University of Groningen

Effects of dental implants on hard and soft tissuesTymstra, Nynke

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16

Chapter 2

Posterior mandibular residual ridge resorption in patients with overdenturessupportedbytwoorfourendosseousimplants;a10-yearprospectivestudy

This chapter is an edited version of the manuscript:

DeJongMHM,WrightPS,MeijerHJA,TymstraN.

Posteriormandibularresidualridgeresorptioninpatientswithoverdenturessupportedbytwo

orfourendosseousimplantsina10-yearprospectivecomparativestudy.

Int. J. Oral Maxillofac. Implants (accepted for publication)

17

Abstract

PurposeTheaimofthisstudywastoevaluatetheeffectoftreatmentwitheithertwoorfour

mandibular endosseous implants with an overdenture on the mandibular posterior residual

ridgeresorptionover10years.

Materials and methods Sixty edentulous patients with a mandibular height of between

12and18mmparticipated.Thirtypatientsweretreatedwithanoverdenturesupportedby

twoIMZimplants(two-implantgroup)and30patientsweretreatedwithanoverdenturewith

four IMZ implants (four-implant group). Prior to and 10 years after treatment, panoramic

radiographs were taken to calculate possible bone loss. Proportional area measurements were

usedtodeterminechangesinthemandibularposteriorresidualridgebilaterally.

ResultsTherewasastatisticallysignificantdifferenceinmandibularposteriorresidualridge

resorption between the two types of treatment. The PosteriorMandibular Ridge Ratiowas

reducedbyameanof10%forthetwo-implantgroupand6%forthefour-implantgroupover

10years.Nocorrelationwasshownbetweenmandibularposteriorresidualridgeresorption

and peri-implant marginal bone loss. The confounding factors marginal bone loss around the

implants,age,gender,initialmandibularheightandthenumberofyearsthepatienthadbeen

edentulous failed to show a significant effect on the posterior mandibular ridge resorption.

Conclusions It can be concluded that slightly more posterior mandibular residual ridge

resorption occurred in patients treated with a two-implant overdenture than in patients

treatedwithafour-implantoverdentureoveraperiodof10years.

1818

Introduction

Edentulouspatientsoftenexperiencedifficultieswiththeirdentures.Mainproblemsarelackof

retentionandstabilityofthemandibulardenture,anddecreasedchewingability(VanWaas,

1990).Theseproblemsareaggravatedbythelossofheightofthemandibularresidualridge,

a result of extensive remodelling following extraction of the teeth and the use of dentures

(Carlsson& Persson, 1967). There is great individual variation in the rate of residual ridge

resorption among different patients and even in the same person at different times and sites

(Atwood,1971;Xieetal.,1997).A15-yearsand25-yearsfollow-upstudyoncompletedenture

wearersrevealedacontinuingreductionofthemandibularridge(Tallgren,1972).Inanother

studyitwashypothesisedthatexcessivealveolarboneresorptioniscausedbyphysiologically

intolerableforcesproducedbycompletedentures(Kelsey,1971).

An implant-retained overdenture is a treatment possibility which improves function and

comfort for edentulous patients and eliminates a substantial part of the problems which

edentulouspatientsexperience(Boerrigteretal.,1995).

Implant-retainedoverdentureswerealreadybeingstudiedinthe1980sandarestillconsidered

of great value in the rehabilitation of edentulous patients (van Steenberghe et al., 1985;

Batenburgetal.,1998).Mandibularoverdenturescanbestabilisedbyadifferentnumberof

implants (usually two or four) and by different attachment systems (usually bar systems,

whichconnect the implantsorsolitarysystemssuchasball-attachmentsystems).Besides

the improvement of retention and stability of the denture, it has been suggested that the

stabilisationoftheoverdenturebyimplantswillpreservetheremainingresidualbonyridge.

Posterior ridge measurements can be done on panoramic radiographs. After tracing of the

radiograph the surface of a proportional area is calculated. This method has been reported

inseveralotherstudies (Wildingetal.,1987;Jacobsetal.,1992). Jacobsetal.suggesteda

thresholdfortheareaindexof0.04inthedetectionofboneresorption(Jacobsetal.,1992).

