Review questions

The review questions were presented in Fig 1.

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Fig 1. The primary question (center) and secondary questions (surrounding) guiding the scoping review.

https://doi.org/10.1371/journal.pone.0338462.g001

Eligibility criteria

The criteria are summarized in Fig 2 and described in the following paragraphs.

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Fig 2. Flow diagram with questions related to eligibility criteria and principal reasons for exclusion.

https://doi.org/10.1371/journal.pone.0338462.g002

It is required that the mesoporous silica serves as a direct carrier of the drug in the given system.

Experimental models

This review includes studies utilizing both in vitro, in vivo, and ex vivo experimental models for osseointegration assessment. In vitro studies are included if they utilize primary cells (such as primary human osteoblasts HOB, primary human endothelial cells, e.g., HUVEC and stem cells (including mesenchymal stem cells and endothelial progenitor cells)), or established cell lines (such as human osteoblast hFOB 1.19, pre-osteoblast MC3T3-E1, osteoblast-like MG-63, SaOS-2, U2OS, and endothelial HMEC-1). For in vivo and ex vivo evaluation, studies using any animal and tissue/organ models to assess bone regeneration or vascularization are considered.

We are interested in biological, cell-dependent studies, as osteoconduction, osteoinduction, and angiogenesis processes depend on cellular activity. If a publication presents only a study of apatite-forming ability in simulated body fluid or other acellular media, it will be rejected, as it will not provide direct evidence of the expected properties of the materials. When it comes to in vivo assays all animal models and implantation sites will be considered, both direct into the bone tissue and ectopic sites. Ex vivo models include any tissue/organ model that is maintained and studied outside the organism.

Intervention

This review will consider studies that examine bone drug delivery systems, based on synthetic mesoporous silica, with reported osteoconductive, osteoinductive, and proangiogenic properties. Engineering strategies may include, but are not limited to:

• Structural or physicochemical alterations affecting material properties
• Doping with ions
• Formation of composites with other biomaterials (synthetic or natural)
• Combining with bioactive molecules

Herein, the term “drug” is not limited to well-established definitions proposed by Food and Drug Administration, but it is defined as any active substance, irrespective of whether it is of synthetic or biological origin, that is directly loaded into the mesoporous silica material, thereby confirming its role as a drug carrier. Studies where the drug is not directly carried by the mesoporous silica will be excluded. Similarly, studies examining non-mesoporous silica materials or bioactive glasses will not be considered.

Control

The review aims to map the effects of MS bone drug delivery systems engineering strategies on osseointegration potential. The controls reported in the publications may vary in type and quality; they will be described to illustrate how the effectiveness of material modifications was assessed.

Typical controls may include, for example:

• Unmodified mesoporous silica
• Commercial bone substitutes
• Standard culture conditions (for in vitro studies)
• Empty defects (for in vivo studies)
• Native tissue without exposure to the material (for ex vivo studies)

The presence or absence of appropriate controls will not be used as an inclusion/exclusion criterion; the primary aim of this review is to map the literature rather than assess the effectiveness of interventions.

Concept and outcomes

This review focuses on mesoporous silica bone drug delivery systems that follow the undermentioned concepts:

1. Mesoporous silica serves as the direct carrier of the drug
2. Mesoporous silica bone drug delivery system was engineered to improve its osseointegration potential
3. One of the key osseointegration-related outcomes has been examined for them in an in vitro cell-based assay, in vivo, and ex vivo study [3]:
   • Osteoinductive properties (e.g., ability to stimulate undifferentiated mesenchymal stem cells to differentiate into osteoprogenitor cells)
   • Osteoconductive properties (e.g., ability to support ingrowth of sprouting capillaries, perivascular tissue, and osteoprogenitor cells)
   • Proangiogenic properties (e.g., ability to promote blood vessel formation)

Studies must report at least one of these outcomes, evaluated through in vitro, in vivo, or ex vivo experiments. The outcomes may be either primary or secondary endpoints of the studies.

Context

This review will consider studies examining mesoporous silica bone drug delivery systems for bone tissue applications, including but not limited to bone defect treatment, bone infection therapy, and bone tumor treatment. No geographical restrictions will be applied. As we anticipate that the majority of relevant publications in this field are published in English, we have decided to include only English literature to maintain consistency in our screening and data extraction processes. Such an approach may cause a language bias that risk of which will be acknowledged in the final review.

Types of sources

This scoping review will consider primary research papers reporting experimental studies. Systematic reviews will be collected to identify relevant references and to map the existence of systematic evidence synthesis in the field, but will not be included in the final data synthesis.

Conference abstracts, narrative reviews, editorials, opinion papers, and articles not reporting original experimental data will be excluded.

Methods

The proposed scoping review will be conducted in accordance with the JBI methodology for scoping reviews and will follow the PRISMA-ScR reporting recommendations [31,32].

