Properties of axially loaded implant–abutment assemblies using digital holographic interferometry analysis

Abstract

Objective

The aim of this study was to (i) obtain the force-related interferometric patterns of loaded dental implant–abutment assemblies differing in diameter and brand using digital holographic interferometry (DHI) and (ii) determine the influence of implant diameter on the extent of load-induced implant deformation by quantifying and comparing the obtained interferometric data.

Methods

Experiments included five implant brands (Ankylos, Astra Tech, blueSKY, MIS and Straumann), each represented by a narrow and a wide diameter implant connected to a corresponding abutment. A quasi-Fourier setup with a 25 mW helium–neon laser was used for interferometric measurements in the cervical 5 mm of the implants. Holograms were recorded in two conditions per measurement: a 10 N preloaded and a measuring-force loaded assembly, resulting with an interferogram. This procedure was repeated throughout the whole process of incremental axial loading, from 20 N to 120 N. Each measurement series was repeated three times for each assembly, with complete dismantling of the implant-loading device in between. Additional software analyses calculated deformation data. Deformations were presented as mean values ± standard deviations. Statistical analysis was performed using linear mixed effects modeling in R’s lme4 package.

Results

Implants exhibited linear deformation patterns. The wide diameter group had lower mean deformation values than the narrow diameter group. The diameter significantly affected the deformation throughout loading sessions.

Significance

This study gained in vitro implant performance data, compared the deformations in implant bodies and numerically stated the biomechanical benefits of wider diameter implants.

Introduction

The surgical placement of dental implants is the most predictable phase in implant dentistry resulting with successful integration in more than 98% of the cases . The long term success of an integrated implant can be assessed by evaluating the surrounding bone level, peri-implant soft tissue, prosthetic superstructure and patient’s overall satisfaction . The objective clinical parameters mentioned are interdependent: a standard two-piece implant has an inevitable microgap at the prosthetic interface, thus facilitating bacterial colonization that may result in soft tissue inflammation and crestal remodeling . Furthermore, functional loading propagates microgap enlargement and can additionally influence the peri-implant tissue physiology.

Research on implant biomechanics associated load distribution with implant size and shape . In finite element analysis (FEA) studies, an increase in implant diameter was demonstrated to cause an exponential decrease of peri-implant bone stress upon loading. In one study , the highest stress reduction exhibited (31.5%) was between the 3.6 mm and 4.2 mm diameter implants. Further enlargement of implant diameter resulted in a continuous drop in stress reduction. The same study, however, reported that increasing the implant length did not influence bone stress in the same order of magnitude; the reduction effect was significantly lower. Another observation pertaining to this issue noted that 80–100% of load induced stress was exhibited in the crestal 40% of the fixture, regardless of the implant length . Collectively, these findings emphasize the importance of analysing the loading properties of the cervical part of an implant in the field of implant biomechanics.

A substantial number of studies have analysed the peri-implant bone stress; however, they seldom investigated the changes in the implant–abutment assembly itself . Most of the analyses were based upon virtually simulated experiments, thus lacking empirical data pertinent for actual performance. In spite of many advantages, FEA can describe the behavior of real 3D structures only to an accuracy depending on the appropriate complexity of the virtual model, size of the mesh, displacement gradient variations and errors in input data. Digital holographic interferometry (DHI) enables a direct insight in rough surface deformation upon loading in vitro with submicron accuracy . The method overcomes the mentioned problems by providing continuous, non-destructive and whole-field deformation measurements of actual specimens. Term ‘digital’ means the employment of a CCD sensor for recording holograms which can then be stored in computer memory and reconstructed either numerically or optically .

To our knowledge, only a small number of studies used an interferometric approach for evaluating implant biomechanics . Wahl et al. studied the influence of load application on titanium implant–superstructure interface dislocation using electronic speckle pattern interferometry (ESPI). Three 4.1 mm diameter implants were connected to three different prosthetic superstructures and loaded with a 41.6 N force applied to the palatal cusps of the crowns at a 45° angle using a piezoelectric device. The screw retained superstructure dislocations at the implant–superstructure interfaces were analysed and quantified, resulting with a mean value of 0.27 μm gap formations. Chorres et al. used holographic interferometry (HI) to compare the load distributions between fully-implant and tooth-implant supported fixed pontics. The authors used three 4.0 mm diameter titanium implants previously placed in the dog mandibles and compared the two types of fixed prostheses loaded with a 10 N force at anterior, midpoint and posterior positions. The qualitative analysis of the holograms obtained showed uniform deflection fringes in fully-implant supported pontics, whereas in case of tooth-implant pontics fringes were not present in dental abutments due to the higher displacement of the teeth than the capacity of the holography set up. Matsumoto et al. investigated implant-induced stress concentration in monkey mandibles using HI. The authors placed a ceramic implant in the second molar position, supporting a fixed partial denture between the fixture and the first and second premolars. The double exposure technique was used to obtain holographic interferograms for various force applications to the superstructure. When vertically loaded at the second premolar point, the bone exhibited the highest stress concentration on the mesial aspect of the pontic, causing a detrimental effect on the bone.

