research on the maintenance of osseointegration under forces 
 transmitted by occlusal load is as important as the study of the initial process of bone 
 formation in different implant surfaces .
2002 ) , in a literature review of the 
 contributing factors to early peri - implant bone loss , pointed to occlusal overload as 
 the most likely cause of this problem .
occlusal overload beyond the threshold of bone 
 homeostasis leads to progressive marginal bone resorption , and eventual osseointegration 
 failure . in order to correlate the forces transmitted by occlusal overload to the degree of bone 
 remodeling in the tissues surrounding osseointegrated implants ,
the mechanostat theory 
 proposed by frost ( 1990 ) may be used 
 to determine the maximum tension bearable by bone . in his theory
, frost proposes that 
 bone mechanical adaptation is governed by a mechanical strain threshold , which he called 
 the minimum effective strain ( mes ) .
if local strains within the bone are above mes , the 
 adaptative response occurs , but if they are below that threshold , bone remains 
 stable .
several methods of investigation and biomechanical analyses have been developed for the 
 study of implant supported prostheses . according to spiekermann , et al .
( 1995 ) , in vitro 
 techniques such as finite element analysis and strain - gauge testing can be used to 
 measure bone strain .
the use of such methods requires previous knowledge of the density 
 and modulus of elasticity of bone .
however , according to katz ( 1995 ) , bone is not homogenous and its physical 
 properties vary greatly according to species , age , gender , type of bone ( e.g. femoral , 
 mandibular , cortical , cancellous ) , and even according to the bone site from where the 
 sample is taken .
( 2000 ) measured the modulus of elasticity of trabecular bone , with 
 and without cortical plates , and concluded that human mandibular trabecular bone 
 presents a significantly higher modulus of elasticity in the anterior mandibular region , 
 and that the absence of cortical plates decreases bone modulus of elasticity .
( 1996 ) , studying the modulus 
 of elasticity of small bone specimens from four dry adult human mandibles , found that 
 elastic properties varied with both site and orientation of the specimen , reflecting the 
 complexity of the mandibular bone structure .
fresh mandibular specimens are inadequate for in vitro biomechanical 
 studies , as they show great variability for modulus of elasticity and density and 
 anisotropy .
moreover , fresh mandibular specimens have a natural viscosity that hinders 
 the attachment of strain gauges .
because of these characteristics , the application of 
 artificial test materials for in vitro biomechanical research has been 
 reported in the literature and 
 mathematical models have been developed to simulate the bone remodeling process under 
 mechanical stimulus in implant supported prosthesis . in vitro studies
require isotropic specimens with elastic 
 characteristics similar to those found in the target mandibular region .
the homogeneity 
 of polyurethane ( pu ) could favor its use in biomechanical studies of force distribution 
 on implant supported prostheses aimed at establishing correlations between strains 
 generated in the periimplant region and physiological strains as proposed by frost 's 
 theory .
based on these grounds , the purpose of this study was to validate the use of an 
 experimental polyurethane model in in vitro biomechanical studies of 
 implant - supported prostheses .
forty - five barbell - shaped , polyurethane ( axson ; cergy , france ) test specimens ( 18 mm 
 in length and 3.0 mm in diameter ) were used in this study . a 2-part male / female 
 stainless steel ( 1010/20 )
polyurethane 
 specimens were obtained by mixing 2 reagents , a and b ( a : polyol - catalyst and b : 
 diisocyanate - base ) . a / b ratio was previously determined in each group .
the mixture , 
 still in its viscous form , was injected into the mold with a hypodermic syringe .
as 
 soon as hardening was complete , specimens were removed from the mold , according to 
 the manufacturer 's instructions .
specimens were initially divided into 3 groups of 15 specimens each according to the 
 ratio of polyurethane reagents ( a / b , with a : polyol and b : diisocyanate ) . following 
 destructive tension testing ,
some specimens were excluded due to the fact that they 
 failed under considerably low forces , what was attributed to the internal bubbles 
 observed under visual analysis , which could have weakened them . as a result ,
tension testing was performed for the measurement of the modulus of elasticity in 
 each specimen .
each one of the specimens was fixed to a kratos universal testing 
 machine ( model k - 2000 mp ; kratos equipamentos industriais ltda . , so paulo , 
 sp , brazil ) , where a 500 kgf load cell pulled the specimen at a crosshead speed of 
 1.0 mm / min until rupture occurred ( figure 1 ) . 

