between january 2006 and december 2009 , a total of 50 consecutive patients fulfilling the inclusion criteria were included in a randomized controlled trial ( fig .  1 ) .
fully edentulous patients , referred to the department of oral and maxillofacial surgery ( university medical center groningen , the netherlands ) , suffering from lack of retention and stability of the upper denture and lower denture , were considered for inclusion if they fulfilled the following criteria : flow diagram of the study . 

at least 18  years of age;capable of understanding and giving informed consent;at least 1  year edentulous in the maxilla and mandible;bone dimensions in the region between the bicuspids in the anterior area of the maxilla had to be at least 12  mm in height and at least 5  mm in width to reach initial stability of the implant;sufficient interocclusal space for a barsupported attachment system in this region . 
 
at least 18  years of age ; capable of understanding and giving informed consent ; at least 1  year edentulous in the maxilla and mandible ; bone dimensions in the region between the bicuspids in the anterior area of the maxilla had to be at least 12  mm in height and at least 5  mm in width to reach initial stability of the implant ; sufficient interocclusal space for a barsupported attachment system in this region . excluded
were patients with american society of anesthesiologists score ( asa score ) iii ( smeets et  al .
1998 ) , who were smoking , with a history of radiotherapy in the head and neck region or a history of preprosthetic surgery or previous implant placement .
the patients were informed about the study on overdenture treatment with insertion of four or six dental implants in the maxilla and about the extra efforts associated with the study ( questionnaires , evaluation visits ) before they signed an informed consent .
the study was approved by the medical ethical committee of the university medical center groningen ( abr nl32503.042.11 ) .
rotational panoramic radiographs , lateral cephalograms and posteroanterior oblique radiographs were taken to assess the volume of the maxillary alveolar bone , the dimensions of the maxillary sinus and the anteroposterior relationship of the maxilla to the mandible .
in addition , to assess the bone volume of the maxillary processus , a ridgemapping procedure was performed as described by ten bruggenkate et  al .
a diagnostic setup of the planned overdenture was made to obtain insight into the available dimensions for the barsupported attachment system and overdenture .
the treatment and evaluation procedures have been described in detail with the reporting of 1year results of the study by slot et  al .
four or six dental implants with a length of at least 11  mm and a diameter of 4  mm were inserted in the maxillary anterior region ( osseospeed 4.0 s dental implants , astra tech ab , mlndal , sweden ) .
the implants were placed at crestal bone level in predefined positions ( positions 15 , 13 , 11 , 21 , 23 , 25 in the six implants group and positions 13 , 11 , 21 , 23 in the four implants group ) with the help of a surgical template following a submerged healing protocol . in some cases ,
the most distal implant position may be 16 or 26 ( in the six implants group ) , but it is always without a large sinus floor elevation .
small dehiscences or fenestrations were covered with bone harvested from the maxillary tuberosity and organic bovine bone ( biooss ; geistlich pharma ag , wolhusen , switzerland ) and subsequently covered with a resorbable membrane ( biogide ; geistlich pharma ag , wolhusen , switzerland ) . if the most distally placed implants ( usually in the six implants group ) were partially placed in the anterior part of the maxillary sinus , a small sinus floor elevation surgery was performed in that region to prevent a perforation of the sinus membrane by the implant . after a 3month osseointegration period ,
secondstage surgery was performed and healing abutments ( uni healing abutments , astra tech ab ) were placed . after the secondstage surgery , prosthetic procedures were initiated .
healing abutments were replaced by 20 uni abutments ( astra tech ab ) and with impression copings and polyether material ( impregum f ; 3 m espe , st . paul , minn ) , a complete arch impression was made .
a composite resin record base ( lightplast base plates ; dreve dentamid gmbh , unna , germany ) with a wax occlusion rim was used to determine the occlusal vertical dimension and to record the maxillomandibular relationship .
acrylic resin artificial teeth ( ivoclar sr orthotyp dcl and ivoclar vivodent pe , ivoclar vivadent ag , schaan , liechtenstein ) were selected and arranged on the record base for a trial arrangement .
