Marginal ridge strength of teeth with tunnel preparations.

The aim of this study was to investigate how the cavity design of tunnel preparations influences the strength of the marginal ridge and to assess the ability of a restoration to support it. Extracted upper premolars were divided into eight experimental groups and one control group of intact teeth. The size and the position of the tunnel preparation in relation to the marginal ridge differed in four of the groups, the preparations remaining unfilled, while teeth in four corresponding groups were filled. The teeth were subjected to incremental dynamic forces until fracture of the marginal ridge occurred at which point a 'maximum energy-index' was calculated. The relationship between this index and the experimental variables was analysed by the Cox proportional hazards model. The results for both filled and unfilled teeth indicated that the distance from the preparation to the marginal ridge is more influential on weakening the ridge than is the buccopalatinal size of the opening. A conservative tunnel restoration situated 2 mm from the marginal ridge, does not significantly weaken an otherwise intact tooth.