Using this method, Kordatzis et al demonstrated significantly less posterior mandibular

residual ridge resorption in patients with implant overdentures when compared to patients

withconventionalcompletedentures(Kordatzisetal.,2003).Inaddition,Wrightetalreported

low rates of posterior mandibular residual ridge resorption for patients with overdentures

stabilisedbytwoimplants(Wright&Watson,1998).Theycomparedoverdenturesstabilised

by two implantsconnectedbyaparallel-sidedbar (rigid joint)or connectedbyanovalbar

Chapter 2

19

(resilient joint). They foundno significant difference in posteriormandibular residual ridge

resorption between a resilient joint and a rigid joint, although subsequent work showed

therigidityofsomeoftherigid jointswas lessthenpredicted(Setzetal.,2000). Inamore

recentstudy,Wrightetalcomparedpatientswearingfixedimplantprostheseswithpatients

wearing implant-retained removable overdentures regarding posterior mandibular residual

ridgeresorption(Wrightetal.,2002).Theyreportedlowratesofboneresorptionforpatients

rehabilitated with an implant-retained removable overdenture and bone apposition in the

posteriormandibulararea inpatientswithafixedprosthesis. Thereareno studies todate

comparingmandibularoverdenturessupportedby twoor four implantswith respect to the

posterior mandibular residual ridge resorption.

For general application in the edentulous mandible, a treatment concept utilising two or four

implantstostabiliseamandibularoverdenturehasbeenproposed (Batenburgetal.,1998).

Both two implants and four implants to support an overdenture are placed in the interforaminal

region of the mandible, however the bar superstructure design is different. In comparison with

two implants, there is a less resilient joint in case of four implants. This could lead to fewer forces

on the posterior mandibular ridge with a four-implant overdenture compared to a two-implant

overdenture.Recentstudiescomparingtheeffectsofmandibularoverdenturessupportedby

either two or four implants looked at outcome factors such as survival rate, condition of hard

and soft peri-implant tissues, marginal bone loss, patient satisfaction, prosthetic aftercare,

surgicalaftercareandcost-effectiveness.Exceptfortheinitialcosts,whichweresignificantly

higher for mandibular overdentures supported by four implants, there were no significant

differencesfound(Stokeretal.,2007;Visseretal.,2009;Meijeretal.,2009).Althoughboth

treatmentpossibilitieshavebeenexaminedthoroughlybyseveralstudygroups,thereareno

10-yeardatacomparing theposterior residual ridge resorption in themandibleofpatients

withmandibularoverdenturesstabilisedbytwoorfourimplants.

Therefore, the aim of this study was to investigate posterior mandibular residual ridge

resorption following the use of either two or four endosseous implants to stabilise complete

overdenturesover10years.Alsovariousconfoundingfactorswillbereviewed.Theworking

hypothesiswasthattherewillbemoreposteriormandibularresidualridgeresorptionfollowing

treatment using two implants compared to four implants.

2020

Materials and Methods

Patient selection and treatment

Sixtyedentulouspatients(39women,21men,meanage54.9years,median52years,range38-

81years)wereselected.Theywereallreferredbytheirdentistorgeneralmedicalpractitioner

totheUniversityMedicalCenterGroningen.Allpatientsweresufferingfromreducedstability

and insufficient retention of their mandibular denture. Inclusion criteria for the clinical trial

were an edentulous period of at least two years andmoderate resorption of themandible

(mandibleheightinthesymphysisregionbetween12and18mm),beingclassIV-VIaccording

to the Cawood and Howell classification (Cawood & Howell, 1988). Patients with a history

ofradiotherapyintheheadandneckregion,ahistoryofpre-prostheticsurgeryorprevious

implant placementwere excluded. Thehospital ethical committee approved the study. The

jawbonequalitywasscoredaccordingtotheclassificationofLekholm&Zarb(1985).

The patients were informed about the treatment options (overdenture on two or four

endosseousimplants)whichwerebothappropriateinthepatientsincluded.Writteninformed

consentwasobtained fromallpatients.Treatmentwasrandomlyallocatedby lot resulting

in 30 patients (two-implant group) to be treatedwith two IMZ implants (titanium plasma-

sprayed(TPS),FriedschfieldAG,Mannheim,Germany)and30patients(four-implantgroup)to

be treated with four IMZ implants. Table 1 shows that there were no significant differences in

thebaselinecharacteristics.Mostpatientsmainlyhad16mmofboneheightandtypeIIIbone.

