The effect of 2 versus 4 implants on implant stability in mandibular ... · CLINICAL RESEARCH The...

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THE JOURNA

The effect of 2 versus 4 implants on implant stability inmandibular overdentures: A randomized controlled trial

Wafa’a R. Al-Magaleh, BDS, MS, PhD,a Amal A. Swelem, BDS, MS, PhD,b and Iman A. W. Radi, BDS, MS, PhDc

ABSTRACTStatement of problem. Dental research is rich with articles that investigated the influence of host-site variables, some implant-related variables (implant length, diameter, taper, design, location, andsurface topography), different loading protocols or surgical procedures, and measurementmethodology on dental implant stability. However, the number of implants and its effect onimplant stability remain unclear.

Purpose. The purpose of this randomized clinical trial was to investigate the influence of implantnumber on implant stability by comparing 2 versus 4 implants in mandibular implant overdentures.

Material and methods. The trial included 20 participants with edentulous mandibular ridges.Participants were randomly assigned to 2 equal groups, a 4-implant (experimental) group consistingof 4 implants installed in lateral-canine and premolar regions; and a 2-implant (control) group,consisting of 2 implants in lateral-canine regions. Implant stability was measured usingresonance frequency analysis at implant placement and then at 1, 3, 6, 9, and 12 months. TheStudent t test was used to compare the implant stability quotient (ISQ) values of the anteriorimplants in the 4-implant and 2-implant groups. One-way ANOVA followed by the post hocBonferroni test was used to compare ISQ values among the different follow-up periods withineach group (a=.05).

Results. Mean ISQ values for anterior implants in the 4-implant group were slightly higher thanthose recorded for the 2-implant group at all follow-up periods. However, these differences werenot statistically significant (P>.05). Within-group comparison revealed an initial decrease inimplant stability for all implants. This decrease was statistically significant for the 2-implantgroup (P<.001) and for posterior implants in the 4-implant group (P<.001). This was thenfollowed by a gradual increase in ISQ values for all implants in both groups.

Conclusions. Increasing the number of implants from 2 to 4 in mandibular implant overdenturesdid not have a significant influence on implant stability. (J Prosthet Dent 2017;118:725-731)

Implant stability is an essentialfactor for achieving successfulosseointegration and, therefore,the long-term success of dentalimplants.1e4 Accurate measure-ment and quantification ofimplant stability has become afundamental procedure as itprovides valuable diagnosticinsight that helps dictate thetime of functional loadingand project a long-term prog-nosis to ensure successfultreatment.5e7 Implant stability isusually divided into 2 stages:primary and secondary.3 Pri-mary implant stability hasproved to be a mechanicalphenomenon as a result of me-chanical engagement of animplant in the surroundingbone immediately after place-ment; secondary implant sta-bility is attributed to thebiological stability gained with
bone regeneration and remodeling (osseointegration)around the implant.6 Several methods have been imple-mented to assess implant stability. Resonance frequencyanalysis (RFA) has gained popularity over the past years.8 Ithas proved to be straightforward, reliable,7,9e12 noninva-sive,13,14 andmore sensitive thanother diagnosticmethods,such as damping capacity assessment or the periotest.15,16
ofessor, Quality Assurance Committee, Prosthodontic Department, Facultyofessor, Oral and Maxillofacial Prosthodontic Department, Faculty of Dentrosthodontic Department, Faculty of Oral and Dental Medicine, Cairo Univrosthodontic Department, and Evidence Based Dentistry Centre, Faculty o

L OF PROSTHETIC DENTISTRY

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The dental publications are rich with articles that haveinvestigated the influence of host-site variables, such asbone quality and quantity9,10,17e20 and the anatomic re-gion of the jawbone21,22; implant-related variables, suchas implant length,18,20,23 implant diameter,24,25 implanttaper,26 implant design, and surface topography12,25e30;different loading protocols24,31e34; different surgical35e39

of Oral and Dental Medicine, Sana’a University, Yemen.istry, King Abdulaziz University, Jeddah, Saudi Arabia; and Associate Professor,ersity, Egypt.f Oral and Dental Medicine, Cairo University, Egypt.

