## 4 Discussion  
The present study aimed to investigate predictive factors of primary and secondary implant stability for future risk stratification models. The present study provided a large dataset and included outcome measures for both primary and secondary implant stability. Furthermore, we stratified our results by two frequently-used implant systems. We did not observe an influence of the different surgical techniques on implant stability, as already described by other authors [30; 31; 32; 33]. However, we cannot make a conclusion regarding other surgical techniques not included in our study design. In our regression model, implant and bone characteristics also showed no significant prognostic relevance for the increase of ISQ after implant insertion. However, the quality and quantity of jaw bone have been reported to have a significant impact on implant success [34; 35; 36]. Interestingly, different operation techniques are expected to have a negligible influence on implant survival [7]. The clinical significance of primary stability is expressed by the resistance of the bone during the insertion of an implant [37]. As also seen in our patient cohort, mandible implants are often reported to achieve better primary implant stability [38]. In contrast, maxillary implants resulted in higher  
Figure 6: Illustration of changes in ISQ between primary (implant insertion) and secondary (implant exposure) RFA measurements, grouped by predictive factors (left). Implant localization (maximillary vs. mandibular) showed statistical significance in the pairwise analysis (* \(p\) < 0.05). However, healing time (in weeks) was the only predictive factor to show statistical significance in the multivariable linear mixed regression model when adjusted for other study variables (right). STR: Straumann implants. SPI: Thommen implants. ISQ: implant stability quotient. ISE: internal sinus elevation. F: female. M: male. Reg. coeff. : regression coefficient. se: standard error of the mean. increases between primary and secondary measurements in our study cohort, which probably resulted from the lower primary implant stability values of maxillary implants. Additionally, healing time was significantly associated with higher ISQ values in our regression analysis, which is in accordance with the results provided by other authors [39; 40; 41]. A healing time of 120 days, which considers the initial stability gap in the first weeks after implant insertion, seems to fulfill optimal requirements for successful osseointegration of dental implants [8; 42; 43; 44; 45]. Age and sex did not reveal significance for the prediction of primary and secondary implant stability in our study. These findings were also controversially discussed by other authors [46; 47; 48]. Age is not considered to be a reliable parameter for the success of osseointegration, as shown previously [49; 50; 51]. Our results further revealed that Straumann implants were significantly associated with higher ISQ compared to Thommen implants. This was seen in jaws as well as all tooth regions, particularly in the premolar region. Regardless, Thommen implants showed a higher increase in ISQ values between implant insertion and exposure. One explanation could be that the already high primary stability values of Straumann implants compared to Thommen implants resulted in lower ISQ increases between primary and secondary measurements, as described before [52]. In contrast, a recent study with 15 implants each (Straumann implants vs. Thommen implants) reported higher ISQ values for Thommen implants for primary stability, whereas no significant differences were found for secondary implant stability [53]. However, the small study size did not allow for an adequate comparison. We also found significant associations between implant length and primary implant stability, though not for secondary implant stability, as confirmed by other authors [54; 55; 56].