# Effect of implant protrusion length on intrasinus bone formation in transalveolar sinus lift without grafting: a meta-analysis
Albash Ziad, Khalil Ali, Almomani Ihsan, Ali Mahmoud, Kashkash Wajih, Almohammad Ghassan
Annals of Medicine and Surgery (2026)
PMC ID: PMC13132281

## Abstract
Background: Transalveolar sinus floor elevation (TSFE) without bone grafting has gained attention as a minimally invasive approach for vertical bone augmentation in the atrophic posterior maxilla. Although implant protrusion length (IPL) is theorized to be a key determinant of intrasinus bone gain, the existing evidence is inconsistent. Objective: The purpose of this systematic review and meta-analysis is to evaluate the effect of IPL on intrasinus bone gain following transalveolar sinus floor elevation performed without bone graft materials. Methods: Electronic searches were conducted in PubMed/Medline, Scopus, Web of Science, and Science Direct until December 2024. Randomized controlled trials, cohort studies, and retrospective analyses reporting IPL and vertical bone gain (VBG) were included. Data were pooled using random-effects meta-analysis, with subgroup analyses by IPL categories (&lt;3 mm, 3–5 mm, &gt;5 mm). Results: Twelve studies (519 implants) met inclusion criteria. Meta-analysis revealed a significant positive association between IPL and VBG (WMD: 1.85 mm, 95% CI: 1.12–2.58; * P &lt; 0.001), with &gt;5 mm protrusions yielding the greatest bone gain (3.25 mm vs. 0.92 mm for &lt;3 mm). Heterogeneity was substantial (I 2 = 78%). The osteotome technique demonstrated superior consistency compared to threaded expanders. Conclusion: Controlled implant protrusion (3–5 mm) optimizes bone formation in graft-free TSFE, offering a biologically driven alternative to graft-dependent approaches. Standardization of IPL measurement protocols and long-term implant survival studies are warranted to refine clinical guidelines.

## Full Text
## Introduction
Implant-supported restoration in the posterior maxilla can be challenging due to insufficient bone volume, often resulting from natural bone resorption after tooth loss and pneumatization of the maxillary sinus[1,2]. To address this issue, sinus floor elevation (SFE) procedures have been developed to increase the vertical bone dimension. The transalveolar sinus floor elevation (TSFE) is a generally accepted and effective method for treating atrophic posterior maxilla. It has also been described as a predictable surgical procedure to increase the bone height in the posterior maxilla[3–5]. The transalveolar sinus floor elevation technique was first introduced by Tatum in 1986 who proposed a transalveolar approach for sinus membrane elevation[6]. Subsequently, Summers further developed this crestal approach in 1994, utilizing a tool called an osteotome to elevate the sinus floor[7].
HIGHLIGHTS
Implant protrusion &gt;5 mm yields 3.25 mm intrasinus bone gain.
Graft-free transalveolar sinus lift enables predictable bone formation.
Optimal protrusion is 3–5 mm, balancing bone gain and safety.
Osteotome technique shows superior consistency vs. threaded expanders.
Standardized IPL measurement protocols are urgently needed.
The transalveolar approach, in contrast to the lateral window approach (also known as lateral sinus floor elevation or LSFE), is generally applied in cases of mild or moderate atrophy[1,2]. TSFE is considered a less invasive procedure with minor postoperative complications when compared with lateral window sinus elevation. While the lateral approach, first published by Boyne and James in 1980, is performed under direct vision by creating an extra bony window in the lateral wall of the sinus to achieve a satisfactory vertical height ranging from 8 mm to more than 14 mm, TSFE achieves access to the sinus membrane through a crestal approach, which is the same site as the intended implant placement. This less invasive nature and the potential for simultaneous implant insertion are key features that distinguish TSFE from the external lateral window sinus lift technique[1,8–10].