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Table 1. Baseline characteristics of included participants

Characteristic 4-Implant Group 2-Implant Group P*

Males/females 5/5 6/4 .861

Age (y) 53.7 ±6.3 51.4 ±5.7 .614

Bone density (HU) 760 ±150 779 ±170 .129

Anterior bone height (mm) 18.4 ±1.9 18.6 ±2.01 .743

Posterior bone height (mm) 14.5 ±2.4 15.2 ±1.7 .552

Anterior bone thickness (mm) 6.9 ±0.7 7.1 ±0.4 .431

Premolar bone thickness (mm) 10.6 ±1.2 10.9 ±1.01 .768

Data presented as mean ±SD, unless stated otherwise. *Based on chi-square test for sexand age and t test for bone parameters.

Clinical ImplicationsThe placement of 2 implants to support and retain amandibular overdenture seems to provide acost-efficient treatment approach that does notjeopardize implant stability over a 1-year follow-upperiod.

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and augmentation procedures40e45; and the effect ofmeasurement methodology3,18,46e48 on dental implantstability. However, the number of implants and its effecton implant stability remains unclear. The distribution offunctional load on a greater number of implants mayhave a favorable influential effect on the biomechanicalstability of each individual implant. Nevertheless, thevalidity of this assumption has not yet been tested.Therefore, the objectives of this randomized controlledtrial were primarily to determine the influence of implantnumber on implant stability by using RFA to compare 2versus 4 implants in mandibular implant overdenturesand secondarily to monitor longitudinally the develop-ment of implant stability throughout a 1-year studyperiod. The null hypothesis was that increasing thenumber of implants from 2 to 4 would have no significantinfluence on implant stability.

MATERIAL AND METHODS

The study was a randomized controlled trial with a par-allel design, an equivalence frame, and a 1:1 allocationratio in which the 20 included participants wererandomly allocated to 1 of 2 equal groups, a 4-implant(experimental) group, in which 4 implants were placed,consisting of 2 anteriors (lateral incisor-canine region)and 2 posteriors (premolar region) on both sides of thearch; and a 2-implant (control) group, in which only 2implants (the anteriors) were installed. The study pro-tocol was approved by the Research Ethical Committee,Faculty of Oral and Dental Medicine, Cairo University(approval number 15/9/21).

The study was conducted in women and men (mean52.5 years of age; median 54 years of age; range, 47 to 60years of age) in whom 60 tapered screw vent implants(TSVB11; Zimmer Dental Inc) were placed. The baselinecharacteristics are shown in Table 1. Participants wererandomly selected from 43 eligible individuals whovisited the Prosthodontic Department, Faculty of Oraland Dental Medicine, Cairo University (June 2013 to June2014). Eligibility of the participants was based on clinicaland panoramic radiographic examinations. Eligible par-ticipants were those with edentulous mandibular ridgesopposed by a complete maxillary dentition (naturalmaxillary teeth or restored with fixed restorations)assuming normal maxillomandibular relationship (class I

THE JOURNAL OF PROSTHETIC DENTISTRY


Angle classification) and with adequate mandibular bonethat allowed the placement of implants, 11.5 mm inlength and 3.7 mm in diameter. Ridge mapping was doneat the proposed implant sites. Diagnostic tooth ar-rangements for all participants were also prepared toensure a crown height space of at least 12 mm toaccommodate the ball attachments at the suggestedareas. Participants with parafunctional habits or withsystemic diseases that contraindicated implant placementand heavy smokers (>10 cigarettes/day) or who hadreceived local radiotherapy were excluded. All partici-pants signed an informed written consent.

Initially, all participants received a conventionalmandibular complete denture fabricated with a bilaterallybalanced occlusal concept against the adjusted occlusionof the existing or restored dentition. For proper planningof implant placement at the proposed sites, a cone beamcomputed tomography scan of the mandible of eachparticipant was made while the participant was wearing aradio-opaque duplicate of the mandibular denture asa radiographic stent, which was then modified to act as asurgical guide.

