Discussion

The treatments evaluated in the present study were equally effective in increasing the amount of available bone tissue, allowing the placement of implants with satisfactory primary stability. Furthermore, the composition of the grafted areas and the stability of implants placed in these sites did not differ between groups. However, the β-TCP grafted areas exhibited a greater volume reduction than the areas grafted with β-TCP combined with autogenous bone specially in more atrophic sinus. Accordingly, the null hypothesis of this study, which stated that the addition of autogenous bone to β-tricalcium phosphate (β-TCP) would not influence healing outcomes, was partially rejected.

Tomographic analysis revealed a substantial gain in mineralized tissue following maxillary sinus floor elevation in both groups, supporting the predictability of this technique in achieving successful increases in bone availability [8, 23, 24]. A reduction in the volume of the grafted area was also observed 10 months after the surgical procedure. Indeed, previous clinical studies reported considerable volumetric resorption in maxillary sinuses grafted with particulate autogenous bone (e.g., 24–45% at 6 months post-surgery) [25, 26], as well as in areas grafted with β-tricalcium phosphate, which on average showed approximately 20% greater volume loss at 6 months post-surgery [13]. The association of the bone substitutes and autogenous bone grafts presented higher resorption, related to higher rates of autogenous bone graft [16], and the proportion of 1:1 was related to a 23% reduction in grafted area. Data presented by previous studies are in line with our findings, showing reductions in the volume of the grafted area within the range of 15–25%Some other factors, such as the healing period, the size of the grafted sinus, the purity of β-tricalcium phosphate, and the source of autogenous bone, may contribute to variations in volumetric resorption between sinus floor elevation and subsequent implant placement. Indeed, in our study, larger maxillary sinuses were associated with higher levels of grafted volume loss. These findings highlight the need for careful planning and the use of larger amounts of graft material in wider maxillary sinuses. However, caution is warranted regarding the excessive use of bone substitutes in the maxillary sinus, as greater distances from the grafted area to native bone are associated with lower new bone formation, regardless of sinus size [27, 28]. Furthermore, it is noteworthy that in the current study, sinuses grafted exclusively with β-TCP showed a greater reduction in graft volume compared to those in which β-TCP was combined with autogenous bone. It has been reported that β-TCP exhibits higher resorption rates than other types of osteoconductive bone substitutes, such as synthetic or natural hydroxyapatite [29]. These higher resorption rates of β-TCP may also explain the differences between the present findings and those of other studies, which demonstrated that the use of autogenous bone grafts increases grafted volume loss in direct proportion to the amount of autogenous bone relative to the total graft material volume. [16, 26]. In addition, it is important to note that the autogenous bone used in the current study was harvested from the posterior region of the mandible, which, due to its highly cortical nature, shows less volumetric loss compared to more trabecular donor sites, such as the iliac crest [30]. Interestingly, graft stability was significantly correlated with the initial residual bone height only in the β-TCP-alone group. This finding suggests that the dimensional stability of β-TCP may depend more strongly on the amount of residual native bone available at baseline. Therefore, although β-TCP alone resulted in similar histological outcomes after 10 months, its clinical performance may be less predictable in severely atrophic maxillary sinuses, where the addition of autogenous bone could contribute to maintaining graft volume during healing.

The grafted areas in the present study showed bone formation above 40% in both groups, with a similar amount of soft tissue and a low proportion of residual bone substitute particles. This tissue composition pattern is closely related to volumetric loss, as materials or associations of materials that induce greater formation of bone and soft tissues, with a lower proportion of residual bone substitute particles, tend to result in volume reduction [31, 32]. Another notable finding in this study was the lack of additional benefits from combining autogenous bone grafts with β-TCP compared to the use of β-TCP alone regarding the histological bone formation after 10 months of follow-up. The autogenous bone graft has been considered the best bone graft, as confirmed by previous studies. Systematic reviews showed that pure autogenous bone grafts exhibited a higher mean percentage of mineralized bone compared to substitutes or mixtures, demonstrating that autogenous bone may promote more pronounced histomorphometric outcomes. However, this advantage is accompanied by greater volumetric resorption and morbidity associated with donor site harvesting [31, 33].

Due to these inherent limitations, combinations of bone substitute materials with autogenous bone have been proposed, as exemplified in the present study. These combinations aim to reduce donor-site morbidity, while ensuring adequate bone formation and volumetric stability [11, 34]. This approach has previously demonstrated favorable outcomes, with enhanced bone formation and lower volumetric loss compared to the use of autogenous bone grafts alone [34, 35], findings that are consistent with those observed in our investigation. Previous studies reported that mixtures of autogenous bone and deproteinized bovine bone tend to promote greater bone formation as the proportion of autogenous bone increases [33, 36, 37]. However, these results remain controversial, as other investigations showed that the combination of autogenous bone with deproteinized bovine bone and β-TCP does not confer additional advantages in bone formation when compared with grafted areas treated with these materials individually [37, 38]. Moreover, clinical studies demonstrated that the survival and success rates of implants placed in maxillary sinuses grafted with deproteinized bovine bone, with or without the addition of autogenous bone, were comparable after 1–5 years of functional loading, [39–41]. Only a limited number of studies have assessed the combination of autogenous bone with β-TCP [11, 26, 37], and in all of these, the use of autogenous bone did not provide significant benefits regarding bone formation or volumetric maintenance of the grafted area compared with the use of β-TCP alone.

