How to intervene in proximal lesions
A recent consensus statement  established guidelines on when to intervene on proximal lesions. It was agreed that for non-cavitated lesions, minimally invasive (restorative) strategies should be avoided, and non- or micro-invasive strategies are preferable. Non-invasive strategies do not remove dental hard tissue and involve, for example, topical fluorides and other chemical agents for controlling mineral balance, biofilm control measures, and dietary control. Micro-invasive strategies remove the dental hard tissue surface at the micron level, usually during an etching step, such as used in sealing or infiltration techniques. Minimally invasive operative strategies remove a limited amount of gross dental hard tissue, through the use of hand excavators, rotary instruments, or other devices. In most cases, this process is associated with the subsequent placement of restorations. The specific types of non- and micro-invasive strategies have been laid out in more detail in the systematic review underlying this consensus statement, together with the evidence supporting them (see Table 1 and below). Cavitation status should be assessed using visual and/or tactile methods, and/or the radiographic lesion depth as a proxy, with cavitation being unlikely in lesions radiographically confined to enamel, likely in those clearly entering the middle third of dentine, and variable for lesions around the enamel-dentine junction/outer third of the dentine.
The systematic review  identified mainly studies combining oral hygiene measures and fluoride application for managing proximal lesions, either as part of long-term cohort studies or as control arm of randomized trials comparing non- and micro-invasive strategies for managing proximal lesions. Overall, in most low caries risk/susceptible populations, lesion progression was slow, taking several years. Progression was more rapid in lesions extending radiographically into dentine than in enamel alone, in molars rather than premolars, and in adolescents rather than adults. Notably, in highly caries susceptible individuals, lesion progression can be faster, with non-cavitated lesions progressing to cavitated dentine lesions over a 1–2-year period. Generally, non-invasive measures depend on the potential for patients’ behavioral modification and adherence which is not often considered in such studies . Overall, the evidence supporting non-invasive management of proximal lesions as a therapy vs. doing nothing was graded weak. However, as non-invasive measures also aim to modify a patient’s caries risk/susceptibility, they are relevant clinically as part of fundamental disease prevention.
For micro-invasive management of proximal lesions, a larger and more robust body of evidence was available, with a number of randomized trials assessing micro-invasive treatment combined with non-invasive measures vs. non-invasive management alone. The studies found that micro-invasive treatment combined with non-invasive measures arrests non-cavitated enamel and initial dentine lesions (limited to the outer third of dentine based on radiograph and clinically non-cavitated) and is significantly more effective than non-invasive management alone. However, it remained unclear which micro-invasive technique offered the greatest benefit. There was some evidence that both sealing and resin infiltration can arrest enamel-limited lesions and those around the enamel-dentine junction, while only infiltration techniques were effective for lesions involving the dentine. Overall, the evidence supporting micro-invasive treatment combined with non-invasive measures of proximal lesions was graded as moderate.
For the minimally invasive restorative/operative management of proximal lesions, a larger number of studies were available. Mean annual failure rates varied between 1.2 and 3.8% (after weighting for sample size). When restricting to only proximal restorations, conventional resin composites showed a significantly higher risk of failure than amalgams; all other materials did not perform significantly differently than conventional resin composites. Notably, though, studies were conducted at different times, with different materials and different operator experience, which makes comparisons difficult. Overall, the evidence was graded as weak. Amalgams appear to show a lower risk of failure in proximal cavities than most other materials. In light of many patients wishing to receive tooth-colored restorations, but also because the aim of minimally invasive tooth tissue preservation, by biologically oriented lesion preparation, as well as considering the current global phase-down of dental amalgam associated with the Minamata agreement , adhesive bio-interactive restorations are common in many countries. For extensive defects, especially in endodontically treated teeth, indirect restorations may also be considered on an individualized basis.
Based on these findings for proximal lesions, the consensus group developed the following recommendations:
Overall, the management of proximal lesions should be provided under the tenet of a preventively oriented approach (dietary advice, oral hygiene motivation). Managing a patient’s caries risk/susceptibility will also help support managing specific proximal lesions and assist in the prevention of new lesions on unaffected surfaces.
For non-cavitated lesions:
Non-invasive measures (e.g., interdental cleaning, topical fluoride application) could be applied to arrest proximal lesions. This may be sufficient for lesion arrest in low caries risk/susceptible individuals or for lesions radiographically confined to enamel (weak recommendation, agreement 88%, median: 10)
In high-risk/susceptible individuals or for lesions extending radiographically into dentine, micro-invasive strategies should be considered additionally (moderate recommendation, agreement 83%, median: 10).
The decision between sealing and resin infiltration should be guided by individual considerations, including applicability, clinical experience, or costs (moderate recommendation, agreement 88%, median: 10).
