The purpose of this study was to find out the difference in the stresses induced by the one piece and the two piece dental implants supporting the All-on-4® implant supported prosthesis under simulated lateral occlusal schemes using non linear Finite Element Analysis. The null hypothesis was rejected as the one piece dental implant induced less stress values compared to the two piece dental implant for all simulated scenarios. The canine guidance simulation also resulted in less stress values compared to the group functional one.
The posterior implants were distally inclined 17 degrees in the current study as this inclination induced less stress values compared to the 45 degrees one . Owing to its biocompatibility, low density and favorable mechanical properties, titanium alloy was selected to be the implant bridge material in this study . Zirconia was also selected for its esthetical outcome. Furthermore, stress analysis studies showed comparable results between the different occlusal materials regarding the stress pattern induced by them on the supporting structures in the implant supported fixed restorations [35–37].
The magnitude, distribution and direction of loads used in this study were based on previous studies [33, 38, 40]. Delayed loading was adopted in the current study so, complete bone osseointegration with the dental implants were assumed. For more realistic simulation, nonlinear static analysis was adopted in the implant abutment complex design and the friction coefficient was set to 0.2 . The Von Mises stress values were used in this study to display the results as it is the most commonly used measurement for evaluating the yielding behavior of the materials . The maximum value of the Von Mises stress was recorded in the crestal region of the dental implants on the loaded side in the model TP during both loading scenarios. For the model OP, it was observed in the junction of the implant-abutment complex of the dental implants during both loading scenarios. However, the average stress values recorded posteriorly were higher than anteriorly in the model TP and higher anteriorly rather than posteriorly in the model OP. Similarly, the highest value for the maximum principal stress in the bone was recorded in the crest of the bone surrounding the posterior implants. The results of the current study for the model TP matches the results of several studies performed on the All-on-4® implant supported prosthesis as that published by Kucukkurt et al, Horita et al, Ayali et al, Moreira et al, Sannino et al, Lofaj et al, Liu et al and Turker et al [4, 5, 33, 38, 41–44, 46]. Regarding the stresses induced in the prosthetic screw, the maximum value for the Von Mises stress was recorded in the posterior screws for the model TP; matching the results of Ozan et al and Oh et al [6, 45]. However, for the model OP, the maximum value for the Von Mises stress was recorded in the anterior screw during both loading scenarios. Also as mentioned earlier, the anterior implants showed higher stress values than the posterior implants in the model OP. However, the difference in the average stress values between the anterior and posterior implant-abutment complexes in the model OP was less compared to the difference in the model TP. This might give a speculation for an improved load sharing between the anterior and posterior components in the model OP compared to the model TP. Such a speculation might be related to the uniform one body design of the one piece dental implant.
The higher stress value recorded posteriorly rather than anteriorly in the model TP could be attributed to the fact that the posterior implants were present in the region of load application in the group functional scenario . Furthermore, distal inclination of the posterior implant was another reason mentioned by Liu et al . On the other hand, the lever arm effect and the fact that the stress value becomes more maximized as it moves further from the fulcrum may explain the reason for the maximum stress values recorded posteriorly during the canine guided scenario . The higher stress values observed in the anterior implant system compared to the posterior one in the model OP could be due to a lesser bending moment of the implant framework in this model compared to the model TP thus a lesser stress was delivered to the posterior implant and more stress was delivered to the anterior one. The lesser bending moment of the implant bridge might be related to the stronger body design of the underlying one piece dental implant in addition to the short lever arm.
The canine guidance loading scenario showed less stress value compared to the group functional loading scenario in this study. Gore et al showed similar result in their dynamic Finite Element Analysis study comparing both occlusal schemes in implant supported fixed prostheses as well . Similar findings were also reported by Turker et al when they compared different occlusal schemes in the All-on-4® implant supported prosthesis [44, 46]. They related the lower stress values in the canine guided simulation to the anterior and posterior disocclusion of all the teeth except the canines during lateral movements of the mandible. Moreover, Abdou et al stated in their systematic review that the group functional occlusion had double the stress values of canine guidance during lateral movements . Moreover, more marginal bone loss was reported to take place in the implant supported fixed partial dentures having the group function occlusal scheme compared to those with the canine guidance occlusion. This was attributed to the greater occlusal stresses exerted in group functional occlusion. Another reason was the increased possibility of contact with the opposing teeth in the non functional lateral movements .
In all loading scenarios, the model TP showed higher stress values compared to the model OP regarding the implants, bone, and screws. Such a result came in line with Cehreli et al and Hajimiragha et al who showed lower stress values induced on the bone and the implants for the one piece dental implants in comparison to the two piece ones. This can be explained in the fact of the strong one body design and the improved mechanical properties for the one piece implant compared to the two piece one. The absence of the abutment screw contributes to such improved mechanical properties in the one-piece implant as well [14, 15, 17].
Regarding the clinical relevance of the current study results, the difference in the pattern of periimplant load distribution between both virtual models in the current study may account for the different bone remodeling that occurs clinically between them in addition to the crater like defects that appeared around the one piece dental implant. Furthermore, in the clinical setting the lower stress values recorded for the one piece monophasic dental implant may reduce the risk of component fracture; especially in the posterior region where the highest stress value was recorded in all Finite Element Analysis studies related to the All-on-4®implant supported prosthesis. Moreover, the lesser stress value recorded in the region of periimplant crestal bone in the model OP may help to reduce the rate of marginal bone loss. However, it has to be mentioned that the stress level is not the only factor that affects marginal bone loss since further factors as the design of the implant platform, presence of cantilevers, occlusal forces in addition to the implant number and diameter have their effect too . Moreover, systematic reviews dealing with the issue of marginal bone loss in the one piece and two piece dental implants reported controversial results and concluded that both implant types showed no difference in their effect on marginal bone loss [16, 17]. However, such results should be held with caution due to the possible heterogeneity of the studies enrolled in the systematic reviews. The less stress values induced in the prosthetic screw in the one piece dental implants may be accompanied clinically with a reduced risk of screw loosening and thus a decreased incidence of prosthesis movement, soft tissue irritation and patient apprehension. The reduced levels of the marginal bone loss in the implant supported fixed prosthesis having canine guided occlusion reported by Koller et al might be related to the lower stress values for a such occlusal scheme compared to the group functional one. So, the canine guided occlusion could be suggested clinically to have lesser biological and mechanical complications in the light of the results of the current study and that published by koller et al and Abdou et al [30, 32]. Moreover ,in the light of the current results for the dental implant design, the authors also prospect that there may be a lesser bending moment of the prosthetic superstructures when the one piece dental implant is used in the All-on-4® implant supported prosthesis.
Although standardization and variables control can be achieved in Finite Element Analysis studies yet, this study has several limitations. Static loading was applied for simplification even though loading is dynamic during chewing functions. Moreover, it was proposed that the dental implants were completely osseointegrated with bone however, this does not simulate the natural situation. Also, the material properties of bone were linearly elastic and isotropic in this study yet, this does not come in line with consistent living tissue simulation. Furthermore, when clinically compared to the two piece dental implants, the one piece ones did not have a reduced rate of marginal bone loss and showed an increased risk of screw loosening when an intermediate abutment was used. So, the results of the current study which showed higher stress values for the two piece dental implants compared to the one piece monphasic ones do not suggest more biological and mechanical complications clinically. However, randomized clinical trials are needed to compare both implant designs regarding their biological and mechanical complications in addition to the technical limitations when used in the All-on-4® implant supported prosthesis.