the final superstructure consisted of a milled titanium eggshaped bar with distal extensions , screwretained to abutments , and an overdenture with builtin cobalt chromium reinforcement structure and gold retentive clips attached to it ( slot et  al .
the design of the overdentures was with full coverage of the alveolar process , but with limited palatal coverage .
the patient was instructed in hygiene procedures associated with the dentures and the bars ( fig .  2 ) .
fiveyear panoramic radiograph of a patient with four implants connected with a bar in the maxillary anterior region .
secondary outcome measures were implant survival , overdenture survival and soft tissue conditions ( plaque index , presence of calculus , gingiva index , sulcus bleeding index and pocket probing depth ) .
these parameters were scored at placement of the overdenture , and 1  year and 5  years after loading .
patients ' satisfaction was also scored before treatment , and 1  year and 5  years after loading .
thereafter , the patients were followed up for routine checkup visits at 2 , 3 and 4  years .
if plaque and/or calculus were present at an evaluation period , cleaning was performed with additional hygiene reinstructions .
standardized intraoral radiographs were taken according to a longcone paralleling technique with an individualized xray holder as described by meijndert et  al .
the digital images were analysed using computer software ( biomedical engineering , university medical center groningen , the netherlands ) to perform linear measurements on digital radiographs .
the known implant length was used as a reference to transform the linear measurements into mm .
reference line for bone level evaluation was the outer border of the neck of the implant .
implant survival was defined as the percentage of implants initially placed that was still present and not mobile at followup .
survival of maxillary overdentures was defined as the percentage of overdentures initially placed that was still present at followup .
remake or adjustment of the maxillary overdenture was scored any time after placement . for presence of plaque ,
the presence of calculus ( score 1 ) or the absence of calculus ( score 0 ) was scored . to qualify the degree of periimplant inflammation ,
probing depth was measured at four sites of each implant ( mesial , labial , distal and lingual ) using a manual periodontal probe ( williams colourcoded probe ; hufriedy , chicago , il , usa ) .
periimplant mucositis and periimplantitis was calculated at patient level . as definition for periimplant mucositis and periimplantitis ,
the consensus reached at the vi and vii european workshop on periodontology ( lindhe & meyle 2008 , lang & berglundh 2011 ) and reconfirmed at the viii european workshop on periodontology ( sanz & chapple 2012 has been used , being : 
 periimplant mucositis ( radiographic bone loss < 2  mm ) : bop+ and/or suppuration andperiimplantitis : bop+ and/or suppuration in combination with marginal bone loss 2  mm . 
 
periimplant mucositis ( radiographic bone loss < 2  mm ) : bop+ and/or suppuration and periimplantitis : bop+ and/or suppuration in combination with marginal bone loss 2  mm .
patients ' satisfaction with their overdenture was assessed using a validated questionnaire ( vervoorn et  al .
this questionnaire focused on complaints and consisted of 54 items and was divided into six scales : 
 nine items concerning functional problems of the upper denture;eighteen items concerning functional complaints in general;three items concerning facial aesthetics;three items concerning accidental lip , cheek and tongue biting (  neutral space);twelve items concerning aesthetics of the denture . 
 
nine items concerning functional problems of the upper denture ; eighteen items concerning functional complaints in general ; three items concerning facial aesthetics ; three items concerning accidental lip , cheek and tongue biting (  neutral space  ) ; twelve items concerning aesthetics of the denture .
the extent of each specific complaint could be expressed on a fourpoint rating scale ( 0  =  no complaints , 1  =  little , 2  =  moderate , 3  =  severe complaints .
chewing ability  questionnaire ( stellingsma et  al . 2005 ) . in this questionnaire
, patients gave their opinion about the ability to chew nine different kinds of food on a threepoint rating scale ( 0  =  good , 1  =  moderate , 2  =  bad ) .
the items were grouped into three scales , being soft food , tough food and hard food .
next to these questionnaires , the patients ' overall denture satisfaction was expressed on a 10point rating scale ( 1  =  very bad to 10  =  excellent ) .
it was assumed that an implantsupported overdenture on four implants was not inferior to an overdenture supported by six implants .
sample size calculation of the study and allocation to the groups is described in the study of slot et  al .
