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 .
 objective : retention and resistance of the implant - tissue supported overdenture may be affected by the type of attachment . the aim of this research was to compare the retention and resistance of nobel biocare ball ( nbb ) , nobel biocare bar and clip ( nbbc ) and sterngold era red ( erar ) attachments on an implant - tissue supported overdenture model.materials and methods : the attachment samples were divided into 3 groups of nbb , nbbc , and erar ( 5 samples in each group ) . 
 two parallel nobel biocare branemark implants were placed symmetrically at the symphysis region of a mandibular test model . 
 a metallic overdenture was fabricated precisely adapted to the model and attached to a zwick testing machine ( crosshead speed of 51 mm / min ) . 
 dislodging tensile forces were applied in three vertical , oblique , anterior - posterior directions and two situations , at the beginning and after 100 times of insertion / removal of the overdenture , for each sample . 
 the maximum dislodging force was measured . a one - way anova test 
 was employed followed by tukey s test.results:erar was the most retentive and resistant in both situations . 
 nbb and nbbc showed the same anterior - posterior resistance at the beginning . 
 all test groups represented a large amount of retention and resistance loss after the insertion / removal of the overdenture , while nbbc showed a higher loss of anterior - posterior resistance than nb.conclusion:a highest level of retention and resistance was seen in erar . 
 the retention and resistance were affected by the wear of attachments . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION