In both the experimental group and the control group, bone resorption at the IBSH site was the least, and bone resorption at the MBH and DBH sites was nearly equivalent. When comparing the experimental group with the control group, the bone resorption at the IBSH site was 0.20 ± 0.43 mm in the experimental group and 1.95 ± 0.39 mm in the control group, with a statistically significant difference between the two groups (p < 0.01); the bone resorption at the MBH site was 0.43 ± 0.30 mm in the experimental group and 1.72 ± 0.58 mm in the control group, with a statistically significant difference between the two groups (p < 0.05); the bone resorption at the DBH site was 0.38 ± 0.32 mm in the experimental group and 1.75 ± 0.37 mm in the control group, with a statistically significant difference between the two groups (p < 0.01); the bone resorption of BW was 0.13 ± 0.12 mm in the experimental group and 0.06 ± 0.03 mm in the control group, with no statistically significant difference between the two groups (p > 0.01) (Table 3, Fig. 7).

For patients with residual bone volume less than 3 mm in the posterior maxilla, maxillary sinus floor elevation is typically employed clinically to augment bone volume in the implant area prior to the placement of implants. This study was a prospective randomized controlled clinical trial. For patients with residual bone height less than 3 mm in the maxillary molar area, the experimental group underwent simultaneous maxillary sinus floor elevation and implant placement. Compared with the conventional surgical treatment in the control group, the results indicated that the implant survival rate was 100% in both groups. Imaging comparisons of bone height changes revealed that both groups experienced increases in bone height and width following sinus elevation, but both subsequently decreased, with changes in bone height being more pronounced than those in bone width. The experimental group received implant placement during the initial surgery, which reduced treatment time, the number of surgeries, and patient discomfort, and demonstrated superior advantages in terms of the amount of bone graft material resorption in the later stages.

The favorable bone remodeling outcomes observed, particularly in the simultaneous group, must be interpreted in the context of the grafting material's properties. Xenografts, derived from different species, have been widely used in maxillary sinus floor elevation as semi-permanent and slowly resorbing bone-conductive grafts [16]. This is supported by a recent Bayesian network meta-analysis which concluded that xenografts are a reliable choice for sinus augmentation, showing favorable outcomes [17]. Further reinforcing this, a systematic review on graft volume changes concluded that while all materials (including allografts such as DBM or FDBA, synthetic biomaterials like β-TCP, and autografts) undergo some resorption at 6 months, combinations involving xenografts (e.g., with allografts) demonstrated the largest net volumetric gain, underscoring their efficacy in space maintenance [18]. The selection of this material for the present study was deliberate, as its proven efficacy and exceptional capacity for long-term space maintenance are paramount for ensuring the stability of the grafting site in cases of severe bone atrophy (RBH < 3 mm) -- the primary focus of this investigation. These properties effectively minimize a key variable, allowing for a clearer evaluation of the surgical protocol (simultaneous vs. staged implantation). In line with the objective of achieving optimal space stability, large-particle xenograft bone powder was uniformly used in all cases. While controversy exists regarding the impact of graft particle size on osteogenesis -- with some studies suggesting small particles of allografts may form more active bone [19], while others, such as Testori et al., report superior results with larger particles [20] -- the choice of large particles in this experimental design was motivated by their demonstrated clinical efficacy in the authors' previous bone augmentation surgeries and their theoretical advantage for volume stability. Therefore, the use of large-particle xenograft was considered the most appropriate choice to address the research question at hand.

Ensuring the integrity of the Schneiderian membrane is crucial for the success of maxillary sinus external lift surgery [21]. Schneiderian membrane perforation is the most common surgical complication in maxillary sinus floor elevation, with an incidence ranging from 20% to 44% in the lateral window approach [22]. In the experimental group, one patient experienced perforation due to the excessively small distance between the medial and lateral walls of the maxillary sinus, as well as the excessively small angle formed between the medial wall and the floor of the maxillary sinus. During the external sinus lift procedure, the medial mucosa of the maxillary sinus was damaged, leading to the patient's withdrawal from this experimental study. Currently, there is no optimal method to ensure the safety of mucosal elevation in this area, and the only measure is to perform gentle manipulation. Even if visual inspection and the Valsalva maneuver show no abnormalities in this area, maxillary sinus mucosal rupture may still occur at a later stage. For such patients, the author places a trimmed absorbable membrane in this area after mucosal elevation, followed by the placement of bone graft material. In the surgical cases over the past five years, no loss of bone graft material due to mucosal damage has occurred in the above-mentioned situations.

