Finite Element Analyses of Two Antirotational Designs of Implant Fixtures

Finite Element Analyses of Two Antirotational Designs of Implant Fixtures
March 2005
Akour, Salih N. BSc, MSc, PhD*; Fayyad, Mohammed A. BDS, PhDÇƒÜ; Nayfeh, Jamal F. BSc, MSc, PhDÇƒ?
Implant Dentistry: Volume 14
Abstract TOP
The purpose of this study was to compare the effect of cyclic compressive forces on loosening of the abutment retaining screw of dental implant fixtures with two different antirotational designs using the finite element analysis. A three-dimensional model of externally hexed and trichannel dental implant fixtures with their corresponding abutments and retaining screws was developed. Comparison between the two designs was carried out using finite element analysis. The results revealed that the externally hexed design has significantly higher overall stress, contact stress, and deflection compared with the trichannel design. The trichannel antirotational design has the least potential for fracture of the implant/abutment assembly in addition to its capability for preventing rotation of the prosthesis and loosening of the screw.
High success rates of root-form dental implants1-8 coupled with the preferences of patients for fixed restorations9 have encouraged dentists to use implants as an alternative to conventional restorative procedures and, in some instances, to take greater risks.8,10 Although problems do occur at the implant-bone interface, the most common failures involve prosthetic components and restorative materials. Loosening or fracturing of abutments and retaining screws occurs frequently.11 It has been reported in several clinical studies3,10,12 that the most common complication of the implant-supported restorations is loosening of the abutment retaining screw. The highest rate of screw loosening was reported with single tooth implant-supported restorations.13
Antirotation, provided by the coronal hex, is necessary to stabilize angulation-correcting abutments as well as for the stabilization of abutments in single-tooth restorations.14 Historically, the nonhexed implants for single-tooth replacements required flattening the contacts of the adjacent teeth or adding wings to the crown to provide stability. These changes result in compromise in access for plaque control or alteration of adjacent tooth structure to compensate for an inadequacy in the implant system.14 However, the external hexagonal design of the coronal portion of the root-form implants was first introduced to drive the implant in place. It was not intended for antirotational purposes originally.14,15 Later, Ohrnell et al 15 suggested that the external hex of the implant should be extended to 1.2 mm to prevent loosening of the retaining screw.
Today, there are many coronal designs intended to provide antirotation of single-tooth replacements. These include external hex (Branemark System, Nobel Biocare, Chicago, IL), internal hex (ScrewVent, Zimmer Dental, Carlsbad, CA), internal octagon (T.B.R.Ide@Conic, Toulouse, France), and trichannel (Replica Select, Nobelpharma, Chicago, IL). However, in addition to the accuracy of the clinical and laboratory procedures, these antirotational features are important factors in achieving a stable connection and accurate transfer. Manufacturing variations can result in up to 100 ¨µm space between mating parts.16 An independent study has reported the Hex Lock (Zimmer Dental, Carlsbad, CA) to have zero rotation compared with rotational stability for Nobelpharma abutments of 198 ¨µm.17
The purpose of the present investigation was to use finite element analysis to compare two different antirotational designs and to study the effect of the cyclic compressive forces on loosening of the abutment retaining screw.
Please visit the web site to view the article in its entirety.