Dimensional tolerances and assembly accuracy of dental implants and machined versus cast-on abutments

The clinical application of prosthetic components obtained by different manufacturing processes lacks technological foundation: the dimensional tolerance of individual parts and their assembly accuracy are not known. The rotational misfit (RM) of the hexagonal connection is critical in single-tooth implant restorations, but no standard control procedures are available for its evaluation.

The research aimed at proposing a new protocol for the dimensional assessment of implant-abutment connections, based on noncontact measurement and statistical data processing. The procedure was applied to machined- and cast-on abutments, as well of the matching implants.

Three groups of five abutments each were studied: machined titanium abutments, pre-machined calcinable abutments before casting procedures and the same specimens after casting. A group of five corresponding implants was considered as well. Twice the apothem was measured on each hexagon through an optical measuring microscope. The data were processed to obtain the international tolerance (IT) grade. The RM was then calculated using the apothems of the external and the internal hexagon.

All the components were classified between IT8 and IT9, and the maximum RM was around 3-4° for all the assemblies, inferior to the critical limits for the screw joint stability.

An original measuring protocol was developed, independent of parts assembly and based on ITs. An objective dimensional characterization of prosthetic components and assemblies has been achieved, which is the basis for their reliability in clinical applications.