a difference of at least 0.4  mm in bone height ( measured on standardized radiographs , with a standard deviation of 0.5  mm ) between the four implants group and six implants group after 12  months was expected to differentiate between the two groups , based on the findings of a study on maxillary implantsupported overdentures ( raghoebar et  al .
a ttest given a  =  0.05 with a power 80% combined with the expected effect size for two independent means gives a sample size of 21 persons in each group . to deal with withdrawal of individuals in the study ,
data collection and analysis of the radiographs were done by the same observer . for clinical and radiographical parameters ,
the implant has been taken as statistical unit ( with multiple scores per implant , the worst score was taken ) . for calculation of percentage of perimucositis and periimplantitis ,
the patient has been as taken as statistical unit ( worst implant per patient ) . and
data were analysed using the statistical package for social sciences ( version 22.0 ; spss inc . , ibm corporation , chicago , il , usa ) . in all tests ,
a significance level of 0.05 was chosen . to see whether the data were normally distributed ,
the frequency distribution was plotted in a histogram . to test whether the result from the frequency analyses differed significantly from a normal distribution , kolmogorov 
the potential effects of imbalances between the study groups on the outcomes of this study were analysed by regression analysis . as no such effects
were observed , differences between study groups were tested with an independent student 's ttest and between evaluation periods were tested with a paired student 's ttest .
between january 2006 and december 2009 , a total of 50 consecutive patients fulfilling the inclusion criteria were included in a randomized controlled trial ( fig .  1 ) .
fully edentulous patients , referred to the department of oral and maxillofacial surgery ( university medical center groningen , the netherlands ) , suffering from lack of retention and stability of the upper denture and lower denture , were considered for inclusion if they fulfilled the following criteria : flow diagram of the study . 

at least 18  years of age;capable of understanding and giving informed consent;at least 1  year edentulous in the maxilla and mandible;bone dimensions in the region between the bicuspids in the anterior area of the maxilla had to be at least 12  mm in height and at least 5  mm in width to reach initial stability of the implant;sufficient interocclusal space for a barsupported attachment system in this region . 
 
at least 18  years of age ; capable of understanding and giving informed consent ; at least 1  year edentulous in the maxilla and mandible ; bone dimensions in the region between the bicuspids in the anterior area of the maxilla had to be at least 12  mm in height and at least 5  mm in width to reach initial stability of the implant ; sufficient interocclusal space for a barsupported attachment system in this region . excluded
were patients with american society of anesthesiologists score ( asa score ) iii ( smeets et  al .
1998 ) , who were smoking , with a history of radiotherapy in the head and neck region or a history of preprosthetic surgery or previous implant placement .
the patients were informed about the study on overdenture treatment with insertion of four or six dental implants in the maxilla and about the extra efforts associated with the study ( questionnaires , evaluation visits ) before they signed an informed consent .
the study was approved by the medical ethical committee of the university medical center groningen ( abr nl32503.042.11 ) .
rotational panoramic radiographs , lateral cephalograms and posteroanterior oblique radiographs were taken to assess the volume of the maxillary alveolar bone , the dimensions of the maxillary sinus and the anteroposterior relationship of the maxilla to the mandible .
in addition , to assess the bone volume of the maxillary processus , a ridgemapping procedure was performed as described by ten bruggenkate et  al .
a diagnostic setup of the planned overdenture was made to obtain insight into the available dimensions for the barsupported attachment system and overdenture .
the treatment and evaluation procedures have been described in detail with the reporting of 1year results of the study by slot et  al .
four or six dental implants with a length of at least 11  mm and a diameter of 4  mm were inserted in the maxillary anterior region ( osseospeed 4.0 s dental implants , astra tech ab , mlndal , sweden ) .
the implants were placed at crestal bone level in predefined positions ( positions 15 , 13 , 11 , 21 , 23 , 25 in the six implants group and positions 13 , 11 , 21 , 23 in the four implants group ) with the help of a surgical template following a submerged healing protocol . in some cases ,
the most distal implant position may be 16 or 26 ( in the six implants group ) , but it is always without a large sinus floor elevation .
