Effect of metal type and surface treatment on in vitro tensile strength of copings cemented to minimally retentive preparations

Statement of problem: Due to the potential lack of ideal preparation form, the type of alloy and its surface pretreatment may have clinically relevant correlations with the retentive strength of castings to minimally retentive preparations. Purpose: The purpose of this study was to evaluate the effect of alloy type and surface pretreatments of base and noble metal copings on their tensile strength to minimally retentive preparations. Material and methods: Minimally retentive, standardized crown preparations were made on recently extracted human third molars (n=68). Noble (IPS d.SIGN 53) and base metal (Rexillium NBF) copings were fabricated. All copings received heat treatment for oxide formation. Three experimental groups were then developed for each metal type (groups ranging from 10 to 12 specimens each): oxide only, airborne-particle abraded, or metal-primed. Copings were cemented using a self-adhesive universal resin cement (RelyX Unicem) and were thermal cycled (500 cycles between 5 and 55°C) and stored (24 hours, 37°C) before debonding using a universal testing machine. Frequency of debond location was compared among specimen groups. A 2-way ANOVA was used to test for interaction between the metal type and surface treatment, and, if no significant interaction was found, to test the main effects for metal type and surface treatment (α=.05). A multinomial logit model using the likelihood ratio test was used to describe the effect of metal type and surface treatment on failure site location (α=.05). Results: The 2-way ANOVA indicated no significant influence of any factor on debond load: metal type (P=.885), surface treatment (P=.555), or their interaction (P=.644). The multinomial logit statistical model showed that noble metals and metal primers significantly (P<.05) shifted debond failures to occur more frequently at the resin/tooth interface or within the tooth itself. Conclusions: Neither metal type nor surface pretreatment affected bond strength. However, alloy type and surface treatment affected site of debond location. (J Prosthet Dent 2007;98:199-207).