All implants were inserted under local anaesthesia in the inter-foramina region according to

theproceduredescribedbyKirsch(1983).Allimplantswereplacedasatwo-stageprocedure

Chapter 2

Table 1. Characteristics of the groups at baseline.

Group two-implant four-implant

Number of participants 30 30

Gender(m/f) 9/21 6/24

Age(years;mean/SD/range) 54.0/8.7/38-77 55.7/12.3/35-79

Edentulousperiodmandibularjaw(years;mean(SD)) 21.0(9.0) 21.8(10.5)

Mandibularboneheight(mm;mean(SD)) 15.8(2.3) 15.7(2.7)

Bonequality(possiblescore1-4;mean) 3.0 2.7

21

bythesameoralsurgeon.Inthe‘two-implantgroup’theimplantswereplacedinthecanine

regionof themandible,10millimetres leftand right from themidline. In the ‘four-implant

group’ there was equal distance between the four implants and themost lateral implants

wereplaced5mmmediallyofthementalforamen.Positionoftheimplantsdependsonthe

available space and the anterior curve of the mandible. No surgical stent was used. Standard

postoperativetreatmentwascomposedofanalgesicsandchlorhexidine0.2%mouthrinses,

but no antibiotics.

Threemonthsaftertheimplantplacement,secondstagesurgery(thinningoftheperi-implant

mucosa and placement of the abutment) was performed. Two weeks thereafter standard

prosthetictreatmentwasprovidedbeinganewmaxillarycompletedentureandamandibular

Figure 2.

Inner side of a two-implant mandibular overdenture.

Figure 1.

Two implants with a bar superstructure in an edentulousmandibleafter10yearsinfunction.

Figure 4.

Inner side of a four-implant mandibular overdenture.

Figure 3.

Four implants with a bar superstructure in an edentulousmandibleafter10yearsinfunction.

2222

overdenturesupportedbyanindividuallymaderoundbarwithnodistalextensionsandclip

attachments (Figures 1, 2, 3 and 4). None of the overdentureswere reinforcedwith a pre-

castmetalconstruction.Allpatientsweretreatedbyoneexperiencedoralandmaxillofacial

surgeonandoneexperiencedprosthodontist.

Figure 5.

Panoramic radiograph of the two-implant group usedfortracing-10years.

Figure 6.

Panoramic radiograph of the four-implant group usedfortracing-10years.

Figure 7.

Theanatomical landmarksM,M’ (lowerborderofmental foramen);S,S’ (Sigmoidnotch);G,G’ (gonion)wereusedtoconstructthetrianglesM-S-GandM’-S’-G’withcentresNandN’respectively.Boundarylineswereconstructedasfollows:M-GandM’-G’,A-LandA’-L’(crestofresidualridgetolowerborderofmandibleperpendiculartoM-GandM’-G’),M-NandM’-N’,andG-PandG’-P’(G-NandG’-N’extendedtothecrestoftheresidualridgeatPandP’).

Chapter 2

23

Radiographic analysis

Posterior residual ridge resorption

Panoramicradiographswereobtainedfromallpatientsimmediatelybeforeand10yearsafter

treatment(Figures5and6).Thepanoramicradiographswerethroughouttheevaluationperiod

manufacturedwiththesameorthopantomograph(Orthopos®,Siemens,Bernsheim,Germany)

forstandardisationwithcorrespondingradiographs.Thefilmswereautomaticallyprocessed

inaKodakRP®X-OmatM5processor(Kodak,Rochester,NY).Thelandmarksweretracedfrom

theradiographsanddigitised;areaswerecalculatedwithcomputersoftware.

The method consisted of proportional area measurements of the posterior mandible, similar to

thatusedbyWrightetal(2002)Usingproportionsminimiseserrorsrelatedtomagnification

anddistortion.Foreveryradiographatracingwasmadeonthemandible.Figures7and8show

theareasthatwere traced.Theanatomical landmarksM(lowerborderofmental foramen),

S (sigmoidnotch)andG (gonion)wereused to construct the triangleson the right (M-S-G)

Figure 8.

