prostheses utilizing screws are largely utilized for multiple and single units because the screws can be easily accessed without damage to the prostheses1 . this is important because although we have historical clinical success of implantsupported rehabilitations , some adverse outcomes still occur , such as loosening or fracture of the prosthetic screws2 . the stability of the retention screw is related to several factors , including the geometrical shape , thread format , fit of the prosthetic component , frictional coefficient of the screw , amount and properties of lubricant , speed of tightening , tightening force used , and occlusal loads applied3 . when torque is applied to screws that retain a non - passive framework , after the initial torque is applied , a compressive force is generated on the screw . the tension created at the thread surfaces of the retention screw , the so - called preload , is responsible for keeping the components together . the stability is directly related to the stretching of the screw and the maintenance of the retaining screws . in such cases , if the preload decreases below a critical level , the stability of the screw joint will be compromised4 . the aim of the study was to evaluate the stability of prosthetic screws used within three types of craniofacial prostheses retention systems ( bar - clip , ball / o - ring , and magnet ) when submitted to mechanical cycling . to conduct the present study , twenty - four craniofacial implants of dimensions 46 mm ( habe implants - facie line ; inp system , so paulo , brazil ) utilized for facial prosthesis were used . three retained auricular prosthesis implant - supported systems , the bar - clip system , magnet system , and ball / o - ring system , were considered as study factors . twelve cylinders 2.03.8 cm ( heightdiameter ) filled with self - polymerized colorless acrylic resin ( vipi flash ; dental vipi , so paulo , brazil ) were used to replicate the fixation of two craniofacial implants ( habe implants - facie line ) and its connection components . the craniofacial implants were positioned parallel at 20 mm apart in a polyvinyl chloride cylinder . to capture the retained auricular prosthesis systems , twelve cylinders with similar characteristics were filled with self - polymerized acrylic resin , and then the respective retention system was positioned . three groups were present based upon the retention systems tested ( 4 devices with 8 implants in each group).(fig . 1 ) two craniofacial implants and a bar - clip retention system were used with a bar of 20 mm for the overdenture and three straight plastics clips ( inp system ) . the custom bars were fixed to the implants via ucla ( inp system ) and cast in nickel - chromium alloy . the clips were placed on the metal bars and fixed to the holders with self - polymerized acrylic resin . two craniofacial implants and an o - ring retention system with two retaining balls , two spacers rings , and two rubber rings retained in a metal capsule ( inp system ) were required . the retaining balls were screwed to the implants with a torque of 30 ncm using a torque wrench ( inp system ) . the o - rings were placed over the spacers , and then captured with self - polymerized acrylic resin . the magnet system contained craniofacial implants and a magnet retention system with three 4.02.0 mm neodymium - ironboron magnets coated with nickel ( metalmag , so paulo , brazil ) . a plastic structure containing three apertures was fixed to the implants via ucla ( inp system ) , and then sent to the lab for casting of nickel - chromium alloy . three magnets were glued to the metallic structure and another three magnets where positioned over the glued magnets in order to capture the holders . for torque and detorque readings , all retention systems were screwed onto their respective implants using a digital torque meter with 0.1 n / cm precision ( torque meter tq-8800 ; lutron , taipei , taiwan ) to 30 ncm ( torque recommended by the manufacturer inp system ) for each screw . 2 ) the retention systems were submitted to cyclical mechanical loading using the servo - hydaulic machine mts 810-flex test 40 ( mts systems , eden prairie , mn , usa ) , which was calibrated to operate with a 1.5 kn load cell at 0.5 hz and performed movement along the long axis of the implants . to adapt the samples to the servo - hydraulic machine , a two - piece metal device was prepared . each part has a cylindrical portion , where the samples were positioned and secured by four screws located around the cylinder . in the lower portion of these devices , a cylindrical metal bracket allowed for attaching the samples to the mts machine . in the device located at the bottom of the machine , after this procedure , the samples were first inserted into the lower cylinder component , and then the load cell was slowly lowered to the upper cylinder locking position , thus guaranteeing adaptation between the prosthetic components . therefore , both cylinders containing the samples were fixed in their respective devices . each replica was subjected to the pull - out test to determine the tensile strength corresponding to three years of prosthesis usage , which assumes that an auricular prosthesis is removed three times a day and the change of prosthesis should be performed after two to three years of use due to progressive change in texture and color567 . the lifetime of a prosthesis was based on results from hooper et al.8 that reported a mean lifetime of 14 months for implant - retained extraoral prostheses . another study demonstrated a mean time of 17 months for implant - retained auricular prosthesis9 and a