the modulus of elasticity was then calculated based on the generated tension and the 
 linear deformation of the specimen .
generated tension was calculated as follows : polyurethane bell - shaped specimen positioned for tension tests where : t = tension [ pa ] ; so = original cross section [ m ] .
deformation was calculated as follows : where :  = deformation [ nondimensional ] ; lo = reference initial length ( load zero ) [ m ] ; l = reference length for load p [ m ] .
finally , the modulus of elasticity was calculated as follows : where :  = modulus of elasticity [ pa ] .
the values of p and l were calculated according to the elastic deformation of 
 the specimen , represented in figure 2 by the 
 curve of generated tension versus linear deformation during the 
 tension test .
area i corresponds to specimen accommodation , area ii to elastic 
 deformation , area iii to plastic deformation and area iv corresponds to fracture of 
 the specimen .
tension x deformation curve of polyurethane specimen subjected to tension 
 test anova was performed to determine statistically significant differences among groups , 
 and the tukey 's test ( p0.05 ) was used to show differences among groups .
forty - five barbell - shaped , polyurethane ( axson ; cergy , france ) test specimens ( 18 mm 
 in length and 3.0 mm in diameter ) were used in this study . a 2-part male / female 
 stainless steel ( 1010/20 )
polyurethane 
 specimens were obtained by mixing 2 reagents , a and b ( a : polyol - catalyst and b : 
 diisocyanate - base ) . a / b ratio was previously determined in each group .
the mixture , 
 still in its viscous form , was injected into the mold with a hypodermic syringe .
as 
 soon as hardening was complete , specimens were removed from the mold , according to 
 the manufacturer 's instructions .
specimens were initially divided into 3 groups of 15 specimens each according to the 
 ratio of polyurethane reagents ( a / b , with a : polyol and b : diisocyanate ) . following 
 destructive tension testing ,
some specimens were excluded due to the fact that they 
 failed under considerably low forces , what was attributed to the internal bubbles 
 observed under visual analysis , which could have weakened them . as a result ,
tension testing was performed for the measurement of the modulus of elasticity in 
 each specimen .
each one of the specimens was fixed to a kratos universal testing 
 machine ( model k - 2000 mp ; kratos equipamentos industriais ltda . , so paulo , 
 sp , brazil ) , where a 500 kgf load cell pulled the specimen at a crosshead speed of 
 1.0 mm / min until rupture occurred ( figure 1 ) . 
 the modulus of elasticity was then calculated based on the generated tension and the 
 linear deformation of the specimen .
generated tension was calculated as follows : polyurethane bell - shaped specimen positioned for tension tests where : t = tension [ pa ] ; so = original cross section [ m ] .
deformation was calculated as follows : where :  = deformation [ nondimensional ] ; lo = reference initial length ( load zero ) [ m ] ; l = reference length for load p [ m ] .
finally , the modulus of elasticity was calculated as follows : where :  = modulus of elasticity [ pa ] .
the values of p and l were calculated according to the elastic deformation of 
 the specimen , represented in figure 2 by the 
 curve of generated tension versus linear deformation during the 
 tension test .
area i corresponds to specimen accommodation , area ii to elastic 
 deformation , area iii to plastic deformation and area iv corresponds to fracture of 
 the specimen .
tension x deformation curve of polyurethane specimen subjected to tension 
 test anova was performed to determine statistically significant differences among groups , 
 and the tukey 's test ( p0.05 ) was used to show differences among groups .
figure 3 shows the modulus of elasticity ( mpa ) of 
 each specimen , means and standard deviation for each group .
maximum and minimum tension 
 forces ( fmax / fmin ) were 323.59 n/110.97 n for pu-1 ; 384.77 n/233.87 n for pu-2 ; and 
 384.91 n/263.86 n for pu-3 .
mean modulus of elasticity values and standard deviation for 
 groups pu-1 , pu-2 and pu-3 .
there was statistically significant difference between groups 1 and 2 , as well as 
 between groups 1 and 3 ( p<0.05 ; anova , tukey 's test ) .
no statistically significant 
 difference was observed between groups 2 and 3 ( p>0.05 ) .
using pu to build test specimens presented difficulties related to material viscosity , 
 bubble formation , the time length of the process and the heterogeneity found in some 
 reagent mixtures .
the greatest difficulties were encountered in pu-1 as it was more 
 prone to bubble formation than the other groups .
in addition , the altered reagent 
 proportion caused some of the specimens of this group to be rubbery . on the other hand , 

polymerization time was shorter for pu-1 than for the other groups , indicating a greater 
 amount of catalyst in its composition .
these facts together contributed to the lowest 
 mean modulus of elasticity 192.98 mpa ( sd=57.20 ) seen in pu- 1 , as compared to pu-2 and 
 pu-3 .
the best handling conditions were found in pu-2 whose polymerization time was 
 adequate and similar to that in pu-1 , whereas the modulus of elasticity was higher 
 347.90 mpa ( sd=109.54 ) than in pu-1 .
pu-3 showed the longest curing time , probably 
 because this group had the smallest amount of catalyst in the mixture , which contributed 
 for difficulties to build the specimens .