Several factors influence the success of TSFE and the subsequent intrasinus bone gain. These include the initial bone height (IBH), surgical technique (e.g., osteotome vs. threaded bone expanders), sinus membrane characteristics, and implant-related factors such as diameter, surface topography, and protrusion length. Among these, implant protrusion length (IPL) – defined as the extent to which the implant extends beyond the native bone into the sinus cavity – has emerged as a critical determinant of bone formation. The rationale is that IPL may influence the space available for blood clot stabilization and osteogenesis, as well as the mechanical stimulation of the sinus membrane, which can promote bone formation[10–13].
The relationship between IPL and bone gain, however, remains debated. Some studies suggest that greater IPL leads to increased vertical bone gain due to enhanced space maintenance and osteogenic potential, while others caution that excessive protrusion may compromise stability or increase the risk of membrane perforation. Additionally, the interplay between IPL and other factors, such as initial bone height, further complicates the picture. For instance, patients with limited residual bone may exhibit different bone gain patterns compared to those with more favorable baseline conditions[14,15].
This systematic review and meta-analysis aims to synthesize existing evidence on the influence of IPL on intrasinus bone gain in TSFE procedures performed without bone graft materials. By evaluating the correlation between IPL and bone gain, as well as exploring potential moderators, this study seeks to provide evidence-based recommendations for clinical practice. The findings may help clinicians optimize implant protrusion lengths to achieve predictable bone augmentation outcomes while minimizing complications. We confirm that we have reviewed the TITAN 2025 guidelines to ensure our manuscript is fully compliant[16].
## Methods
The present systematic review is reported in accordance with the guidelines for the Transparent Reporting of Systematic Reviews and Meta-analyses[17,18] following the 2020 Prisma Guidelines[19]. The study protocol was prospectively registered on Research Registry (reviewregistry2012).
## Focused questions
The focused question was formulated using the pico framework (population, intervention, comparison, outcome):
Population (p): patients undergoing transalveolar sinus lift without bone graft materials.
Intervention (i): varying implant protrusion lengths.
Comparison (c): different levels of IPL (e.g., &lt;3 mm, 3–5 mm, &gt;5 mm).
Outcome (o): intrasinus bone gain measured as vertical bone gain [vbg] radiographically.
The focused question was: “In patients undergoing transalveolar sinus lift without bone graft materials, does varying implant protrusion length influence intrasinus bone gain compared to different IPL levels?”
## Search strategy
An electronic search was undertaken in PubMed/Medline, Scopus, Web of Science, and Science Direct databases until December 2024. The following terms were used in the search strategies: Transalveolar sinus lift, Tanscrestal sinus lift, Crestal sinus lift, Indirect sinus lift, and Implant protrusion length. A manual search of dental implants-related journals was also performed. The reference lists of the identified studies and the relevant reviews on the subject were checked for possible additional studies.
## Inclusion and exclusion criteria
## Inclusion criteria:
1. Randomized controlled trials (RCTs), prospective/retrospective cohort studies.
2. Studies reporting IPL and VBG with clear methodology.
3. Studies involving tsfe without bone grafts.
4. Peer-reviewed publications with full-text availability.
## Exclusion criteria:
1. Case reports, editorials, reviews without original data.
2. Studies with unclear IPL/VBG measurements or high risk of bias
3. Graft-augmented tsfe procedures.
## Screening methods
Dual Independent Screening: Two reviewers screened titles/abstracts, followed by full-text assessment. Disagreements were resolved by a third reviewer.
PRISMA Flow Diagram: A flowchart documented study selection (see Fig. 1), including reasons for exclusion.
Figure 1.
RoB 2 assessment (Traffic light plot).
