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 .