the implant - supported prostheses are attached to implant abutments either by cementation , or by retaining screws . because of the advantages of cement - retained implant - supported restorations , such as passive fit , reduced cost , ease of fabrication ,
superior esthetic , fewer components , and uncomplicated laboratory technique , these restorations have gain popularity compare to screw - retained restorations.[13 ] even though cement wash outs occur in cement - retained restorations , they do have high clinical success rates .
retrievability may be a critical aspect of implant - supported restorations because of problems such as loosening or fracture of the abutment screws , mechanical failures , treatment of peri - implant tissues , and evaluation of ailing implants mobility .
although using provisional cements are considered to achieve retrievability of implant - supported prostheses , retrievability of screw - retained prostheses is more secure than cement - retained ones . retention of implant - supported restorations plays an important role in success of the treatment .
uncemented restorations may cause problems such as inhalation of the restorations , increased bone loss around the implant , prosthesis failure breakage , trauma to antagonistic teeth , food impaction , accumulation of microorganisms resulting in bad odor and soft tissue response .
extra practitioner time and patient embarrassment have also been noted . on the other hand ,
luting agents that are too retentive may damage the osseointegration of the implant during removal of the restoration .
behavior of permanent and provisional luting agents in cementation of implant - supported prostheses differs from cementation on natural teeth .
in particular , water - based cements such as zinc phosphate , zinc polycarboxylate , and glass ionomer , have shown a wide variety of retentive values , which can sometimes be unpredictable.[61115 ] some authors suggested the use of permanent and provisional cements for luting single - unit and multi - unit implant - supported restorations , respectively .
provisional cements have been recommended for restorations that may require intervention . whereas more retentive cements , such as resin cement , are more appropriate when future retrievably is not necessary .
the use of different cements , protocols , and implant systems may alter the retentive strength of implant - supported restorations .
in addition , different aging processes , such as thermal cycling and mechanical loading , as well as different pretreatment techniques can also affect the retentive strength.[2022 ] the aim of this study was to evaluate the retention values of cement retained implant - supported restorations with different luting agents .
the null hypothesis was that there is no differenece in retentiveness of cement retained implant - supported restorations with different luting agents .
twenty iti solid abutments with 5.5 mm length and 8 taper ( 048.541 , iti dental implant system , straumann ag , basel , switzerland ) and twenty iti implant analogs ( 048.124 , iti dental implant ) were used .
the implant - abutment complex were embedded vertically in a block of autopolymerizing acrylic resin ( meliodent , heraeus kulzer , hanau , germany ) using a dental surveyor for precise alignment .
a loop of wax was added to the occlusal surface of each coping for retention test .
the wax patterns were invested in a phosphate - bonded investment ( ceravest quick , gc , tokyo , japan ) and cast in a base metal alloy ( rexillium iii , pentron , wallingford , ct ) . after divesting and cleaning with an ultrasonic cleaner and hydrofluoric acid , the inner surface of the castings
were inspected under magnification ( 4 ) , and surface irregularities were removed with a small round carbide bur .
the metal copings were checked for fitness using a silicon disclosing medium ( fit checker , gc co , tokyo , japan ) , and further potential interferences of castings were evaluated and adjusted if necessary .
each casting was randomly numbered and paired with an implant - abutment assembly for further procedures .
eight commercially available luting agents , including provisional ( temp bond , gc - free eugenol , tempspan ) and definitive ( panavia f2.0 , fuji plus , fleck 's , poly f , and fuji i ) cements were evaluated in this study [ table 1 ] .
each cement was mixed according to the manufacturer 's instruction and applied to intaglio surface of the copings ( n=10 ) .
the copings were gently seated on the abutments and held in place under a 5 kg load for 10 minutes . after initial setting of the cement , the excess cement was removed with an explorer .
the specimens were stored in a 37c incubator for 24 hours , immersed in artificial saliva for 7 days and thermocycled for 5000 cycles ( 5 - 55c ) with a 30-s dwell time . after aging process , the dislodging force of the copings was measured using a universal testing machine ( 4302 instron ltd .
