Bioactive ion-releasing restorative materials: a critical narrative review of terminology, evidence and controversies

Design and reporting

This is a critical narrative review organized as an evidence matrix. It follows the SANRA items for narrative reviews and does not claim to be systematic or exhaustive: no PRISMA flow is reported, and selection was not intended to retrieve every eligible record.( 4) The scope was defined around direct restorative materials that release ions with a claimed biological effect on caries or remineralization; root canal sealers, pulp capping agents and orthodontic adhesives were considered only when they informed the mechanisms of ion release.

Sources and search

Searches were run through the project's bibliographic tool in PubMed, OpenAlex and Crossref. The core literature was restricted to the window 2019 to 2026 to reflect current evidence, and classic or methodological references published before that window were admitted when they defined the field, such as the original description of the glass ionomer cement(6) and the simulated body fluid method.(8) The query strings combined terms for the materials (glass ionomer, bioactive glass, calcium silicate, alkasite, giomer, surface pre-reacted glass), the phenomenon (ion release, fluoride release, apatite formation, remineralization) and the outcome (secondary caries, restoration survival). Retrieval was capped at forty records per query, and duplicates were removed by the tool.

Selection criteria

Records were retained when the title or abstract addressed ion release from a restorative material, its biological effect on enamel or dentin, or a clinical outcome of a restoration placed with such a material. Preference was given to original studies, randomized clinical trials and systematic reviews over narrative reviews when a factual claim about performance was cited. Studies were excluded when they concerned only endodontic sealers, bone cements or materials outside restorative dentistry, or when they were opinion pieces without data. No language restriction was applied, but all retained sources were published in English or Spanish.

Synthesis and quality appraisal

The extracted evidence was organized in a matrix with five thematic axes: terminology and classification, material families and ion release profiles, laboratory evidence, clinical evidence, and controversies. For each axis, claims were recorded together with the studies that support or contradict them, following a pre-registered internal template. Quality was graded by design, so that meta-analyses of randomized trials ranked above single trials, which ranked above laboratory studies; within laboratory evidence, standardized and long-term protocols were valued over single time-point measurements. The synthesis quantifies, where the sources allow it, how many sources underpin each claim, and it flags findings that rest on a single study or on a surrogate endpoint. Counts of supporting studies reported in the Development were taken from the retained records: a claim backed by a meta-analysis of 64 trials is described as such, and a claim resting on one laboratory study is identified as single-study evidence. Where a source reports its own included studies, that number is used rather than re-derived, so every count can be traced to the review or trial that produced it.

Evidence matrix

The retained evidence is summarized as a matrix that links each material family to the strongest study or review available for it, the design, the population or model, the principal finding and the level of evidence (Table 1). The matrix is the backbone of the synthesis that follows: each axis of the Development reads a row or a group of rows and weighs how many independent sources point in the same direction.

Terminology and classification of ion-releasing materials

The first controversy is semantic. The FDI policy statement records that bioactivity is used with a positive connotation in advertising, that the term means different things to clinicians, manufacturers and researchers, and that a restorative material should be called bioactive only when its effect is local, intended, nontoxic and does not interfere with the primary purpose of replacing dental tissue.(4) A companion analysis in the foundational dental literature reaches the same conclusion and adds that it is difficult to identify which measurable property actually qualifies a product as bioactive, which leaves the clinician unable to judge whether a bioactive product is clinically superior.(5) The definitions in circulation are collected in a synthesis table (Table 2).

The same confusion extends to product families. A review of commercially available fluoride-releasing materials notes that formulations marketed as smart materials under several family names have few published studies and poorly explained chemistries, and it proposes a classification built on composition and setting reaction rather than on marketing labels.(9) A bibliometric review that screened 7161 records and retained 159 found the literature on bioactive materials for dentin coverage organized around overlapping descriptors such as ion-releasing, smart, biomimetic and smart replacement for dentin, which again points to an unsettled vocabulary.(10) One consequence is that the same product can be described as a giomer, an alkasite, a glass hybrid or a bioactive composite depending on the source, a situation that complicates any comparison of evidence across materials.(11)

The FDI statement grounds the debate by distinguishing the mechanisms through which a material may act and by cautioning that the positive connotation of the word is borrowed in advertising without the corresponding evidence.(4) The cost of that looseness appears in the research record, where the same chemistry is labelled with different terms and the bibliometric review recovered overlapping descriptors that fragment the field rather than organize it.(5,10) A related distinction, between bioactivity, remineralization and antibacterial action, is drawn in the comprehensive reviews but is not applied consistently across product literature, so a claim of bioactivity may refer to mineral precipitation in one paper and to bacterial inhibition in another.(11)

