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1. Introduction
Dental implants are currently accepted as a predictable treatment option for the rehabilitation of both partial or total edentulism. Moreover, immediate and early loading protocols have been introduced into clinical practice in the attempt to shorten treatment time and minimize patient discomfort, with positive results [1]. During the early phases of healing, dental implants should be protected from detrimental micromovements [2, 3] which, according to the literature, should not exceed values ranging between 50 and 150 μm to avoid risks for the osseointegration process [4, 5]. When exceeding this threshold, there is a concrete possibility that the bone-implant interface could be colonized by fibroblasts from the overlying connective tissue, with consequent implant encapsulation in fibrous tissue and clinical failure [6]. In this scenario, the role of primary stability has become extremely important and, in recent years, many studies focused on this crucial topic [7–9]. Primary stability is a surgical outcome due to the mechanical engagement between implant and host bone, being influenced by surgical technique and by fixture and recipient bed characteristics. Numerous noninvasive methods have been proposed to evaluate implant stability, including Periotest [10] and Dental Fine Tester and Implatest conventional impulse testing [11], but the most widespread techniques are implant insertion torque measurement (IT) [12] and resonance frequency analysis (RFA) [13]. Insertion torque is the measure of the frictional resistance encountered by the implant while moving forward apically through a rotatory movement on its axis. RFA is performed by measuring the response of a magnetic device screwed on the implant when excited by a vibration consisting of small sinusoidal signals. The peak amplitude of the response is recorded and encoded into a parameter called implant stability quotient (ISQ), ranging from 0 (minimum stability) to 100 (maximum stability).
The correlation between IT and ISQ has been investigated in numerous studies but it is still unclear: according to some authors the two parameters are in a direct relationship [14, 15], and other studies demonstrated no statistically significant correlations between them [16, 17].
Furthermore, it must be considered that implants with different characteristics...