To study the effect of 2 (control) versus 4 (experi-mental group) implants on implant stability in mandib-ular overdentures with an allocation ratio of 1:1, thesample size was calculated using software (G* Powerv3.1.9.2; Universität Düsseldorf). This was based on astudy conducted by Barewal et al,10 in which implantstability within each group was normally distributed witha standard error of 0.7. If the true mean difference be-tween the experimental and control groups was 1.6, 14participants were needed, 7 in each group, to be able toreject the null hypothesis that the population means ofthe experimental and control groups were equal withprobability (power) of .85. The Type I error probabilityassociated with this test was .05. In an attempt to over-come the attrition bias, a 30% increase was accountedfor, dictating a sample of 20, 10 in each group. A com-puter-generated random sequence for the numbers 1 to20 was developed in a 2-column table using http://random.org, a site for creating tables with randomnumbers. Each column contained 10 numbers and rep-resented a treatment group. Allocation concealment was

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http://random.org

http://random.org

Figure 1. Ball abutments in place in 4-implant group. Figure 2. Implant stability measurement after tightening smart peg toimplant.

December 2017 727

ensured by allowing each participant to draw a numberthat was enclosed within an opaque sealed envelope.This was done at the time of surgery, after the 2 anteriorimplants had been placed, to eliminate the possibility ofperformance bias and to overcome the problem ofblinding by the operator.

All participants were given 2 g of antibiotic (amoxi-cillin capsules; Teva Canadal Ltd) 1 hour before surgeryin addition to an analgesic and anti-inflammatorymedication (ibuprofen, 400 mg tablets; Shasun Chem-icals & Drugs Ltd) to be taken every 8 hours for 2 daysafter the surgery and 0.2% chlorhexidine (Chlorhexidine;Kahira Pharm & Chem Ind Co) mouth wash for 1 weekafter surgery. All implants were placed under localanesthesia by 2 experienced prosthodontists (W.R.A.,I.A.R.). As dictated by the random numbers, either 2 or 4implants were placed using the surgical guides. Beforethreading the covering screw, the baseline implant sta-bility measurement was made. All participants weregiven similar written postoperative instructions andmedications. Three months after insertion, the implantswere exposed, and healing collars (HC333; ZimmerDental Inc) were fastened to the implants for 2 weeks.They were then removed, and ball attachments (BAC4;Zimmer Dental Inc) were screwed in (Fig. 1). While theparticipants closed in centric occlusion, standard directpickup procedures were done to incorporate nylon capsand metal housings within the previously fabricateddentures.49

Implant stability was measured by using an externalassessor for all implants of both groups immediatelyfollowing implant installation (0 month) and then at 1, 3,6, 9, and 12 months postoperatively. It was measuredusing the Osstell (Osstell ISQ; Osstell AB Gamles-tadsvägen) implant stability quotient (ISQ) measurer byinserting a standardized transducer (a magnetic peg;Smartpeg; Integration Diagnostics AB, Gamlestadvägen3B) of fixed length into each implant (Fig. 2). The device’s

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probe was held perpendicularly to the implant’s long axisat a distance of 2 to 3 mm from the transducer to avoidmeasurement inconsistencies. This was carried out at thebuccal, lingual, mesial, and distal aspects to measurerespective implant stability from all implant sides. Tosimplify the data, the average of the readings of the 4sites was then recorded to represent the stability of eachimplant. The records of all average readings for bothgroups at the different follow-up periods were tabulated.The participants, operators, and assessor were not blin-ded because of the obvious difference between thenumbers of installed implants in the groups.

The statistical analysis was conducted by blindedresearcher (A.A.S.) using statistical software (SPSS forWindows, v16.0; SPSS Inc). Descriptive statistics (arith-metic mean, standard deviation) were used to summarizethe data. The ISQ values of the right and left anterior andthen posterior implants were compared using a paired ttest. Because no statistical significance was found be-tween the right and left implants, the ISQ values of bothsides were averaged for further analysis. The data weretested using the Shapiro-Wilk test and found to benormally distributed. Accordingly, the t test was used tocompare the ISQ values of anterior implants in the4-implant group with those of the implants in the2-implant group. A paired t test was used to compare theanterior and posterior implants in the 4-implant group.Two-way ANOVA was performed to investigate group-time interaction. Because no interaction was found(F=2.207; P=.070), 1-way ANOVA followed by the posthoc Bonferroni test was performed to investigate theeffect of time on the ISQ values among the differentfollow-up periods within each group (a=.05).