Search strategy

The search strategy has been designed to locate both published and unpublished studies. Initially, a limited search of PubMed, Scopus, and Embase was undertaken to identify articles on the topic. A complete search strategy for PubMed was developed based on the text words contained in the titles, abstracts, and keywords of relevant articles, as well as synonyms found during manual searches, deducted during discussions between authors, and found in the MeSH and Emtree thesauri. The developed high-sensitive search strategy contains 3 parts:

1. (1) High-sensitive search of source related to silicon dioxide-based materials – it contains directly stated mesoporous silica materials [both descriptive terms (e.g., “mesoporous silica”, “mesoporous SiO₂”), names of commonly used materials (e.g., “SBA-15”, “MCM-41”), and brand names of materials of interest (e.g., “AEROPERL 300”, “Syloid XDP 3050”)], terms related to silica-based materials (e.g., “silica scaffold”, “silica xerogel”), and terms related to surface-modified silica materials (e.g., “functionalized silica”, “modified silica”).
2. (2) Filter for studies related to drug delivery systems (to exclude studies examining silica particles without any drug) – it contains terms such as “drug”, “deliver”, “carrier”, “loaded”, or “release”.
3. (3) High-sensitive search of sources related to outcomes – it contains synonyms of osteoconduction (e.g., “osteoconductivity”, “osseoconduction”), osteoinduction, and angiogenesis, as well as names of bone cells related to these concepts (e.g., “osteoblast”, “preosteoclast”), bone cell lines used in such studies (e.g., “hFOB”, “MC3T3-E1”), endothelial cell lines used in such studies (e.g., “HUVEC”, “EA.hy926”), proteins related to the concepts of interest (e.g., “osteopontin”, “osteoprotegerin”), genes coding these proteins (e.g., “SPP1”, “TNFRSF11B”), transcription factors related to the concepts of interest (e.g., “RUNX2”, “Sp7”), and methods used to assess outcomes of interest (e.g., “Alizarin Red S”, “von Kossa”)

The source must contain at least one term from each of the three parts to be found by the search strategy.

The search strategy, including all identified keywords and index terms, has been adapted for each included database and/or information source.

The complete search strategy for PubMed has been prepared and is included in S1 Appendix.

Studies published in any language will be included. Since we are interested in up-to-date engineering strategies used to enhance osteoconductive, osteoinductive, and angiogenic properties of MS bone drug delivery systems. The review will include articles published from January 2015 up to the date of the search, which will be conducted immediately following the publication of this protocol.

Study/Source of evidence selection

Following the search, all identified citations will be collated and initially deduplicated using SR Deduplicator (relax mode). The results will then be imported into Rayyan for a second round of deduplication, followed by screening. Using Rayyan’s screening interface, two independent reviewers will conduct both stages of screening: first titles and abstracts, and subsequently full texts, against the inclusion criteria. Reasons for the exclusion of sources in full text that do not meet the inclusion criteria will be recorded and reported in the scoping review. Any disagreements between the reviewers at each stage of the selection process will be resolved through discussion. If consensus cannot be reached, a third reviewer will provide arbitration. The results of the search and the study inclusion process will be fully reported in full in the final scoping review and presented in a PRISMA flow diagram.

Data extraction

Each article will undergo data extraction by at least one human reviewer (https://forms.gle/CHYNyndGDCG9fBeE9) and Elicit, an AI-based tool [33]. Any discrepancies found between human- and AI-assisted extractions will be resolved through a second human review. Elicit will be used solely for technical assistance in extracting data from full-text articles. The tool will not perform any interpretation or automated assessment of study outcomes. For reproducibility, the instructions used in Elicit for data extraction will be documented in the supplementary materials of the final manuscript.

The extracted data will include specific details about:

1. 1. Material characteristics:
   • Type of mesoporous silica (e.g., MCM-41, SBA-15, KIT-6).
   • The method of confirming the presence of mesopores in the material (e.g., based on data from another publication or experimental data).
   • Porosity parameters (specific surface area, pore diameter, pore volume, pore arrangement).

Materials in which this has not been confirmed will also be included in the review, but the lack of this information will be noted.

