In the report by Tahmaseb et al. [1], the error of the placement position was as follows: Angle, 3.5° (95% confidence interval, 3.00–3.96°); Base, 1.2 mm (95% confidence interval, 1.04–1.44 mm); and Apex, 1.4 mm (95% confidence interval, 1.28–1.58 mm). These error values are larger than those in the present study. This is considered to be because the present study is limited to partial edentulism, and since the measurement method of implant validation is different, a simple numerical comparison is not possible.
Missing teeth-derived factors
Errors in the placement position caused by differences in the type of tooth and type of edentulism did not differ significantly in all measurement outcomes, but this result differs from previous reports. This is probably because Vasak et al. [14] classified the type of tooth into two groups (anterior teeth and molars), and Behneke and Burwinkel, [25] distinguished between the type of edentulism by a single missing tooth versus multiple missing teeth. These studies did not exclude the effects of other confounding factors by statistical processing.
The error in the placement position caused by the difference in the distance from the remaining teeth to the placement position was significantly different only for the Angle. Thus, if the distance from the remaining teeth to the placement position is large, the placement direction is likely to be incorrect even if guided surgery is used because the adjacent tooth serves as an indicator of the placement direction.
Of the three types of missing teeth-derived factors, the only significant outcome was the Angle at the distance from the remaining teeth to the placement position. This result suggests that in guided surgery, the effect of the missing teeth-derived factor on the error of the placement position is smaller than the effects of other factors.
Implant-derived factors
Errors in the placement position caused by differences in the type of implant were not significantly different in all measurement outcomes. In past reports, a smaller difference in diameter between the sleeve and the drill was associated with a smaller error [17]; however, there is a concern that heat generation may occur by friction between the sleeve and the drill [30]. The difference in diameter between the sleeve and the implant mount of the implant system used in this study was designed to prevent the torque caused by their contact, and the difference tended to be larger in the larger gap group. However, no statistical significance was found.
The error in the placement position caused by the difference in the implant length was significantly different only for the Apex. Therefore, although using guided surgery can reduce errors near the platform, errors at the tip are more likely to occur as the implant lengthens, and more careful planning and surgery are required when using long implants. In past reports, the error was larger for long than short implants [17, 18]; however, the number of implant body samples in these reports is considered too small (8 and 14, respectively). In contrast, some reports have indicated that the implant length does not affect the error of the placement position [26, 31], and no clear conclusion has been obtained. The present study involved as few as 10 implant bodies of ≥ 13 mm; thus, care must be taken in interpreting the results. However, since the total number of samples was 188 and a nonlinear analysis was performed, more reliable results seem to have been obtained.
As the number of implants per patient increased, the error in the placement position significantly increased for all measurement items. This suggests that if the number of implants is large, motion and deflection of the surgical guide occur and the error increases. In past reports [13, 22, 32, 33], the number of implants per subject was about 5; in contrast, this number was 1.71 in the present study. This is because the present study focused only on patients using a method called SmartFusion (Nobel Biocare), in which a surgical guide is produced based on the scan data of a model, and six or more remaining teeth are needed in this method [34]. This might have affected the number of implants. Because the maximum number of implants per patient in this study was four, it is necessary to take into consideration that the results obtained in this study are applicable to patients with few missing teeth.
In addition, the error in the placement position caused by the method of guidance showed a significant difference in all measurement outcomes. The error in the placement position was smaller in the fully than partially guided group. Thus, the accuracy of guided surgery performed with full guidance in the present study was as high as in previous reports [27, 35,36,37,38,39].
Guide design-derived factors
The errors in the placement position caused by the difference in the number of teeth supporting the surgical guide were significantly different in all measurement outcomes. For the Base and Apex, the error decreased as the number of support teeth increased at around 10 teeth and then increased as the number of support teeth increased. Because the gradient of the graph also changed at around 10 teeth for the Angle, the optimal number of teeth supporting the guide appears to be around 10 teeth. Most studies on the precision of guided surgery have included the mucosal support to their data [40, 41] and few previous studies focusing on tooth-supported guides have examined the number of these supportive teeth. Kholy et al. [42] reported guides supported by 4 teeth were not significantly different from the accuracy of full-arch-supported guides; however, this report does not treat the number of teeth supporting the guide as a continuous variable. In the present study, the influence of the type of surgical guide support was excluded by limiting it tooth-supported guide, and the influence according to the number of supportive teeth was examined in detail. Too much or too little teeth supporting the guide are considered unfavorable to balance fit and stability. However, further research is needed to confirm this.
The error in the placement position caused by the number of anchor pins was significantly different for the Angle. The error in the placement position tended to decrease as the number of anchor pins increased. Setting the anchor pin seems to be effective in preventing deviation of the angle, because operators easily fix the surgical guide during surgery [31].
The error in the placement position caused by the presence or absence of the reinforcement structure showed a significant difference for the Angle. This suggests that setting the reinforcement structure was effective in preventing deviation of the angle. In particular, because errors of the Apex tend to be smaller with than without a reinforcement structure, the errors at the tip of the implant can likely be reduced by suppressing the deflection of the surgical guide by the reinforcement structures. Van Assche et al. [16] and Tatakis et al. [43] also noted that deflection or fracture of the surgical guide is a factor that causes an error in the placement position, and this can be interpreted as a supportive result.
Based on past studies [14,15,16,17,18,19,20, 23,24,25,26,27,28] and clinical experience, this study has enumerated the factors that seem to affect the error of the placement position, including the design of the surgical guide. By correcting the confounding of each factor using the multivariate analysis, the magnitude of the influence of each factor on the placement position error was calculated. Although there are many other factors that cause errors in addition to the factors listed in this study, the number of samples is limited, so no further explanatory factors were added to maintain the power of statistics. Certainly, the factors related to the so many surgeons are not included in the explanatory variables, so it is undeniable that differences in operators may affect the results. However, it is generally reported that the guided surgery is not easily affected by the difference in the experience value of the surgeon [44], and the effect is considered to be limited.
The above findings indicate that the error in the placement position caused by the missing teeth-derived factor does not differ significantly in most of the measurement outcomes, while the error in the placement position caused by the implant-derived factor and the guide design-derived factor significantly differs in many measurement outcomes. As a result, factors that can be controlled by the operator have a greater effect on the error of the placement position than the oral condition of each patient. Above all, these findings suggest that the smaller errors could be present by performing guided surgery with full guidance and devising the design of the surgical guide. However, because the results include selection bias, research involving a more advanced study design is necessary, such as performing random assignment to obtain a higher level of evidence.