Previous studies have shown that direct correlations exist between microstrain magnitudes and bone stability/instability conditions. This has been summarized by Frost, when bone is loaded below about 2000 microstrains, bone can easily repair what little microdamage occurs. Yet, when pathologic overloading occurs (over 4000 microstrains), stress and strain gradients exceed the physiologic tolerance threshold of bone and cause micro-fractures at the bone-implant interface [17]. Thus, the maximum normal stress criterion, 3000 μɛ, was used to evaluate the extent of the regions where the normal stresses were beyond the allowable tensile and compressive values in the cortical bone [12].

So the implant design that experienced the least overall amount of strain was thought to represent the best design, at least in terms of stress distribution.

Results revealed that all implant designs with different superstructure materials and under different loading conditions resulted in peri-implant microstrain values which were within the physiologic loading zone, below 3000 μɛ, except for the standard sized implants supporting Lava Ultimate crowns under axial and off-axial loading.

Regarding the effect of implant design on peri-implant microstrains induced in the present study, standard diameter implant showed the highest microstrain values regardless of other variables (3362.4 ± 757.4 μɛ). Microstrain value exceeded the physiologic limit in the standard implant supporting Lava Ultimate crowns. That was in approval with Balshi et al. who stated that replacing a lost molar with only one implant represents a biomechanical challenge [18]. This might be attributed to the differences in the size and morphology of natural tooth roots and the standard size implants (3.75 or 4 mm), thus providing insufficient support [19]. In the present study regardless of other variables, double mini implant showed statistically significantly lower mean microstrains (801.6 ± 251.4 μɛ) than standard implant. Moreover, the double mini implant showed statistically significantly lowest microstrain values with metal and Lava Ultimate crowns under axial loading. Under off-axial loading, it also showed statistically significant lower microstrains value than standard implant. Moreover, the use of two implants provides more surface area for osseointegration and spreads the occlusal loading forces over a wider area while reducing the potential bending forces that would exist in a single-implant molar restoration [1, 18, 20, 21]. The one-piece design of small-diameter implants (1.8–3.0-mm diameter) provides strength to the implant in comparison with small diameter two-piece implants [22]. According to Misch [23], a solid implant with a 1.23-mm diameter has the same resistance to bending fracture as the annulus region of a 3.75-mm traditional design. Moreover, a solid 3-mm implant has an approximately 340% increase in moment of inertia over the 3.75-mm traditional two-piece root form at the annulus position. Generally, short-wide implant resulted in the lowest microstrain values in comparison with the other two implant designs. The reduced strains associated with wider implants may be due to the increased structural capacity and the enlarged resin-implant contact area offered by these implants, resulting in lower torque effect in conjunction with off-axial loading [20]. Accordingly, Balshi et al. [18] indicated that a molar crown supported by a standard or narrow size implant can easily introduce large bending moments to bone because the dimensions of crown are usually greater than the diameter of the implants. Thus, the wide implant is suggested for placement at the molar region to reduce the possibility of overload. The area that transfers the compressive and tensile loads to bone, that is, functional surface area, was proved to be confined to the crestal 5–7 mm [24,25,26,27]. Thus, short implant with a wider diameter provides both improved primary stability and increased functional surface area as it allows engagement of a maximal amount of bone and better distribution of stress in the surrounding bone compared with increases in implant length [28,29,30,31,32]. An increase in diameter by 1 mm will increase the surface area by 30–200% depending on the implant design [33]. Moreover, according to Misch [34], the large-diameter implants which have a larger prosthetic platform exhibit less force transmission.