Axial displacement of abutments in implants with internal tapered connection after cyclic loading: Linear mixed model analysis

Abstract

Purpose: The purpose of this study was to analyze the patterns of the axial displacement of implant-abutment assembly after cyclic loading, and to figure out the plateau in the amount of axial displacement in internal tapered connection.

Materials and methods: External butt-joint connection implant and internal tapered connection implant were connected with three types of abutment for cement-retained prostheses. Three groups of external type abutment (Ext group), internal tapered one-piece type abutment (Int-1 group), and internal tapered two-piece type abutment (Int-2 group) were prepared. For each group, 7 implants and abutments were tested. The implant-abutments assemblies were clamped into the implant holder for vertical loads. A dynamic cyclic loading was applied for 150 N at a frequency of 3 Hz. The amount of axial displacement after tightening was measured before cyclic loading, and the Periotest® values (PTVs) and removal torque values (RTVs) were measured for each group before and after the cyclic loading. The amount of axial displacement of the abutment into the implant was measured at each cycle of 0, 10, 100, 1,000, 10,000, 100,000, 500,000, and 1,000,000. A repeated measures analysis of variance (ANOVA) for the overall effect of cyclic loading. Thereafter, the pattern analysis by linear mixed model were used for statistical analysis of longitudinal data. Differences at P < 0.05 were considered statistically significant. Microscopic computed tomography (micro-CT) was used to evaluate the internal structure, and scanning electronic microscopy (SEM) was used to evaluate the surface of implant-abutment connection.

Results: The mean axial displacement after tightening were 4.0 ± 1.41 μm in Ext group, 31.1 ± 9.48 μm in Int-1 group, and 48.3 ± 8.44 μm in Int-2 group. Before the cyclic loading, the PTVs were not different in each group. After the cycles, the PTV decreased in Int-2 group. The RTV in Int-1 group showed significantly higher value before the cyclic loading. After the cycles, each group showed significant decrease in RTV values, and Int-2 group showed the lowest RTV value. The mean axial displacement after 1 million cycles were 0.6 ± 0.54 μm in Ext group, 3.7 ± 0.76 μm in Int-1 group, and 9.0 ± 2.38 μm in Int-2 group. In the pattern analysis, the breakpoint was found at 171 cycles. There were no declining pattern of axial displacement for the Ext group and the after of the breakpoint (171 cycles) of Int-1 group. However, Int-2 group showed continuous axial displacement. In the SEM evaluation, the surface wear were found in all groups.

Conclusions: Within the limitations of this study, the following conclusions can be drawn: 1. The axial displacements according to the tightening torque occurred in all groups. 2. The surface wear of the implant-abutment connection after cyclic loading were noticed in all groups. 3. The periotest values remained stable, but the removal torque value decreased after cyclic loading in all groups. 4. In the analysis by linear mixed model, Ext and Int-1 group showed the plateau of axial displacement at early phase of cyclic loading. The exact point of plateau was hardly figured out in Int-2 group, but the rate of axial displacement slowed after 100,000 cycles. This study demonstrated the patterns of axial displacement in internal implant system and the decrease in removal torque value. In the clinical aspect, it is recommended that the screw be retightened after at least one month to minimize the screw loosening and clinical prosthetic errors.