, high wycombe , uk ) at a crosshead speed of 5 mm / min [ figure 1 ] .
the failure modes were recorded after the dislodgment of the copings . luted metal coping during pull out test by universal testing machine the same copings and abutments were used with each of the eight cements evaluated .
to remove the residual cement , the castings were heated to 600c for 90 minutes and allowed to cool at room temperature .
a spoon excavator was used to remove any residual cement from the intaglio surface of castings and the abutments were cleaned with a plastic explorer .
the castings and abutments were immersed in ultrasonic cleaner containing cement removal agent ( removalon - i , premier dental products co , norristown , pa ) for 30 and 15 minutes , respectively .
mann - whitney test with a bonferroni adjusted significance level of 0.001 was used for each pairwise comparison to control the overall error rate for all comparisons at an error rate of 0.05 .
mann - whitney test with a bonferroni adjusted significance level of 0.001 was used for each pairwise comparison to control the overall error rate for all comparisons at an error rate of 0.05 .
table 2 represents the mean retentive values and standard deviations in newton ( n ) , as well as the mean rank of the studied luting agents .
kruskal - wallis test revealed a significant difference between the retentive values of the studied luting agents ( p<0.001 ) .
mann - whitney test showed no significant difference between fuji plus , fleck 's , ploy f , and panavia f2.0 and these cements were superior to the provisional and fuji i cements ( p<0.001)which showed statistically the same retentive strength .
mean ( standard deviation ) dislodging forces of the studied cements ( n ) and mann - whitney groups cement failure occurred in the cement - abutment interfaces in all of the fuji plus , panavia f2.0 , and fleck 's samples .
zinc polycarboxylate and zinc oxide with and without eugenol cements remained on both the copings and abutment surfaces whereas tempspan completely remained on the abutment surfaces .
in this study , the null hypothesis that the retentiveness of the metal copings would not affect with the studied cements was rejected .
the resin modified glass ionomer cement ( fuji plus ) had the highest retention , but there were no significant differences between definitive cements .
some studies indicate that the retentive strength of resinous cement is superior to that of , zinc phosphate and zinc polycarboxylate cements .
the higher retentive strength of the resin cements was attributed to the adhesive system which was not used in the current study .
furthermore , resin modified glass ionomer cement adheres to metal by chelating metallic ions , but the retentive strength may be weakened by early water contact . in this study
, the copings were incubated for 24 hours at 37c before the aging process and there was no early water contact .
in a study on titanium copings over short iti solid abutments , the mean retentive values of zinc phosphate cement was comparable to panavia 21 , but the retentive strength was higher than in the present study .
a combination of panavia 21 and titanium alloy was reported to have higher bond strength compared to other alloys . in another study on the retention of base metal copings with dental implants ,
zinc phosphate and resin modified glass ionomer cements showed same retention strength . with regard to the ceraone system , zinc phosphate cement had highest retention value , and resinous cement presented statistically similar retentive strength . in a similar investigation on retention values of metal copings on iti solid abutments ,
interestingly , zinc phosphate and zinc oxide eugenol cements had similar retentive strength . the different results with the present study maybe due to using different cements and thermal cycles .
long - term thermal cycling has been shown to reduce the retentive strength of luting agents .
squier et al . reported a higher retentive value for panavia than zinc phosphate and resin modified glass ionomer , which differs from the current study .
this discrepancy may be related to sandblasting and surface conditioning in this study . surprisingly , glass ionomer cement had the lowest retention among the definitive luting agents , which was comparable to the temporary cements .
this finding is consistent with a previous study that demonstrated glass ionomer cement showed significantly lower retention than zinc phosphate and resinous cements , and similar retention to zinc oxide cement without eugenol . in the present study ,
no pretreatment was performed on either the copings or the abutments , and glass ionomer cement does not adhere to an inert surface .
furthermore , the solubility of glass ionomer cement is more than that of other cements , and it is very susceptible to early water contact and desiccation , which can dramatically reduce the mechanical properties of the cement .
