To promote the success of implantation with RAIs in the anterior region, any disadvantageous factors, for example, excessive stress concentration, should be avoided[20, 21]. Upper incisors have different sagittal root positions in the alveolar bone. However, whether a certain sagittal root position can lead to stress concentration in peri-implant bone when RAIs are under loading remains unknown. This study was designed to analyse the stress around RAIs in different sagittal root positions according to Kan’s classification to provide a prognosis for surgery preoperatively.
As stated by Kan, the Class IV position was unsuitable for immediate implantation because there was a limited amount of bone with which appropriate implant stability could be obtained after tooth extraction. Therefore, we evaluated only Class I, II, and III sagittal root positions in this study.
The labial lamella in the upper anterior region is thin. Bone resorption in the labial and neck cortical bone areas around the implant could be more likely to occur when the implant is exposed to excessive stress. The results showed that the sagittal root position affects the von Mises stress distribution in the peri-implant bone. Among the three sagittal root positions, stress concentration was most apparent in the labial cortical bone area for implants in the Class Ⅰ position, both under oblique loading and vertical loading. The reason was that, RAIs in the Class I position contacted the labial cortical bone directly. Then, when the stress distribution around the implant neck area was analyzed, stress concentration was most evident in the Class Ⅲ position. At the same time, stress concentration in the palatal lamella was also most evident in the Class III position. It was the result from that the cortical bone directly contacted RAIs in the palatal apical area, when implants were in the Class Ⅲ position. When implants were under oblique loading, they tended to show a rotational movement. The center of rotation of implants in the Class Ⅰ and Class II positions might be located in the middle of the root, while the center might be in the apical region in the Class Ⅲ position. Hence, there was a greater rotation amplitude for implants in the Class Ⅲ position. That was the reason why the implant labial neck area in the Class Ⅲ position exhibited the greatest stress concentration in the cortical bone, when implants were under oblique loading.
Bone resorption is one of the key factors affecting the success of implantation. The higher that the stress concentration occur in the bone around the implant is, the higher the risk of bone resorption is . Thus, with respect to the stress concentrations in the labial lamella and neck areas around implants, among the three sagittal root positions, the Class II position is better suited for immediate implantation with RAIs. This finding is different from those with traditional implants. When applying traditional implants to complete the immediate implantation in the aesthetic area, the Class Ⅰ position is thought to be the best position for implantation because it can provide sufficient bone on the palatal side for implantation. In this way, the initial stability of the implants is guaranteed[14, 26].
Although from this study, the Class II position was found to be the most appropriate site for applying RAIs in the maxillary anterior region, according to Kan’s research, there were only 6.5% of maxillary central incisors in the Class II sagittal root position. Similarly, in Xu’s study, the percentage was only 4.4%. Most of the maxillary central incisors were in the Class I position. To broaden indications for RAIs, measures should be obtained to reduce the stress concentration. It was found that, adding threads or a targeted press-fit geometry to the surface of RAIs could decrease the stress concentration in the peri-implant bone [15, 19, 35].
In this research, stress distribution conditions in the smooth group and the threaded group were also compared. In both the cortical bone and the cancellous bone, stress concentrations in peri-implant bone were lower for the threaded implants than for the smooth implants, regardless of the sagittal root position and the loading form. These findings were similar to those of studies exploring the influence of thread design on the stress distribution in peri-implant bone around traditional implants[27–29] and RAIs[15, 30]. The results of this study once again confirmed that a threaded design would result in a lower stress concentration in the alveolar bone around RAIs.
Then, according to the results of Tables 2 and 3, the maximum von Mises stress in the cortical bone was greatest in the Class Ⅲ position and least in the Class Ⅱ position. In addition, it could be found that the maximum von Mises stress in the cortical and cancellous bone was lower in the threaded group than in the smooth group, especially under oblique loading. Many previous studies have assumed maximum bone strength to be a biological limit to bone failure and activation of bone resorption[16, 31, 32]. In addition, it has been reported that overloading of cortical bone occurs when the maximum von Mises stress exceeds 25–28 MPa. Similarly, cancellous bone overloading will occur when the maximum von Mises stress exceeds 6 MPa. According to the results of this study, it could be found that none of the maximum von Mises stress in the cortical and cancellous bone exceeded the stress criterion. Only the maximum von Mises stress in the cortical bone when implants were under an oblique load in the Class Ⅲ position (24.9 MPa) was near the stress criterion of cortical bone overloading. Hence, from the perspective of the maximum von Mises stress, there was no excessive stress concentration in peri-implant bone when RAIs were in these three sagittal root positions.
This research could provide guidance for the clinical application of RAIs. It was found that all of these three sagittal root positions included in this study were suitable for immediate implantation with RAIs while the Class II position would be best suited. If RAIs are placed in the Class I position, attention should be paid to the thickness of the labial lamella, and immediate oblique loading after implantation should be avoided. At the same time, measures such as adding a thread design to the implant could be undertaken to reduce the stress concentration on bone around RAIs. Furthermore, as promoted in other studies, the diameter of the implant next to the buccal cortical bone could be reduced in an effort to avoid fracture of the bony wall and pressure-induced bone loss[9, 34]. When implanted in Class II and Class III positions, the thickness of the alveolar bone in implant labial neck areas should be noted. In addition, when implanted in the Class III position, the thickness of the remaining alveolar bone in the palatal apical areas should be noted. Although in general, the soft and hard tissue on the palatal side was thicker than that on the labial side, excessive local stress should also be avoided to prevent bone resorption and even fenestration on the palatal side, which can lead to implantation failure [35, 36].
There are some limitations to this study. First, the study analyzed the stress distribution conditions after osseointegration, but the conditions could be more concerning in immediate implantation when osseointegration is not yet complete. In a future study, we will explore this problem. Second, the study only concerned the influence of sagittal root positions at the level of the stress distribution. Micromotion analysis should be considered in a future study, which is important for the implant’s initial stability. Third, the resorption modelling of the alveolar ridge in post-extraction sites was not considered. We plan to further modify the alveolar bone model to achieve more accurate results.