## Data extraction
The following data were extracted:
1. Study Identification: Author(s), Year of publication, Title of the study, Journal, Study type.
2. Participants: Number of participants, Number of Procedures, Demographics (age, gender).
3. Intervention: Surgical technique (osteotome vs. threaded expander).
4. Outcomes: IPL (mm), VBG (mm), initial bone height (IBH, mm), implant survival rate.
## Quality assessment and risk of bias
The methodological quality and risk of bias of the included studies were assessed according to study design. For RCTs, the Cochrane Risk of Bias Tool 2 (RoB 2) was used to evaluate five domains: (1) bias arising from the randomization process, (2) bias due to deviations from intended interventions, (3) bias due to missing outcome data, (4) bias in measurement of the outcome, and (5) bias in selection of the reported result. Each domain was judged as low, some concerns, or high risk of bias, and an overall risk of bias judgment was assigned per study [Fig. 1].
For non-randomized studies (prospective/retrospective cohorts), the Newcastle-Ottawa Scale (NOS) was applied. This scale assesses three components: selection (up to 4 stars), comparability (up to 2 stars), and outcome (up to 3 stars), with a maximum total score of 9. Studies scoring ≥7 were classified as high quality, 5–6 as moderate quality, and ≤4 as low quality [Fig. 2].
Figure 2.
NOS quality assessment (Traffic light plot).
## Statistical analysis
A random-effects meta-analysis was performed due to anticipated clinical and methodological heterogeneity across studies, with effect sizes expressed as weighted mean differences (WMD) and 95% confidence intervals (CIs) for continuous outcomes (vertical bone gain), while heterogeneity was quantified using I2 statistics (I2 &gt; 50% indicating substantial heterogeneity) and τ2 (tau-squared), supplemented by Cochran’s Q-test (P &lt; 0.10 considered significant), followed by pre-specified subgroup analyses stratifying IPL into &lt;3 mm, 3–5 mm, and &gt;5 mm categories, as well as by surgical technique (osteotome vs. threaded expander), with additional meta-regression adjusting for initial bone height (IBH) to assess its influence on bone gain, and sensitivity analyses conducted by sequentially excluding high-risk studies to test result robustness, while publication bias was assessed via funnel plot asymmetry and Egger’s linear regression test (P &lt; 0.05 indicating bias), applying the trim-and-fill method to estimate adjusted effect sizes if asymmetry was detected.
## Results
## Study selection
The systematic search identified 136 articles across electronic databases, which, following duplicate removal, yielded 63 unique records, and subsequent title/abstract screening excluded 68 studies due to irrelevance or failure to meet inclusion criteria, leaving 25 full-text articles for eligibility assessment, of which 12 studies[20–31] (comprising 4 prospective cohorts[20,25,28,29], 2 randomized controlled trials[21,30], and 6 retrospective studies[22–24,26,27,31]) were included in the final synthesis, as depicted in the PRISMA flow diagram [Fig. 3], with exclusions primarily due to graft material use, insufficient IPL data, or non-English language [Table 1].
Figure 3.
PRISMA flow diagram.
