Grey areas in restorative dentistry: part 5. Mind the gap 2: the restoration with a deficient margin

In an ideal world, a restoration would be invisible to the eye, blending in with the colour of the tooth where aesthetics was a consideration, the transition from the restoration to the tooth would be imperceptible to a probe, and the restoration would not show any degradation over time, maintaining a perfect seal with the tooth tissue. In the real world, perfect colour matching is extremely difficult to achieve, as is a restoration without some detectable margin. And of course, degradation of most materials can be expected, possibly resulting in a detectable gap between restoration and tooth.

This reality poses difficult questions for restorative dentists because where there are existing restorations, the practitioners are constantly having to make decisions about the tooth–restoration interface; is it acceptable or not? Is the existing restoration failing, or has it failed, where fail means ‘to lose strength, to stop functioning normally’.¹ The role of a restoration is to restore form, function and features (considered in more detail in Part 6 of this series) and implicit in these parameters is that the restoration will have as close a relationship as possible with the cavity or preparation surfaces to reduce the risk of recurrent caries and subsequent failure. And this applies equally to direct and indirect restorations that are to be fitted. How much of a gap is too big?

Recurrent caries

The most frequently cited reasons for the replacement of restorations are recurrent caries, fracture of the restoration, marginal deficiency, lack of a contact point, and occlusal discrepancy,² and of these the clinical diagnosis of secondary caries is the most common.³ Secondary or recurrent caries has been defined as ‘lesions at the margins of existing restorations’ or ‘caries associated with restorations or sealants’ (CARS),⁴ and the vast majority (up to 90%) are found at the gingival margin of restorations regardless of the restoration material.^5,6 This is not unexpected because plaque accumulation is most likely here in the absence of effective approximal cleaning resulting either from surfaces that are not easily cleansable (such as overhanging restorations) or lack of ability or effort by the patient.

Gaps around restorations may be the result of a variety of operator or material failures, such as inadequate placement of the restorative material, or insufficient light-curing with subsequent washout of uncured resin.⁷ In the longer term, defects and gaps may also form from by hydrolytic degradation of the hybrid layer, and hence the interface in the case of adhesive (resin-based) restorations.^8,9

While the early detection of secondary caries will obviously reduce the extent of any reparative work, the actual detection is fraught with difficulties, leading to over or under detection. While there is a correlation between the presence of defective margins in restorations and recurrent caries, it should be borne in mind that there will also be many such teeth that will not have evidence of recurrent caries. Kidd et al¹⁰ studied amalgam restorations and found that plaque samples from margins with ditches greater than 0.4 mm harboured significantly more bacteria (mutans streptococci and lactobacilli) than did clinically intact margins and margins with narrow ditches. Another study¹¹ examined the prediction of secondary caries around tooth-coloured restorations, grading the margins as either intact, having a narrow ditch, or having a wide ditch. It was found that less than 25% of sites without frankly carious cavities had soft dentine at the EDJ and that none of the clinical criteria chosen would reliably predict the presence of this soft dentine. There is a variety of methods available to diagnose secondary caries,¹² and the authors noted that ‘despite being a significant clinical and dental public health problem, detection of secondary caries lesions has been assessed in only a few studies with limited validity and applicability’.

For the clinician, visual, tactile and radiographic assessment are the methods available. Visual assessment is of discolouration or staining; tactile assessment is of ditching around the restoration margins and/or softness; and radiographic assessment is used on its own or combined with visual assessment.¹³

The implication of recurrent caries around a crown is more serious, especially if detected in the approximal region where access can be very limited. The only option may be replacement, which clearly carries with it high risks. The crown must be removed, which can involve traumatizing the tooth and pulp, and the extent of the caries may make replacement non-viable. Because of the radiopacity of crowns, radiographs may not give the full extent of any recurrent caries (Figure 1) and so the patient should be warned beforehand that a final decision regarding the future of the tooth can only be made once the crown is removed and direct visualization is possible.