A limited surgical field is another significant cause of Schneiderian membrane perforation. A larger bone window can offer improved surgical visibility and operational space, particularly when there are bony protrusions at the maxillary sinus floor or chronic inflammation in the maxillary sinus, which reduces the likelihood of maxillary sinus mucosal rupture. Many scholars have attempted to create larger bone windows, typically measuring 10-15 mm in width and 8-10 mm in height [23, 24]. However, some scholars argue that small bone windows can also provide sufficient vision without compromising surgical procedures [25]. Although small bone windows have benefits such as minimal surgical trauma, limited flap reflection, and mild postoperative complications, the author only employs small bone windows under optimal conditions, which include a flat maxillary sinus floor, the absence of cysts or cystic lesions, and no chronic mucosal inflammation in the maxillary sinus.

Surgical technique and instrument selection are also crucial factors influencing the risk of membrane perforation. Piezoelectric surgery has been widely advocated for sinus elevation procedures due to its selective cutting properties for mineralized tissue, which minimizes the risk of soft tissue damage and may thereby reduce the incidence of Schneiderian membrane perforation [26]. In the present study, however, a conventional rotary instrument was employed for all lateral window preparations. This decision was based on the senior surgeon's extensive experience and proven proficiency with this technique, which has yielded a consistently low perforation rate in their clinical practice. The primary aim of this study was to investigate the timing of implant placement rather than to compare surgical devices. By standardizing the protocol with a well-controlled, traditional technique, we aimed to isolate the variable of interest. Nonetheless, the use of piezoelectric devices represents a valuable technological advancement that may further enhance the safety margin, particularly for less experienced operators or in cases with challenging sinus anatomy, and should be considered a recommended option in future clinical practice and studies.

The main focus of perforation management is to provide stable coverage for the perforated area, allowing it to accommodate the graft materials [27]. The first step in treating mucosal perforation is to relax the surrounding mucosa, reduce tension in the area, and avoid further tearing. The size and location of the perforation should then be evaluated. For small perforations, self-repair may be achieved through blood clot formation or folding of the maxillary sinus membrane. For larger perforations ( > 5 mm), an absorbable membrane should be used to cover the site, serving as a barrier between the sinus cavity and the graft materials. In cases of extensive perforations ( > 10 mm), it is recommended to use a large absorbable membrane that extends to the lateral wall and is fixed with membrane nails or sutures [28]. Absorbable sutures should be avoided when repairing ruptured membranes, especially for large perforations, as suturing often exacerbates the rupture area.

Intraoperative hemorrhage is a common surgical complication. It is mainly caused by the rupture of well-known blood vessels and other tiny blood vessels involved in the surgical area, especially the alveolar antral artery (AAA) [29]. The alveolar antral artery can be completely intraosseous, partially intraosseous, or located under the periosteum of the lateral wall of the maxillary sinus. The diameter of this blood vessel is usually < 1 mm, while anastomotic branches with a diameter > 2 mm are relatively rare [30]. Rupture of an artery with a larger diameter (2.5-3.0 mm) can lead to severe intraoperative hemorrhage [31]. A large amount of bleeding will seriously obscure the surgical field of view, thereby affecting the duration of the surgical operation and the precision of surgical manipulation, and increasing the risks of postoperative swelling and infection. In this study, partially intraosseous blood vessels and those under the maxillary sinus periosteum usually did not rupture during the operation through careful surgical techniques. Even if partial rupture of these blood vessels occurred, the self-constriction of the blood vessels would not cause massive bleeding. However, when intraosseous blood vessels with a diameter >3 mm cannot be avoided during the operation, methods such as electrocoagulation hemostasis need to be used to seal the blood vessels.

