Effects of abutment screw preload and preload simulation techniques on dental implant lifetime

Abstract

Background: This study aimed to investigate how the predicted implant fatigue lifetime is affected by the loss of connector screw preload and the finite element analysis method used to simulate preload.

Methods: A dental implant assembly (DI1, Biomet-3i external hex; Zimmer Biomet) was scanned using microcomputed tomography and measured using Mimics software (Materialise) and an optical microscope. Digital replicas were constructed using SolidWorks software (Dassault Systèmes). The material properties were assigned in Abaqus (Dassault Systèmes). An external load was applied at 30° off-axial loading. Eight levels of connector screw preload (range, 0-32 Ncm) were simulated for DI1. This assembly and an additional model (DI2) having a longer and narrower screw were compared regarding their fatigue limits (using fe-safe software [Dassault Systèmes]) for 2 preloading methods: (1) adding preload torque or (2) adding bolt axial tension.

Results: The maximum von Mises stresses of DI1 (on the connector screw threads) with and without preload were 439.90 MPa and 587.90 MPa. The predicted fatigue limit was the same for preloads from 100% through 80% of the manufacturer^'s recommendation and dropped precipitously between 80% and 70% preload. Adding a preload torque on the screw resulted in a more uniform stress distribution on the screw compared with bolt axial tension, especially for DI2, which had a longer and narrower screw than DI1.

Conclusions: A substantial loss of preload can be accommodated without compromising the fatigue resistance of this dental implant. Computer models should be constructed using torque instead of a bolt axial tension.

Keywords: Implants; failure; fatigue; finite element analysis; friction; mechanical testing.

Figure 1

Preload application on a screw to fix 2 components. Application of the torque develops a tensile force on the surface of the screw.

Figure 2

The von Mises stress distribution on the DI1 model with and without preload application.

Figure 3

The locations of fatigue failure on the DI1 model, as predicted by fe-safe (Dassault Systémes) software. The fatigue limit of the model with preload applied was substantially higher than the fatigue limit of the model without preload. Under an external load of 116.39 N, the lifetime of the model with preload (9 × 10⁶ cycles) was substantially longer than the lifetime of the model without preload (33,411 cycles).

Figure 4

The von Mises stress distribution on the connector screw of DI1 when preload is simulated in torque and bolt axial tension.

Figure 5

The von Mises stress distribution on the connector screw of DI2 when preload is simulated in torque and bolt axial tension.