small dehiscences or fenestrations were covered with bone harvested from the maxillary tuberosity and organic bovine bone ( biooss ; geistlich pharma ag , wolhusen , switzerland ) and subsequently covered with a resorbable membrane ( biogide ; geistlich pharma ag , wolhusen , switzerland ) . if the most distally placed implants ( usually in the six implants group ) were partially placed in the anterior part of the maxillary sinus , a small sinus floor elevation surgery was performed in that region to prevent a perforation of the sinus membrane by the implant . after a 3month osseointegration period ,
secondstage surgery was performed and healing abutments ( uni healing abutments , astra tech ab ) were placed . after the secondstage surgery , prosthetic procedures were initiated .
healing abutments were replaced by 20 uni abutments ( astra tech ab ) and with impression copings and polyether material ( impregum f ; 3 m espe , st . paul , minn ) , a complete arch impression was made . a composite resin record base ( lightplast base plates ; dreve dentamid gmbh , unna , germany ) with a wax occlusion rim was used to determine the occlusal vertical dimension and to record the maxillomandibular relationship .
acrylic resin artificial teeth ( ivoclar sr orthotyp dcl and ivoclar vivodent pe , ivoclar vivadent ag , schaan , liechtenstein ) were selected and arranged on the record base for a trial arrangement .
the final superstructure consisted of a milled titanium eggshaped bar with distal extensions , screwretained to abutments , and an overdenture with builtin cobalt chromium reinforcement structure and gold retentive clips attached to it ( slot et  al .
the design of the overdentures was with full coverage of the alveolar process , but with limited palatal coverage .
the patient was instructed in hygiene procedures associated with the dentures and the bars ( fig .  2 ) .
fiveyear panoramic radiograph of a patient with four implants connected with a bar in the maxillary anterior region .
four or six dental implants with a length of at least 11  mm and a diameter of 4  mm were inserted in the maxillary anterior region ( osseospeed 4.0 s dental implants , astra tech ab , mlndal , sweden ) .
the implants were placed at crestal bone level in predefined positions ( positions 15 , 13 , 11 , 21 , 23 , 25 in the six implants group and positions 13 , 11 , 21 , 23 in the four implants group ) with the help of a surgical template following a submerged healing protocol . in some cases ,
the most distal implant position may be 16 or 26 ( in the six implants group ) , but it is always without a large sinus floor elevation .
small dehiscences or fenestrations were covered with bone harvested from the maxillary tuberosity and organic bovine bone ( biooss ; geistlich pharma ag , wolhusen , switzerland ) and subsequently covered with a resorbable membrane ( biogide ; geistlich pharma ag , wolhusen , switzerland ) . if the most distally placed implants ( usually in the six implants group ) were partially placed in the anterior part of the maxillary sinus , a small sinus floor elevation surgery was performed in that region to prevent a perforation of the sinus membrane by the implant . after a 3month osseointegration period ,
secondstage surgery was performed and healing abutments ( uni healing abutments , astra tech ab ) were placed .
healing abutments were replaced by 20 uni abutments ( astra tech ab ) and with impression copings and polyether material ( impregum f ; 3 m espe , st . paul , minn ) , a complete arch impression was made . a composite resin record base ( lightplast base plates ; dreve dentamid gmbh , unna , germany ) with a wax occlusion rim was used to determine the occlusal vertical dimension and to record the maxillomandibular relationship . acrylic resin artificial teeth (
ivoclar sr orthotyp dcl and ivoclar vivodent pe , ivoclar vivadent ag , schaan , liechtenstein ) were selected and arranged on the record base for a trial arrangement .
the final superstructure consisted of a milled titanium eggshaped bar with distal extensions , screwretained to abutments , and an overdenture with builtin cobalt chromium reinforcement structure and gold retentive clips attached to it ( slot et  al .
the design of the overdentures was with full coverage of the alveolar process , but with limited palatal coverage .
the patient was instructed in hygiene procedures associated with the dentures and the bars ( fig .  2 ) .
fiveyear panoramic radiograph of a patient with four implants connected with a bar in the maxillary anterior region .