1. 2. Loaded drug.
   • The method confirmed that the drug had been directly loaded into the mesoporous silica material, which served as its carrier (e.g., nitrogen porosimetry (BET, BJH), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD, SAXS), thermogravimetric analysis (TGA), nuclear magnetic resonance spectroscopy (NMR, solid-state NMR), Raman spectroscopy).
   • Drug type. In the context of this study, any active substance – regardless of its origin – will be considered a drug when loaded into mesoporous silica. This includes both synthetic compounds (e.g., antimicrobial agents, chemotherapeutics) and biologically derived agents (e.g., proteins, peptides).
   • Indications for the use of a loaded drug in a specific disease or indications for the use of the obtained carrier (e.g., bone infection, osteoporosis, bone tumors, periodontal disease).
2. 3. The strategy used to improve osseointegration of drug-loaded mesoporous silica (e.g., structural alterations, composite formation, incorporation of bioactive molecules).
3. 4. Method used for preparing the final formulation of bone drug delivery system (e.g., sol-gel, co-precipitation, solvent evaporation, melt blending, 3D printing, electrospinning, solvent casting, molecules incorporation)
4. 5. Methods of assessment of the used approach:
   • In vitro studies:
     • Cell viability/proliferation assays
     • Migration assays
     • Gene expression analysis (mRNA level)
     • Protein quantification assays
     • Enzymatic assays
     • Angiogenesis assays
     • Microscopic imaging
   • In vivo studies:
     • Histological analysis or immunohistochemistry
     • Radiological imaging: X-ray, CT and MRI
     • Biomechanical tests for implant stability (e.g., percussion test, reverse torque)
   • Ex vivo studies:
     • Viability/proliferation assays
     • Gene expression analysis (mRNA level)
     • Protein quantification assays
     • Histological or immunohistochemistry
     • Microscopic imaging
     • Mechanical testing of tissue samples (e.g., compression or indentation tests)
     • Biomaterial-tissue interface analysis
5. 6. Experimental models:
   • Cellular models
   • Animal models and implantation sites
   • Organ/Tissue models
6. 7. Reported outcomes:
   • Osteoinductive properties (e.g., osteogenic differentiation factors, bone formation at ectopic sites)
   • Osteoconductive properties (e.g., cell attachment, bone-material contact, bone tissue ingrowth into implantable material)
   • Proangiogenic properties (e.g., capillary/vessel formation, angiogenic factors)

The draft data extraction tool will be modified and revised as necessary during the process of extracting data from each included evidence source. Engineering strategies will be detailed in the scoping review. Since we are interested in the current scope of the literature, authors of papers will not be contacted to request missing or additional data.

Critical appraisal

Since the critical appraisal, as well as risk of bias assessment, is generally not recommended in the updated methodological guidance for conducting scoping reviews [34], only basic methodological features from included studies will be extracted and reported, specifically: the number of cell lines (for in vitro studies), animal species/subspecies together with total number of animals (for in vivo/ex vivo studies), and the number of replicates/repetitions. Based on the obtained results, the basic descriptive statistics will be calculated, reported narratively and optionally summarized in a table or figure to support interpretation of studies.

Data analysis and presentation

The extracted data will be presented in tabular form in a manner that aligns with the objective of this scoping review. A narrative summary will accompany the tabulated results and will describe how the findings relate to the review objective and questions. The main characteristics and results of individual studies will be tabularized as follows:

1. Type of MS used (e.g., MCM-41, SBA-15, KIT-6) with porosity parameters.
2. Name of loaded drug.
3. Applied strategy for improving osseointegration (type and method).
4. Methods and markers used for the evaluation of outcomes.
5. Reported outcomes for osteoinductive, osteoconductive, and proangiogenic properties.

To address which cell lines and in vivo or ex vivo animal models are most commonly used in studies evaluating osseointegration, we will present the distribution of each cell line, in vivo/ex vivo animal model (rows) across osteoconduction, osteoinduction, and angiogenesis research categories (columns) in separate tables. For each cell line, in vivo or ex vivo animal model we will report both the number of studies and the corresponding percentage. Analogous tables will be prepared for cross-tabulation of methods and markers that are used for the evaluation of osteoconduction, osteoinduction, and angiogenesis. To analyze the drugs used in the included studies, we will (1) calculate the percentage of studies reporting confirmation of drug loading into the MS, (2) categorize drugs into groups (e.g., antimicrobial agents, chemotherapeutics, bisphosphonates), and (3) list the proposed clinical indications for each group of drugs along with the number of studies and the corresponding percentage.

Moreover, we will construct a visual evidence map with MS modifications as rows and outcome domains as columns; within each cell, the shape of markers (triangles, circles, squares) will represent the type of MS (two most common and merged other), color will indicate study design (green for in vivo, yellow for ex vivo, red for in vitro studies), and marker size will reflect the number of studies identified for each combination. Finally, the identified trends across tables and the evidence map will be narratively reviewed.

The analysis of the specific MS bone drug delivery system modification impact on osseointegration will proceed in a structured sequence. Firstly, included studies will be grouped according to the type of MS bone drug delivery system modification and subsequently by the class of loaded drug to determine whether the observed bone-related effect is driven by the intrinsic bioactivity of the drug or by enhancements introduced through modification. This evaluation will only be feasible when appropriate experimental controls are provided (drug-loaded MS system with vs without modifier). In cases where a clear conclusion regarding the relative contribution of the material modification versus the loaded agent cannot be drawn, this will be explicitly acknowledged. The consistency of osseointegration enhancement of the given MS bone drug delivery system modification across various drug types will prevent the overinterpretation of osseointegration properties resulting from highly osteoinductive agents loaded onto MS.

Ethical declaration

This scoping review protocol and subsequent scoping review do not involve any studies with human participants or animals conducted by authors. Therefore, ethical approval is not required.