Retention strength of metallic posts with ball-attachments for tooth-supported overdentures

Aim: The aim of this study was to evaluate the retention strenght (pull-out) of metallic posts with ball attachment (Pivot Block, Rhein 83, Bologna, Italy), undergone to three different surface treatments and luted with a self-etching, self-adhesive cement. The null hypothesis was that neither the sandblasting of the post surface, nor the retentive notches made on the post, nor both treatments increase the pull-out force (N) of the post from the tooth (p = 0.05). Methods: Sixty human teeth extracted for orthodontic or periodontal reasons were selected and stored in 0.02% thymol solution. The teeth, cleaned of calculus and debris, were sectioned with a diamond bur at the level of the most coronal cement-enamel junction. Once the root canals were shaped, the post-spaces were prepared at a depth of 10 mm. The roots were embedded in resin cylinders so that the canals were coaxial to the cylinders. The Pivot Block metallic posts were randomly divided into 4 groups: A) untreated surface (control); B) sandblasted surface; C) retentive notches; D) retentive notches and sandblasted surface. The notches of groups C and D were 3 semicircular shaped notches at 3 mm, 5 mm and 7 mm from the coronal plate of the post; they were 0.4 mm. deep, made with a 0.8 mm diamond bur. The sandblasting of groups B and C was performed with 50\u3bcm particles (aluminum oxide powder). The posts were luted with RelyX Unicem 2 Automix cement, light cured for 40 seconds. The pull-out test was performed with Instron 4301 machine at a pulling speed of 0.5 mm/min; the maximum values for post removal were recorded. The Kolmogorov-Smirnov (KS) test was used for normality of distribution; ANOVA and Bonferroni-Dunn test for pairwise comparisons were used to determinates differences in the pull-out force measurements. The level of significance was set at p <0.05. Results: The mean values of pull-out strength were the following: group A (control): 113.12\ub151.32 N; group B: 224.63\ub142.54 N; group C: 485.37\ub168.36 N; group D: 355.80\ub1118.47 N. The KS test showed a normal distribution of values for the 4 groups (group D: p = 0.089, groups A, B and C: p = 0.15). The ANOVA and Bonferroni-Dunn test showed significant differences (p