secondary outcome measures were implant survival , overdenture survival and soft tissue conditions ( plaque index , presence of calculus , gingiva index , sulcus bleeding index and pocket probing depth ) .
these parameters were scored at placement of the overdenture , and 1  year and 5  years after loading .
patients ' satisfaction was also scored before treatment , and 1  year and 5  years after loading .
thereafter , the patients were followed up for routine checkup visits at 2 , 3 and 4  years .
if plaque and/or calculus were present at an evaluation period , cleaning was performed with additional hygiene reinstructions .
standardized intraoral radiographs were taken according to a longcone paralleling technique with an individualized xray holder as described by meijndert et  al .
the digital images were analysed using computer software ( biomedical engineering , university medical center groningen , the netherlands ) to perform linear measurements on digital radiographs .
the known implant length was used as a reference to transform the linear measurements into mm . reference line for bone level evaluation
implant survival was defined as the percentage of implants initially placed that was still present and not mobile at followup .
survival of maxillary overdentures was defined as the percentage of overdentures initially placed that was still present at followup .
remake or adjustment of the maxillary overdenture was scored any time after placement . for presence of plaque , the index according to mombelli et  al .
the presence of calculus ( score 1 ) or the absence of calculus ( score 0 ) was scored . to qualify the degree of periimplant inflammation ,
the modified le and silness index ( 1963 ) was used . for bleeding , the bleeding index according to mombelli et  al .
probing depth was measured at four sites of each implant ( mesial , labial , distal and lingual ) using a manual periodontal probe ( williams colourcoded probe ; hufriedy , chicago , il , usa ) .
periimplant mucositis and periimplantitis was calculated at patient level . as definition for periimplant mucositis and periimplantitis ,
the consensus reached at the vi and vii european workshop on periodontology ( lindhe & meyle 2008 , lang & berglundh 2011 ) and reconfirmed at the viii european workshop on periodontology ( sanz & chapple 2012 has been used , being : 
 periimplant mucositis ( radiographic bone loss < 2  mm ) : bop+ and/or suppuration andperiimplantitis : bop+ and/or suppuration in combination with marginal bone loss 2  mm . 
  periimplant mucositis ( radiographic bone loss < 2  mm ) : bop+ and/or suppuration and periimplantitis : bop+ and/or suppuration in combination with marginal bone loss 2  mm .
patients ' satisfaction with their overdenture was assessed using a validated questionnaire ( vervoorn et  al .
this questionnaire focused on complaints and consisted of 54 items and was divided into six scales : 
 nine items concerning functional problems of the upper denture;eighteen items concerning functional complaints in general;three items concerning facial aesthetics;three items concerning accidental lip , cheek and tongue biting (  neutral space);twelve items concerning aesthetics of the denture . 
 
nine items concerning functional problems of the upper denture ; eighteen items concerning functional complaints in general ; three items concerning facial aesthetics ; three items concerning accidental lip , cheek and tongue biting (  neutral space  ) ; twelve items concerning aesthetics of the denture . the extent of each specific complaint could be expressed on a fourpoint rating scale ( 0  =  no complaints , 1  =  little , 2  =  moderate , 3  =  severe complaints .
chewing ability  questionnaire ( stellingsma et  al . 2005 ) . in this questionnaire
, patients gave their opinion about the ability to chew nine different kinds of food on a threepoint rating scale ( 0  =  good , 1  =  moderate , 2  =  bad ) .
the items were grouped into three scales , being soft food , tough food and hard food .
next to these questionnaires , the patients ' overall denture satisfaction was expressed on a 10point rating scale ( 1  =  very bad to 10  =  excellent ) .
standardized intraoral radiographs were taken according to a longcone paralleling technique with an individualized xray holder as described by meijndert et  al .
the digital images were analysed using computer software ( biomedical engineering , university medical center groningen , the netherlands ) to perform linear measurements on digital radiographs .
the known implant length was used as a reference to transform the linear measurements into mm . reference line for bone level evaluation
implant survival was defined as the percentage of implants initially placed that was still present and not mobile at followup .
survival of maxillary overdentures was defined as the percentage of overdentures initially placed that was still present at followup .
