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The relationship between gingival health and the margin of the restoration has also been researched.^206–208 Newcomb showed in 51 subjects that the more apical the margin of the restoration, the more gingival inflammation was present according to a gingival index.²⁰⁹ Four groups were classified depending on the distance of the crown margin to the base of the crevice (CM-BC) and found that margins as close as 0.75 mm or less can induce gingival inflammation.

A human histologic study²¹⁰ showed that restorations placed below the gingival margin were likely to accumulate plaque subgingivally, even when routine oral hygiene was performed. It was also noted that individual sites can re-form plaque as soon as 6 weeks on subgingival restorations, while others can be free of plaque for as long as 2 years.

A 26-year longitudinal study examined the long-term effects of restorations with supra- and subgingival margins on periodontal health.²¹¹ It was concluded that subgingival margins exert a detrimental effect to gingival and periodontal health. Additionally, a “burn-out” effect was suggested as loss of attachment in teeth with subgingival margins was clinically detectable 1 to 3 years after the placement of the restoration.