While the longevity and favorable adaptation of metal margins have been demonstrated for years, marginal adaptation of ceramic crowns needs further evaluation because poor adaptation and gap formation will negatively affect the long-term prognosis of these crowns. A study by Comlekoglu M, et al evaluated 4 different types of marginal finish lines to determine which design achieves the best marginal adaptation of a zirconia ceramic crown to the tooth. The study involved 28 maxillary right central incisors which were divided into 4 groups of 7 teeth. Teeth were then prepared with one of the following designs of cervical finish lines: circumferential rounded-shoulder, chamfer, mini-chamfer, and feather edge. A new diamond bur was used to prepare each specimen. Standard tooth preparations were accomplished with a 2.5-mm incisal reduction and a 6°-convergence angle, after which impressions with a polyvinylsiloxane material were taken. A zirconia framework for each tooth was fabricated on a resin die model using a copy-milling system. After veneering and glazing, the crowns were tried onto the dies and cemented with polycarboxylate cement. After specimens were sectioned with a diamond disc perpendicular to their long axis, marginal openings at different points of the crown-tooth interface were measured using a stereomicroscope with image processing software.
The study found that the shoulder finish line and the mini-chamfer exhibited the smallest marginal opening values, with no statistically significant differences between them. Each of these 2 types of preparations was more resistant to distortion, as each allowed more adequate room for the core and veneering ceramic than did the chamfer or feather edge type of finish lines. The study did not evaluate the amount of ceramic shrinkage that may occur during the subsequent veneering of the zirconia. This would affect the marginal fit of the crowns, as would any effect of the cementation procedure. The study concludes that shoulder and mini-chamfer types of cervical finish lines result in superior marginal adaptation of zirconia ceramic crowns. The limitations and properties of ceramic materials require modification of the types of finish lines that were previously used in porcelain-metal and all metal crowns.
(Source: Operative Dentistry, September/October 2009, Volume 34, Number 5)