implant - supported overdentures provide a good opportunity for dentists to improve the quality of life and oral health .
although ridge augmentation can help to restore ridge volume , grafting procedures can significantly increase patient morbidity , costs , and treatment time [ 35 ] .
narrow - diameter implants are generally used for alveolar ridges that are thin for regular implants with a diameter of approximately 4.0  mm to avoid advanced surgical procedures , such as local bone augmentation [ 69 ] .
they are also indicated when the bone deficiency is circumferential around an implant or the interdental space is limited , as in the replacement of mandibular incisors and maxillary lateral incisors [ 6 , 10 ] .
caution in the use of narrow - diameter implants has been advocated because of the concern regarding the negative impact of loading in these implants , with lower stability when compared to regular platform implants , and increased probability of fracture in clinical practice . moreover
, a nonlinear finite element analysis has shown that the neck of the implant represents a potential zone of fracture when subjected to high bending force , making it mandatory to increase the implant support to improve the biomechanical outcome of the treatment with narrow - diameter implants .
( 94% ) , morneburg and prschel   ( 95.5% ) , jofre et al .  
this attachment is self - aligning , has dual retention , and is available in different colors with different retention values [ 21 , 22 ] .
they are resilient , retentive , and durable and have some built - in angulation compensation .
there is limited evidence for the use of narrow - diameter implants for rehabilitation of the completely edentulous atrophic mandibles with locator attachments to support overdentures .
therefore , the purpose of this 2-year study was to compare treatment outcome ( survival rate and condition of hard and soft peri - implant tissues ) and prosthodontic maintenance requirements of two versus three narrow - diameter bone level implants with locator attachments supporting mandibular overdentures .
twenty completely edentulous patients , 11 men and 9 women , ranging from 54 to 68 years of age ( mean age 60.4 years ) were included in the study .
these patients were treated in dammam dental centre , dammam medical complex ( dammam , saudi arabia ) in the period from march to october 2009 .
ethical approval for the project was granted by the human research ethics committee of the dammam medical complex , dammam , saudi arabia .
inclusion criteria dictated that the patient is completely edentulous for at least 1 year , has no previous denture experience , and has sufficient bone for an implant of at least 10  mm length and 3.3  mm diameter .
exclusion criteria included any medical condition contraindicating implant surgery , logistic or physical reasons that could affect follow - up , psychiatric problems and disorders to the implant site related to a history of radiation therapy to the head and neck , or bone augmentation .
the patients were informed about the treatment options ( overdenture on two or three implants ) .
treatment was randomly allocated by lots resulting in ten patients ( group a ) to be treated with two narrow ( 3.3  mm diameter ) bone level implants ( straumann ag , basel , switzerland ) and ten patients ( group b ) to be treated with three narrow ( 3.3  mm diameter ) straumann bone level implants .  
thorough preoperative clinical assessment was carried out for the quantity and morphology of the bone that would host the implants .
preoperative panoramic and periapical radiographs were used for radiographic evaluation of the placement sites to avoid potential complications with important anatomy in these regions .
the components used were narrow bone level implants with a diameter of 3.3  mm and ranged between 10 and 14  mm in length .
locator attachments ( zest anchors llc , escondido , ca , usa ) were used for prosthetic anchorage . in group
a , the implants were placed in the canine region of the mandible , at equal distance from the midline , while , in group b , one central implant was placed in the midline and the two lateral implants were placed at equal distance from the central implant in the canine regions .
one - stage surgical approach was followed throughout the whole study ( figures 1(a ) and 1(b ) ) . under local anaesthesia , a minimal crestal incision ( envelope type )
was made and a mucoperiosteal flap was raised , both on the labial and the lingual aspects , to enable adequate visualization of the lingual aspect of the mandible and to evenly divide the available keratinized tissue .
the osteotomy was prepared using a standard bone drilling protocol , according to the manufacturer 's directions .
bone quality was identified , and bone tap was used in types 1 and 2 .
initial implant stability was tested manually by hand , and insertion torques 35  ncm were acceptable .
healing abutments of appropriate length were connected , and the mucosa was adjusted and sutured ( 4 - 0 vicryl , ethicon , johnson & johnson , brussels , belgium ) . antibiotic ( augmentin 625  mg ) and nonsteroidal anti - inflammatory ( ibuprofen 400  mg ) medications were given to the patients every 8 hours for 5 days postoperatively .
the patients were instructed in a plaque control protocol at the time of implant placement , and this was reinforced at subsequent reviews .
the healing abutments were replaced by locator attachments ( figures 2(a ) and 2(b ) ) 10 weeks after implant placement .
the height of the attachments ( ranging from 2 to 6  mm ) was selected according to the height of the gingiva .
preliminary impressions for maxillary and mandibular arches were taken with stock trays using irreversible hydrocolloid ( hydrogum , zhermack , italy ) .
secondary impressions were taken with autopolymerized acrylic resin special trays using vinyl polysiloxane impression material ( express , 3  m espe dental products , usa ) .
