## 4 Discussion  
This study has evaluated the influence of implant dimensions (length and diameter) on the primary and secondary stability of implants placed in the upper arch. The results of the present study suggest that the ISQ values significantly increase during the three months of follow-up. The findings also include significant differences between some parameters analysed (implant diameter, implant length and maxilla regions). Bone density seems to strongly influence implant stability and long-term success, as demonstrated by the higher implant survival rates in the mandible compared to the maxilla, especially the posterior maxilla [24]. Bone density can be objectively measured with different methods, including microCT [25] that may define the bone quality, even if the concept of \"bone quality\" is not clearly defined in literature. Currently the most accepted method to assess the bone quality is the one proposed by [PERSON] and [PERSON] [23], which give a scale from 1 to 4 based on the amount of cortical and trabecular bone \begin{table}
\begin{tabular}{c c c} \hline \hline
**Maxilla Region** & **ISQ1** & **ISQ2** \\ \hline Frontal & 75.59 (5.57) & 80.05 (4.97) * \\ Posterior & 75.21 (5.62) A * & 79.44 (6.24) B \\ \hline \hline \end{tabular}
\end{table} 
Table 4: [PERSON]’s multiple comparison test of ISQ1 and ISQ2 values for different maxilla regions (frontal and posterior). Numbers are means and values in brackets are standard deviations. Uppercase letters indicate statistically significant differences between the ISQ 1 or 2 values within the maxillary region. * indicates statistically significant differences between ISQ 1 and 2 values across the maxillary regions. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Implant Length (mm)** & **ISQ1** & **ISQ2** \\ \hline
9 & 73.5 (5.91) aA * & 80.93 (4.28) aB ** \\
11 & 74.95 (5.95) aA ** & 79.25 (4.49) aB * \\
13 & 77.88 (3.13) aB & 79.44 (9.31) aA \\ \hline \hline \end{tabular}
\end{table} 
Table 3: [PERSON]’s multiple comparison test of ISQ1 and ISQ2 values for 9-, 11- and 13-mm implants. Numbers are means and values in brackets are standard deviations. Lowercase letters indicate statistically significant differences among the length within the ISQ 1 or 2 values. Uppercase letters indicate statistically significant differences between the ISQ 1 or 2 values within the length. * and ** indicate statistically significant differences between ISQ1 and 2 values across the lengths. evaluated in preoperative radiographs and the tactile sensation of resistance experienced by the clinician during the drilling procedure of implant site preparation. This method, however, is rather subjective [23]. Depending on the bone quality, surgeons may adapt the surgical protocol in order to increase implant primary stability. Adapted surgical protocols includes the preparation of undersized implant size, the use of osteotomes for bone condensation, the use of different specific drills such as countersink or screw tap drills [26; 27; 28]. Implant stability can be measured clinically with different more objective quantitative methods, such as the insertion torque, or electronic devices such as the Periotest (Medizintechnik Gulden, Germany) and the Ossetl (Integration Diagnostics, Sweden). The insertion torque provides a reliable assessment of the implant stability, but it can be evaluated only at the implant insertion time and cannot be repeated in the follow-up. A recent systematic review concluded that there is no correlation between ISQ and insertion torque values [29]. The Periotest device produces vibrations on the implant and gives a value (PTV) from 8 to 50, while the Ossetl device profits from RFA and gives a value (ISQ) from 1 to 100. Among these two devices the first one raised some criticism since it seems to have a lower sensitivity and is more susceptible to the operator [30]. Implant stability quotient values recorded at the time of implant installation do not reflect the nature of the bone/implant interface and hence the degree of mechanical anchorage.