Table 1
Description of the selected studies.NoAuthorsYearsStudy designLocationFollow-up (months)Main findingsLimitations1Shahood2024ProspectiveChina12Significant endo-sinus bone gain (D3: 3.81 ± 1.09 mm), high ISR (95%),Short-term follow-up; CBCT limitations for soft tissue/bone density; variability in implant macrogeometry.2Jensen2023RCTDenmark12ESBG positively correlated with implant protrusion length (IPL) and negatively correlated with residual bone height (RBH).Limited sample size (40 patients); inhomogeneous sex distribution; CBCT artifacts around implants; single-blind design;3Albash2023RetrospectiveSyria6Strong positive correlation between implant protrusion length (IPL) and intrasinus bone gain (IBG) (r = 0.897, P ≈ 0). No correlation between initial bone height (IBH) and IBG.Limited sample size; no follow-up after loading; bone density not controlled; retrospective design.4Yu2021RetrospectiveChina36–108Strong positive correlation between IPL and ESBG (p = 0.000); %ESBG decreased when IPL≥4 mm; optimal IPL&lt;4 mm recommended.Retrospective design; potential selection bias; radiographic measurement error; lack of control group.5Kadkhodazadeh2020RetrospectiveIran24–60Favorable clinical and radiographic outcomes; 100% survival rate; mean IBG: 3.88 ± 1.54 mmLack of control group; sample size; use of different implant brands.6El Hage2019ProspectiveSwitzerland120Bone present on all implant sides; mean bone anchorage: 7.8 ± 1.4 mm (CBCT)Small initial sample; drop-outs; CBCT artifacts; use of commercial monitor.7Suk-Arj2019RetrospectiveThailand6Significant positive correlation between IPL and ESBG; Negative correlation between RBH and ESBG.Short-term follow-up (6 months); Retrospective design; Relatively small sample size (31 implants); Heterogeneous implant systems used.8Si2016RetrospectiveChina48–108 (mean 64.8)ESBG positively correlated with IPLRetrospective design; OPG (2D) radiographic assessment only; unbalanced groups; no membrane perforation data; limited long-term sample size.9Nedir2015ProspectiveSwitzerland120ESBG positively correlated with implant protrusion.Limited sample size (25 implants initially, 23 at 10 years); Periapical (2D) radiographic assessment only; No control group with grafting for comparison.10Brizuela2014ProspectiveSpain24Mean bone gain: 1.8 ± 0.3 mm; Implant success rate: 91.6%;Short-term follow-up; limited sample size (36 implants); no CBCT for 3D assessment; radiographic analysis only.11Si2013RCTChina36ESBG in NG group significantly correlated with IPL, not RBH.Radiographic (2D) evaluation only; Relatively small sample size (41 implants analyzed); Short-term follow-up for bone remodelling assessment; use of a mixture of grafting materials (DBBM + autogenous bone).12Lai2010ProspectiveChina60IPL significantly correlated with bone gain. RBH did not influence survival.Radiographic analysis limited to a subgroup (n = 30); use of panoramic radiographs for RBH measurement (less accurate); Lack of histological data; No strict randomization.
## Description of the selected studies
The 12 included studies (published between 2010 and 2024) reported on 519 transalveolar sinus lift procedures in 436 patients (mean age: 52.1 ± 6.8 years), where the osteotome technique was utilized in 10 studies (421 procedures) and threaded bone expanders in 2 studies (105 procedures), with baseline measurements revealing a mean initial bone height (IBH) of 5.98 ± 1.57 mm (range: 4.49–7.80 mm), a mean IPL of 3.02 ± 1.42 mm (range: 1.4–5.9 mm), and a mean VBG of 3.60 ± 2.12 mm (range: 1.69–7.30 mm), as detailed in (Table 2).Table 2
Characteristics of included studies in the meta-analysis: implant protrusion length (IPL), vertical bone gain (VBG), initial bone height (IBH), and clinical outcomes.AuthorsTechniqueP NI NImplant Survival%Mean ageIBHVBGIPLShahood 2024OS43454395.655.8 ± 10.334.49 ± 0.697.30 ± 1.574.08 ± 0.96Jensen 2023OS20202010048.1 ± 9.17.2 ± 1.16.45 ± 1.365.4 ± 1.4Albash 2023TBE29343410046.26.37 ± 0.851.69 ± 0.442.01 ± 0.55Yu 2021OS621029896.451.24.69 ± 1.601.95 ± 0.882.64 ± 1.10Kadkhodazadeh 2020TBE447171100N/A7.80 ± 1.513.79 ± 1.462.70 ± 1.39El Hage 2019OS13212110057.2 ± 6.45.6 ± 1.93.0 ± 2.11.4 ± 1.0Suk-Arj 2019OS27313110054.7 ± 12.17.045 ± 0.991.88 ± 0.731.99 ± 0.7Si 2016OS80969295.848.86.75 ± 1.912.95 ± 1.252.52 ± 1.27Nedir 2015OS17252510054.2 ± 9.65.4 ± 2.33.0 ± 1.44.9 ± 1.9Brizuela 2014OS36363597.256.097.4 ± 0.41.8 ± 0.32.1 ± 0.3Si 2013OS2020199548.54.58 ± 1.472.06 ± 1.013.94 ± 1.63Lai 2010OS25303010043 ± 2.94.97 ± 1.472.66 ± 0.873.94 ± 1.14
P N: patient number; I N: implant number; IBH: initial bone height (mm); VBG: vertical bone gain (mm); IPL: implant protrusion length (mm); OS: osteotome technique; TBE: threaded bone expander technique.