For cavitated lesions, restorative strategies will often be needed. For restoring proximal lesions, adhesive direct restorations allow minimally invasive, tooth-preserving preparations, are tooth-colored, and hence are already the material of choice in many cases. Amalgams, however, come with a lower risk of secondary lesions and since their placement is less technique-sensitive, they may be preferred in more clinically complex scenarios, dependent on specific national guidelines. According to legal regulations, the use of amalgam may be restricted in some countries or populations, now and in the future (weak recommendation, agreement 84%, median: 10).
In structurally compromised teeth, especially when endodontically treated, indirect cuspal coverage restorations may be indicated (weak recommendation, agreement 92%, median: 10).
How to intervene in secondary lesions
Secondary lesions may be primary carious lesions associated with existing restorations, as a result of the caries process not having been sufficiently addressed on a patient level [12,13,14], or may be caused by significant margin defects (interfacial gaps, ditches, etc.), allowing acidic by-products or a dysbiotic biofilm to enter the interface between the restoration and the tooth structure. Marginal gaps may be the result of imperfect initial placement of the restoration/management of the tooth surface , but also hydrolytic degradation of the hybrid layer of adhesive restorations [16, 17]. In any case, secondary lesions are found more often in high caries risk/susceptible patients [18, 19], at the gingival margin of restorations [20, 21], and in posterior teeth .
The second review underlying this consensus statement assessed a number of aspects related to secondary carious lesions:
(1) Can secondary lesions be managed using specific restorative techniques or materials?
(2) How can secondary lesions be detected, allowing their adequate management?
(3) If/when detected, which management strategy can be recommended?
Regarding the risk of secondary carious lesions next to restorations placed using different adhesive strategies and restorative materials, the review yielded only a few robust findings . Overall, amalgams appear to be associated with a lower risk of secondary caries , which is in agreement with amalgams showing an overall lower risk of failure in most cavities (see above). Again, however, many dentists will be interested in knowing which non-amalgam materials are least prone to secondary lesions. For these materials (different resin composite types, but also resin-modified or conventional glass-ionomer cement or poly-acid modified resin composites/compomers), only very few differences emerged. Overall, patient- and operator-level factors seem to be decisive in controlling secondary carious lesions, while adhesive strategies and/or restorative materials may play a lesser role. This conclusion, however, is limited by the short follow-up periods, limited sample sizes, and the potential of low caries risk/susceptibility individuals enrolled in most trials.
Early detection of secondary lesions may allow the application of less invasive treatment options like surface refurbishment, re-sealing, or repair instead of complete restoration removal and replacement. A variety of early detection methods for secondary carious lesions are available, including visual, tactile, radiographic, laser fluorescence, and quantitative light-induced fluorescence assessments. These were assessed in a recent review , which found that visual, radiographic, and laser fluorescence detection had similar sensitivities and specificities (abilities to detect carious and sound surfaces, respectively), while tactile assessment and light-induced fluorescence are not as useful at present, at least not if used in isolation. There is further data demonstrating that in most low caries risk populations, avoiding false-positive readings and ultimate over-detection and treatment should be prioritized, as secondary lesions progress slowly (see above) and missed lesions may be detected at the next recall consultation . Overall, it seems advisable to combine bitewing radiography (allowing the detection of clinically inaccessible lesions) with visual-tactile assessment (allowing confirmation of findings). If detected as suspect, but not treated, lesions should be monitored regularly. Moreover, dentists should consider to refurbish, reseal, or repair partially defective restorations if possible, as this allows the preservation of tooth structure and reduces the risk of subsequent treatment-related complications . The evidence quality supporting the decision between refurbishment/reseal/repair vs. replacement, however, was found to be weak. Despite the obvious advantages of this minimally invasive approach (reduced tissue removal, reduced harm/pulp damage, lifetime of existing tooth-restoration complex is prolonged), there may be some other factors to consider (possibly higher long-term costs, careful case selection is required). Overall, the decision will need to consider the specific clinical situation (is repair even possible?), the patient’s wishes, and the dentist’s experience and expertise with different treatments.
Based on these findings for secondary carious lesions, the consensus group developed the following recommendations:
By managing a patient’s overall caries risk/susceptibility, the risk for secondary lesion occurrence is also managed to a certain extent (weak recommendation, agreement 87%, median: 10).
Detection methods for secondary lesions should be tailored according to patients’ caries risk/susceptibility. Especially in low-risk patients, false-positive detection, and subsequent over-treatment should be avoided. This may be achieved by combining bitewing radiography and visual-tactile assessment/confirmation when screening for secondary lesions (weak recommendation, agreement 88%, median: 10).
When managing detected secondary lesions, dentists should adopt the minimally invasive approaches of refurbishment/reseal/repair over replacement of restorations, on a case-by-case basis (weak recommendation, agreement 100%, median: 10).