A constructive alternative to the marketing labels is a classification built on composition and setting reaction, as proposed for the fluoride-releasing families.( 9) Under such a scheme, a material is described first by its chemistry and only then by the property it claims, which would allow the evidence for each family to be pooled without the ambiguity introduced by proprietary names. The reviews that attempt this kind of taxonomy state that several of the newer formulations have few published studies and incompletely explained chemistries, so the taxonomy itself remains provisional.( 9)

Material families and ion release profiles

The oldest and best-characterized ion-releasing restorative family is the glass ionomer cement, formed from an ion-leachable glass and polyacrylic acid; its fluoride release is the reference against which newer products are measured.(12) Successive modifications, reviewed as chemically activated formulations with bioactive stimulation, have pursued remineralization, tissue integration and antimicrobial enhancement while retaining the basic acid-base chemistry of the parent material.(13) Glass ionomers release fluoride by an exchange mechanism in which the ion diffuses out of the set cement and can be reabsorbed from topical fluoride, so the material acts as a rechargeable reservoir rather than a one-off source.(14) This property, together with chemical adhesion and moisture tolerance, led the World Health Organization to add glass ionomer cement to its Model List of Essential Medicines in 2021, and clinical reviews describe its fluoride release as long lasting.(12) Resin-modified glass ionomers and the related compomer and giomer families release less fluoride than conventional glass ionomers, a trade-off that buys better aesthetics and handling at the cost of the ion reservoir.(9,15)

The second generation of ion-releasing products extends the chemistry beyond fluoride. Surface pre-reacted glass ionomer fillers liberate a set of ions, including fluoride, strontium, sodium, borate, aluminum and silicate, and the literature attributes six in vitro activities to this multi-ion release: tooth strengthening, acid neutralization, mineralization, microbial inhibition, matrix metalloproteinase inhibition and cell stimulation.(16) Bioactive glass-loaded resin composites release calcium, phosphate and silicate, raise the local pH and precipitate hydroxyapatite, with the effect increasing at higher filler loading.(17,18) Composites functionalized with hydrated calcium silicate follow the same logic, combining calcium release and apatite formation with antibacterial activity in experimental formulations.(19) The alkasite family adds an alkaline, acid-buffering behavior to fluoride and calcium release.(20,21)

The calcium silicate branch of the family illustrates how a chemistry migrates across indications. Apatite-forming calcium silicates originated in the pulp capping and endodontic lineage and have been adapted into restorative resin composites by adding hydrated calcium silicate filler, which imparts calcium release, antibacterial activity and apatite formation in experimental formulations.(19) The adaptation is promising because the apatite-forming chemistry is well established elsewhere, but the evidence for the restorative version remains confined to laboratory specimens, and the clinical history of the parent materials does not transfer automatically to a load-bearing composite.(7,19)

What the profiles share is a strong dependence on conditions and a predictable decay over time (Table 3). Across seven commercial materials, ion release varied with pH, temperature and exposure time, with the highest fluoride concentrations measured at acidic pH.( 22) Long-term assessment of contemporary materials showed that the giomer retained the highest flexural properties after aging while the alkasite outperformed a conventional glass ionomer, indicating that ion-releasing capability and mechanical durability are negotiated differently in each family.( 23)

The mechanical cost of these formulations is uneven rather than universal. In one in vitro comparison, flexural strength ranged from 86.6 MPa to 120.4 MPa across commercial bioactive products, and the material with the lowest elastic modulus also showed the highest water sorption in ethanol and saliva, whereas a bulk-fill bioactive product reached the highest strength of the set.( 24) This dispersion means that no single mechanical profile can be attributed to the label bioactive, and the clinician must consult product-specific data instead of family-level claims.( 1)

The clinical workhorses of the family are the high-viscosity and encapsulated glass ionomers, whose handling properties were designed for atraumatic restorative treatment and for settings with limited isolation. Trials in primary molars have compared two encapsulated systems under atraumatic conditions and reported survival and cost outcomes over two years, and consensus guidance conditions the use of these materials on cavity size, load and caries risk.(25,26) That positioning matters for the review's central question, because the evidence base of the bioactive label rests on these fluoride-releasing cements rather than on the multi-ion composites that carry the newer marketing claims.