In the present study, implant insertion torque values were equal to or higher than 30 Ncm, whereas the ISQ values ranged between 30 and 40, which are considered low. The absence of significant differences between the groups can be explained by the fact that both received grafts with similar compositions, resulting in comparable bone characteristics at the implant sites. Moreover, insertion torque values above 30 Ncm indicate that, even in grafted areas, the implants achieved satisfactory mechanical retention, which may be associated with the presence of peripheral native bone. On the other hand, the low ISQ values can be attributed to the predominance of regenerated bone in the region, which, although capable of providing initial stability, exhibits lower rigidity compared to native bone. This occurs because resonance frequency analysis assesses not only the cortical bone stability at the time of implant placement but also the condition of the bone surrounding the entire implant [42]. Therefore, when part of the implant is surrounded by newly formed bone or by bone substitute material still undergoing remodeling, ISQ values tend to be lower, even when insertion torque is considered clinically acceptable [43].

The macrogeometry of the implants used in this study may also influence the primary stability data. Conical implants tend to engage more tightly during placement, due to the lateral compression of cortical bone, resulting in higher insertion torque values compared to cylindrical implants [44, 45]. This effect occurs because conical implants are designed to provide good initial stability even in low-density bone [46]. In the context of grafted areas, where regenerated bone presents lower rigidity and density, the conical design may compensate for these less favorable conditions, by improving mechanical interlocking with the surrounding bone. Although a substantial portion of implant stability depends on the grafting material, conical implants typically achieve adequate torque values (≥ 30 Ncm) even under these circumstances, reinforcing their suitability for use in reconstructed maxillary sinuses or areas augmented with bone substitutes. In the present study, despite satisfactory and comparable insertion torque values between groups, ISQ values were low, as abovementioned. This finding indicates that good mechanical engagement, partly provided by the conical design, does not necessarily translate into higher rigidity as measured by resonance frequency analysis [44]. These findings confirm that although the conical implant design contributes positively to insertion torque and initial mechanical stability, these gains may not be fully reflected in ISQ values, particularly in grafted areas.

Previous studies have already demonstrated that β-TCP yields favorable outcomes when used in grafting procedures for maxillary sinus floor elevation [11, 26, 47]. The main strength of the present study is that, to the best of our knowledge, it is the first split-mouth study to demonstrate that the addition of autogenous bone (AB) does not provide higher bone formation than in maxillary sinuses grafted with β-TCP after 10 months of healing. This finding is particularly relevant because the severe maxillary atrophy observed in the patients included in this study limited the use of more conservative autogenous bone harvesting approaches, such as obtaining bone from the maxillary tuberosity or from the lateral window created during sinus access. Therefore, the posterior mandible was selected as the donor site to provide the standardized amount of autogenous bone required by the surgical protocol adopted in this study [48]. Indeed, this procedure is associated with additional morbidity, including postoperative edema and trans- and postoperative discomfort. Therefore, the present findings are clinically important, as they suggest that the addition of AB did not increase the amount of newly formed bone after 10 months in this sample, even in more challenging cases requiring maxillary sinus floor elevation.

Among the limitations of the present study, the 10-month follow-up period between the grafting procedure and implant placement should be highlighted. Although this interval was longer than that reported in similar studies—which generally adopt a 6-month period—the decision was justified by the use of a graft material that has been clinically less explored, thereby warranting an extended observation time to ensure adequate integration. Although the addition of autogenous bone did not result in greater bone formation in the β-TCP-grafted areas after 10 months, it is possible that shorter evaluation periods could reveal a faster pattern of bone formation [49, 50]. Such findings could potentially reduce the waiting time required before implant placement. However, this aspect could not be assessed in the present study and should be investigated in future studies with earlier follow-up intervals. Another limitation is that the secondary stability of the dental implants was not assessed stability at the time of prosthetic delivery. In addition, no clinical follow-up of the implants was conducted after prosthetic rehabilitation, which precludes the evaluation of long-term parameters, such as the maintenance of stability, bone remodeling, and functional success rates. Another important aspect to consider is that part of the implant placement sites involved native bone and were therefore not exclusively composed of grafted bone. Consequently, the stability values observed in the current study reflect the combined contribution of both regions rather than the grafted area alone. Finally, it should be noted that the maxillary sinus represents a highly favorable environment for bone formation; thus, the findings of this study may not be directly applicable to other, more challenging clinical situations—such as horizontal ridge augmentation in severely atrophic alveolar ridges.