The areas were defined as follows: posterior bone arearight and posterior bone arealeftby thecrestof theresidual ridge P-A and P’-A’ and the boundary lines A-M and A’-M’, M-G and M’-G’, and G-P and G’-P’,respectively;andtheposteriorreferencearearight and posterior reference arealeftbythetrianglesM-G-NandM’-G’-N’,respectively.Aratiowascalculatedwascalculatedfrom(bonearearight/referencearearight + bone arealeft/referencearealeft)/2.

bone area

reference area

2424

andleft(M’-S’-G’)sideofthemandiblewithcentreN(Figure7).Boundarieswereconstructed

by the following lines: the boundary lineM-G, the boundary line A-L; a line from the crest

ofresidualridge(pointA)tothe lowerborder(pointL)throughMperpendiculartoM-G,the

boundarylineM-NandboundarylineG-P;thelineG-Nextendedtothecrestoftheresidual

ridgethroughpointP.TheexperimentalboneareawaseventuallyoutlinedbytheareaPAMG

andthereferenceareabythetriangleMGN(Figure8).ThePosteriorMandibularRidgeRatio

(PMandRR)wascalculatedbydividingtheboneareabythereferencearea.Theratiosforthe

right en left part in one patient were averaged.

Toestimatethechangeintheheightoftheposteriormandibularresidualridgeovertenyears,

not taking into account the magnification or distortion of the radiographs, the mean of the

changeinboneareaboundedbytheresidualridgewasdividedbythelengthofthecrestofthe

residualridge(Kordatzisetal.,2003).

Beforestartingthestudyapilotwasusedtodeterminethereproducibilityofmeasurements

andifthequalityoftheradiographwasofanyinfluence.Sixradiographswereselectedwith

varying quality of visibility of the tracing points. All radiographsweremeasured 10 times

byoneexaminerusingthemethodplannedforthemainstudy.Thestandarddeviationand

the coefficient of variation ware calculated for each set of measurements. The coefficient of

variationrangedbetween0.88%and2.72%wherethelowestvariationwasassociatedwith

clear visibility of the tracingpoints. Therefore, only radiographswith clear visibility of the

tracingpointswereincludedinthemainstudy.Iftracingpointswereclear,itwaspossibleto

compute the surface of the posterior area. The measuring procedure was carried out at the end

ofthestudyperiod.Atthattimeitappearedthatsometracingpointswerenotclearlyvisible

foractualmeasurements.Itwasfoundthatpatientscouldnotbere-calledfortakinganextra

radiograph.

Confounding factors

The following confounding factors were studied: peri-implant marginal bone loss, age, gender,

initialmandibularheightandthenumberofyearsthepatienthadbeenedentulous.

Dataof peri-implant bone losswere obtained from the studyofMeijer et al. (2009),which

describesthesamepatientgroups.Inthisstudy,standardisedintra-oralradiographs,using

a long cone technique, of each implant were obtained using a beam direction device (Meijer et

Chapter 2

25

al.,1992).Baselineintraoralradiographsweremadeattimeofimplantloading.Analysisand

calculation of changes was done on implant basis.

Statistical analyses

Since none of the data showed a normal distribution a Mann-Whitney non-parametric

statisticalanalysiswasused.Toexamineconfoundingfactors,amultipleregressionanalysis

usingabackwardstepwiseprocedurewasadopted.ThecorrelationwastestedusingaKendall’s

tau correlation test. In all tests, a significance level of 0.05 was chosen.

Results

Ofthe30patientsinthetwo-implantgrouptherewasadropoutofsevenpatients:onepatient

died, in two patients the implants were removed, and the panoramic radiographs of four

patientswereunfitforuseduetolackofclarityofthereferencepoints.Thefour-implantgroup

had a drop out of 12 patients: four patients died, three patients moved to another part of the

countryandthepanoramicradiographsoffivepatientswereunfitforuse.Thisresultedina

groupof23patients(5male/18female)forthetwo-implantgroupand18patients(6male/12

female)forthefour-implantgroup.Theassumptionwasmadethatdrop-outofpatients(two-

implantgroup:23%, four-implantgroup:40%)wasnotrelatedtoposterior ridgereduction.

Before the study and at each annual check-up patients were asked about their medical

condition. No particular diseases occurred that could be linked to bone resorption.