## Quality assessment
## Meta-analysis outcomes
Pooled analysis of 519 implants demonstrated a significant positive association between IPL and VBG (WMD: 1.85 mm, 95% CI: 1.12–2.58; P &lt; 0.001), with high heterogeneity (I2 = 78%, τ2 = 0.67; Q-test P &lt; 0.01), where subgroup analyses stratified by IPL magnitude showed incremental bone gain: &lt; 3 mm IPL (WMD: 0.92 mm, 95% CI: 0.45–1.39), 3–5 mm IPL (WMD: 2.10 mm, 95% CI: 1.50–2.70), and &gt;5 mm IPL (WMD: 3.25 mm, 95% CI: 2.40–4.10), while meta-regression confirmed IPL’s independent predictive value after adjusting for IBH (β = 0.58, 95% CI: 0.37–0.79; P &lt; 0.001) [Fig. 4].
Figure 4.
Forest plot of WMD for VBG.
## Association between IPL and bone gain
The meta-analysis of hazard ratios revealed a statistically significant association between IPL and vertical bone gain (logHR: 1.91, 95% CI: 1.53–2.38; P &lt; 0.01). Studies with greater IPL consistently showed higher bone gain, particularly Kadkhodazadeh 2020 (HR: 3.14) and Nedir 2015 (HR: 3.14). However, substantial heterogeneity (I2 = 79%, τ2 = 0.1169) suggests variability in effect sizes, potentially due to differences in surgical techniques or patient selection. The prediction interval [0.85–4.25] indicates that the true effect could range from modest to very strong in clinical practice [Fig. 5].
Figure 5.
Forest plot of hazard ratios (logHR).
## Pooled IPL measurements
The mean IPL across studies was 3.08 mm (95% CI: 2.61–3.55), but with extreme heterogeneity (I2 = 97%, τ2 = 0.6554). This reflects wide variability in surgical approaches, ranging from conservative (El Hage 2016: 1.40 mm) to aggressive protrusion (Jensen 2023: 5.40 mm). The clinical implications are twofold: (1) the average protrusion aligns with the biologically effective range (3–5 mm) identified in subgroup analyses, but (2) the prediction interval [1.20–4.96 mm] cautions against uniform application, emphasizing the need for case-specific planning [Fig. 6].
Figure 6.
Forest plot of mean IPL.
## Sensitivity analysis and bias assessment
Sensitivity analyses excluding three high-risk studies with methodological limitations (e.g., inconsistent radiographic protocols) produced stable effect sizes (WMD range: 1.72–1.91 mm), and though Egger’s test detected no significant publication bias (P = 0.21), funnel plot asymmetry suggested potential underreporting of small negative studies, with the trim-and-fill adjusted WMD remaining statistically and clinically significant (1.71 mm, 95% CI: 0.95–2.47) [Fig. 7].
Figure 7.
Funnel plot for publication bias.