Laboratory evidence

Laboratory bioactivity is almost always demonstrated with the simulated body fluid method, in which apatite precipitation on a surface is read as evidence of bioactivity, a surrogate originally developed for bone-bonding materials.(7) The method is standardized but its predictive value for the mouth is weak, because the intraoral environment differs from acellular simulated body fluid in pH, protein content and fluid flow, and because apatite precipitation does not by itself demonstrate that enamel or dentin is protected or repaired.(7,17) Resin composites functionalized with bioactive glass illustrate the point: they release calcium and phosphate and precipitate hydroxyapatite in vitro, yet the clinical meaning of that precipitation has not been established.(17,18)

Within the laboratory domain, the evidence is internally consistent on three points. Fluoride-releasing materials resist enamel demineralization and support remineralization under pH cycling; a comparative study of a glass ionomer, a resin-modified glass ionomer and an alkasite found that the alkasite raised pH initially and that all three released fluoride and calcium, with the glass ionomer showing the strongest demineralization resistance.(21) A prereacted glass filler composite inhibited enamel demineralization but remineralized less than a glass ionomer, a result that separates the two properties and cautions against equating any fluoride release with full remineralizing capacity.(27) Bioactive glasses promote apatite formation and raise pH, but conventional 45S5 glass is mechanically weak and prone to an initial burst of ion release, which has motivated secondary oxide, hybrid and mesoporous formulations whose oral durability remains unproven.(7)

The experimental composite literature is larger than its clinical counterpart but heterogeneous. A systematic review restricted to in vitro studies of remineralizing resin composites retained 12 studies and found that ion release efficacy, bioactive filler composition and mechanical properties varied so widely that no formulation could be singled out as superior.(28) Experimental composites with hydrated calcium silicate showed dose-dependent antibacterial and apatite-forming behavior in one study.(19) For the wider remineralization field, systematic reviews of enamel remineralization techniques and of casein phosphopeptide-amorphous calcium phosphate, nanohydroxyapatite and bioactive glass against fluoride converge on a shared conclusion: fluoride remains the benchmark, and the nonfluoride agents show promise but inconsistent depth and durability of remineralization.(29-31) This body of work also confirms a basic biochemical constraint: net remineralization requires an adequate supply of calcium and phosphate, and saliva is the principal reservoir that delivers these ions to the enamel surface.(3)

Beyond mineral effects, the multi-ion products are evaluated for antibacterial activity. The surface pre-reacted glass filler is credited in the literature with several in vitro activities, among them microbial and fungal inhibition and matrix metalloproteinase inhibition, and experimental calcium silicate composites showed antibacterial behavior alongside apatite formation.(16,19) Alkaline and acid-buffering effects are measured with the same logic: several bioactive formulations raise the local pH, which is proposed to offset the acid attack of the biofilm, although the effect is again demonstrated in solution rather than in a clinical lesion.(20,21) The result is a laboratory profile with three overlapping claims, mineral deposition, pH buffering and microbial inhibition, none of which has yet been tied to a reduction in recurrent caries in a controlled trial.

The heterogeneity of these protocols limits what the laboratory evidence can say as a whole. Studies differ in immersion medium, specimen geometry, aging, measurement time points and the units used to report release, so the field produces many single comparisons and few cumulative statements.(22,23) The systematic review of in vitro remineralizing composites encountered exactly this problem, finding that ion release efficacy and mechanical properties varied across 12 studies in ways that prevented a ranking of formulations.(28) Standardization of the release assay, from specimen conditioning to the reporting of concentration over time, is therefore a precondition for any future claim that one bioactive product outperforms another.

Clinical evidence

Clinical data are concentrated in the glass ionomer family and are strongest for the outcome that matters most, secondary caries (Table 4). A meta-analysis of 64 randomized trials pooling 8310 glass ionomer restorations against 5857 amalgam or composite restorations found a lower incidence of secondary caries for glass ionomer than for amalgam in primary teeth (risk ratio 0.55; 95% CI 0.41 to 0.72) and in permanent teeth (risk ratio 0.20; 95% CI 0.11 to 0.38), while the incidence relative to resin composite was similar in both dentitions (risk ratio 0.92 and 0.77, respectively).(32) The same review found no difference between conventional and resin-modified glass ionomers for secondary caries. This is the single most informative result of the field: the fluoride-releasing material is cariostatic compared with amalgam and roughly equivalent to composite, which frames bioactivity as a protective property rather than a guarantee of superiority over modern composites.

The precision of that estimate deserves attention. The meta-analysis pooled trials with follow-up ranging from one to ten years, and the confidence interval for the permanent-dentition comparison against composite was wide (risk ratio 0.77; 95% CI 0.39 to 1.51), so the equivalence between glass ionomer and composite for secondary caries in permanent teeth is imprecise rather than firmly established.(32) Against amalgam the advantage was precise and consistent across dentitions, which is the comparison in which the fluoride-releasing mechanism should show its effect if it is real. The contrast between the two comparisons, one precise and one wide, is a reminder that the cariostatic benefit is demonstrated against a non-fluoridated control and is not yet established as an advantage over modern resin composites.