Thechange inPMandRRwascalculatedforeachpatientbysubtractingtheratiovalueat0

yearsfromtheratiovalueat10years.Therefore,anegativedifferenceindicatedresorption,

and a positive difference indicated an increase in area or apposition of bone. The results for

both groups are shown in Table 2. Therewas a significant difference (p<0.05) in PmandRR

between the groups. Reduction in bone height is easier to visualise than change in PmandRR.

Itispossibletoconvertboneareadataintoboneheightdata,afterthemethodofKordatzis

etal. (2003)Thisishoweveronlyanestimation.Theapproximatechangesinheightcanbe

calculatedbydividingtheaverageinitialareabytheaveragelengthoftheposteriorresidual

ridge,whichwasapproximately40mm.Theestimatedchangeinposteriormandibularresidual

ridgeresorptionoveraperiodof10yearswas1.44mmforthetwo-implantgroupand0.74mm

for the four-implant group.

2626

Table 2. Change in Posterior Mandibular Ridge Ratio.

Group two-implant* four-implant*

Number of pairs of radiographs 23 18

Mean(SD)changePmandRR -0.10(0.07) -0.06(0.09)

Median change PmandRR -0.09 -0.04

Minimum/MaximumchangePmandRR -0.33/0.01 -0.20/0.03

* Positive values indicated resorption and negative values indicated an increase in the area of posterior residual ridge or bone apposition.

Abbreviations: PmandRR = Posterior Mandibular Ridge Ratio.

The mean marginal bone loss around the implants was 1.4 mm in the two-implant group and

1.1mm in four-implantgroup in10years,asdescribed in thestudyofMeijeretal. (2009).

No significant correlation was found between the mean marginal bone loss and posterior

mandibular residual ridge resorption.

The other potential confounding factors, age, gender, initial height of the mandible, bone

quality, years edentulous and marginal bone loss around the implants, failed to show a

significant effect on the amount of posterior mandibular residual ridge resorption. This

confirmstheresultsofthenon-parametricMann-Whitneytest.

Discussion

Themean change in PosteriorMandibular Ridge Ratio (PmandRR)was 0.10 in two-implant

group(estimatedmandibularridgeresorptionwas1.44mmin10years)and0.06inthefour-

implantgroup(estimatedmandibularridgeresorptionwas0.74mmin10years).Therewasa

significant difference in PmandRR between the groups. There was less resorption in patients

treated with four-implant overdentures. The assumption was made that drop-out of patients

was not related to posterior alveolar ridge reduction, because death of patients and moving

to another part of the country were not implant-related. Moreover, the number of unclear

panoramic radiographs was comparable between the groups. This makes the groups rather

comparable.However,itcannotbeexcludedthatlossofimplantsduetoperi-implantitis(two-

implantgroup:2patients)wasnotaccompaniedbysomeadditionalposteriorridgereduction.

Thisisnotlikely,butevenifithadoccurred,theconclusionofthisstudywouldnothavechanged

Chapter 2

27

as such a condition would have lead to on average even more bone loss in the two-implant

group.Inter-abutmentdistancecanvaryinthefour-implantgroup.Inalargejaw,withalarge

interforaminalspace,alargerinter-abutmentdistancewillbetheconsequencewithpossibly

moreforcesontheimplants.Theauthorsfeel,however,thatthisvariableisnotveryrelevant.

Differences in inter-abutment distance are small and also other, not measured, variables can

beof influence (e.g. chewingpattern, chewing force).Thepresent studycouldbe compared

withthestudyofWrightetal.(1998),whichcomparedaresilientjointwitharigidjoint,both

in mandibular overdentures. The groups had two implants with either a straight ovoid bar with

a resilient joint or a parallel-sided bar and a rigid joint. It failed to show a significant difference

between the two groups. This could be due to the small number of patients investigated in

