Continuous efforts are exerted to maximize the survival rate of the implant itself, abutment screw, implant abutment connection, and the superstructure and also to minimize the problems that frequently accompany the treatment with dental implants.
The clinician must recognize the possible forces that will be acting on the screw joint so that screw loosening and other possible complications can be minimized or avoided.
The implant system used in this study provides the criteria of having various screw diameters, for standard and narrow implant fixtures; this allows accurate evaluation of the biomechanics of SIs and NDIs [36]. Most of implant systems introduce all implant diameters with the same dimensions of the screw; for this reason, most of NDIs showed many complications when they are tested in many researches.
Implants were placed into acrylic resin as according to De Carvalho et al. [37] who used acrylic resin to be subjected to cyclic loading because acrylic resin has enough flexural strength making it sufficiently tough to allow cyclic testing .Also, its modulus of elasticity (3.4 × 105 lb/in.2) is quite close to that of cancellous bone (3.6 × 105 lb/in.2).
Conical hybrid connection was selected because it was reported that, among the different internal connection types, the conical hybrid connection showed the best stress distribution as it has a mechanical friction grip that enhance resistance to the lateral forces decreasing the probability of screw loosening, so it was the best connection to be used [38, 39].
The desired design and dimensions of metal tube were designed by the CADCAM software (Dentcreate, Exocad) in wax (CopraDur, White peaks dental solution, Germany), with flat occlusal surface (10 mm in diameter) which was parallel to the horizontal plane and perpendicular to the implant fixture long axis to permit contact with the testing machine piston in a flat horizontal plane. In the center of the flat occlusal surface, a small rounded hole was designed exactly opposite to the abutment screw hole that facilitates screw driver accessibility for easy tightening and removal. Then, this accurately designed wax pattern was casted to a nickel chromium alloy tube [38].
CAD/CAM system was to ensure standardization as CAD/CAM system has the ability to produce physical models using digital methods instead of traditional impression techniques with high error rate, time consuming procedures, and lack of accuracy and standardization [40].
Ten minutes interval was left after first torque application, and all screws were retightened to the same tightening torque (30 Ncm), to compensate for the preload loss due to settling effect of screw thus ensure achieving optimal preload.
Tightening and removal of abutment screws in all the groups was done with a digital torque gauge instead of a hand torque wrench to eliminate the possibility of deviations from exact torque value which gives decimal precision for accuracy and standardization [41, 42]. The samples were placed in a rigid mounting jig to ensure solid fixation without rotation during tightening and removal of the screws [37].
Load was applied eccentrically at a distance of 5 mm away from the center of abutment [43, 44] to simulate lateral component of intraoral forces that have critical effects on joint instability [45, 46]. A better stress distributions when lateral external force components act on the prosthetic abutment.
For all test groups, there were significant differences in RTL ratios before and after application of DCL. These findings emphasize on the occurrence of screw loosening process.
The screws of NDIs are comparable with standard ones because the implant system used in this study provided narrow screws with narrow fixtures and standard screws for standard fixtures on contrast with most of implant systems that provide the same screw diameter for both narrow and standard implants. Standard screw within narrow fixtures (relatively thin walls) makes the force transmission more destructive and shows more biomechanical complications that were reported in most of studies worked on NDIs. This finding is supported by Patterson and Johns [47] who stated that failures are due to metal fatigue and occur under repeated loads at levels below the maximum strength of the material when worked on metal fatigue failure of the gold screw used to retain a fixed prosthesis to Brånemark osseointegrated fixtures/abutments; they emphasized on the necessity of screw design and applying the correct torque to achieve a long fatigue life for the screw.
The results of this study showed that the screw loosening process occurred in both SIs and NDIs with non-significant difference; this can be due to use of various abutment screw diameter. The retained preload inside the standard screw provided more screw stability due to the relative increase in material thickness for the standard screw. In other words, when forces are greater than usual, a larger diameter screw will decrease the risk of loosening or fracture, so the standard screw has superior biomechanics over the narrow one; this finding matches the statements of Byrne et al. [48] who states that, the greater the joint preload, the greater the resistance to loosening, and the more stable the joint that was after using the same screws that were loosened and retightened; tightening was on three occasions to the three insertion torques; they revealed the higher preloads generated using the gold-coated screw with both abutment types; the screw design was the crucial factor not the abutment.