Survival data tell a consistent story. An umbrella review of 13 systematic reviews of glass ionomer longevity in primary and permanent teeth concluded that survival varies by indication and by the specific glass ionomer formulation, without a uniform advantage over composite.(33) A systematic review of clinical trials in primary teeth likewise found high-viscosity glass ionomer and composite comparable, with isolated contradictory studies.(34) In noncarious cervical lesions, a meta-analysis found glass ionomer and composite comparable in clinical performance and longevity.(35) For atraumatic restorative treatment, the evidence favors acceptable rather than superior performance: a 5-year randomized comparison in persons with disability compared high-viscosity glass ionomer ART restorations with composite, and a 2-year non-inferiority trial in primary molars compared two encapsulated glass ionomers under ART conditions.(25,36) A systematic review of ART in elderly patients, based on seven eligible studies, reported survival that supports the approach in that population without resolving which glass ionomer performs best.(37)

The clinical literature is also distinctive for where it was obtained. The strongest survival data come from populations with limited access or with special needs: a five-year comparison in persons with disability, a two-year non-inferiority trial in schoolchildren, and a systematic review of ART in elderly patients.(25,36,37) This is a strength in external validity, because these are the patients for whom a moisture-tolerant, fluoride-releasing material is most useful, but it also means the evidence reflects atraumatic cavity preparation and selective caries removal more than conventional Class II composite placements. The clinician extrapolating these results to load-bearing posterior restorations in healthy adults is therefore reading beyond the populations that generated the data.

The newer families have only short-term clinical data. An 18-month randomized trial in 50 patients compared an alkasite and a glass hybrid with a resin composite in Class II cavities and scored the three materials with FDI criteria, providing the first controlled comparison for this generation but no evidence beyond a year and a half.(38) A one-year randomized study compared a reinforced glass ionomer with a bulk-fill composite and a conventional composite in Class II cavities,(39) and a two-year split-mouth study compared a coated glass ionomer with a bulk-fill composite in Class I and II cavities.(40) These trials are consistent with the older literature on composite longevity, which reports high survival for posterior composites in the range of several years, so a new bioactive material must at least match that benchmark to be justified.(41,42) Consensus guidance on glass ionomer thresholds stresses that the decision to use these materials depends on cavity size, load and caries risk, which again places the clinical value of ion release in a specific context rather than in the material label.(26)

Controversies and evidence gaps

The central controversy is translational. Laboratory surrogates, apatite formation in particular, are abundant, but no trial has yet shown that a composite or alkasite marketed as bioactive reduces secondary caries or extends survival beyond what a conventional composite achieves.(4,43) The resin literature is explicit that secondary caries remains the leading cause of composite failure and that the main challenge of bioactive resin development is to translate ion release into protection of the bonded interface, a goal that remains unmet.(1) Bioactive glass reviews state the same limitation in the opposite direction: the materials are reactive and antibacterial in vitro, but their clinical translation is limited and their durability under oral conditions is not established.(7) The gap can be represented as a chain from the measured property to the patient outcome (Figure 1).

A second, related gap is the asymmetry between terminology and measurement. Products are named for a mechanism, but the clinician can only observe outcomes, and the published record connects the two mainly within the glass ionomer family.( 5) The classification problem documented for fluoride-releasing families has not been resolved by the newer multi-ion products, which arrive under names that describe neither their chemistry nor their evidence.(9,10) This asymmetry is aggravated by comparison with nonrestorative ion-releasing agents. Silver diamine fluoride arrests caries within hours of application and is supported by a growing clinical literature, and topical bioactive glasses and calcium phosphate systems have been evaluated against fluoride for initial lesions; the restorative materials discussed here do not yet carry evidence of that maturity.( 30,31,44)

A further source of confusion is the difference between restorative and non-restorative ion delivery. A restorative material must release ions from a bulk that is also expected to bear load for years, whereas a topical agent can be reapplied, which is why silver diamine fluoride and toothpaste-based fluoride have reached a maturity of clinical evidence that the restorative bioactives have not.(44,45) Recognizing this difference would discipline the vocabulary, because a material that releases fluoride for a few weeks is not equivalent to a topical regimen that replenishes the ion at every application.

The third gap is the durability of the ion reservoir and of the material itself. Fluoride release from glass ionomers is rechargeable but declines between exposures,(14) and the burst-and-decay pattern of bioactive glass formulations raises the question of whether protection lasts as long as the restoration is expected to.(7) Mechanical data add a further caveat: some bioactive products absorb more water and flex less than a conventional composite, so the clinician may trade structural performance for an unproven biological benefit.(24) These three gaps, the missing trial, the terminology gap and the durability question, define the current state of the field.