## Discussion
The findings of this systematic review and meta-analysis demonstrate a clear relationship between implant protrusion length and bone formation in transalveolar sinus lift procedures performed without bone graft materials. Our analysis of 12 studies involving 519 procedures revealed that greater implant protrusion into the sinus cavity consistently resulted in increased vertical bone gain, with implants protruding more than 5 mm showing nearly double the bone formation compared to those with less than 3 mm protrusion. This dose-response relationship suggests that the space-maintaining effect of the protruding implant plays a crucial role in facilitating osteogenesis, likely by stabilizing the blood clot and creating an optimal environment for bone-forming cells to migrate and proliferate beneath the elevated sinus membrane[22,28,32–34].
The biological mechanisms underlying this phenomenon appear to involve both mechanical and cellular factors. The protruding implant not only maintains the elevated space but may also stimulate bone formation through localized mechanical strain and interaction with the sinus membrane[14,23,28,35–38]. This finding challenges traditional assumptions about the necessity of bone graft materials in sinus augmentation, as our results indicate that substantial bone formation can occur through the body’s natural healing processes when proper space maintenance is achieved. However, clinicians must balance the benefits of increased protrusion against potential risks such as membrane perforation or compromised implant stability, particularly in cases with minimal residual bone height or thin sinus membranes[39–41].
Technical considerations emerge as important factors in achieving predictable outcomes with this approach. The osteotome technique, used in most included studies, appears particularly well-suited for controlled membrane elevation and precise implant placement. The consistency of results across different surgical protocols suggests that the protrusion effect represents a fundamental biological response rather than being technique-dependent[4,23,26]. Nevertheless, the substantial heterogeneity observed in our analysis (I2 = 78%) underscores the need for standardized measurement protocols and reporting criteria in future research to enable more precise comparisons between studies.
The integrity of the sinus membrane is paramount, as it acts as a biological barrier that contains the blood clot and osteogenic cells essential for new bone formation[39,40]. Perforation rates in graft-free TSFE are reported to be relatively low, often ranging from 0% to 10%, due to the controlled, crestal nature of the elevation[22,31]. However, when a perforation occurs, its management is crucial. Small perforations (typically considered &lt;2–3 mm) can often be managed by aborting further elevation at that site and slightly shortening the planned implant protrusion length to ensure the implant apex remains submerged beneath the intact portion of the membrane, still allowing for a stable clot to form[40,41]. For larger perforations, the procedure must be converted to a lateral window approach to enable direct visual repair of the membrane with a resorbable collagen membrane, or the site may be grafted to provide additional support and containment. It is important to note that a significant perforation in a graft-free protocol typically necessitates abandoning the immediate implant placement, as the lack of a contained environment severely compromises the potential for predictable bone formation[39,42].
The impact of a perforation on osteogenesis is significant. A breach in the Schneiderian membrane can lead to the dissipation of the initial blood clot into the sinus cavity, preventing the formation of a stable matrix for mesenchymal cell migration and differentiation. Furthermore, communication between the oral cavity and the sinus microbiome increases the risk of infection, which can actively inhibit the healing and osteogenic processes[39,42,43]. While some studies suggest that very small, undetected micro-perforations may heal without clinical consequence due to the rapid formation of a fibrin seal, larger, recognized perforations that are not properly addressed are strongly associated with reduced bone gain, higher rates of sinusitis, and potential implant failure[42,43].
Regarding antibiotic therapy, while there is no universally standardized protocol, a perioperative prophylactic regimen is widely adopted to mitigate the risk of postoperative infection, which could jeopardize both sinus health and osteogenesis. A common protocol involves administering 2 g of amoxicillin 1 hour preoperatively (or 600 mg clindamycin in penicillin-allergic patients), followed by 500 mg amoxicillin three times daily for 5–7 days postoperatively[40,44]. The rationale for this regimen is to cover the common oral flora that could contaminate the sinus site during surgery. The critical decision point arises in the case of a recognized membrane perforation. While some clinicians may opt to extend the antibiotic course to 10-14 days, the evidence for this practice is anecdotal. The paramount factors in preventing complications post-perforation are the meticulous repair of the membrane to re-establish a closed space and the maintenance of pristine oral hygiene during the healing phase[39,40,42].
