Clinical Pearls: A JOMI review by AO's YCC
As an occasional feature of AO CONNECT, articles from The International Journal of Oral and Maxillofacial Implants (JOMI) — the official journal of AO— are reviewed from the viewpoint of a member of the Academy’s Young Clinicians Committee. This “JOMI Clinical Pearl” by Dr. José Antonio Garcia was one of top three abstracts from Volume 32, Number 4, 2017 as selected by AO’s Website Education Committee.
Misfit of Three Different Implant-Abutment Connections Before and After Cyclic Load Application: An In Vitro Study
(Sergio Alexandre Gehrke, DDS, PhD1/Rafael Arcesio Delgado-Ruiz, DDS, PhD2/Juan Carlos Prados Frutos, DDS, PhD3/María Prados-Privado, PhD5/Berenice Anina Dedavid, PhD4/Jose Manuel Granero Marín, DDS, PhD5/José Luiz Calvo Guirado, DDS, PhD5)
The present study evaluates the misfit of the interface between the abutments and implants of three different connections (External Hexagon, Internal Hexagon, Morse Taper) before and after the application of mechanical load cycling. They analyzed two sections to understand how the load cycling affects different levels of the interfaces.
The transversal section allowed a 360-degree view and analysis of the implant-abutment connection and had information about the rotational freedom. The analysis was significant at the Morse Taper connection, where the connection showed full contact even before the application of the cycling load.
Different studies show the benefits of implants with a Morse Taper connection over the implants with an Internal Hexagon connection; nevertheless, these implants are used frequently with high rates, even in unit rehabilitations.
Several techniques were used to examine the implant-abutment interface, including human studies of samples under magnification, measurements of the cross sections, In the present examination, SEM (Scanning Electronic Microscopy) measurements were chosen for their precision
After the application of mechanical cyclin microgaps were reduced or disappeared between the abutments and the implants in all groups, which confirmed that the abutment contact increases after load application.
The mechanical cycling altered the fit between the implant and the abutment in all groups. Thus, increased contact between the implant-abutment connection might improve the resistance to loosening and prevent the entry of bacteria and fluids.
Vertical misfits was reported between 2.3 and 6.4 mm for machined abutments with different connections. The results revealed that all the connections exhibited better fits after the load applications.
Misfits between the components of screwed connections have been considered to be possible causes of mechanical complications, screw loosening or fractures. Misfit has been indicated as a possible cause of the loosening of retention screws.
The two factors are responsible for the microgap changes and variations in micromovement of the implant-abutment connection: first, the differences in the torque of the internal and external hexagon implants compared with the torque of the Morse taper implants, which are directly related to the micromovement of the assembled parts and second, the assembly adjustments caused by the deformations after load.
The stability of a screwed connection is directly related to the preload achieved during the torqueing. In addition, the lack of adaptation between the implant-abutment components will facilitate the rotation of the abutment screw or the abutment part.
Some studies have suggested a correlation between the misfit at the implant-abutment interface and screw loosening,but no studies have examined the correlation between unscrewing and the level of vertical misfit, and few studies have evaluated the inter- action between the external hexagon of the implant and the internal hexagon of the abutment or rotational freedom.
Fatigue tests were used to simulate masticatory loading on the implants and to deter- mine the stabilities of the interfaces. Variations in the numbers of load cycles, frequencies of loading, direction of the load, forms of application, and other factors vary greatly, and the comparison with this study was not possible.
The first time that an abutment screw is tightened within an implant, contact between the implant and the threads of the screw occurs only via the microroughnesses of the surfaces. A reduction of 2% to 10% in preloading should occur within the first few seconds or minutes after clenching due to the relaxation phenomenon that is termed “settling.”1Therefore, in this study, the abutment screws were retightened to the initial torque value 10 minutes later.
The different style of the Morse taper connection system has advantages over hexagonal connections because this connection type works with the friction between two walls, A screw receives the tightening torque to settle the implant and control the friction between the walls. The intimate contact between the implant and the abutment reduces micromotion because the two parts behave as a single structure.
Within the limitations of this experimental study, it can be concluded that the application of cycling load produces an accommodation of the implant-abutment connection in internal, external, and Morse taper connections. In the longitudinal direction, the accommodation decreases and eliminates the gap observed initially (before load). In the transversal cuts, Morse cone implant-abutment connections experience a complete accommodation with the elimination of the gap. The presence of implant-abutment misfit results in an increment of the wear and deformation of the implant-abutment components after cycling loading plus screw loosening after preload loosening. To reduce the wear, deformation, and risk of fracture of the implant-abutment components, as well as reduce the screw loosening, implant-abutment connections with a Morse taper connection are recommended.
About the Author
Dr. José Antonio Garcia graduated from Universidad Autonoma de Nuevo Leon in 2004 and from Instituto Dental de Implantologia in 2007.