RESULTS

All participants attended all follow-up visits, and theirresults were analyzed (Fig. 3). No implant was lost, giving



Individuals assessed for eligibility, N=70

Individuals were excluded (n=27):required an opposing removable

prosthesis (n=7), uncontrolleddiabetes (n=5), crown

height space less than 12 mm(n=3), smokers (n=9), Angle Class III(n=1), and muscle hypertrophy and

severe attrition in artificial teethfrom previous dentures (n=2)

Eligible individuals, n=43

Random selected participants, n=20

Participants underwent randomization, n=20

4-Implant Group:10 participants (40implants) with noloss to followup

2-Implant Group:10 participants (20implants) with noloss to followup

Implants includedin the analysis, n=40

Implants includedin the analysis, n=20

Figure 3. Participants at different study stages.

Table 2. ISQ implant stability values of anterior implants in 4-implantgroup compared with those in 2-implant group at different follow-upperiods (mean ±SD)

Time ofMeasurement(mo)

4-Implant Group(n=10)

2-Implant Group(n=10) P

95% CI of MeanDifference

At insertion (0) 68.2 ±4.2 66.5 ±3.2 .354 -1.94 to 5.16

Post Insertion

1 65.6 ±2.6 64.3 ±2.9 .294 -1.26 to 3.94

3 67.0 ±3.4 66.6 ±3.6 .787 -2.86 to 3.72

6 68.9 ±3.7 67.5 ±4.0 .419 -2.19 to 5.03

9 71.6 ±3.7 68.8 ±5.4 .203 -1.62 to 7.10

12 71.8 ±2.1 68.6 ±5.8 .116 -0.88 to 7.30

CI, confidence interval; ISQ, implant stability quotient; m, month; SD, standard deviation.Not significant at P<.05.

Table 3. ISQ implant stability values of implants at different follow-upperiods within each group (mean ±SD)

Follow-up Period(mo)

4-Implant Group

2-Implant Group(n=10)

Anterior(n=10)

Posterior(n=10)

0 68.2 ±4.2abc 68.0 ±5.5ac 66.5 ±3.2a

1 65.6 ±2.6a 64.5 ±4.7b 64.3 ±2.9b

3 67.0 ±3.4b 67.0 ±4.9a 66.6 ±3.6a

6 68.9 ±3.7c 69.6 ±5.4c 67.5 ±4.0a

9 71.6 ±3.7d 71.9 ±5.2d 68.8 ±5.4a

12 71.8 ±2.1d 72.0 ±5.3d 68.6 ±5.8a

ISQ, implant stability quotient; SD, standard deviation. Different superscript letters arestatistically significant; those with similar letters are insignificant (P<.05), based onBonferroni pairwise comparison test.


a 100% survival rate for both groups. At any givenobservation period, the mean ISQ values of all implantsranged between 66.5 and 72.0.

The mean ISQ values recorded for anterior implantsin the 4-implant group were slightly higher than thoserecorded for implants in the 2-implant group at allfollow-up periods. However, these differences were notstatistically significant (P>.05) (Table 2). Statistical anal-ysis revealed nonsignificant differences between meanISQ values recorded for anterior and posterior implantsin the 4-implant group (P>.05).

Table 3 shows the within-group changes in implantstability of all implants in both groups. For all implants,and regardless of their site, initial (0 month) implantstability values were higher than those of the 1-monthISQ values. This apparent decrease in implant stabilitywas statistically significant for the 2-implant group(P<.001) and for the posterior implants in the 4-implantgroup (P<.001). This decrease was then followed by agradual increase in implant stability values for implants ofboth groups. At the end of the follow-up period(12 months), ISQ values in the 4-implant group reached71.8 ±2.1 for anterior implants, 72.0 ±5.3 for posteriorimplants, and 68.6 ±5.8 for the 2-implant group. This



increase was statistically significant for implants in the4-implant group, whether anterior (P<.001) or posterior(P<.001), at the 9 month follow-up period, after whichthe increase became nonsignificant (P>.05). Neverthe-less, at the 12th month, it maintained a significant dif-ference compared with baseline for both anterior(P<.024) and posterior (P<.001) implants. For the2-implant group, the closest mean ISQ value to baselinewas reached at 3 months after insertion; it then graduallyincreased but not significantly (P>.05) until the end of thefollow-up period.