In addition to intraoperative hemorrhage, various complications may arise following maxillary sinus external elevation surgery, such as hematoma, edema, acute maxillary sinusitis, postoperative infection, wound dehiscence with exposure of graft material, and postoperative hemorrhage. In both the experimental and control groups, only edema and facial ecchymosis were observed in patients post-surgery, with none of the aforementioned complications occurring. The factors contributing to postoperative complications are often multifactorial. The primary factors for preventing maxillary sinusitis and postoperative infection include the use of aseptic techniques during surgery and maintaining the integrity of the maxillary sinus mucosa. Furthermore, obstruction of the maxillary sinus ostium is also a significant factor that can lead to maxillary sinusitis. Proper hemostasis at intraoperative bleeding sites and postoperative pressure dressing on the surgical area are also important measures to reduce postoperative edema and hemorrhage.

Another important consideration in planning sinus floor elevation is the management of pre-existing maxillary sinus cysts. Cysts and cyst-like lesions in the maxillary sinus can affect the complexity and prognosis of maxillary sinus floor elevation surgery. Asymptomatic cysts and cyst-like lesions are predominantly maxillary sinus pseudocysts and retention cysts. The accumulation of mucus in these cysts within the lamina propria of the maxillary sinus mucoperiosteum often stretches and thins the mucoperiosteum, thereby increasing the risk of perforation during maxillary sinus floor elevation [18, 32, 33]. For patients without clinical symptoms of the maxillary sinus included in this study, there are two treatment options. First, if maxillary sinus floor elevation is performed and the cyst does not interfere with the maxillary sinus opening, maxillary sinus external elevation can be directly conducted without addressing the cyst. Second, if the cyst hinders the drainage of the maxillary sinus opening after maxillary sinus floor elevation, the cyst must be aspirated or removed prior to the elevation procedure. Maxillary sinus cysts or cyst-like lesions are no longer considered absolute contraindications for maxillary sinus floor elevation; however, surgeons should devise appropriate treatment plans based on X-ray films and patients' clinical symptoms.

Some scholars have attempted to perform maxillary sinus external lift with simultaneous implant placement in the context of residual bone height (RBH) < 3 mm, but this is limited to maxillae with intact double-layer cortical bone, as intact double-layer cortical bone can provide good primary stability [34]. Studies have also demonstrated that simultaneous implant placement in maxillae with RBH < 3 mm can achieve good primary stability and osseointegration effects through measurements of implant stability quotient (ISQ) values and CBCT imaging [35]. However, some scholars question whether severely atrophied maxillae with RBH < 3 mm can provide good primary stability for implants [36]. The authors also believe that when RBH < 3 mm, it is impossible to ensure that every patient will have good primary stability after implant placement. In this study, some patients had primary implant stability <10 N due to maxillary sinus cortical bone loss and residual bone density classified as Type IV bone, but this did not affect implant osseointegration or subsequent loading. In addition, successful implant osseointegration is related to multiple factors, such as general health status, oral hygiene habits, surgeon experience, implant design and surface treatment, and the influence of bone augmentation materials [37].

When performing maxillary sinus floor elevation and simultaneous implantation in patients with RBH < 3 mm, some scholars believe that the success rate (the 10-year cumulative survival rate of osseointegrated implants is only 53.3%) is much lower than the survival rate (92.9%) of implants simultaneously placed in the maxilla with RBH > 3 mm [38]. Some scholars have also put forward similar views, that is, the long-term survival rate of implants is positively correlated with the remaining bone mass [39]. However, another study with a longer follow-up period and a larger number of included cases showed that the 20-year cumulative survival rate of implants in this procedure was 78.8% [4].

This study has several limitations that should be considered when interpreting the results. First, the sample size, although adequate for the primary outcome, remains relatively small and limits meaningful subgroup analyses (e.g., based on bone density or specific anatomic variations). Second, the single-center, single-surgeon design may affect the generalizability of the findings, and the results should be validated in multi-center settings with larger cohorts. Third, the 9-month follow-up period is sufficient for assessing bone remodeling and implant stability before loading but is insufficient to evaluate long-term outcomes, such as graft resorption and implant success over decades. Furthermore, the study lacked a non-grafted control group for ethical and clinical rationale, which would have provided direct insight into the intrinsic osteogenic capacity of the maxillary sinus under these conditions. Future studies with larger sample sizes, longer follow-up periods, and multi-center designs are warranted to confirm these preliminary findings. Furthermore, the absence of a non-grafted control group limits our ability to precisely delineate the extent of bone loss specifically attributable to the resorptive characteristics of the xenograft material versus the baseline resorption resulting from the surgical trauma of the sinus elevation procedure itself.