denture stability is believed widely to be related to resistance against other forces like oblique and anterior - posterior forces . the patient s satisfaction is directly influenced by the amount of denture retention as it has been shown through several studies [ 24 ] . the need for correcting the patient s problems with faulty denture is an inevitable consequence of retention failure and residual ridge resorption . several different strategies have been introduced to overcome the problem , one of which is the use of dental implants . three distinct methods of prosthetic treatments based on implants in the lower jaw are : 
 implant supported fixed prosthesis ( fp-1 , fp-2 , fp-3)removable implant supported overdentures ( rp-4)combined implant - tissue supported over - dentures ( rp-5 ) . implant supported fixed prosthesis ( fp-1 , fp-2 , fp-3 ) removable implant supported overdentures ( rp-4 ) combined implant - tissue supported over - dentures ( rp-5 ) . the third of the above mentioned methods has the advantage of reducing the number of implants with a more simplified prosthetic approach . intra - osseous implants are considered as acceptable abutments in conjunction to attachments for efficient retention of the overdentures . factors involved in choosing the type of attachment include available space , maintenance necessity , spare part availability and ease of change , force distribution to the soft tissue or implant and the level of retention . another important fact is the level of wear seen in attachments following the use of overdenture . the aim of this research was to compare the effects of three different attachments ; nobel biocare ball ( nbb ) , nobel biocare bar and clip ( nbbc ) and sterngold era red ( erar ) on the retention and resistance of implant - supported overdentures . this in vitro investigation was performed on a total of 15 samples divided into three groups of five . each group was designed to include the attachments with erar ( 0 degree era micro np abutment only , 1 mm cuff and micro era male - red overdenture 5 , sterngold , usa ) , the attachments with nbbc ( gold adapt & gold abutment bar and clip attachment , nobel biocare , sweden ) and attachments with nbb ( ball abutment titanium , branemark system , rp 1 mm and plastic cap , nobel biocare , sweden ) . all samples were tested for the retention and resistance of overdenture on vertical , oblique and anterior - posterior tensile force load . recordings were made after the first insertion of the overdenture ( sit . 1 ) and also following 100 times of insertion / removal process ( sit . a pair of implants , 3.7510 mm , ( mk iii , branemark system , nobel biocare , sweden ) was inserted at both sides of the midline at the symphysis region in a parallel position . a milling machine ( paraskop m , bremen , germany ) was employed to avoid any fault in the angulation of insertion ( fig 1-a ) . a framework of cobalt chromium alloy ( wiironit , bego , bremen , germany ) was fabricated to fit on the two implants and the model . metallic structure was used for its durability and stability in all stages of the experiment . four stainless steel bands were soldered to the framework , which was positioned at the anterior part of the overdenture close to the implants . the others were established on the midline in the buccal and lingual side of the ridge . the overdenture was attached to the testing machine through the hooks ( fig 1-b ) . a prototype of overdenture housing was then constructed over the metallic overdenture by use of a light cure acrylic resin ( vlmc , vita , badsackingen , germany ) ( fig 1-c ) . all overdenture housings were duplicated from the prototype and fabricated by use of a self cure acrylic resin ( orthocryl , dentaurum , ispringen , germany ) ( fig 1-d ) . therefore , an acrylic overdenture housing was made for each sample ( totally 15 ) . one part of the attachment was attached into the overdenture housing and the other part , the abutment , was screwed on the implant ( fig 1-e ) . after the confirmation of appropriate sitting of the overdenture and adaptation of attachments , a series of measurements were started for each sample . recordings were made at 51 mm / min speed similar to that of denture move at bite and mastication . vertical force was measured to assess the retention while oblique and anterior - posterior forces were evaluated for resistance testing . maximum dislodging force ( mdf )   was recorded at the point of complete detachment of the overdenture from the model . data were finally subjected to an analysis using one - way anova test as well as tukey s test . based on the results derived from the collected data , the clinical behavior of three different attachments tested were not similar during application of different forces . as the data was normally distributed , one - way anova test was performed to compare the differences among three attachments for their force tolerance . at the beginning ( sit . 