The main contribution of this study was the finding that the success rate of the MIs was higher with a greater distance between the implant and the alveolar crest, applying to both jaws and to both types of MIs. The rate of success (88.1%) corresponded to the mid-range of the success rates reported in other studies (70.7% to 95.2%) [1, 2] and is close to the 87.8% weighted mean survival rate of maxillary MIs related in a meta-analysis [3].
The clinical implication would be to insert the screw away from the crestal edge at a level where a thicker layer of bone would account for the observed stability. Combining this directive with the indication to position the MI within the attached keratinized gingiva for stability and long-term maintenance without inflammation [9, 19, 20], an optimal apical angulation of the MIs (30–35° to the horizontal) would be warranted. Although such an angulation has been proposed to avoid root damage during placement [25, 26], our findings suggest that it would also be essential to maximize MI stability. Further research focused on this aspect is indicated.
Root proximity has been widely associated with MI failure, more in the mandible than in the maxilla [22, 27]. In 2 prior studies using CBCT technology, MI success was also associated with a greater distance from root surface [24, 27]. In our study the distance (DT) from MI to root surface did not differ between success and failure groups irrespective of jaw or MI type (Table 2). The discrepancy with our findings may relate to the available space and operator-sensitive method as the MI insertion is usually within a limited inter-radicular space and is planned to allow leeway for movement of a tooth towards its adjacent, such as the placement of the MI closest to the mesial surface of the first molar prior to its distalization.
Despite the higher success rate in males than females (91.7% and 84.1%, respectively), the lack of statistical significance suggests that gender is not a factor in the failure of MIs, supporting prior conclusions [3, 7, 8, 25]. As corroborated in most studies [3, 5, 7, 25], patient age also did not impact the MI success rate, although this rate increased with age (from 87.5% under age 20 years to 93.3% over age 30 years). Yet, when controlling for other variables, age emerged as a predictor of implant failure in the logistic regression analysis, joining the conclusion of Yao et al. who ascribed greater risk of failure to MIs placed in patients younger than 35 years, using a generalized estimating equation [25].
Success rates were not statistically significantly different between the maxilla and the mandible (90% and 80.4% respectively), in agreement with previous findings [4, 6, 15, 26], but also conflicting with systematic reviews suggesting greater failure rates in the mandible compared to the maxilla [3, 7]. The conclusions may have been affected by the disproportion in sample sizes in favor of greater success in the maxilla [7].
Our findings of no statistically significant difference in MI success between right (88.8%) and left (87.2%) sides are in concordance with the most recent systematic review [3] and do not concur with reports of better success on either the left [4, 8] or the right side [13]. The results relating a higher success rate for MIs placed between the second premolar and the first molar (91.6%) and a lower success rate between the two molars (82.4%) may be associated with different bone density existing between the second premolar and first molar compared to that between the two molars, along with the possibility of better hygiene more anteriorly than posteriorly. While the bivariate analysis showed placement between the premolars comparable to other sites, the multivariate analysis depicted the interpremolar and premolar-molar sites as predictors of success.
The investigated MIs had the same length (8 mm), but their diameters differed. The wider MI (1.8 mm, Imtec®) showed greater success rate (90.5%) compared to the other (1.4 mm, AbsoAnchor®; 85%), but the difference was not statistically significant. Published reports are contradictory. One meta-analysis suggested no significant effect of implant thread diameter on failure rate [3]; yet another meta-analysis indicated that MIs of smaller diameter (1–1.1 vs 1.5–2.3 mm) had significantly lower success rates [7]. Research should be focused on whether co-variates rather than diameter alone impact the success rate.
The outcomes are specific to the conditions applied in the present population, whereby the MI was inserted at about 35o. However, should the implant be at a different angle, the tip and neck would be at different distances from the alveolar crest, possibly influencing the success rate of the MI.
Measurements on periapical two-dimensional images may be affected by potential projection errors, as variations in the mesio-distal and vertical directions of the X-ray beam may modify DT and DC, respectively. To counter such errors, we used standardized methods with properly positioned X-ray holders to best approximate the axes of the teeth to real anatomy. Imaging would be best with 3-D technology; however, the caveats about increased radiation with CBCT records precluded consideration of this tool by the Institutional Review Board. However, the high intracorrelation coefficients regarding measurement reproducibility on properly taken periapicals reflect the adequacy of these radiographs, which are universally used in similar studies. Also, consideration of the magnification effect disclosed minimal differences in the magnification of brackets and MIs, close 1:1 ratio of measurements that were made to a single point, the tip of the MI, not a line or surface.
The retrospective nature of this study imposed several inevitable limitations, including the inability to control for various factors known to affect MI stability such as insertion torque, patient oral hygiene, local gingival inflammation, and smoking [25, 28]. The possible over-representation of MIs with an acceptable distance from adjacent roots may be indirectly related to the discrepancy in sample size between the success (n = 229) and failure (n = 31) groups. Lower failure representation reflects clinical realities, related to increasing adherence to proper MI placement. Yet, validation of our findings through future research should generate a solid base for the above-inferred clinical recommendations.