pressurevalue did not exceed 200 kPa [36]. Also, several studies examiningthe tissues and bone surrounding implants have reported that noattachment had unfavorable effects [37–39]. Those resultsindicated that all the values acquired in our experiment shouldbe reasonable and reflect the clinical situation.

Various factors including the morphology of residual ridge, theposition and angle of the implants, retentive force, the ease ofmaintenance, patient’s ability to clean, and the patient’s lifestyleshould be considered for the attachment selection. Pressure on theresidual ridge mucosa was examined in this study, and the resultssuggested that BA could be the first choice to reduce oral mucosapressure value during mastication.

5. Conclusions

Significant decrease in oral mucosa pressure value and increasein support and bracing ability were observed when 2-IOD wasapplied, compared with CD. Our in vitro study indicates that theeffect of BA on the reduction of oral mucosa pressure is greaterthan LA, MA and R-BA in the supportive and bracing regions.

Acknowledgements

We thank Mr. Shingo Kurosu of Iwate Industrial ResearchInstitute for his helpful instruction and operating the precisionuniversal testing machine.

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