Experimental study on the compressive strength of fixed prosthetic restorations on tooth preparations with or without horizontal preparation

Fixed prosthetic restorations, particularly those made entirely of zirconia, are widely used in clinical dental practice for the restoration of natural teeth. Their success largely depends on the design of the tooth preparation, especially the type of finish line, as well as the restoration's ability to withstand occlusal forces during mastication. Over 12 months, a total of 21 extracted teeth were initially collected, from which six were selected for this study. To achieve the objective of the study, three teeth were prepared with a horizontal finish line (shoulder), and the remaining three were prepared without a defined finish line (vertical preparation). For each prepared tooth, three full-contour zirconia crowns were fabricated. To ensure accurate comparison and standardization of occlusal force application during mechanical testing, all crowns were designed with identical morphology. The force application device used in mechanical testing was digitally designed to replicate the anatomy of the antagonistic teeth corresponding to each tested crown. Samples I and III showed significant structural changes, including horizontal fractures at the cervical level. Samples II, 1, and 2 exhibited no visible damage. The maximum recorded compressive forces at which structural failure occurred varied widely, ranging between 180 N and 2537 N. Consequently, all samples that recorded fracture values below 1000 N were analyzed separately for interpretation. The findings of this experimental study indicate that tooth preparation design significantly influences the compressive strength of monolithic zirconia crowns. Both preparation types-shouldered and shoulderless-demonstrated high resistance values, with a slight advantage observed in favor of the preparations with a defined finish line (shoulder).