1 ) , tukey s test revealed that there was significant differences between erar / nbb , erar / nbbc and nbbc / nbb for vertical and oblique tensile strengths ( p<0.001 ) . this result was reemphasized by one - way anova test ( p<0.001 ) . therefore , erar had the highest retention and oblique resistance and nbb had the least . for anterior posterior tensile strength , tukey s test showed significant differences between erar / nbbc and erar / nbb ( p<0.001 ) , while there was no significant difference between nbbc and nbb ( p=0.528 ) . so erar had the highest anterior - posterior resistance but nbbc and nbb had the same . figs 24 show mdf measured for these attachments under vertical , oblique and anterior - posterior tensile forces in both situations ( sit . 1 and 2 ) . 2 , these data were statistically significant in their differences among different attachments for vertical and oblique tensile strengths , using one - way anova test ( p<0.001 ) . however , tukey s test on nbbc / nbb showed no statistically significant difference for vertical ( p=0.198 ) and oblique tensile strengths ( p=0.462 ) . erar had the highest retention and oblique resistance , while nbbc and nbb had the same . a further kruskal - wallis test performed on three groups in anterior - posterior tensile strength showed significant differences ( p<0.01 ) . the paired t - test performed to evaluate the condition of the three attachments in two different situations ( sit . 1 and sit . this indicated that the insertion / removal of overdenture for 100 times caused the differences between sit . 1 and sit . 2 in all three groups ( erar , nbbc and nbb ) . over the past three decades the use of implant - tissue supported overdenture has raised considerably among dentists . it is mainly related to the rise in patient awareness and availability of attachments to the dentist . several force directions are considered as applied in the mouth during function of the overdenture with retention being the most critical issue . two important points of view are considered on the retention of prosthesis , including the patient s feeling and the dentist s assessment . the latter which is measured during maximum load force application was surveyed in this research . 1 , erar has the highest resistance against vertical , oblique and anterior - posterior tensile forces . this means that overdenture has a higher retention and resistance potential with a significant difference from the other two attachments . this difference could be due to the design of erar as being intra - radicular . this is explained further by a female segment on the abutment of the attachment in addition to high and parallel abutment walls ( 5 mm length ) . the nbb showed the least mdf against vertical , oblique and anterior - posterior tensile forces . this is concluded to be due to the extra - radicular design in addition to free movement of the male component inside the female component . retention and resistance of nbb was in the least level while erar was at the most with significant differences ( p<0.001 ) . anterior - posterior resistance of erar was also at the highest level as mentioned before , while no differences were established between nbbc and nbb ( p=0.528 ) . petropoulos   has reported a roughly similar result on nbb and nbbc characteristics while results reported on erar were different from the result achieved in this research . this could be due to the use of sterngold era of intra - radicular type which was not the case in the earlier mentioned study . 2 ) showed a considerable reduction in retention and resistance of overdenture on attachments possibly due to constant wear ( p<0.01 ) . this difference was statistically significant even in several insertion/ removal tests which the level of mdf against vertical , oblique and anterior - posterior forces was higher in erar than the others ( p<0.001 ) . in sit . 2 , there were no differences in retention and oblique resistance of nbb and nbbc ( p=0.198 , p=0.462 ) , but the anterior - posterior resistance of nbb was more than nbbc ( p<0.01 ) . it could also be explained by the material the attachments were made of , resulting in a higher resistance to wear in nbb compared to nbbc . based on the results of this research , clinical behaviors of these attachments ( erar , nbbc , nbb ) have been well identified . therefore , the dentist can choose an appropriate attachment in implant - tissue supported overdentures according to the clinical situation . it is recommended to use erar attachment when a high amount of retention and resistance is necessary . for example , for a patient who has severe residual ridge resorption or used to remove and insert the denture frequently , erar attachment should be applied . nbb and nbbc attachments would be the choice when there is low demand of retention and resistance for overdenture . insertion and removal of overdenture gradually causes loss of retention and resistance due to the attachment s wear .