the presence of calculus ( score 1 ) or the absence of calculus ( score 0 ) was scored . to qualify the degree of periimplant inflammation , the modified le and silness index ( 1963 ) was used . for bleeding , the bleeding index according to mombelli et  al .
probing depth was measured at four sites of each implant ( mesial , labial , distal and lingual ) using a manual periodontal probe ( williams colourcoded probe ; hufriedy , chicago , il , usa ) .
periimplant mucositis and periimplantitis was calculated at patient level . as definition for periimplant mucositis and periimplantitis ,
the consensus reached at the vi and vii european workshop on periodontology ( lindhe & meyle 2008 , lang & berglundh 2011 ) and reconfirmed at the viii european workshop on periodontology ( sanz & chapple 2012 has been used , being : 
 periimplant mucositis ( radiographic bone loss < 2  mm ) : bop+ and/or suppuration andperiimplantitis : bop+ and/or suppuration in combination with marginal bone loss 2  mm . 
 
periimplant mucositis ( radiographic bone loss < 2  mm ) : bop+ and/or suppuration and periimplantitis : bop+ and/or suppuration in combination with marginal bone loss 2  mm .
patients ' satisfaction with their overdenture was assessed using a validated questionnaire ( vervoorn et  al .
this questionnaire focused on complaints and consisted of 54 items and was divided into six scales : 
 nine items concerning functional problems of the upper denture;eighteen items concerning functional complaints in general;three items concerning facial aesthetics;three items concerning accidental lip , cheek and tongue biting (  neutral space);twelve items concerning aesthetics of the denture . 
 
nine items concerning functional problems of the upper denture ; eighteen items concerning functional complaints in general ; three items concerning facial aesthetics ; three items concerning accidental lip , cheek and tongue biting (  neutral space  ) ; twelve items concerning aesthetics of the denture .
the extent of each specific complaint could be expressed on a fourpoint rating scale ( 0  =  no complaints , 1  =  little , 2  =  moderate , 3  =  severe complaints .
chewing ability  questionnaire ( stellingsma et  al . 2005 ) . in this questionnaire
, patients gave their opinion about the ability to chew nine different kinds of food on a threepoint rating scale ( 0  =  good , 1  =  moderate , 2  =  bad ) .
the items were grouped into three scales , being soft food , tough food and hard food .
next to these questionnaires , the patients ' overall denture satisfaction was expressed on a 10point rating scale ( 1  =  very bad to 10  =  excellent ) .
study analysis was performed according to a noninferiority design . it was assumed that an implantsupported overdenture on four implants was not inferior to an overdenture supported by six implants .
sample size calculation of the study and allocation to the groups is described in the study of slot et  al .
( 2013 ) . a difference of at least 0.4  mm in bone height ( measured on standardized radiographs , with a standard deviation of 0.5  mm ) between the four implants group and six implants group after 12  months was expected to differentiate between the two groups , based on the findings of a study on maxillary implantsupported overdentures ( raghoebar et  al . 2003 ) .
a ttest given a  =  0.05 with a power 80% combined with the expected effect size for two independent means gives a sample size of 21 persons in each group . to deal with withdrawal of individuals in the study ,
data collection and analysis of the radiographs were done by the same observer . for clinical and radiographical parameters ,
the implant has been taken as statistical unit ( with multiple scores per implant , the worst score was taken ) .
for calculation of percentage of perimucositis and periimplantitis , the patient has been as taken as statistical unit ( worst implant per patient ) . and
data were analysed using the statistical package for social sciences ( version 22.0 ; spss inc . , ibm corporation , chicago , il , usa ) . in all tests ,
a significance level of 0.05 was chosen . to see whether the data were normally distributed ,
the frequency distribution was plotted in a histogram . to test whether the result from the frequency analyses differed significantly from a normal distribution , kolmogorov 
the potential effects of imbalances between the study groups on the outcomes of this study were analysed by regression analysis .
as no such effects were observed , differences between study groups were tested with an independent student 's ttest and between evaluation periods were tested with a paired student 's ttest .
two patients died during the followup period ( one patient in the four implants group before the 1year evaluation and one patient in the six implants group between the 1year and 5year evaluation ) .
