Group C: custom-made post-core of PEEK (one piece)
fracture, volume reduction
A plastic post (PinJet, Angelus) was placed into the canal to obtain negatives of the post cavities by injecting auto-mix light body addition silicone impression material (Empress XT, 3 M ESPE) into the root canal space. The impression was completed with the heavy-body material placed in a cellulose crown form of A4 (Dentsply, Switzerland). The exact size used for group P. Digital images of the impression were obtained with an intraoral scanner (Digital photos of the impression were obtained with an intraoral scanner CEREC Omnicam, Sirona, Germany). The post-and-core was modelled with design software (Inlab CAM SW18.0 software 2.0, Dentsply Sirona, Germany). A 2.5% volume reduction in the mesiodistal and buccolingual axes was defined to allow room for the cement layer. After developing the milling plan, 29 posts were milled from PEEK discs like the previous group. The post-cementation was carried out using Rely Xtm U200 Automix (3 M ESPE, USA) self-adhesive resin cement according to manufacture instructions as in the previous group.The specimens from each group with cemented posts were randomly divided into two subgroups according to the types of tests utilized. Twenty-eight samples (14 from each group) for the fracture strength test (P1, C1) and 30 samples (15 from each group) for the pullout resistance test (P2, C2). The crown preparation steps were only made for the 28 samples (14 from each group) in the fracture strength test.