Thanks to modern minimally invasive surgical techniques and optimized implant systems, the lack of sufficient bone in the maxillary posterior region can be counteracted by bone augmentation so that initial sufficient implant stability can be ensured later [3]. The BLC technique was developed to avoid rupture of the Schneiderian membrane during surgery and thus to simplify the process of membrane detachment. The mucous membrane of the maxillary sinus is separated from the bony bottom of the maxillary sinus by a minimally invasive, atraumatic and controlled balloon filled with liquid [30]. Because of this gentle procedure, the strain on the patient is extremely low [25].

However, the necessary minimum layer thickness of the residual bone in the case of simultaneous implantation and augmentation remains a controversial issue. In addition, disagreement exists over the choice of surgical approach, the most suitable augmentation material and the necessary healing time [22].

The long-term success of alternative therapy options, which do not require bone augmentation for the same indication position (e.g., subperiosteal framework implants, tilted implants or implants that have been anchored in the pterygoid process for a long time), is questionable, and considerable surgical risks are involved. Because of possible complications, such as massive inflammation and substantial consequential damage during implant removal, their use is questionable. Additionally, perforations of the maxillary SF, bleeding from the major palatinal artery and the pterygoid plexus, and because of the unfavorable implant axis in the medial-posterior direction, these techniques are not recommended [8].

In this study, we focused on the two techniques of SF elevation that are the easiest to implement and the most common and most successful today: the classic OSFE according to Tatum and the BASF technique.

For the evaluation of bone growth in the upper jaw, an animal species should be selected with human-like chewing forces and bone vascularization [11]. The current results are almost completely transferable to humans. Great importance should be attached to reproducibility under the same conditions. Therefore, the mini-pig is described as the animal model of choice [31, 34].

A variety of data have been presented on normal and pathological bone healing in mini-pigs, describing the comparable physiology of the bone tissue and the anatomical conditions similar to those of humans [10, 13, 31].

In contrast to the general surgical technique described in the introduction, surgical access was chosen via the lateral maxillary sinus to leave all teeth undamaged. In addition, the augmentation area was not exposed to chewing forces or "misbehavior" (compliance), which makes it reliable to compare the results. For this reason, the NB formation within the augmentations may have behaved differently from the NB formation in humans, in which the BAM or the implants are inserted at a site exposed to the physiological load of chewing.

Although even the loosening of the Schneiderian membrane triggers the formation of NBs as a stimulus, the necessity for BAMs to be used for the BLC approach has not yet been clarified [3, 15].

We chose the synthetic bone regeneration material “easy-graft™” for this experimental work because of the strong demand for alloplastic BAMs in routine clinical work, its ability for subsequent modeling and its simultaneous dimensional stability after healing. Furthermore, ß-TCP, which is the underlying compositional part of easy-graft™, seems to be a suitable graft material because of its wide range of applications, low complication rates and satisfying long-term results [5, 9, 12]. Furthermore, it is clinically approved and investigated in multiple experimental and clinical studies.

We assumed that the inserted osteoconductive BAM merely acts as a placeholder or functions as a framework. The material introduced, therefore, plays a less significant role with regard to a periosteal reaction than the technique or the surgical procedure.

The aim of the present study was to quantify the periosteal response after the respective surgical method had been performed to compare the two SF elevation techniques. A realistic evaluation of the morphology of the cells and the differentiation of the various tissues was ensured by Giemsa eosin staining.

If one considers the periosteal reaction as an area, then a difference can be shown in the available healing time of the augmentations (Fig. 1). Although not significant, for the BLC technique, the periosteal reaction was demonstrably more pronounced than after OSFE. More NB formation could also be detected for the longer survival time point (T2).

This means that the longer healing time of the augmentations (56 days) with the applied technique of the OSFE in relation to the periosteal reaction did not allow the bone growth values that were achieved with the BLC technique by the same healing time (56 days). Even a comparison of the techniques with a shortened healing time (28 days) showed that the area of the periosteal reaction was larger in the BLC group than in the OSFE group. The BLC technique is thus superior to OSFE in terms of bone turnover. Of note, despite the low number of cases, the gradient indicates a definite trend (Fig. 1).

To show whether the surgical technique had a significant influence on the maximum or minimum expected bone growth area, values were compared by FL (Fig. 1). Especially in situations in which rapid initial healing of the augmentation or quick bone regeneration in the first days after the operation is desired, the BLC technique seems to provide better initial results in terms of NB than OSFE. Thus, the BLC technique induces faster bone growth.