two patients of the six implants group did not attend the 5year evaluation because of severe illness .
no implants failed in the four implants group , while one implant in the six implants group was lost during the osseointegration period . because a barsupported overdenture could still be made on the remaining five implants , there was no need to replace the implant .
fiveyears ' survival rate of implants was 100% in the four implants group , 99.2% in the six implants group .
baseline characteristics ( at placement of overdenture ) of the groups with four implants ( four implants group ) and six implants ( six implants group ) differences between study groups were tested with the independent student 's ttest ( p  <  0.05 ) .
the mean loss of marginal bone between baseline ( placement of the overdenture ) and the 5year evaluation was 0.50    0.37  mm in the four implants group and 0.52    0.43  mm in the six implants group ( p  >  0.05 , table  2 ) . mean scores of the indices for plaque , calculus , gingiva and bleeding were very low at all evaluation periods and did not differ between the groups . also no timedependent significant changes in probing depth were observed between the groups ( p  >  0.05 , table  3 ) .
patientlevel incidence of periimplant mucositis was 41.7% and 45.5% , and of periimplantitis was 8.3% and 4.5% in the four and six implants groups respectively .
mean values and standard deviations of marginal bone loss in mm , and frequency distribution of bone loss 1 and 5  years after placement of the overdenture of the four and six implants groups .
mean marginal bone loss did not differ significantly between the groups differences between study groups were tested with the independent student 's ttest ( p  <  0.05 ) .
mean change from baseline to 1 and 5  years of followup for plaque index , calculus index , gingival index , bleeding index and probing depth .
no timedependent significant changes were observed between the groups differences between study groups were tested with the independent student 's ttest ( p  <  0.05 ) .
in both groups , all scores had improved significantly ( p  <  0.05 ) between pretreatment and posttreatment assessments , but the scores did not differ between the groups ( p  >  0.05 ) .
after 5 years , patients continued to be very satisfied with their overdentures ; the results are rather similar to the 1year data .
mean score of 5 scales concerning the denture complaints ( possible range 03 ) , mean scores of chewing ability of soft , tough and hard food ( possible range 02 ) and overall satisfaction score ( possible range 110 ) before , and 1 and 5  years after treatment .
no significant differences between the four and six implants group were observed at all evaluation periods differences between study groups were tested with the independent student 's ttest ( p  <  0.05 ) .
the surgical and prosthetic aftercare during 5 years of followup revealed a small number of events , mostly repair of the denture base or teeth .
surgical treatment of periimplantitis was not performed during the 5year followup ( table  5 ) . at the 5year followup evaluation ,
a number of patients were identified as having periimplant mucositis ( treatment : cleaning and reinstruction ) or periimplantitis ( treatment : cleaning , reinstruction and openflap surgical treatment ) .
this study revealed that both four and six dental implants placed in the anterior region of the edentulous maxilla , connected with a bar , form a proper base for the support of an overdenture . in none of the parameters studied , significant differences were observed between the four and six implants group .
fiveyears ' survival rate of implants was high in both the four and six implants group .
these results are on the high end compared to the implant survival reported in the 5year followup case series of jemt et  al .
( 1997 ) and bergendal & engquist ( 1998 ) who used implants with a machined surface ( 5year implants survival 72.479% ) and similar to those reported by mangano et  al .
the latter studies also used implants with a roughened surface , as in this study .
although a study comparing various implant designs was not analysed , apparently implants with a roughened surface perform better than implants with a machined surface in the maxilla .
the mean marginal bone loss between baseline ( placement of the overdenture ) and the 5year evaluation was limited in both groups and again comparable to those reported by mangano et  al .
the very limited marginal bone loss might be due to the neck design of the implants used ( hermann et  al .
mean indices for plaque , calculus , gingiva and bleeding were very low at the 5year evaluation and probing depth was not deviating .
there might be some extra effort in performing hygiene tasks with six implants , but this does not result in worse results .
incidence of periimplantitis after 5  years was 8.3% and 4.5% in the four and six implants groups respectively .
because there was no significant change in pocket probing depth at an earlier stage , no extra radiographs were made .
nevertheless , bone loss as a sign of periimplantitis could not be reliably excluded in some patients before the 5year evaluation period . in a systematic review of de waal et  al .