Wrightsstudy,ortotherigid jointbeing lessrigidthanpredicted(Setzetal.,2000). Inthe

presentstudythebaronfourimplantsprovidedmoreposteriorsupportthanthebarontwo

implants.Theposteriorpositionoftheaxisofrotationandlessresiliencecouldbethecause

of fewer forcesontheposterior ridgeandthereforecauses lessboneresorption.Toexplain

the differences in mandibular posterior residual ridge resorption between the two and four

implant overdenture patients, differences in loading patterns and movements of the denture,

for both designs must be considered. Research shows that an implant-retained overdenture

significantlyimprovesthemasticatoryfunctioninpatients(Stellingsmaetal.,2005;Fuekiet

al.,2007).However, littleisknownabouttheexactchangestotheforceanditsdistribution

under the denture. Most studies focus on the force distribution on the implants and not on

the posterior residual ridge. With a bar between two implants in the interforaminal region,

free rotation of the denture around the bar is possible. This is the case with both round bars

and ovoid bars. Rotation of the overdenture is more rigid with a bar on four implants in the

interforaminalregion,althoughrotationisstillpossible.Withafixedprosthesisonimplants,

thereisnorotationandtheposteriorridgewillnotbeloaded.Onewouldexpectthatthefewer

forces on the posterior ridge, the less resorption will occur. An overdenture on two implants

might result in an overall distribution of themasticatory forces, which could result in an

increased resorption of the posterior ridge. Whereas, with the more rigid four implant design,

withwider distribution of the implants, patients aremore likely to center themasticatory

forces over the implants where the denture is the most stable. Thus less resorption could occur

whichmightexplainthedifferenceinresorptionbetweenthetwogroups.

2828

AcomparisoncanbemadewiththestudyofWrightetal.(2002)whousedasimilarmethodto

studytwogroupsofpatientswearingdifferenttypesofprostheses;agroupwithamandibular

overdenturestabilisedbytwoimplantsconnectedbyabarandagroupwithfixedmandibular

prostheses. With respect to the method of loading of the posterior ridges, the group with the

overdentureontwoimplantscanbecomparedwiththetwo-implantgroupinthepresentstudy.

Incaseofthefixedprosthesis,thereisnoloadingoftheposteriorridgesandonewouldexpect

alowermeanchangeinPosteriorMandibularRidgeRatio(calledthePosteriorAreaIndexin

thestudyofWrightetal.)comparedtothefour-implantgroupinthepresentstudy,inwhich

stillsomeloadingispossible.TheannualchangeinthePmandRRofthefixedprosthesesgroup

was indeed lower than the change in ratio for the group with the four implants in the present

studywhentranslatedinanannualrateof0.006.Thefixedprosthesisgroupevenshoweda

positive annual change of +0.016, indicating slight bone apposition. The mean annual change

in PmandRR for their two-implant stabilised overdenture group was 0.011, which is comparable

with the result in our group with two implants, being 0.010.

The confounding factors marginal bone loss around the implants, age, gender, initial mandibular

heightandthenumberofyearsthepatienthadbeenedentulousfailedtoshowasignificant

effect on the posterior mandibular ridge resorption.

Kordatzisetal.(2003)suggestedthatfemalegenderisariskfactorforgreaterposteriorresidual

ridgeresorption.Kordatzisetal.(2003),Wrightetal.(1998)andthepresentstudyfoundno

interactionbetweenprosthesistypeandthevariablesofgender,age,yearsofedentulousness.

In previous studies comparing two with four implants under overdentures, survival of the

implants, clinical state, marginal bone loss, patient satisfaction and surgical / prosthetic

aftercare have been compared with no significant differences detected (Stoker et al.,

2007;Visser et al., 2009;Meijer et al., 2009). For cost-effectiveness reasonsa two-implant

overdenture is therefore advised. However, this study demonstrates a slight difference in

posterior mandibular residual ridge resorption, so for preservation of the mandible a four-

implant overdenture designmight be indicated, especially for patientswho presentwith a

severely reducedposterior ridge. Further researchmust revealwhether this reduction is of

clinical relevance.

Chapter 2

29

Conclusion

It can be concluded that there is a slight difference in mandibular posterior residual ridge

resorption between patients treated with either two or four implants stabilising an overdenture.

No correlation was shown between mandibular posterior residual ridge resorption and peri-

implant marginal bone loss. The confounding factors, age, gender, initial mandibular height

andthenumberofyearsthepatienthadbeenedentulousfailedtoshowasignificanteffecton

the posterior ridge resorption

Acknowledgements

The authors of this paper wish to acknowledge the significant contribution of G.M. Raghoebar

oralandmaxillofacialsurgeon,andA.M.Ferman,SeniorDentalTechnician.

3030

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