Retention of selected core materials to zirconia posts

Due to their favorable optical and mechanical properties, endodontic posts made of partially stabilized zirconia ceramic (ZrO2-Y2O3) are a promising alternative to those made of metal. Zirconia posts can be combined with various tooth-colored core materials to increase the optical properties of a final esthetic restoration. For stability, a reliable bond between core material and the post should be generated. This in vitro study evaluated the retention of selected core materials to zirconia posts dependent on different surface treatments and bonding procedures. Two types of zirconia posts (CeraPost [CEP], Lemgo, Germany) and CosmoPost [COP], Ivoclar Vivadent, Amherst NY 14228, USA) were employed for the study. Ring-shaped cores were fabricated of either heat-pressed, zirconia-containing glass ceramic (IPS Empress Cosmo [EMC], Ivoclar Vivadent), highly-filled hybrid composite (Tetric Ceram [TEC], Ivoclar Vivadent) or an experimental, high-strength glass ceramic (OHSU-RWTH [EX], Ivoclar Vivadent). The core made of material EX was either directly heat pressed (EXP) or adhesively bonded (EXB) onto the post using a flowable composite. Prior to core application, the post surfaces were preconditioned by alumina abrasion (AA) or tribochemical silicoating and silanation (TCS). Specimens (10 per group) were stored in artificial saliva (pH 5.2) for 150 days. Storage time included 5,000 thermocycles (5/55 degrees C per 30 seconds). Defect analysis was conducted visually using a light microscope and a fiber optic transillumination prior to the testing procedure. The loads required to separate post and core were determined by a push-out test. Following testing, the surfaces of the posts and core materials were evaluated in a scanning electron microscope (SEM). There were no statistically significant differences between the separation loads of groups COP/AA/EMC, COP/TCS/TEC, CEP/AA/EMC and COP/AA/EXB. Group COP/AA/EXP showed significantly higher retention, but also the highest standard deviation and the highest number and diversity of severe defects in the core material prior to testing. Similar defects were detected in the group COP/AA/EXC. In group COP/TCS/TEC, where there were a lower number of minor defects, and in COP/AA/EMC and COP/AA/EMC, no defects were observed. For both post systems tested with the combinations alumina abrasion/zirconia-containing glass-ceramic and tribochemical silicoating and silanation/highly-filled hybrid composite, a reliable retention was achieved. The use of the experimental high-strength glass ceramic as a core material is contraindicated due to a discrepancy in the coefficient of thermal expansion to the zirconia-post.