DISCUSSION

Results of the current study revealed no statistically sig-nificant differences between the 2 groups for both pri-mary stability (represented by ISQ values at 0 month)and secondary stability (represented by ISQ values from1 to 12 months). Hence, the results support the nullhypothesis and suggest that the number of implants maynot be as influential on implant stability as other factors.

ISQ values above 54 represent clinically acceptablestability.18,25,28,50 Therefore, all implants in the presentstudy were and remained clinically stable at placementand throughout the 1-year study period with a meanISQ range of 66.5 to 72.0, indicating that good pri-mary and secondary stability and, hence, successful

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osseointegration had been achieved. This explains the100% survival rate.

The nonsignificant differences found between the 2groups can be attributed to different factors. Primary sta-bility is a mechanical phenomenon6 and is mainly influ-enced by the bone density of the osteotomy site.9,10,18,22,25

It could also be influenced by the surgical technique5,10 andimplant-related factors especially implant length.18,20,25

During healing, this primary stability is replaced by a bio-logical bonding of newly formed bone to the implant sur-face and is then termed secondary stability. The degree ofwhich is determined by the dynamic changes taking placeduring the tissue-integration process, such as bonemodeling and remodeling at the bone-implant interface.6

Evidently, this healing process will also be influenced bythe bone morphology51 and the total surface area of theimplant.20 In the current study, bone density values of bothgroups were not found to be significantly different. Otherpotentially influential factors, namely: implant design,implant size, opposing dentition, loading protocol andsurgical technique, were all standardized in the presentstudy. All implants were installed in the mandible.Consequently, all these aspects may have attributed to thenonsignificant differences between stability readings of thegroups. Considering the elimination of the effect of suchconfounding variables, the results imply that the additionof 2 posterior implants did not play a significant role infavor of the stability of the anterior implants. Even thoughmean ISQ values of anterior implants in the 4-implantgroup were slightly higher than those of the 2-implantgroup throughout the study, yet this cannot be safelyattributed to the presence of the posterior implants. Simplybecause the mean ISQ values of the anterior implants wereinitially higher at baseline in the 4-implant group(68.2 ±4.2) than in the 2-implant group (66.5 ±3.2).

Within-group comparisons revealed a common stabilitypattern for all studied implants, regardless of the group orsite, represented by a considerable initial decreasein implant stability during the first month followed bya gradual increase during the subsequent follow-up pe-riods. This pattern seemed consistent with previousstudies.3,5,10,14,25,26,41,52 The initial decrease was statisticallysignificant for the 2-implant group and for posterior im-plants in the 4-implant group. However, the decrease inimplant stability of anterior implants in the 4-implantgroup during this period was not significant, which couldreflect the relative importance of the posterior implants inthe initial periods of bone healing. The phenomenon of thistransient decrease in implant stability during the initialhealing period has been termed as a “dip” or “drop” instability41 and is thought to correspond to the transitionfrom primary to secondary stability.52 Studies havedemonstrated that this stability dip is commonly observedbetween the second and eighth weeks after inser-tion5,10,14,31,33,41 with a peak drop detected during the third

Al-Magaleh et al


or fourth week5,10,41 and ranged from 2 to 12 ISQvalues.14,25,33,41 The findings of the current study coincidewith these 2 observations. The significant decrease inimplant stability was indeed recorded during the firstmonth, and it was within the reported range, where itdecreased by 2.6 ISQ values (3.8%) for anterior implants inthe 4-implant group, 3.5 ISQ values (5.1%) for posteriorimplants in the 4-implant group, and 2.2 ISQ values (3.3%)for the 2-implant group. This dip phenomenon has beenbroadly discussed andhas been proposed as a consequenceof the dynamic early modeling-remodeling phase whichinvolves resorption of the necrotic bone adjacent to theimplant after surgery and its replacement by a biologicalattachment of woven bone.18,53 This in turn results inminimal bone-implant contact, and consequent reductionin the mechanical anchorage. Results of the present studyconfirm this phenomenon.