Regarding crowns or other indirect restorations, the fit of the item is of great importance, because in the absence of a ‘perfect’ fit, a gap of some proportions will exist between the margin of the restoration and the tooth substance. This gap will be filled with the luting cement, but just how much of a gap is clinically acceptable? Or, how much of an overhang is acceptable? The clinical test is by the tip of a probe and the tactile feel when the probe is run from the restoration onto the tooth or vice versa. However, this is a very crude measure because the size of tip can vary greatly and will be a critical factor in the clinician's decision making.¹⁴ Some can be pinpoint sharp and others quite blunt and rounded. Furthermore, in a laboratory setting, when revisiting a previous decision about the clinical acceptability of a crown, several surfaces were rated differently at a 6-month review.¹⁵ Jacobs et al¹⁶ studied zinc phosphate cement as a luting agent in vitro and found that the dissolution rate of the cement was greater with a 150-micron gap compared with 25-, 50-, and 75-micron gaps, which showed no significant difference between each other. Whether this can be extrapolated to other luting materials is not clear, and at the same time, accurate measurement of a gap between the indirect restoration and the tooth at all points is not possible in the clinical situation owing to restricted access.

Where a gap is detected between the restoration and the tooth tissue the practitioner has four possible options: the four Rs (Table 1, Figure 2).

In Figure 3, the restorations may be considered to have less than ideal margins but nevertheless could be considered as adequately sealing the teeth in the absence of any clinical indication of secondary caries. Replacing these in the hope of producing restorations of the quality shown in Figure 4 would be high risk, first to the teeth themselves, and secondly with the possibility of failing to provide restorations significantly better than those already in place.

There are many articles regarding the repair or replacement of restorations.^18,19,20 As for the actual protocols, Hickel et al²¹ and Loomans et al²² suggest the following:

‘defective parts of the restoration should be removed, and the restoration surface of the cavity roughened by use of diamond burs. Then, depending on the type of restorative material to repair, different surface pre-treatments can be recommended: Resin composite and amalgam surfaces should be air abraded with Al₂O₃; silicate ceramic surfaces can also be air abraded with Al₂O₃.²³ Alternatively, acid etching using hydrofluoric acid can be performed if contamination of gingiva, enamel, and dentin can be avoided. Metallic and oxide ceramic surfaces should be air abraded using either Al₂O₃ or silica coated Al₂O₃.²⁴ Areas of dental hard tissue in the cavity to be repaired should be etched with phosphoric acid although modern universal bonding agents generally can be used in self-etch mode without the need for this additional stage. Phosphoric acid etchant can also be applied on composite or glass-based ceramic surfaces to obtain a cleaning effect. However, contact of phosphoric acid to metallic surfaces and zirconia should be avoided as this might hamper the adhesion of 10-methacryloyloxydecyl-dihydrogen-phosphate (MDP) containing primers. Then, silane coupling can be recommended; alternatively, universal primers containing components (e.g., silanes, MDP, sulphur-containing monomers) for chemical bonding to a variety of surfaces can be used. Simplified approaches involving only conventional etching and bonding procedure without air abrasion or further surface treatment of the remaining restorations are also effective for restoration repair’.

Help in reducing the subjectivity in assessing any given restoration, i.e. determining the acceptability or not of a restoration, has been given in the United States Public Health Service Ryge clinical criteria (Table 2).^25,26 Any restoration, direct or indirect, falling in the Charlie category would be considered as a good candidate for modification. These criteria were updated by Hickel et al,^27,28 and introduced a more comprehensive assessment, the main features of which are presented in Table 3.

The authors give detailed criteria for allocating each aspect of the restoration into one of the clinical observation categories. These may be too detailed for routine practice, but show the depth of scrutiny to which each restoration can be subjected. Indirect restorations should be subjected to scrutiny before cementation, and if considered suitable for cementation, should also be examined after cementation.

While there is sufficient evidence to validate the minimal procedures,^29,30,31 not all practitioners take up non-replacement methods, an attitude that has been identified as originating from negative personal experiences or lack of success, as well as the practice setting, i.e. financial considerations.³²

Conclusion

We live in an imperfect world and the challenge for the dental practitioner is how much treatment should be undertaken to bring distressed teeth and restorations to as near normal as possible. Failing restorations present a unique challenge in decision making. The diagnosis of recurrent caries is difficult and liable to misinterpretation, leading to perhaps unnecessary remedial work. Less is more, and so it is important to consider the less-invasive options such as refreshing, repairing or resealing, as opposed to the complete replacement of the existing restoration. A deficient margin in and of itself is not an indication for clinical intervention. Other factors, such as caries risk, patient co-operation, oral hygiene, occlusion, and the confidence that a better result can be achieved, should form part of the decision-making process.