Reliability, failure mode, and stress distribution in conical connections with different types of retention

The tapered interference fit (TIF) implant system, a screwless system with purely frictional retention, was developed to minimize prosthetic complications associated with screw loosening. However, evidence regarding its biomechanical behavior is scarce.

The purpose of this in vitro study was to compare the reliability, failure mode, and stress distribution of frictional TIF implant systems with straight and custom in-office angled abutments and conventional taper integrated screw-in (TIS) systems.

One hundred and five implants with single crowns were divided into 5 groups (n=21) according to the type of retention (TIF or TIS) and angulation of the abutment, straight (S) or angled (A): straight universal abutment (TIF); 17-degree angled abutment (TIF-A); straight universal abutment (TIS); 17-degree angled abutment (TIS-A); and straight abutment with passing screw (TIS-P). The reliability was assessed by step-stress accelerated life testing (SSALT) (n=21). The failure mode was investigated using fractographic analysis. Finite element analysis (FEA) was performed to analyze stress distribution. The von Mises stress (σvM) and displacement were assessed for implant and abutment, while shear stress (τmax) and minimum principal stress (σmin) were evaluated for cortical and cancellous bone.

All groups presented high reliability at a load of 100 N. The TIS-P (93%) showed lower reliability than the TIF and TIS-A (99%) groups. However, with the increase of load to 150 N, the reliability of the TIS (60%), TIS-P (82%), and TIF-A (81%) groups decreased. No changes were observed for TIF and TIS-A. All specimens failed by fracture after SSALT, with failures restricted to the transmucosal area of the abutment. FEA showed an increase in σvM of at least 60% in the abutment for the TIF groups, while a reduction of 64% was observed in the implant. The displacement was greater for TIF (54.36 mm) in the abutment and higher for TIS-P (6.97 µm) in the implant. Also, the TIF and TIF-A groups exhibited better cortical and cancellous bone stress distribution.

The TIF systems can be safely indicated for implant-supported prostheses in the anterior region because of their high reliability and excellent stress distribution to the bone. Custom in-office angulation may decrease reliability, suggesting a preference TIS systems.