teeth try - in and manufacturing of the acrylic dentures were carried out using standard prosthetic procedures .
the final prostheses were checked in the patient 's mouth and the required adjustments were carried out .
the denture caps with attached black processing males were connected to the mandibular denture using the indirect technique on the mandibular master model .
the black processing males were removed after polishing the denture , and the appropriate locator replacement males were inserted according to the retention required .
the maxillary complete denture and implant - retained mandibular overdenture ( figures 3(a ) and 3(b ) ) were delivered to the participants approximately 12 weeks after implant placement .  
loss of implants was scored after removal of a loose implant any time after placement .
for the presence of plaque , the index according to mombelli et al .   was used ( score 0 : no detection of plaque ; score 1 : plaque can be detected by running a probe across the smooth marginal surface of the attachment and implant ; score 2 : plaque can be seen by the naked eye ; score 3 : abundance amount of plaque ) .
the presence of calculus ( score 1 ) or the absence of calculus ( score 0 ) was recorded . to assess potential peri - implant inflammation ,
the gingival index was used according to the modified le and silness index   ( score 0 : normal peri - implant mucosa ; score 1 : mild inflammation , slight change in color , and slight edema ; score 2 : moderate inflammation , redness , edema , and glazing ; score 3 : severe inflammation , marked redness and edema , and ulceration ) .
for bleeding , the bleeding index according to mombelli et al .   was used ( score 0 : no bleeding when using a periodontal probe ; score 1 : isolated bleeding spots visible ; score 2 : a confluent red line of blood along the mucosal margin ; score 3 : heavy or profuse bleeding ) .
probing depth was measured at four sites of each implant ( mesially , labially , distally , and lingually ) by using a periodontal probe .
the distance between the marginal border of the mucosa and the tip of the periodontal probe was scored as the probing depth .
standardized intraoral radiographs using a long cone technique of each implant were obtained as described by el - sheikh et al .   to provide a geometrically reproducible alignment , an index was recorded for each patient on the inserted mandibular overdenture with the use of vinyl siloxane material . with the aid of hawe 's sensor holder system ( kerr , kerrhawe sa , switzerland ) ,
the radiographs were taken using direct digital imaging system ( trophy rvg , william green pty ltd , australia ) .
images were displayed on a computer screen with such a dimension and brightness that the observer could read comfortably and accurately the image . on each image ,
the implant - locator interface and the first bone - to - implant contact were identified and marked with a cursor on the mesial and distal sides of the implant . the analysis program calculated and reported the distance between the two points with a degree of accuracy of 0.01  mm .
the initial postoperative radiographs immediately after insertion of the final overdentures ( baseline radiography ) were compared with the follow - up radiographs after 6 , 12 , and 24 months of functional loading .
the vertical bone loss was calculated by subtracting the bone heights in the baseline radiographs from those of follow - up radiographs .
any prosthodontic complications / interventions during the 2-year follow - up were recorded according to following events : locator attachment loosening , retention loss , overdenture repair , overdenture relined / rebased , and opposite denture remade / rebased .  
the data collection ( clinical and radiographic outcomes , and prosthodontic maintenance requirements ) of all patients was performed as follows : at the completion of the prosthetic treatment ( baseline ) and after 6 , 12 , and 24 months of functional loading .
probing depth was measured at four sites around each implant and bone height measurement was taken mesially and distally on the radiograph for each implant and the mean was taken .
the data were analyzed using t - tests for the continuous data and mann - whitney tests for the ordinal data .
the correlation was tested using pearson 's correlation tests ( spss for windows , version 10.0 , spss inc . ,
chicago , il , usa ) . in all tests , a significance level of 0.05 was chosen .
during the healing period prior to the locator connection operation , one implant ( 10  mm long ) was lost in group b. after removal of the implant and a bone healing period of 6 months , another implant was successfully placed ; this patient was included in the study for follow - up evaluation . during the functional period , none of the implants were lost .
survival rate of the narrow - diameter straumann bone level implants after 2 years is 98% .
the mean scores for the indices of plaque , calculus , gingival , and bleeding were low at all evaluation periods ( table 3 ) .
no significant differences between the two- or three - implant concepts were observed with regard to plaque , calculus , gingival , and bleeding scores throughout the observation period ( mann - whitney test p > 0.05 ) .
no significant difference between both groups was observed with regard to probing depth ( t - test , p > 0.05 ) ( table 3 ) .
all implants showed less than 1  mm of marginal bone loss during the first - year of the follow - up period .
the average bone loss over the first year was 0.5 and 0.6  mm for groups a and b , respectively . the average bone loss at the end of the two years follow - up was 0.8 and 0.8  mm for groups a and b , respectively .
therefore , the average bone loss over the second year of follow - up was only 0.3 and 0.2  mm for groups a and b , respectively .
no significant differences in bone loss were observed between both groups ( t - test , p > 0.05 ) .
