The primary stability of an implant at the time of placement is considered as one of the key factors for clinical success of implant treatment [1-6]. Orenstein et al. reported that implants that were appropriately stabilized without any mobility at the time of placement had a significantly high survival rate compared with those that were not [7].

The evaluation of the primary implant stability is usually performed after placement. Some of the main methods include mobility test, resonance frequency analysis, and the measurements of the removal torque values and the insertion torque values (ITVs). In particular, the measurement of the removal torque values is an objective evaluation method, but its clinical application is difficult because it is an irreversible and invasive method. Mobility test is useful for the evaluation of an implant whose osseointegration was surely obtained, but there is a possibility that the primary stability could decrease by the impact of the tapping head. On the other hand, the measurement of ITVs and the measurement of implant stability quotient (ISQ) values by using a resonance frequency analyzer are non-invasive, convenient, and objective evaluation methods. Therefore, these methods are used for evaluation in various researches investigating the primary stability including immediate loading implants [8-11].

The primary stability is significantly affected by bone quality. Herrmann conducted a study of the prognosis for as long as more than 5 years and reported that poor bone quality and quantity had a major impact on the long-term failure rate of implants [12]. Jaffin observed the prognosis of implants for 5 years after providing the final restoration [13]. As a result, the failure rate of the implants was 3% when the implant was placed in the alveolar bone having a thick cortical bone or otherwise a dense spongy bone even if the cortical bone was thin, whereas it was 35% when the implant was placed in the alveolar bone having both a thin cortical bone and a sparse spongy bone.