recent study indicated that a new prosthesis should be made every 1.5 to 2 years6 . the test samples underwent 1,080 cycles , after which the detorque value was measured and a 30 ncm re - torque was applied to the connector screw . this procedure was repeated after the corresponding cycles representing two years ( 2,160 cycles ) and three years ( 3,240 cycles ) . data analysis was performed using the computer programs biostat 5.0 ( belm , brazil ) and pasw statistics 18.0 ( ibm co. , armonk , ny , usa ) . the detorque values were submitted to the shapiro - wilk and levene method ; despite having homogeneity of variances p>0.05 , the sample did not show a normal distribution ( p<0.05 ) . thus , the data were evaluated via comparative analysis in relation to the number of cycles ( one year , two years , and three years ) within each type of retention system ( bar - clip , ball / o - ring , and magnets ) by the kruskal - wallis test and post hoc duncan test . to conduct the present study , twenty - four craniofacial implants of dimensions 46 mm ( habe implants - facie line ; inp system , so paulo , brazil ) utilized for facial prosthesis were used . three retained auricular prosthesis implant - supported systems , the bar - clip system , magnet system , and ball / o - ring system , were considered as study factors . twelve cylinders 2.03.8 cm ( heightdiameter ) filled with self - polymerized colorless acrylic resin ( vipi flash ; dental vipi , so paulo , brazil ) were used to replicate the fixation of two craniofacial implants ( habe implants - facie line ) and its connection components . the craniofacial implants were positioned parallel at 20 mm apart in a polyvinyl chloride cylinder . to capture the retained auricular prosthesis systems , twelve cylinders with similar characteristics were filled with self - polymerized acrylic resin , and then the respective retention system was positioned . three groups were present based upon the retention systems tested ( 4 devices with 8 implants in each group).(fig . 1 ) two craniofacial implants and a bar - clip retention system were used with a bar of 20 mm for the overdenture and three straight plastics clips ( inp system ) . the custom bars were fixed to the implants via ucla ( inp system ) and cast in nickel - chromium alloy . the clips were placed on the metal bars and fixed to the holders with self - polymerized acrylic resin . two craniofacial implants and an o - ring retention system with two retaining balls , two spacers rings , and two rubber rings retained in a metal capsule ( inp system ) were required . the retaining balls were screwed to the implants with a torque of 30 ncm using a torque wrench ( inp system ) . the o - rings were placed over the spacers , and then captured with self - polymerized acrylic resin . the magnet system contained craniofacial implants and a magnet retention system with three 4.02.0 mm neodymium - ironboron magnets coated with nickel ( metalmag , so paulo , brazil ) . a plastic structure containing three apertures was fixed to the implants via ucla ( inp system ) , and then sent to the lab for casting of nickel - chromium alloy . three magnets were glued to the metallic structure and another three magnets where positioned over the glued magnets in order to capture the holders . two craniofacial implants and a bar - clip retention system were used with a bar of 20 mm for the overdenture and three straight plastics clips ( inp system ) . the custom bars were fixed to the implants via ucla ( inp system ) and cast in nickel - chromium alloy . the clips were placed on the metal bars and fixed to the holders with self - polymerized acrylic resin . two craniofacial implants and an o - ring retention system with two retaining balls , two spacers rings , and two rubber rings retained in a metal capsule ( inp system ) were required . the retaining balls were screwed to the implants with a torque of 30 ncm using a torque wrench ( inp system ) . the o - rings were placed over the spacers , and then captured with self - polymerized acrylic resin . the magnet system contained craniofacial implants and a magnet retention system with three 4.02.0 mm neodymium - ironboron magnets coated with nickel ( metalmag , so paulo , brazil ) . a plastic structure containing three apertures was fixed to the implants via ucla ( inp system ) , and then sent to the lab for casting of nickel - chromium alloy . three magnets were glued to the metallic structure and another three magnets where positioned over the glued magnets in order to capture the holders . for torque and detorque readings , all retention systems were screwed onto their respective implants using a digital torque meter with 0.1 n / cm precision ( torque meter tq-8800 ; lutron , taipei , taiwan ) to 30 ncm ( torque recommended by the manufacturer inp system ) for each screw . 