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the prosthodontic rehabilitation of edentulous patients has been used over the years primarily for esthetics purposes after mandible resection .
the development of osseointegrated implants allowed the production of more stable overdentures , improving retention and stability , thereby improving prosthetic rehabilitation prognosis and oral function .       
some researchers demonstrated the use of the photoelasticity to evaluate the stress of bodies subjected to efforts [ 14 ] .
other studies have exalted the success of the osseointegrated implants [ 58 ] pointing out their advantages in supporting overdentures .
some authors also   referred to the improvement of the dental prosthetic therapy with the use of two implants linked through a bar .
other fact , which must be considered , is the biting force , that increases three times more with the use of overdentures based on implants , when compared with conventional dentures .
some studies referred to the use of implant - supported prosthesis to compensate partially problems that arose from partial resection of the mandible [ 7 , 8 , 10 ] .         
it is of great interest to know the tensions of the implants , also in cases of overdentures in patients with partial resection of mandible .
recent studies , using the method of photoelasticity [ 1 , 2 , 4 , 11 ] , have demonstrated the tensions in implant - supported prostheses , in which   the largest tensions were found in the distal crests of the more distal implant .  
the obtained result , with the implant supported by overdentures , leads the authors to recommend the use of this technique with their patients with partial resection of mandible , but also suggested different retention mechanisms   [ 2 , 11 ] to the implants by using ball / o - ring and bar - clip attachments .
they showed that ball / o - ring attachments transferred less stress to implants , than the bar - clip when the photoelastic model was subjected to a posterior vertical load .        
the objective of this study was to evaluate , by the photoelastic method , the tensions around osseointegrated implants and ridge , used as overdentures support in patients with partial resection of jaw .  
   a human jaw was duplicated and sectioned at the level of the left mental foramen .
three perforations corresponding to the 33 ( cal ) , 42 ( lir ) , and 43 ( car ) teeth were made in the model , where laboratory analog implants were installed , connected to molding cylinders , reproduced using silicone ; the laboratory analogs were substituted by 13  mm implants ( 3i implant innovations , west palm beach , fl . ) ; by the use of   photoelastic resin ( pl-2 ; photoelastic division , measurement group , raleigh , nc ) , the final model was obtained , and allowed the investigation of different retention mechanisms .
evermore , the evaluation of maximum tension was made in the left canine ( cal ) , right lateral incisor ( lir ) , right canine ( car ) , and alveolar ridge below the overdenture .      
six different groups of investigation were studied , with two implants : ( 1 ) t ( traditional ) , implants in the positions of the canines , linked with hader bar and yellow clips ( figure 1(a ) ) ; ( 2 ) f ( fulcrum ) , implants in the positions of the left canine and right lateral incisor , linked with hader bar and yellow clips ( figure 1(b ) ) ; with three implants , both canines and right lateral incisor : ( 3 ) implants in the three positions with  o  
rings ( figure 1(c ) ) ; ( 4 ) two eras and clip , implants in the three positions with two eras in the canine teeth , linked for hader bar and yellow clips ; ( 5 ) two eras ( without clip ) , similar to the anterior but without use of yellow clips ; ( 6 )   retainer type  oring in the left canine and era in the right canine , linked with hader bar and yellow clips ( figure 1(d ) ) .       
the overdentures were made by traditional method and fixed in position for the tests ( figure 1(e ) ) .
the applied load was 15 pounds ( 6.8  kgf ) on the oclusal of 1st pm , 2nd pm , and 1st m surfaces .
the values of the tensions in the implants were listed in five arrangements , ( 1 ) the support / group system , ( 2 ) the angle of incidence of the force , ( 3 ) the place of force incidence , ( 4 ) the place of the tension , and ( 5 ) the direction of the tension .
the values corresponding to the tensions in the alveolar ridge consisted of 3 factors : ( 1 ) the support group , ( 2 ) the incidence angle , and ( 3 ) the incidence place .         
the analyses were made separately for the tensions in the implant apex ,   and the alveolar ridge .
the analysis of variance related to the tension on the implants , in the alveolar ridges and their arrangements with the various retention devices , showed variable statistical significant interaction .      
the 22 experimental conditions showed statistical significant differences for the factor support / group system , but not for place or angle of incidence of the force .
the interactions between the support / group system and the place of incidence of the force , and also between the support / group and the angle of incidence of the force , were also statistically significant ( table 1 ) . 

table 1 presents the averages of the maximum tensions for the support / group system and their counterparts , as demonstrated by the tukey test , and it also shows the averages amongst the tensions around implants for the six studied groups . concerning the place and angle of incidence of the load , it was demonstrated that practically no group of the left canine tooth ( cal ) presented stress .
the stress was identified only on group six (  o  cal ) ( era car ) ( cal ) , and even so , with a very low value ( 0.07 ) and when the angle of load was inclined ( 0.10 ) , when the place of the load was over the 1st pm , in the right lateral incisor ( lir ) , only in group f the average value was relatively high ( 1.215  fringes / cm ) ; even so it was smaller than in most other groups . in the right canine tooth ( car ) , the concentration of stress in the groups with 2 eras was high , which reached 4.9  fringes / cm . 