the retentive strength of restorations is influenced by taper , surface area , height and surface roughness of the abutment , restoration characteristics , type of cement , marginal accuracy and the treatment method .
thus a direct comparison with other studies may be difficult because of differences in the implant systems , specimens , and methodology .
adhesion of the cement to abutment may be difficult to remove and damage the abutment surface . in this study ,
the resinous and resin modified glass ionomer cements completely adhered to the intaglio surface of the copings .
the surface of implant abutment is relatively smooth , which caused remaining zinc phosphate cement in the inner surface of the copings .
the primary failure mode for the polycarboxylate cement was adhesive , but there was also some cohesive failure .
the temporary cements , with the exception of tempspan , adhered to both inner surface of the copings and the abutment surfaces .
our results suggest that the tested definitive cements are not desirable if retrievability of single - unit metal restorations is sought .
the dislodgement force for an implant with 4-mm diameter and 6 mm height directly out of the socket along its long axis was about 290 n. in this study the snap - on mechanism of the burn - out cap was removed by a reamer from the inner surface of the castings , which resulted in a passive fit .
the force that is used to retrieve the implant - supported restoration is high impact and of short duration . on the other hand , in this study
the failures of the restorations result in several comparative small dynamic loadings and the axial dislodging of cemented implant crowns is a rather seldom clinical event .
the results of this in vitro study should be interpreted with caution and confirmed with clinical studies .
within the limitations of this study , the following conclusions can be drawn : 
 the most retentive luting agent was resin - modified glass ionomer , but there was no significant difference between this cement and panavia 2.0 , zinc phospate , and zinc polycarboxylate cements .
these definitive cements are recommended for luting single - unit implant - supported metal restorations.the temporary and glass ionomer cements were the least retentive cements and might not be suitable for luting single - unit implant - supported restorations . 
 
the most retentive luting agent was resin - modified glass ionomer , but there was no significant difference between this cement and panavia 2.0 , zinc phospate , and zinc polycarboxylate cements .
these definitive cements are recommended for luting single - unit implant - supported metal restorations .
the temporary and glass ionomer cements were the least retentive cements and might not be suitable for luting single - unit implant - supported restorations .
 background : with regard to potential retrievability of cement - retained implant restorations , the retentive strength of the luting agents is critical . the aim of this study was to evaluate the retention values of implant - supported metal copings using different luting agents.materials and methods : twenty iti implant analogs and solid abutments of 5.5-mm height were embedded vertically in autopolymerizing acrylic resin blocks . 
 metal copings with a loop on the occlusal surface were fabricated using base metal alloy ( rexillium iii ) . 
 the copings were luted using eight cements with different retention mechanisms ( panavia f2.0 , fuji plus , fleck 's , poly f , fuji i , temp bond , gc - free eugenol , and tempspan ) under static load of 5 kg ( n=10 ) . 
 all specimens were incubated at 37c for 24 hours , conditioned in artificial saliva for 7 days and thermocycled for 5000 cycles ( 5 - 55c ) . 
 the dislodging force was measured using a universal testing machine at a crosshead speed of 5 mm / min . 
 statistical analyses were performed using kruskal - wallis ( =0.05 ) and mann - whitney tests with bonferroni correction ( =0.001).results : fuji plus and tempspan had the highest and the least mean retentive strength , respectively ( 320.97161.47 , 3.392.33 ) . 
 there was no significant difference between fuji plus , fleck 's , ploy f , and panavia f2.0 . 
 these cements were superior to provisional cements and fuji i ( p<0.001 ) which showed statistically same retentive strength.conclusion:within the conditions of this study , the resin modified glass ionomer , zinc phosphate , zinc polycarboxylate , and panavia f2.0 had statistically the same retentive quality and are recommended for definitive cementation of single implant - supported restorations . 
 the provisional cements and glass ionomer may allow retrievability of these restorations . 
INTRODUCTION
MATERIALS AND METHODS
Statistical analysis
RESULTS
DISCUSSION
CONCLUSIONS