the tests in pu-3 showed mean modulus of elasticity and standard deviation values of 
 304.64 mpa ( sd=25.48 ) .
the broad variance of measured values of modulus of elasticity in 
 group pu2 compared to groups pu 1 and pu 3 can be explained by the low concentration of 
 base ( diisocyanate -pu1 ) and low concentration of catalyst ( polyol - pu-3 ) resulting in 
 a more brittle material .
anova 
 showed difference among groups and the tukey 's test showed no statistically significant 
 difference between pu-2 and pu-3 .
therefore , the increase in the b reagent concentration 
 can not be said to be directly proportional to the increase in modulus of elasticity .
the modulus of elasticity is extremely important to the validation of the material used 
 in the building of experimental models as the comparison between the values obtained 
 with those reported in the literature for mandibular bone is the basis for building 
 reproducible , easy - to - handle models of isotropic characteristics .
( 2000 ) found 
 different modulus of elasticity in different mandibular regions , which ranged from 47 to 
 2.283 mpa .
( 2000 ) 
 reported modulus of elasticity values ranging from 24.9 to 240.0 mpa , with a mean value 
 of 96.2 mpa ( sd=40.6 ) in the mandibular trabecular bone with its cortical plates . 
 without cortical plates ,
elasticity ranged from 3.5 to 125.6 mpa , with a mean value of 
 56.0 mpa ( sd=29.6 ) .
( 1996 ) , observed 
 that the modulus of elasticity of the mandible varied with bone site and orientation , 
 and that the mandibular bone presented anisotropic characteristics , reflecting the 
 complexity of its structure . in their study , these authors obtained the following 
 modulus of elasticity values : 16.9 gpa ( sd=2.7 ) ; 15.4 gpa ( sd=4.9 ) and 13.9 gpa ( sd=3.4 ) 
 in the lower , medium and upper incisal regions , respectively ; 19.4 gpa ( sd=2.5 ) , 18.8 
 gpa ( sd=3.5 ) and 12.6 gpa ( sd=4.2 ) in the lower , medium and upper premolar region , 
 respectively .
scwartz - dabney and dechow ( 2002 ) , also noted a great variation in modulus of elasticity 
 according to the mandibular region , confirming the bone heterogeneity seen by 
 katz ( 1995 ) and o'mahony , et 
 al .
( 2000 ) . by comparing the modulus of elasticity values observed in this study to those reported 
 in the literature for mandibular bone , where modulus of elasticity is known to be 
 greatly variable ,
the mean modulus of elasticity values reported here are consistent 
 with those found in the literature .
according to the highest value of 
 modulus of elasticity found in group pu 2 and based on handling conditions , where the 
 group pu-3 showed the longest curing time what contributed for difficulties to build the 
 specimens , pu-2 was the group chosen for the building of the experimental model . 
 altering the reagents ratio also resulted in excessively rubbery specimens . the reagent 
 ratio suggested by the manufacturer ( 1:1 ) proved to be the most adequate for obtaining 
 the target modulus of elasticity .
thus , the use of this material in further experimental 
 studies was considered adequate .
based on the results obtained under the proposed conditions , it seems valid to conclude 
 that : modulus of elasticity values varied according to reagent concentration in the test 
 groups studied .
however , the increase in concentration of reagent b was not directly 
 proportional to the increase in modulus of elasticity ; the 1:1 concentration for reagents a and b ( pu-2 ) showed the best mechanical and 
 handling characteristics , and should be the concentration of choice for building of 
 experimental models to be used in upcoming biomechanical studies of implant - supported 
 prostheses in the mandibular region .
 objectivesthe complexity and heterogeneity of human bone , as well as ethical issues , 
 frequently hinder the development of clinical trials . 
 the purpose of this 
 in vitro study was to determine the modulus of elasticity of a 
 polyurethane isotropic experimental model via tension tests , comparing the results 
 to those reported in the literature for mandibular bone , in order to validate the 
 use of such a model in lieu of mandibular bone in biomechanical studies.material and methodsforty - five polyurethane test specimens were divided into 3 groups of 15 specimens 
 each , according to the ratio ( a / b ) of polyurethane reagents ( pu-1 : 1/0.5 , pu-2 : 
 1/1 , pu-3 : 1/1.5).resultstension tests were performed in each experimental group and the modulus of 
 elasticity values found were 192.98 mpa ( sd=57.20 ) for pu-1 , 347.90 mpa 
 ( sd=109.54 ) for pu-2 and 304.64 mpa ( sd=25.48 ) for pu-3.conclusionthe concentration of choice for building the experimental model was 1/1 . 
INTRODUCTION
MATERIAL AND METHODS
Test Specimens
Study groups
Testing
RESULTS
DISCUSSION
CONCLUSIONS