To this date, there has not been an interferometric study that thoroughly analysed loading properties of different implant-abument assemblies. The purpose of this study was to obtain the interferometric patterns and analyse the deformations in the cervical portions of several implant–abutment assemblies under incremental axial loading by means of digital holographic interferometry. Additionally, our objective was to compare the loading properties of different implant–abutment assemblies based on their diameters.

Materials and methods

Experiments included 5 different titanium dental implant brands: Ankylos (Friadent, GmbH, Mannheim, Germany), Astra Tech (Astra Tech AB, Mölndal, Sweden), blueSKY (Bredent GmbH & Co. KG, Senden, Germany), MIS (MIS Implants Technologies Ltd., Bar Lev Industrial Park, Israel) and Straumann (Institut Straumann AG, Basel, Switzerland). Each brand was represented by a narrow and a wide diameter implant connected to a corresponding titanium abutment, as listed in Table 1 .

Table 1
Implant–abutment assemblies used in the study.
Brand Fixture Abutment Manufacturer
Ankylos C/X A11 Regular/X Friadent GmbH, Mannheim, Germany
Ø 3.5 mm/L 11 mm GH 3.0 mm/A0
C/X B11 Regular/X
Ø 4.5 mm/L 11 mm GH 3.0 mm/A0
Astra Tech Osseospeed TX S TiDesign 3.5/4.0 Astra Tech AB, Mölndal, Sweden
Ø 3.5 mm/L 11 mm GH 3.0/4.5 mm
Osseospeed TX S TiDesign 4.5/5.0
Ø 5.0 mm/L 11 mm GH 3.0/5.5 mm
blueSKY Ø 3.5 mm/L 12 mm Esthetic L 0° Bredent GmbH & Co. KG, Senden, Germany
GH 3.0 mm
Ø 4.5 mm/L 12 mm Esthetic L 0°
GH 3.0 mm
MIS C1 Transgingival MIS Implants Technologies Ltd., Bar Lev Industrial Park, Israel
Ø 3.75 mm/L 11.50 mm GH 3.0 mm
C1 Transgingival
Ø 5.0 mm/L 11.50 mm GH 3.0 mm
Straumann Bone level RC ST Anatomic Institut Straumann AG, Basel, Switzerland
Ø 3.3 mm/L 12 mm GH 3.5 mm
Bone level RC ST Anatomic
Ø 4.8 mm/L 12 mm GH 3.5 mm

Loading device

A special set of devices and accessories was designed, fabricated and tested at the Mechanical Properties Research Laboratory (Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia) throughout one year time period prior to this research ( Fig. 1 a). The basis for the implant loading assembly was a precision fabricated vertical vice with a heavy metal base that minimized undesired movements. First, a metal cylinder was mounted onto the vice bar. Secondly, a high precision digital dynamometer (HBM Kraftaufnehmer U1, Hottinger Baldwin Messtechnik GmbH, Darmstadt, Germany) was attached to the metal cylinder with a large diameter screw. The dynamometer was connected to a precision amplifier (GTM LT-Digitizer, Gassmann Testing and Metrology GmbH, Bickenbach, Germany) with an output to a personal computer. Furthermore, a custom made bearing table was screwed onto the dynamometer ensuring an even and straight platform for load bearing purposes. The clamping device, seated on the table, had a sliding metal plate which, when tightened, firmly secured an implant in its corresponding diameter hole. This way, the implant position was always perpendicular to the surface of the clamping device. Every implant hole had an adjustable depth to ensure that all the implants protruded above the surface evenly, with 5 mm of their cervical portions exposed to the object beam ( Fig. 1 b). The abutments were tightened to their matching implants according to manufacturers’ instructions using their corresponding torque wrenches. Individually crafted flat surface pins were seated into all the abutments studied in order to ensure an even pressure when loading implant–abutment assemblies. Gentle rotation of the screw press handle caused the ram to load an implant–abutment assembly with a force that was instantly recorded and visualized by the VN-Digitizer software (Gassmann Testing and Metrology GmbH, Bickenbach, Germany) on a personal computer.

