The Journal of Prosthetic Dentistry Asvanund and Morgano Clinical Implications The internal-implant abutment connection could potentially reduce stresses within the connection when off-center loads are applied. Statement of problem. 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. Purpose. This study compared the load transfer characteristics of a complete-arch restoration supported by 4 im- plants with external and internal implant-abutment connections. Loads were applied to the prostheses in 3 positions. Material and methods. Two-dimensional photoelastic models were used to simulate bone. Two types of implants (Re- placeSelect 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 ante- rior load; and 2-point lateral load). The numbers of fringe orders were recorded and compared. Results. 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. Conclusions. When loaded off-center, the internal-implant abutment connection produced less stress when compared with the external-implant abutment connection. (J Prosthet Dent 2011;106:266-271) Photoelastic stress analysis of external versus internal implant-abutment connections Pattapon Asvanund, DDS MSD, a and Steven M. Morgano, DMD b Mahidol University, Bangkok, Thailand; Goldman School of Dental Medicine, Boston University, Boston, Mass a Instructor of Prosthodontics, Director Prosthodontic Residency Training Program, Faculty of Dentistry, Mahidol University. b Professor and Director, Division of Postdoctoral Prosthodontics, Department of Restorative Sciences and Biomaterials, Goldman School of Dental Medicine. The success of implant treatment depends partly on the biomechanical load on the dental implants. 1 Func- tional loads applied on the prosthesis are transferred through the implants to the surrounding bone. Factors such as location and angulation of the im- plant, as well as geometry of the im- plant-abutment connection have been reported to influence stress and strain around implants. 2-5 The increase in the load on an implant is directly pro- portional to stress and strain devel- oped at the bone-implant interface, 6 and some studies have reported a relationship between occlusal over- load and loss of osseointegration. 7,8 The microgap between the abutment and the implant has been studied. 9 Steinebrunner et al 10 evaluated micro- bial leakage of different designs of the implant-abutment connections in an in vitro model. The authors reported that bacterial penetration occurred at the implant-abutment interface during mastication. This microgap may allow micromovement between the implant- abutment connection during clinical function, and this micromovement could result in localized inflammation with subsequent crestal bone loss. 11 The stability of an implant-re- tained prosthesis depends on the integrity of the screw connection. 12 Yousef et al 13 reported that off-axis loading may result in screw and abut-