Photoelastic stress analysis of external versus internal implant-abutment connections

Common complications of implant restorations are abutment screw loosening and screw fracture. The geometry of the implant-abutment connection may affect stresses generated from loading, and these stresses may have a role in screw loosening or fracture.

This study compared the load transfer characteristics of a complete-arch restoration supported by 4 implants with external and internal implant-abutment connections. Loads were applied to the prostheses in 3 positions.

Two-dimensional photoelastic models were used to simulate bone. Two types of implants (ReplaceSelect Internal-Interface Tapered Implants and ReplaceSelect External-Interface Tapered Implants) were placed in the photoelastic models. Complete-arch metal frameworks were fabricated on the abutments. Artificial teeth were arranged on the framework, and the prosthesis was screwed onto the abutments. The specimens were analyzed at 2 levels (implant-abutment level and apical to the implant level) with 3 loading conditions (4-point load; 2-point anterior load; and 2-point lateral load). The numbers of fringe orders were recorded and compared.

With the 4-point load, no stress differences occurred between the external-implant abutment connection and internal-implant abutment connection at the connection level and at the apical level. With the 2-point anterior load, the internal-implant abutment connection resulted in lower stresses at the connection level both in the loaded and non-loaded areas. With the 2-point lateral load, the internal-implant abutment connection resulted in lower stresses at the connection level at the non-loaded area.

When loaded off-center, the internal-implant abutment connection produced less stress when compared with the external-implant abutment connection.