At present, there are no standards for the measurement of primary stability. However, the most used objective simple methods are resonance frequency analysis (RFA) and the Periotest, but both frequently debated for their limitations and risks. (16; 17; 18). Insertion torque (IT) and resonance frequency analysis (RFA) are the most widely used methods to measure primary stability and they are independent and incomparable methods so their relationship has been extensively analyzed by numerous researchers and yet remains controversial. (19)
Regarding clinical evaluation, the inserted implants were left about 4 months and judged clinically by follow up to see the fate of zero inserted torque implants in comparison with others who made tests aggressive and invasive tests as histologic or microscopic analysis which was considered the gold standard method to evaluate implant stability, but it has ethical issues. Non-aggressive methods as perio test, resonance frequency test and the cutting torque resistance is not accurate enough to judge the predictability of implant success (2). The found similarities with Fu et al., who warns of the limitations of primary stability measurement instruments in bone densities. Therefore, clinical assessment of implant stability is generally subjective, observational and experience-based (20). However, it has been reported in the literature that implants that lacked or had low primary stability have comparable survival rates to those with high primary stability. (21; 22; 23; 24)
The primary stability can be measured at the time of implant placement but never after that time. An increased value of insertion torque could be due to direct friction of the implant with the dense cortical plate and not enough bone to implant contact and this can be deceiving in the giving a rock primary stability. (25)
Although the insertion torque was zero, the biological response lead to a structural direct connection between the vital bone and the surface of an implant. Successful osseointegration that comes from secondary implant stability is the main required criteria for functional dental implants where the low values of insertion torque tended to increase, in the transition to secondary stability (1).
The early osseointegration can be accelerated with the surgical protocol and the new developed implant surfaces. This can reduce the required time for healing, even to a point of immediate/early loading. (26) It is reported from the literature that machined surface implants have a low survival rate (27; 28); however, osseointegration of rough surface implants with no primary stability at placement is predictable. (22; 7; 29) Orenstein and colleagues done a study with 2770 implants that have 6 different designs. They found that the survival after 3 years for mobile implants was 79.8% while it was 93.4% for the stable implants. They concluded that the survival rate for implants could increase for those with modified implant surfaces even with low primary implant stability. (28)
There were some argue about the removal of any implant from the osteotomy site if there was any loss of primary stability and replacing its size with a bigger one. Based on the clinical findings of the small limited sample, very low or even zero insertion torque in immediate implant placement may not statistically significant in the loss of implants if the surrounding sterile socket biology, intact socket walls, thick gingival phenotype, atraumatic extraction, hand skills and absence of lateral eccentric implant movement are guaranteed. (30)
In a study by Veradi and colleagues for 11 implants with insertion torque less than 10 N-cm, he found that the survival rate was 100% and concluded that primary stability is not a prerequisite for osseointegration although there was a slight mobility to lateral force of 250 g at the time of placement. (31)