Fig. 1
Implant loading device. (a) Device components. (b) Loaded implant components exposed to the laser beam (1 – vertical vice, 2 – metal cylinder, 3 – digital dynamometer, 4 – large diameter screw, 5 – bearing table, 6 – implant clamping device, 7 – dental implant, 8 – implant abutment, 9 – loading pin, 10 – deformed state implant, 11 – ram).

Experimental measurements

All the experiments were performed at the Coherent Optics Laboratory (Institute of Physics, Zagreb, Croatia). A quasi-Fourier setup on a vibration-isolating table with a Spectra-Physics 25 mW helium–neon laser light source (wavelength: 632.8 nm) was used for digital holographic interferometry measurements ( Fig. 2 ). The object beam was expanded and collimated to reflect from a part of the base (clamping device), the exposed cervical 5 mm of an implant and the abutment. A double recording technique was used for interferometric measurements. In the first recording, an implant–abutment assembly was always preloaded with a 10 N force in order to avoid unwanted movements upon ram seating on the abutment pin. After obtaining the first hologram, the implant–abutment assembly was loaded with a desired force and instantly recorded, resulting with the second hologram. Both holograms were superimposed and the obtained interferogram was reconstructed. This procedure was repeated throughout the whole process of incremental axial loading. Double exposure holograms of all implant–abutment assemblies were obtained for the following forces: 20 N, 30 N, 40 N, 50 N, 60 N, 70 N, 80 N, 90 N, 100 N, 110 N, and 120 N. Each series of measurements was repeated three times for each implant–abutment assembly, with complete dismantling of the implant-loading device in between series.

Fig. 2
The interferometric setup (La – laser, BS – beam splitter, RB – reference beam, OB – object beam, M – mirror, L – lens, A – attenuator, ILD – implant loading device, O – object, CCD – charge-coupled device).

Digital holographic interferometry

Digital holographic interferometry (DHI) is a technique used for measuring displacement and/or deformation of a surface of investigated objects. Since the relative changes in the topology of the surface affect the phase of the incident coherent light, a series of holograms can be recorded to preserve different surface states. Then, two arbitrary holograms can be superposed and reconstructed. Their restored wave fronts interfere and form a fringe pattern that is modulating the reconstructed irradiance of the surface and also revealing the topology changes. By topology changes we refer to displacement vector, d ( P ), describing a change in location of a point P (new location is P ′) on the investigated surface. Generally, discrete-valued interferogram can be described by

I(m,n)=Ib(m,n)+V(m,n)cos[Δφ(m,n)]+N(m,n)I(m,n)=Ib(m,n)+V(m,n)cos[Δφ(m,n)]+N(m,n)
I ( m , n ) = I b ( m , n ) + V ( m , n ) ⋅ cos [ Δ φ ( m , n ) ] + N ( m , n )

where I b ( m , n ) is the background intensity, V ( m , n ) is the fringe contrast (also, visibility of fringes), Δφ(m,n)
Δ φ ( m , n )
is the phase difference between two holograms, and N ( m , n ) is an additive noise. Thus, the task is to find d ( P ) values for the entire surface from the I ( m , n ) values. Two restrictions concerning the Eq. (1) are obvious: (i) the phase Δφ(m,n)
Δ φ ( m , n )
can only be evaluated up to due to the periodicity of a cosine function and (ii) since the cosine function is even, the phase Δφ(m,n)
Δ φ ( m , n )
can be evaluated merely up to a sign. Less obvious but extremely important is influence of noise N ( m , n ) on the I ( m , n ) values. Many disturbances contribute to noise such as vibrations, thermal electronics noise, speckle noise, etc. Moreover, microscopic variations in the loading conditions affect the stress distribution within a sample and also variations in d ( P ) values. Finally, the insufficient sampling rate conditions (undersampling) lead to the loss of information due to the frequency folding or aliasing errors . All these disturbances imply that more independent measurements for the same situation are needed to minimize the effect of noise.

For quantitative evaluation, the relation between the observed phase difference Δφ(m,n)
Δ φ ( m , n )
and the displacement vector d ( P ) is given by

Δφ(m,n)=2πλS(P)d(P)
Δ φ ( m , n ) = 2 π λ S ( P ) ⋅ d ( P )
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Properties of axially loaded implant–abutment assemblies using digital holographic interferometry analysis
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