( 2013 ) , it was reported that numbers of prevalence of periimplant mucositis and periimplantitis with standardized threshold levels are missing in studies with fully edentulous patients . in this study ,
the patientlevel incidence of periimplant mucositis was 41.7% and of periimplantitis was 8.3% in the four implants group .
in the six implants group , patientlevel incidence of periimplant mucositis was 45.5% and of periimplantitis was 4.5% .
also in fully edentulous patients , but for mandibular overdentures , 51.9% periimplant mucositis and 16.9% periimplantitis was reported after 5  years ( meijer et  al .
existing studies have demonstrated clearly that edentulism per se is no protection from periimplant disease and that continuous supportive care is essential in maintaining periimplant health .
patients ' satisfaction improved significantly when wearing implantsupported maxillary overdentures and results were shown to be favourable at both the 1 and 5year evaluation , again in agreement with data reported by krennmair et  al .
( 2008 ) also mentioned in their case series that patients with an overdenture on four or six implants in the anterior maxilla were equally satisfied .
in other words , patients ' satisfaction seemed to be irrespective of whether the bar was supported by four or six implants in the anterior maxillary region .
the high satisfaction might be due to the fact that the overdenture is supported by a bar , which gives a comparable stability for both treatment options .
the idea that six implants gives more stability than four implants , and resulting in more patients ' satisfaction , turned out to be not true . probably because four and six implants are more or less placed in the same area and the fact that the length of the bar is the same in both designs .
the high satisfaction with maxillary overdentures , including opinions on speech , resemble the results of heydecke et  al .
( 2004 ) in which favourable results of an overdenture were displayed compared with a fixed implant prosthesis .
( 2011 ) reported that the majority of complications were related to the weakness of the anchorage components connecting bar and overdenture .
this is in contrast with this study in which no clip repairs were recorded . in this study , an overdenture with builtin cobalt chromium reinforcement structure and gold retentive clips attached to it were used , which probably led to a minimum of prosthetic complications ( slot et  al .
rotational panoramic radiographs , lateral cephalograms and posteroanterior oblique radiographs were made to assess bone volume and possible pathology .
nowadays , in terms of radiation protection and diagnostic possibilities , cone beam computer tomography is preferred . in the study ,
some studies reveal that periimplant hygiene of solitary attachment systems are superior compared with bar attachment systems ( zou et  al .
longterm studies on comparison of attachment systems would be advantageous . from this 5year followup study
, it was concluded that barconnected maxillary overdentures on four or six implants resulted in a comparable treatment outcome with high implant survival , limited loss of periimplant marginal bone and high patient satisfaction .
 abstractobjectiveto compare a four and siximplant maxillary overdenture after an observation period of 5  years.material and methodsfifty subjects with functional problems concerning their maxillary denture , who had ample bone volume in the anterior region to place four or six implants , were included and randomly assigned to either group . 
 implant and overdenture survival , clinical performance , marginal bone loss and patient satisfaction were assessed.resultsfortysix patients completed the 5year followup . 
 one implant failed in the six implants group ( 99.2% survival ) and none in the four implants group ( 100% survival ) . 
 no overdentures had to be replaced during the observation period and the number of complications was limited . 
 clinical function was good , with no difference in clinical parameters between the groups . 
 mean marginal bone resorption was 0.50    0.37 and 0.52    0.43  mm in the four and six implant group respectively.conclusionin patients with functional complaints of their maxillary denture , barsupported overdentures on four implants in the anterior maxillary region were not inferior to overdentures supported by six implants after 5  years of function . 
 implant survival and patient satisfaction were high , clinical parameters favourable , bone loss and complications to the denture were minor in both groups ( clinical trial registration number : ntr2969 ) . 
Material and Methods
Patient selection
Treatment procedure
Surgical procedures
Prosthetic procedure
Outcome measures
Marginal bonelevel alterations
Implant survival
Overdenture survival
Clinical parameters
Patients' satisfaction
Statistical analysis
Results
Discussion