Commentary

Cast prosthetic restoration in private practice is reported to have a ten year survival rate of over 90%.1 Despite their impressive prognosis, cast prosthetic restorations occasionally fail, typically as a result of caries.2,3 Dental caries progression is conventionally managed with fluoride exposure and diet control. However, it is theorised that dental restoration margins below the gingival crest (Sub-G margins) are protective against caries attack because the biochemical environment in the gingival sulcus – anaerobic, alkaline and void of dietary components – inhibits the activity of carious causing bacteria.4

The objective of Papagreorgiou et al.'s review4 was to answer the following clinical question: are sub-gingival prosthetic margins less susceptible to secondary caries than prosthetic margins at or above the gingival crevice. This review followed a standardised protocol (PRISMA statement). The search strategy was thorough with clearly defined inclusion and exclusion criteria. This led to a generally impressive agreement between the two reviewers (Kappa; 0.715-0.933). However, the search was limited to only English articles and included both prospective and retrospective studies. Ideally, such an intervention-based clinical question would best be answered with randomised controlled trials (RCTs), which may have been published in a language other than English.

Only two of the 22 included articles were prospective studies, reflecting the generally low quality of studies of which this review was comprised. Also, a large majority of the included prostheses (83.9%) had margins at or above the gingival margins. This imbalance between the two groups is largely due to the fact that margin locations were determined at the end of the study. This is significant because the progression of gingival recession around prosthetic margins is a common occurrence.5 Furthermore, significant variation existed in how marginal caries were measured and assessed between included studies. Sub-G crown margins are more difficult to identify visually or tactilely with an explorer. As well, radiographs are unreliable at assessing mild to moderate marginal caries.

The five years pooled risk ratio favoured prosthetic margins at or above the gingival crevice with a RR= 1.25 (95%CI-0.70-2.22) at the surface level. But this was based on only two generally poor quality studies (141 patients) with a dropout rate of 40%.6,7 The derived ten and 15 years pooled risk ratio estimates are based on only one small study (108 patients) with a dropout rate of 45%. Such a large dropout rate threatens the validity of the evidence generated from this review. Interestingly, the pooled 15 years risk ratio favoured the Sug-G prosthetic margin group with an RR of 0.67 (95%CI: 0.45 -1.00). However, statistical significance between the two groups was not reached at five, ten or 15 years.

In conclusion, due to the dearth of quality and quantity in the available evidence, it is not possible to come to a conclusion on the caries protective nature of Sub-G prosthetic margins. However, there is evidence demonstrating the risk of harm that Sub-G prosthetic margins have on the periodontal apparatus compared to prosthetic margins at or above the gingival crevice.5,7,8,9,10