Engaging vs. Non-Engaging Abutments: An In Vitro Study Evaluating Changes in Microgap and Screw Morphology

Abstract

Background: The purpose of this study was to compare the microgap size between engaging (E) and non-engaging (NE) abutments and screw morphology changes between E and NE abutments using scanning electron microscopy (SEM) before and after cyclic loading (CL).

Methods: Thirty-six implants were arranged into four groups as follows: Group 1, single units with E abutments; Group 2, single units with NE abutments; Group 3, three-unit fixed partial dentures with a hemi-engaging design; and Group 4, three-unit FPDs with two NE abutments. The microgap was evaluated using a stereomicroscope. SEM was used to qualitatively evaluate screw morphology. The specimens were subjected to axial loading first and then lateral loading (30°) using the settings; one million cycles (1.0 × 10⁶ cycles) for each loading axis.

Results: There were no significant differences detected in the microgap sizes between the E and NE abutment groups. In addition, there were no significant changes in the microgap sizes after CL in the E or NE abutment specimens. More damage to the screws was noticed after CL compared to before, with no difference in the patterns of damage detected between the E and NE abutments.

Conclusions: No significant difference in microgap size was detected between the E and NE abutments. Furthermore, there was no significant difference in microgap size between the different prosthetic designs. From the SEM qualitative evaluation, there were similar screw morphology changes after CL between the E and NE abutments.

Keywords: SEM; abutments; abutments design; biomechanics; cyclic loading; microgap; morphology; prosthetic design; prosthetics; screw morphology.

Figure 1

Image from the Discovery V12 stereomicroscope used for microgap evaluation. The line refers to the space measured.

Figure 2

SEM images of sites evaluated. Red circle indicates thread site for qualitative evaluation performed and reported in Table 3.

Figure 3

SEM images used to evaluate screw morphology. (A) Screw #1 before, showing a non-homogenous, striated, porous appearance as well as plastic deformation (surface damage) that could be the result of uncareful handling or a machining error in the component since this was before loading and placement. The screw on the thread flank is thinning compared to the other thread, creating a knife-edge appearance (blue arrow) that is also seen. Also noted is the presence of surface cracks (red arrow). (B) Screw #1 after: extensive surface damage and the separation of surface material of the thread, possibly caused by an adhesive wear mechanism or galling (blue arrow), which is described as “the removal or displacement of material from a surface by the welding together and subsequent shearing of minute areas of two surfaces that slide across each other under pressure”. (C) Screw #6 before: surface irregularities are present, probably due to mishandling or machining (blue arrow), and a notably striated surface is visible (red arrow). (D) Screw #6 after: surface chips and delamination and adhesive wear mechanism (galling).

Figure 4

SEM images used to evaluate screw morphology. (A) Screw #7 before: note the smooth, non-striated surface vs. the previously striated surface on screws #1,6; it is non-porous. The thread appears to have plastic deformation where the thread is V-shaped (red arrow) compared to the adjacent thread, probably from the machining of the components. (B) Screw #7 after: no major changes are noted after loading. (C) Screw #12 before: it appears homogenous with a relatively smooth surface that has minimal striations, is non-porous, and has little surface debris that could be the result of machining of the components. (D) Screw #12 after: it appears non-homogenous (red arrow) with a considerable amount of surface debris and surface damage in the form of galling or adhesive wear (blue arrow). Changes in thread height were noted but were minimal.

Figure 5

SEM images used to evaluate screw morphology. (A) Screw #13 before: the surface appears homogenous, with a striated appearance. (B) Screw #13 after: surface debris with adhesive wear (galling) and plastic deformation causing the thread to have a knife-edge appearance (blue arrow) and changes in thread geometry; a reduced thread height measurement was also seen. One area shows the gross separation of the surface material (red arrow). The surface of the screw appears to be smooth compared to its striated appearance before loading.