The success of maxillary sinus floor elevation was due to the effect of new bone formation, which is influenced by anatomical morphology, bone graft substitutes, surgical methods, etc. In the process of new bone formation, extensive neovascularization, migration, and colonization of bone progenitor cells are two key biological steps of postoperative healing. The neovascular network and osteoprogenitor cells may come from the bone or sinus membrane in contact with the graft materials. This signifies that the size of the contact area is likely to affect osteogenesis; however, this is little literature regarding its impact, how it affects, and the specific impact relationship.
This study evaluated the effect of Sbc and Smc on Va, showing that the graft material is in contact with the maxillary sinus bone, the more it will be absorbed, and for every 1 cm² increase in the contact area, the absorbed volume increases by 0.141 cm³. During the healing process, the sub-antral space volume reduction is mainly due to the absorption of the contents, which in addition to the bone substitute, contains CGF, blood, and air (Fig. 5), but the metabolism rate of these contents is much faster than that of the bone substitute. Therefore, when the Sbc is larger, these contents will be absorbed faster, and the Schneiderian membrane does not participate in this absorption process, which indicates that the contribution of bone to the absorption of these contents is much greater than that of the Schneiderian membrane. Although the bone material will also be absorbed during the healing, since Bio-oss is a long-term degradation material that is difficult to be absorbed [28], it can exist in the body for a long time [29], so the volume of this part is negligible. Second, the reduction of sub-antral space volume is also related to the collapse of the three-dimensional structure of bone material, which may be affected by the negative pressure generated by the absorption of contents within the gaps of bone material particles, and the process is associated with the extra pressure on the Schneiderian membrane [5, 30], which is unavoidable due to respiration [31]. However, no effect of Smc on Va was observed in this study, so the effect of pressure from breathing on collapse may be much greater than that of Smc.
This study analyzed the effect of Sbc on V2, showing that there is a considerable positive association between Sbc and V2, illustrating the relationship between Sbc and the osteogenic effect. The larger Sbc can preserve more bone graft after six months of healing. However, there is often a strong correlation between the volume and area values of sub-antral space; in other words, the larger the volume, the larger the area, and vice versa. So, the significant positive correlation between Sbc and V2 may not fully explain the influence of Sbc on osteogenesis. Therefore, ratios were used instead of numerical values to represent the osteogenic effect to avoid the influence of the strong correlation between volume and area on the results. Sbc was significantly positively correlated with V2% and significantly negatively correlated with Va%, and when Sbc increased by 1 cm², V2% increased by 2.27%. This indicates that the larger the Sbc, the higher the volume of graft material maintained, and the more new bone can be obtained. The possible reason for this, as mentioned earlier, is that a larger contact area leads to more osteoprogenitor cell migration, as well as a wider range of neovascularization.
Significant correlation of Sbc with V2% and Va% has important implications for clinical and research work. Clinicians not only need to consider the patient factors (systemic condition, oral condition, maxillary sinus anatomy, etc.), select the appropriate type of graft material and dosage, but appropriately increase the contact area of the graft material with the sinus wall and sinus floor during the operation when faced with a patient who may require sinus bone augmentation to gain more neogenetic bone. In the light of the classification of maxillary sinus contours [32], in the narrow tapered maxillary sinus, the proper elevation of the maxillary sinus membrane and filling of graft materials can achieve a large contact area between the sub-antral space and the bone, resulting in a good osteogenic effect (Fig. 6A). However, in the square maxillary sinus, especially at sites that require delayed implantation, the traditional sinus membrane elevation range often cannot obtain a sufficient Sbc, so it is necessary to consider continuing to elevate the sinus membrane (Fig. 6B); otherwise, the bone height after six months of healing may be lower than expected. In square maxillary sinuses where simultaneous implantation can be performed, an implant may assist in maintaining the volume of the sub-antral space [33], but this was not supported in other studies [34, 35, 36]. As a result, if the scope of the elevated sinus membrane is not excessive in the square maxillary sinus, it may lead to less periapical bone in the implant at the follow-up visit after six months or even the implant in direct contact with the Schneiderian membrane (Fig. 7), which may damage the sinus membrane, leading to perforation [37] and affect the long-term survival of the implant [38]. It should be noted that in the process of increasing Sbc by elevating the membrane, for those maxillary sinuses with an acute angle between the sinus walls or sinus septa and obvious vessels are present, it is also vital to control the angle and strength of the surgical instruments to avoid bleeding and perforation of the Schneiderian membrane [39], or consider using hydraulic pressure [40, 41, 42]. For researchers, Sbc should be considered as one of the variables when studying the factors influencing osteogenesis of the maxillary sinus lifting surgery, and its value should be recorded and inserted into statistical models or controlled as an irrelevant variable when studying other influencing factors.
In addition, this study corroborates the conclusions of previous studies that V1 is significantly positively correlated with Va, which means the more graft material, the more it is absorbed [43]. Simultaneous and delayed implant placement results in similar bone augmentation in line with two separate studies with a short-term follow-up of four months after loading and a long-term follow-up of 24 months after surgery [34, 35]. Thus, the choice of implant timing for patients who need sinus floor lift surgery does not affect the osteogenic effect, so clinicians should choose simultaneous or delayed implant methods based on bone mineral density and the overall cortical bone thickness [44].
This study confirmed that bone formation is affected by the contact area between the sub-antral space and surrounding bone after lateral window sinus elevation surgery. Moreover, the contact area between the sub-antral space and the Schneiderian membrane has no effect; thus, it is possible to reject the null hypothesis. However, this study has some limitations, such as the time point at which CBCT images were collected (6 months after surgery) only provides information on prognosis in the short term, and images at follow-up visits may be needed to confirm these results [45]. Furthermore, only one bone graft material (Bio-oss) was used in this study, so the applicability of the findings to other materials needs to be investigated. Lastly, similar to other studies, only imaging methods were used to analyze the effect of contact area on osteogenesis [39, 46], and it is better to use histological studies to further verify the results [47]. Further larger, longer-term studies are required to confirm these results.