table 2 values show that tensions in the ridge did not reach high values in any of the cases .
it is also possible to observe that , as the load traveled further from the implants , the tension decreased in the implants and increased in the ridge , showing the negative correlation between tensions around implant and ridge .
it was particularly notable that the total tensions in the implants and ridge were practically constant .
photoelastic methods have been applied to investigate biomechanical behavior of dental implants in bone supporting fixed and removable prostheses [ 1214 ] ; however , representing the nonhomogeneous and anisotropic structure of bone by plastic models gives rise to certain limitations in predictions of biological response to applied loads .
nevertheless , photoelastic models have successfully indicated differences between various conditions by comparative evaluation of stress - related outcomes .       
the group t ( traditional ) presented the smallest tension at the alveolar ridge when compared to the other groups .
the left canine of this group was the teeth with the lowest tension value ( zero ) .
in the right canine , the tensions did not get to be very high , despite being superior to the right lateral incisor of the group f ( fulcrum ) . although tensions at the ridge have been larger in group f than in group t , it was smaller in the support element .
maybe this is due to the fact that the model t axes are parallel to the bar and to the ridges , and in the case of model f , axes are parallel only in relation to the remaining ridge . when there is application of load in the f model , even if the patient uses only one side during masticatory function , there is a slight rotation as in any normal expected overdenture and in the model t , there is a decrease of this rotational regular functional and an unexpected torsion of the bar , causing increased load on implants and less load on the ridge .     
this protects the implant , whose loss would be clinically more problematic than an eventual reabsorption in the ridge .       
the lowest stress levels were observed for the group 3 with  o    rings attachment system , which seems to present the more favorable clinical conditions showing a better distribution of tensions when the loads were uniformly distributed in ridge and also in the implants .
this might mean that the ample covering of the basal area allowed for a better distribution of the load per unit area , as suggested by others .        
both era retention groups , with and without clips , presented high stress levels mainly concentrated in the cr but not in the lir and ridge , that maybe be linked to a larger setting in this group , which would be in agreement with authors   that found larger tensions in the distal implant when the load application was on elements in balance .       
the group (  o  cal ) ( era car ) presented quite favorable results .
although the ridge suffers from a little high tension , the implants do not develop very high tension themselves .
kenney and richards   and others [ 14 , 16 , 17 ]   concluded that ball / o - ring attachments transferred less stress to implants than bar - clip attachments when the photoelastic model with 2 implants was subjected to a posterior vertical load .      
although several authors have associated implant - supported overdentures , as an indication for being used with patients without surgical resection problems   [ 6 , 7 , 9 , 12 , 15 ] , it becomes a valuable rehabilitation alternative in patients with partial resection of the jaw [ 5 , 10 ] .        
the negative correlation found between tension in the ridge and implants ,  the more distant from the implants , the more is the load supported by the ridge 
, offers an opportunity of therapy , even though temporarily . the fact is partly confirmed by the almost constant average stress in the implants and ridge .       
osseointegrated implants should be considered as a part of the mandibular rehabilitation plan of partially resected patient .
functional instability during aperture / closure movements might be partially established by implants and overdentures , even though they do not achieve all renowned biomechanical aspects .
the aim is to minimize stability problems which may emerge with an  o  ring retention or bar retention mechanism .
aspects related to irradiated patient might be considered cautiously such as radiation dosage and exact local .    
we may also consider that patient - mediated factors such as retention , jaw morphology , jaw physiology , and unfortunately , financial considerations dictate the number and design of implants and the design of the mandibular overdenture prosthesis .
very low stress level was transferred to the left canine next to the mandible resection .   the contralateral canine ( next to the prosthesis ) accumulated the highest tensions , especially those of the both era groups .   
the more distalized , the application of the load resulted in decreased stress transfer to the implant and increased to the alveolar ridge . considering the group of three implants , the group 06 (  o  cal ) ( era car ) presented the smallest concentrations of tensions in the right canine tooth .
the purpose of this research was to evaluate the maximum stress around osseointegrated implants and alveolar ridge , in a mandible with left partial resection through a photoelastic mandibular model . 
 the first group consisted of two implants : traditional model ( t ) , implants placed in the position of both canines ; fulcrum model ( f ) , implants placed in the position of left canine cl and right lateral incisor lir . 
 both models linked through a bar and clips . 
 the second group was consisted of three implants , with implants placed in the position of both canines ( cr and cl ) and the right lateral incisor ( lir ) , which composed four groups : ( 1 ) model with 3  o  rings , ( 2 ) model 2 eras , bar with clips , ( 3 ) model 2 eras bar without clips ; ( 4 ) model  o  ring bar and era . an axial and an oblique load of 6.8  
 kgf was applied on a overdenture at the 1st pm , 2nd pm , and 1st m. results showed that the area around the left canine ( cl ) was practically free of stress ; the left lateral incisor ( lil ) developed only small tensions , and low stress in all the other cases ; the right canine tooth suffered the largest concentrations of stress , mainly with the era retention mechanism .