2 ) the retention systems were submitted to cyclical mechanical loading using the servo - hydaulic machine mts 810-flex test 40 ( mts systems , eden prairie , mn , usa ) , which was calibrated to operate with a 1.5 kn load cell at 0.5 hz and performed movement along the long axis of the implants . to adapt the samples to the servo - hydraulic machine , a two - piece metal device was prepared . each part has a cylindrical portion , where the samples were positioned and secured by four screws located around the cylinder . in the lower portion of these devices , a cylindrical metal bracket allowed for attaching the samples to the mts machine . in the device located at the bottom of the machine , after this procedure , the samples were first inserted into the lower cylinder component , and then the load cell was slowly lowered to the upper cylinder locking position , thus guaranteeing adaptation between the prosthetic components . therefore , both cylinders containing the samples were fixed in their respective devices . each replica was subjected to the pull - out test to determine the tensile strength corresponding to three years of prosthesis usage , which assumes that an auricular prosthesis is removed three times a day and the change of prosthesis should be performed after two to three years of use due to progressive change in texture and color567 . the lifetime of a prosthesis was based on results from hooper et al.8 that reported a mean lifetime of 14 months for implant - retained extraoral prostheses . another study demonstrated a mean time of 17 months for implant - retained auricular prosthesis9 and a recent study indicated that a new prosthesis should be made every 1.5 to 2 years6 . the test samples underwent 1,080 cycles , after which the detorque value was measured and a 30 ncm re - torque was applied to the connector screw . this procedure was repeated after the corresponding cycles representing two years ( 2,160 cycles ) and three years ( 3,240 cycles ) . data analysis was performed using the computer programs biostat 5.0 ( belm , brazil ) and pasw statistics 18.0 ( ibm co. , armonk , ny , usa ) . the detorque values were submitted to the shapiro - wilk and levene method ; despite having homogeneity of variances p>0.05 , the sample did not show a normal distribution ( p<0.05 ) . thus , the data were evaluated via comparative analysis in relation to the number of cycles ( one year , two years , and three years ) within each type of retention system ( bar - clip , ball / o - ring , and magnets ) by the kruskal - wallis test and post hoc duncan test . the mean and standard deviation of the detorque value according to the type of retention system and the number of cycles are shown in table 1 . it was observed that some screws showed total loosening after completion of the first year cycling test , with a higher number in the bar - clip group ( n=5 ) . there were no statistically significant differences when comparing the detorque values in relation to the number of cycles ( one , two , and three years ) with bar - clip ( kruskal - wallis , p=0.66 ) and magnet retention systems ( kruskal - wallis , p=0.20 ) . however , for the ball / o - ring retention system , a statistically significant difference was observed between the first year and second year , while the highest detorque was observed in the first year ( z=2.72 ; p<0.05 ) . when comparing the torque values to the type of retention system , a statistical difference was observed for the first cycle ( kruskal - wallis , p=0.02 ) ; the detorque value was greater in the ball / o - ring retention system when compared to the bar - clip retention system ( z=2.81 ; p<0.05 ) . for the second cycle , a statistically significant difference was noted between the three groups , with higher values in the bar - clip group as compared with ball / o - ring ( p<0.05 ) and magnets groups ( p<0.05 ) . there was no difference between groups for the third year of cycles ( kruskal - wallis , p=0.102 ) ; results are presented in table 2 . this study evaluated the loosening of screws and the influence of three different retention systems . the hypothesis of the study was that the different prosthesis retention systems did not influence the loosening of prosthetic screw . there were no statistically significant differences when detorque values were compared between the bar - clip and magnet retention systems , but statistical difference was found when comparing the first and second cycles of the ball / o - ring retention system . the detorque value refers to the force required to remove the screw , so when this force is greater , it means that the screw has lost only part of the initial retention torque applied ( 30 n ) . in the first year of cycles , we observed a greater loss of the initial screw torque for the group retained by a bar - clip compared to a ball / o - ring . however , in the second year of cycling , we observed that the group retained by a bar - clip did not lose retention as noted in the first years . this result could be due to the plastic deformation of the clip and consequent loss of retention . for the group retained by a ball / o - ring , the contrary effect was noted with an increase of retention between the first and second years . despite these results , we did not find the same tendency in the third year of cycles , which could be due to possible wear on the internal threads of the implant . the results demonstrate that the more times a screw is removed and reinserted , the less torque that is required to remove it again . these results are similar to those obtained by weiss et al.10 in 2000 and cardoso et al.11 in 2012 , who also found an inverse relationship between the tightening / loosening force and the number of cycles . however , this relationship is not true for the ball / o - ring group , which showed an increase of retention between the first and second year cycles . these results could be due to degradation of the plastic parts , which tend to suffer deformation due to forces generated during the test . these changes may have caused an increase in the retaining force and screw loosening concomitantly , a phenomenon already observed in another study using electron microscopy12 . another probable cause of increased retention is the thermal expansion of plastic during the test13 . in this study , all retention systems had a smaller detorque value than the initial tightening torque ( 30 ncm ) . cho et al.14 confirmed that screw