The FL enabled the temporal course of bone metabolism to be objectified, and thus, the formation of NBs could be assigned to a highly specific formation time.

With regard to the course of time, bone growth behavior does not seem to be related to a specific zone, and no temporal connection could be determined. However, strikingly, the initial NB fractions were significantly superior to those formed later, as revealed by the two dyes that were administered initially. Nevertheless, the duration of the healing phase played a greater role when considering maximum bone growth than the surgical technique.

Following Cb staining, 13 out of 14 samples in the central region showed no bone growth at all. In the other two ROIs, 50% of the samples also showed no bone growth. One assumes that the middle region was inferior to the other two regions with regard to bone turnover at the applied time point (Fig. 5A–D).

This suggests that, at least from the 16th day or from the 30th day after surgery, no significant bone growth occurred in the investigated zone.

Notably, the present study did not show any particular growth behavior in relation to a specific zone. Each zone showed its own behavior, which seemed to be independent of each other in terms of location and time. If NB was formed, it created the impression that this process started at the site of the set stimulus and went into deeper regions. Moreover, the duration of healing had more influence on the maximum NB formation than the surgical technique.

With the BLC technique in combination with a long healing time, better NB formation could be achieved than with the OSFE. Various reasons for this can be proposed.

The maintenance of a sufficient vascular supply system during the elevation of the mucous membrane (mainly the stratum vasculare of the lamina propria) might play a decisive role in the success of SF elevation. Although the lifted mucous membrane has a sufficient vascular network for its supply [21], larger venous blood vessels that pass from the bone of the SF into the Schneiderian membrane are torn off during elevation. Studies have demonstrated that the mucous membrane when directly removed with instruments shows microflap formation with partial detachment or tearing of the periosteum [29]. Additionally, when the mucosa is removed by the seemingly gentle piezo-electric technique, the micro vessels are torn off by constant vibrations, microthrombi are induced, and thus, the blood supply is highly compromised or even interrupted [14, 30]. The mucosa, when lifted off with the balloon, on the other hand, remains absolutely intact and unstressed. This effect can also be seen in the histological images depicted in Figs. 3 and 4.

The BLC technique is, therefore, a promising procedure that facilitates and standardizes surgery even under difficult anatomical conditions (septa) and inaccessible access paths. Furthermore, it supports bone turnover by its gentle method and, therefore, provides demonstrably higher NB formation values compared with the open instrumental technique.

The decision for the "right" access route (lateral sinus wall or transcrestal access) depends on the individual clinical situation and, in particular, on the quantity and quality of the preexisting crestal bone site. The use of BAMs for lateral access is a well-documented procedure with a low complication rate of only approx. 5%. The BLC approach, however, is also a component of many examinations and represents an alternative to external procedures because of its minimal invasiveness [35].

The present study demonstrates that the surgical procedure can contribute to favorable conditions for the formation of NB and moreover for the RBGR or for the healing of augmentations. Although several studies have suggested that the search for the perfect BAM is the sole item that requires adjustment in the system of SF elevations, this is to be questioned and reconsidered because various studies have revealed sufficient bone turnover in the maxillary sinus even without BAMs [35]. In addition to the treatment factors, systemic factors and the condition of the patient's bone position must also be taken into account as factors influencing NB formation.

When comparing clinical studies in which SF augmentations with alloplastic BAMs have been performed, Zerbo et al. [36] and Frenken et al. [7] have shown NB formation rates of approximately 17% after an integration phase of 6–8 months for augmentation with ß-TCP-based BAM and approximately 27% for augmentation with a biphasic calcium-phosphate based BAM.

In summary, the highest bone formation values were seen in the first weeks shortly after augmentation, especially for the BLC technique. However, the bone formation values approached each other over time. In the 56-day group, the treatment group showed significantly faster initial growth behavior than the OSFE group (p < 0.02).

Almost twice as much bone was initially newly formed with the BASF technique, suggesting that a sufficient blood supply is a prerequisite for further bone regeneration. The detachment of the SF mucosa by the BLC technique, which is gentle on the blood vessels, thus has a clearly positive effect on the initial phase of osseointegration of the BAM.

Most authors agree that, for sufficient stable osseointegration of dental implants in the area of the augmented maxillary sinus, NB formation of 20–35% should be achieved at minimum [1, 19, 24, 26].

Both the BLC and the OSFE provide good results in terms of the amount of NB formation based on an osteoconductive BAM. The surgeon's task is therefore to assess the individual clinical situation and then select the appropriate surgical method to achieve optimal results.