The results of this study revealed that mean percentage initial removal torque loss (% RTL) of NDIs is higher than (% initial RTL) of SIs while mean percentage post load removal torque loss after exposure to dynamic cyclic loading is non-significantly different. This can be explained that the same tightening torque (30 Ncm) is applied for both diameters, so the initial tightening torque is relatively high for NDIs, and this is one of the suggested solutions for decreasing the chance of screw loosening process. This explanation is supported by Siamos et al. [49] who stated that increasing the torque value for abutment screws above 30 Ncm can be beneficial for abutment-implant stability and to decrease screw loosening. On the other hand, Jaarda et al. [50] have concluded that altering the preload torque applied to Nobelpharma gold-retaining screws did not affect their ultimate tensile strength. The ultimate tensile strength of the screws from the two lots used in the study differed, suggesting an unannounced change in component specifications. The mechanical integrity of the abutment/implant system depends on two factors: the contact area between the components and the screw’s effectiveness [51]. When two metal surfaces are in contact, adhesion and friction forces do limit the movement between them. When there is full contact between the surfaces, elongation properties of the screw will increase loosening resistance due to higher contact forces over the screw [52].
Another study has shown that retorque does not significantly interfere on the loosening torque when the titanium screws are used in dentures with passive fit. On the other hand, the retorque significantly increased the loosening torque when these screws were used in dentures with misfit [28].
Replacing the old abutment screw with new one showed better results in SIs than NDIs with non-significant difference after two exposures of DCL; this is due that retaining much preload within standard screws will in turn increase the clamping force between screw threads and internal threads of screw channel making it more liable to surface flattening and excessive wear that is considered actual screw failure and requires its replacement with new functioning one. This finding is supported by Haak et al. [7] who discussed the elongation and preload stress in dental implant abutment screws and concluded that tightening the screws beyond recommended levels may be beneficial without producing plastic deformation.
Other authors confined replacing the old screw with new one to the fractured abutment screw that no more performs function under loading. Fracture of the implant abutment screw can be a serious problem, as the fragment remaining inside the implant may prevent the implant from functioning efficiently as an anchor.
Flanagan [53] had introduced many cases of fractured abutment screw and also introduced a technique for abutment, fragment retrieval, crown-abutment separation, crown recementation, and over denture retainer fracture. Nergiz et al. [54] had also introduced a clinical method of removal of a fractured implant abutment screw with successful utilization of the existing prosthesis. Reyhanian et al. [55] also managed the fractured abutment screw with replacing it by new one with care to avoid any fracture of implant abutments and to use the repair kits offered by some implant systems, such as ITI® Dental Implant System (Institut Straumann AG, Switzerland), consists of drills, two drill guides, and six manual tapping instruments, IMZ® (TwinPlus Implant System DENTSPLY Friadent, Germany), only in exceptional circumstances.
This finding is supported by Barbosa et al. [29] who evaluated the screw loosening on new abutment screws and after successive tightening and concluded that loosening percentage of the initial torque is smaller when using screws that already suffered application of an initial torque staying stable after successive tightening procedure. This sign was proven by the SEM (Scanning Electron Microscopic) analysis, which showed removal of the screws spirals irregularities after successive torques. Such event could explain why the values of detorque increased after the second detorque, and the samples remained constant in the subsequent detorques in all body tests. The removal of the surface irregularities must allow less friction between the screw surface and the internal implant surface, favoring the screw sliding and a higher preload transmission.
But this finding is opposed by Bacchi et al. [56] who stated that use of conventional titanium screws for fixation of universal abutments provides higher loosening torque values even after application of DCL, irrespective of the technique applied. The suggested reason for this was that the application of a longer torque period or retorque once again after embedment relaxation or settling would act to regain preload and to increase contact area between the threads. This study evaluated full arch prostheses supported by five implants. Full-arch prostheses are more likely to dissipate cyclic loading along all components, reducing the effect of loads on the screws in comparison to single-crowns.
Another study has shown that retorque does not significantly interfere on the loosening torque when the titanium screws are used in dentures with passive fit. On the other hand, the retorque significantly increased the loosening torque when these screws were used in dentures with misfit [28].