This stability dip has been correlated to primaryimplant stability values. Several investigators came to theconclusion that only implants with high primary stabilityexhibited that decrease in stability during the initialhealing phase, whereas implants with low primary sta-bility showed a significant increase.14,50 Nedir et al50 re-ported that implants with ISQ values <60 at baselineexhibited a stability increase, whereas implants with ISQvalues between 60 and 69 exhibited the stability dip. Theimplants then approached their initial stability values atthe end of their 12-week study period. This was typicallydemonstrated in the current study as the mean primaryISQ values were 68.2 for anterior implants in the4-implant group, 68.0 for posterior implants in the4-implant group, and 66.5 for the 2-implant group, andthe 3-month mean ISQ values were 67.0, 67.0, and 66.6,respectively. However, an increase in stability readings inthe initial 2 weeks has been reported.29 It seems thatparameters of the dip can be influenced by other factors,such as implant surface properties,28,29 and timing offunctional loading,18,24 indicating that stability changesduring the initial healing phase is a multifactorial phe-nomenon that cannot be generalized for all implants andclinical scenarios.

The presumed interval between mechanical stabilityoriginally achieved by macro-retentions of the implants(threads) and direct bonding of newly formed wovenbone onto the implant surface possibly takes place from 2to 4 weeks.54 Subsequently, the osseointegration processand maturation of woven bone into lamellar bone takesplace and progresses, thereby establishing biologicalbonding and increasing secondary stability by time.54

This could have contributed to the gradual increase thatwas observed in the ISQ values at the subsequent follow-up periods of the present study. Compared with valuesfrom the 1-month follow-up, 12-month values reflected amean ISQ increase of 6.2 (9.5%) for anterior implants inthe 4-implant group, 7.5 (11.6%) for posterior implants in




the 4-implant group, and 4.3 (6.7%) for the 2-implantgroup. Compared with baseline values, this increasewas statistically significant (P<.001) for implants in the4-implant group after 9 months of healing, with aninconsiderable increase thereafter. However, this in-crease was not statistically significant (P>.05) in the2-implant group throughout. The distribution of func-tional loading among 4 implants instead of only 2implants, could have been in favor of the remodelingprocess and might have speeded up the biologicalresponse around implants in the 4-implant group.However, this within-group significance was not pro-found enough to create a significant difference betweenthe two groups as previously discussed. This observationhas been confirmed by the nonsignificant group-timeinteraction reported earlier.

Comparison between anterior and posterior implantsin the 4-implant group revealed no significant differences(P>.05) between their mean ISQ values at all time points.Balshi et al5 reported the same conclusion for mandibularimplants but not for maxillary implants. Whereas, Bischofet al24 found that implant location had no influentialeffect on implant stability for either mandibular ormaxillary implants.

The number of participants did not allow for a posthoc subgroup analysis to explore the effect of sex or bonedensity on implant stability. Therefore the results of thecurrent study should be interpreted with caution andcannot be generalized. Including more mandibular im-plants (> 4) or conducting a similar study with maxillaryimplants (where bone density is typically lower) mayreveal significant differences, considering that overallstability levels have been found to be higher in themandible than in the maxilla.5,10,21,22,24 Hence, furtherwork with an increased sample size and subgroup anal-ysis, different clinical scenarios, and longer follow-upsare recommended to provide generalizable conclusions.

CONCLUSIONS

Within the limitations and time frame of this randomizedcontrolled trial, the following conclusions were drawn:

1. Increasing the number of implants from 2 to 4 inmandibular implant overdentures did not have asignificant influence on implant stability.

2. Longitudinal measurement of implant stability us-ing RFA demonstrated a decrease in implant sta-bility during the initial healing period (1 month)followed by a gradual increase in the subsequenthealing periods.

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Corresponding author:Dr Amal Ali SwelemFaculty of DentistryKing Abdulaziz UniversityJeddahSAUDI ARABIAEmail: [email protected]

AcknowledgmentsThe authors thank Dr Ashraf Abdul-Moneim, Professor of RemovableProsthodontics, Faculty of Oral and Dental Medicine, Cairo University, forhis valuable advice.

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











































































































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