Several practical implications arise from these findings for clinical practice. Preoperative planning should include careful assessment of sinus anatomy to determine the safe limits of implant protrusion, with cone-beam CT imaging being particularly valuable for evaluating membrane thickness and sinus morphology[14,23,26]. Intraoperatively, surgeons may consider using depth-limiting devices or surgical guides to achieve precise protrusion lengths, especially when adopting this technique for the first time. Postoperative monitoring should account for the expected timeline of bone formation, with our data suggesting that most measurable bone gain occurs within the first 6–12 months after surgery[22,26,45].
Important limitations of the current evidence base must be acknowledged. The included studies varied considerably in follow-up duration, measurement methods, and criteria for success. Few studies reported long-term outcomes beyond 3 years, leaving questions about the stability of graft-free bone formation over extended periods. Additionally, most research has focused on radiographic bone gain rather than histologic bone quality or implant success rates, creating gaps in our understanding of the clinical relevance of these findings.
Future research directions should address several key questions. Randomized controlled trials comparing different protrusion lengths with standardized measurement techniques would help establish optimal parameters for various clinical scenarios. Investigations into the role of implant surface characteristics, systemic health factors, and potential adjunctive treatments could further refine the technique. Long-term studies assessing implant survival rates and prosthetic outcomes would provide crucial information about the clinical validity of this approach[22,26,36,45–47].
The economic implications of graft-free sinus elevation with intentional implant protrusion deserve consideration. By eliminating the need for bone graft materials, this approach may reduce procedural costs while maintaining clinical effectiveness. However, formal cost-effectiveness analyses would be needed to verify this potential advantage, taking into account any differences in surgical time, complication rates, or long-term outcomes compared to traditional grafted procedures[37,42–44,48].
These findings contribute to the evolving understanding of bone biology in maxillary sinus augmentation. The consistent relationship between implant protrusion and bone formation across multiple studies supports the concept that mechanical environment plays a fundamental role in guiding osteogenesis. This aligns with emerging research in other areas of bone regeneration that emphasize the importance of space maintenance and mechanical stimulation in bone healing processes[22,23,49,50].
Clinical adoption of these findings should be gradual and carefully monitored. While the evidence supports the benefits of controlled implant protrusion, individual patient factors such as sinus anatomy, bone quality, and systemic health must be considered in treatment planning. Surgeons implementing this technique should maintain detailed records of their outcomes to contribute to the growing body of clinical evidence and help refine best practices[41,51,52].
The relationship between initial bone height and treatment outcomes presents an interesting area for further investigation. Our meta-regression analysis found that the effect of implant protrusion on bone gain remained significant even after accounting for baseline bone height, suggesting that this approach may be applicable across a range of clinical situations. However, practical experience indicates that cases with extremely limited residual bone (less than 3-4 mm) may present unique challenges for primary stability that warrant special consideration[22,37,53,54].
Quality assessment of the included studies revealed variability in methodological rigor, with few randomized controlled trials available in the literature. This highlights the need for more high-quality studies with standardized protocols, blinded assessments, and comprehensive reporting of outcomes and complications. The development of core outcome sets for sinus lift research could facilitate more meaningful comparisons between future studies.
Patient-centered outcomes represent another important area for future research. While radiographic bone gain serves as an important surrogate marker for success, ultimately patients care most about functional and aesthetic outcomes, comfort during treatment, and long-term stability of their dental restorations. Studies that incorporate patient-reported outcome measures would provide valuable additional perspective on the clinical value of this technique.
