Effects Of Layers Of Die Spacer On Retention And Adaptation Of Complete Cast Crowns

Objective:The adaptation of complete crowns, which can be divided into internal adaptation and marginal adaptation, means the space between the preparation and the crown, many researchers have agreed that the thickness of die spacer can affect retention and fitting of complete crowns, but they have differences with each other about the coating layers of die spacer. Some believe just one or two layers are good for adaptation, while some believe more layers more retention. But few considers the coating layers of die spacer, combining adaptation and retention. In our study, we evaluated the effect of die spacer of different coating layers on the adaptation and retention of complete cast crowns by using two different cements.Methods:A total of60intact human first premolars extracted freshly were collected. After tooth preparation, impressions were made with polyether, and stone dies were made.60dies were randomly divided into3groups according to different coating layers of diespacer(one layer, two layers and three layers). Ni-Cr alloy was selected to fabricate complete cast crowns which were cemented to their respective teeth with polyether impression material to measure the adaptation with scanning electron microscopy. Then each group was further divided into two subgroups, the samples were made of the crowns and teeth bonded with Fuji glass ionomer and PermaCEM. After cementation, the samples were immersed in artificial saliva for1month and subjected to10000cyclic mechanical loading. The tensile retentive force was measured on a universal testing machine with a crosshead speed of0.5mm/min. The data obtained for the adaptation were recorded in millimeters and the data for the tensile retentive force were recorded in N. The statistical analysis was performed by analysis of variance and SNK-q test (P<0.05).Results:The marginal adaptation of three groups was clinically acceptable. The marginal adaptation decreased as the coating layers of diespacer increased, but there was no significant difference between the groups coated with two and three layers of diespacer on the die(P>0.05). No significant difference was found in internal adaptation between the groups coated with one and two layers of diespacer on the die(P>0.05), while both of them had significant difference with the one coated with three layers(P<0.01).The group coated with two layers of diespacer on the die had bigger retention forces than the other groups(P<0.05; P<0.01), regardless of the cements, Fuji glass ionomer cement or PermaCEM.When dies were coated with the same layers of die spacer(one layer, two layers or three layers), the retention force of complete crowns cemented by PermaCEM was bigger than that of complete crowns cemented by Fuji glass ionomer cement.Conclusions:1. When dies were coated with one to three layers of die spacer, the more coating layers of diespacer are, the better marginal gap is, that is, the better marginal adaptation is. but when the layers were three or more, the internal adaptation will decrease.2. When dies were coated with one to three layers of die spacer, regardless of the cements, crowns whose dies coated with two layers of diespacer had biggest retention forces. And castings luted with PermaCEM cement showed the greater retention force failure than that of Fuji glass ionomer cement.3. When Ni-Cr alloy was selected to manufacture crown, the die coated with two layers of diespacer was recommended in clinic.