loosening is caused by an inadequate tightening torque , settling of implant components , inappropriate implant position , inadequate occlusal scheme or a crown anatomy , poorly fitting frameworks , improper screw design / material , and heavy occlusal forces . the resistance to opening torque is a direct function of the tension in the screw and the frictional resistance of the components , and is inversely related to the vertical force vector that results from the thread 's radial and tangential slopes . because the vertical force vectors resulting from the threads ' radial and tangential slopes tend to open the screw , maintaining the preload relies mainly on the component of friction10 . during this study , the removal and insertion axis were always perpendicular to the base of the retention system . this condition is not necessarily true during clinical use , because the retention system may undergo small dislocations and lead to deformations of the retention system , faster loss of retention , and hence a decrease in clinical longevity1516 . when the prosthetic screw is tightened , a preload is produced , causing tension in components that create friction . in cases where the adaptation is tolerable thus , the screw ultimately receives all of the load and its fatigue resistance becomes low . when an external load is applied to the components that are joined , the tension on the screw increases further . according to the same authors , in cases where the precision of the prosthesis is adequate , the screws provide a fatigue resistance of at least twenty years . in cases where the adaptation of the prosthesis is deficient , this time as presented by weiss , all of the screws showed a progressive decrease in opening torque . to minimize screw loosening , weiss also recommends that the number of closing / opening cycles in clinical and laboratory procedures before final prosthetic closure should be minimized . no other study has assessed the effect of replacing a screw with a new screw . in 2012 , cardoso et al.11 found no evidences that the replacing of a prosthetic screw procedure alone increases the resistance to loosening , perhaps because a modification had already occurred , possibly due to wear of the internal threads of the implant . apparently , it is more important to restrict the number of removals than to replace the screw when inserting the definitive restoration . a previous study evaluated the effect of retightening sometime after the initial tightening torque on the stability of the prosthetic screws , and concluded that retightening represents an easy and fast method to increase stability . however , it has been observed that holding the torque meter for a period of time during the tightening of the screws could provide an elongation of the screw and increased pre - load1819 . the coating of the screw surface with diamond - like carbon film is one of the methods employed and already commercially available . being better than non - coated screws due to its capacity to reduce stresses during tightening and offering a higher preload , it may equally reduce the frictional resistance of the screw to removal . however , there is no consensus in the literature regarding the benefits of coated screws for prosthetic fixation1 . this study concluded that as follows : ( 1 ) all prosthetic screws will have a loosening trend after initial tightening torque . ( 2 ) the bar - clip retention system yields more loosening of the screws when compared with ball / o - ring and magnet retention systems during the first year of prosthesis usage . ( 3 ) the ball / o - ring system seems to have a better retention of prosthesis over the time , which means a lower detorque . however , it was not possible to establish whether this trend is stable over the long - term , as it was not present in third year of cycles .
objectivesthis study aimed to evaluate the stability of prosthetic screws from three types of craniofacial prostheses retention systems ( bar - clip , ball / o - ring , and magnet ) when submitted to mechanical cycling.materials and methodstwelve models of acrylic resin were used with implants placed 20 mm from each other and separated into three groups : ( 1 ) bar - clip ( sistema inp , so paulo , brazil ) , ( 2 ) ball / o - ring ( sistema inp ) , and ( 3 ) magnet ( metalmag , so paulo , brazil ) , with four samples in each group . 
 each sample underwent a mechanical cycling removal and insertion test ( f=0.5 hz ) to determine the torque and the detorque values of the retention screws . a servo - hydraulic mts machine ( 810-flextest 40 ; mts systems , eden prairie , mn 
 , usa ) was used to perform the cycling with 2.5 mm and a displacement of 10 mm / s . 
 the screws of the retention systems received an initial torque of 30 ncm and the torque values required for loosening the screw values were obtained in three cycles ( 1,080 , 2,160 , and 3,240 ) . 
 the screws were retorqued to 30 ncm before each new cycle.resultsthe sample was composed of 24 screws grouped as follows : bar - clip ( n=8 ) , ball / o - ring ( n=8 ) , and magnet ( n=8 ) . 
 there were significant differences between the groups , with greater detorque values observed in the ball / o - ring group when compared to the bar - clip and magnet groups for the first cycle . 
 however , the detorque value was greater in the bar - clip group for the second cycle.conclusionthe results of this study indicate that all prosthetic screws will loosen slightly after an initial tightening torque , also the bar - clip retention system demonstrated greater loosening of the screws when compared with ball / o - ring and magnet retention systems .