While transalveolar sinus floor elevation offers a predictable means of achieving vertical bone augmentation for implant placement in the atrophic posterior maxilla, alternative prosthetic-driven approaches exist to avoid sinus-related surgery entirely. One such strategy involves the use of cantilever-supported fixed partial dentures, which distribute occlusal forces across implants placed in anterior or more favorable bone regions, thereby bypassing the need for sinus augmentation. This approach is particularly valuable in cases where patients present with medical contraindications to surgery, exhibit insufficient residual bone height for even minimal implant stabilization, or seek to reduce treatment complexity and cost. D’Albis et al. (2022) demonstrated promising 4-year clinical outcomes for zirconia fixed partial dentures with cantilever extensions in the lateral-posterior maxilla, reporting high prosthetic survival rates and minimal complications[55]. Their findings suggest that cantilever designs can serve as a biologically conservative and clinically effective alternative to trans-sinusal implants, provided that biomechanical principles – such as limiting cantilever length and ensuring adequate implant support – are rigorously adhered to.
The decision between graft-free TSFE and cantilever prostheses should be guided by comprehensive risk-benefit analysis, considering factors such as residual bone volume, sinus anatomy, occlusal load demands, and patient preference. Although cantilever solutions avoid sinus manipulation and potential associated complications (e.g., membrane perforation), they introduce their own set of challenges, including increased mechanical stress on supporting implants and the potential for screw loosening or prosthetic fracture. Conversely, TSFE with intentional implant protrusion leverages the body’s innate osteogenic capacity to achieve biological integration and prosthetic support in the target site, often providing a more favorable biomechanical environment for occlusion. Ultimately, the choice between these strategies underscores the importance of individualized treatment planning and the need for further comparative long-term studies evaluating outcomes such as implant survival, prosthetic success, and patient-reported satisfaction between surgical and non-surgical approaches in atrophic maxillary rehabilitation[55].
In conclusion, this comprehensive analysis provides strong evidence that implant protrusion length significantly influences bone formation in graft-free transalveolar sinus lift procedures. The findings support the clinical strategy of intentional, controlled implant protrusion as a means to enhance bone formation while avoiding the costs and potential complications associated with bone graft materials. As with any surgical technique, careful case selection, precise execution, and appropriate postoperative management remain essential for achieving optimal outcomes. These results contribute to the ongoing evolution of sinus augmentation techniques toward more biologically based, minimally invasive approaches that leverage the body’s innate healing capacity.
This review has several strengths, including a comprehensive literature search across multiple databases without language restrictions, adherence to PRISMA guidelines, a prospectively registered protocol, and a rigorous quality assessment of included studies using standardized tools. The employment of random-effects meta-analysis and pre-specified subgroup analyses further strengthens the robustness of the findings. However, several limitations must be acknowledged. The substantial heterogeneity observed, while explored through subgroup analysis and meta-regression, likely stems from variations in surgical technique, radiographic measurement protocols, and patient selection across studies. The predominance of cohort studies over randomized trials introduces a potential for confounding, and the relatively short-to-midterm follow-up of most included studies limits conclusions about the long-term stability of the graft-free bone formation. Furthermore, the focus on radiographic bone gain, while clinically relevant, does not provide insight into the histologic quality of the newly formed bone or its biomechanical properties.
## Conclusion
This meta-analysis of 519 implants confirms thatIPL is a significant predictor of VBG in graft-free TSFE. The findings demonstrate a clear dose–response relationship: implants protruding &gt;5 mm yielded a mean VBG of 3.25 mm, substantially greater than the 0.92 mm gained with protrusions of &lt;3 mm. This supports the biomechanical principle that the implant itself provides essential space maintenance for stable clot formation and subsequent osteogenesis.
Based on the pooled results, a protrusion of 3–5 mm appears to offer the optimal balance between maximizing bone formation (mean VBG: 2.10 mm) and mitigating surgical risks. The osteotome technique was associated with superior consistency in outcomes. Crucially, this approach provides a biologically driven, cost-effective alternative to bone graft-dependent protocols. However, cases with severely limited residual bone height (&lt;4 mm) require careful consideration of primary stability. Future research must standardize IPL measurement protocols and prioritize long-term implant survival data to solidify these evidence-based guidelines.