there was no significant difference with regard to peri - implant bone loss between lateral and central implants in group b during the evaluation period .
there was no correlation between the radiographic findings and the peri - implant clinical parameters ( pearson 's correlation test , p > 0.05 ) .
the retention values were increased after 12 months in only two cases of group a by replacing the pink replacement males with the clear ones .
there was no need for relining the overdentures in group b , while only one overdenture required relining in group a after 18 months of follow - up .
maxillary denture rebasing was required in only one patient in group a and in two patients in group b.
in cases where bone width is narrow , local bone augmentation to enable the use of standard - size implants is an option .
the main advantage of the narrow - diameter implants is the ability to apply less invasive surgical procedures when there is circumferential bone deficiency around the implants .
a reduced diameter means a reduction in the contact surface between the implant and the bone , and one might ask whether osseointegration is sufficient to withstand loading forces . decreasing the diameter also means increasing the risk of implant fracture due to reduced mechanical stability and increasing the risk of overload .
it has been suggested that narrow - diameter implants are less prone to stand against stress structurally and could increase the stress transmitted to the bone [ 2931 ] .
for example , it was estimated that fracture resistance of the implant decreases approximately 25% when implant diameter reduces from 3.75 to 3.3  mm . in the present study ,
the 2-year survival rate of narrow - diameter straumann bone level implants in this study is 98% .
this percentage is comparable with other clinical studies , which have reported survival rates of narrow - diameter implants supporting mandibular overdentures ranging from 94% to 100% [ 1420 ] .
these findings support the hypothesis that narrow - diameter implants can be used in prosthetic rehabilitation of the atrophic mandibles with predictable positive outcomes .
the mean indices for plaque , calculus , gingival , and bleeding were low at all evaluation periods with no significant difference between the two groups .
the strict oral hygiene regime to which the patients were subjected provided healthy peri - implant tissues .
these findings are in agreement with other studies [ 20 , 28 , 32 ] .
the overall mean marginal bone loss after 1 year of function in the present study was less than 1  mm for both groups which is in agreement with previous studies [ 19 , 20 , 33 ] .
the values of marginal bone resorption recorded in the present study over the second year of follow - up were within the accepted standard success criteria for implants .
the findings in the present study are in agreement with the study of cakarer et al . , which reported no prosthetic complications with locator attachments in comparison with the ball and bar attachments .
reported that locator attachments of titanium nitride - coated patrices and nylon matrices showed extensive deformation and deterioration with a substantial need for maintenance .
evtimovska et al .   demonstrated that retentive values of the locator attachments are reduced significantly after multiple pull .
the retention values were increased after 12 months in two cases of group a by changing the replacement males .
there was no need for relining any of the overdentures in group b , while only one overdenture required relining in group a. this could be explained by the fact that the support in group a is mainly soft tissue support .
since there were no significant differences with regard to any of the studied clinical or radiographic parameters of the peri - implant tissues between the two groups , placement of two narrow - diameter bone level implants in the interforaminal region of the atrophic mandible to support an overdenture with locator attachments seems to be sufficient .
the use of three implants should be restricted to patients with dentate maxilla who will have increased bite forces .
with the limited observation period and the number of patients included in this study , it may be concluded that the use of narrow - diameter bone level implants appears to be predictable if clinical guidelines are followed and appropriate prosthetic restorations are provided .
it may also be concluded that there seems to be no need to insert more than two narrow - diameter implants with locator attachments in cases of atrophic mandible to support an overdenture ; however , further investigations and long - term prospective studies are certainly required to confirm the encouraging results of this clinical study .
 
 purpose . to compare treatment outcome ( survival rate , condition of hard and soft peri - implant tissues ) and 
 prosthodontic maintenance requirements of two versus three narrow - diameter bone level implants with locator attachments supporting mandibular overdentures . materials and methods . 
 twenty completely edentulous patients with atrophic mandibles were treated . 
 ten patients ( group a ) were treated with overdentures supported by two narrow ( 3.3-mm diameter ) implants ( straumann ag , basel , switzerland ) and ten patients ( group b ) were treated with overdentures supported by three narrow implants . 
 locator ( zest anchors , usa ) attachments were used for prosthetic anchorage . 
 standardized clinical and radiographic parameters ( survival rate , plaque index , calculus index , gingival index , bleeding index , probing depth and marginal bone loss ) were evaluated at the time of the completion of the prosthetic treatment ( baseline ) and after 6 , 12 and 24 months of functional loading . 
 prosthodontic maintenance requirements were also scored . results . 
 only one implant was lost ( group b ) during the healing period . 
 there were no significant differences with regards to any of the studied clinical and radiographic parameters between the two groups ( p > 0.05 ) . 
 few prosthetic complications were recorded . 
 conclusions . 
 no need to insert more than two narrow - diameter bone level implants with locator attachments in cases of atrophic mandible to support an overdenture , however , long - term prospective studies are required to support this notion . 
1. Introduction
2. Materials and Methods
3. Results
4. Discussion
5. Conclusions