Development of a new quantitative gas permeability method for dental implant-abutment connection tightness assessment
Most dental implant systems are presently made of two pieces: the implant itself and the abutment. The connection tightness between those two pieces is a key point to prevent bacterial proliferation, tissue inflammation and bone loss. The leak has been previously estimated by microbial, color tracer and endotoxin percolation.
A new nitrogen flow technique was developed for implant-abutment connection leakage measurement, adapted from a recent, sensitive, reproducible and quantitative method used to assess endodontic sealing.
The results show very significant differences between various sealing and screwing conditions. The remaining flow was lower after key screwing compared to hand screwing (p = 0.03) and remained different from the negative test (p = 0.0004). The method reproducibility was very good, with a coefficient of variation of 1.29%.
Therefore, the presented new gas flow method appears to be a simple and robust method to compare different implant systems. It allows successive measures without disconnecting the abutment from the implant and should in particular be used to assess the behavior of the connection before and after mechanical stress.
KeywordsColor Marker Screw Hole Differential Pressure Gauge Leakage Study Initial Pressure Difference
and, more recently, studying tightness against endotoxins .
These techniques hardly provide a quantitative and reproducible way to measure the leakage, as it was demonstrated in endodontics . Indeed, many techniques have been used to investigate the sealing ability of root filling procedures and materials. Some leakage investigations, like dye spectrometry , fluid filtration , and electrochemistry , are considered to provide pure quantitative data. Other studies using bacteria are essentially qualitative [23, 24]. However, the majority of leakage tests are related to linear measurement of tracers like coloring agents and radioisotopes , which give semi-quantitative data . Despite the long experience of measuring tightness in this field, in vitro assessment of sealability has lost its credibility . Among the leakage studies, tracer diffusion studies are the more frequent but have been demonstrated to have false conclusions  and results to be very driven by presence of air entrapped. These critics could be applied to implants leakage studies using tracers or bacteria entrapped in the inner part of the implant.
Recently, a gas flow test was shown to be a sensitive, reproducible and quantitative method to assess endodontic sealing . The aim of the present engineering contribution was to adapt this gas permeability technique to implant-abutment connection leakage, in order to provide a new quantitative and reproducible tool for further investigations. Such a method has never been used in the field of implantology for assessing implant-abutment microgap before.
a blind-ended glass test tube was used to assess the gas tightness of the experimental chamber;
a double-ended opened glass tube sealed on one side with a plug of epoxy glue to assess the gas tightness between the glue and the glass;
a double-ended opened glass tube sealed on one side with a plug of wax to assess the gas tightness of wax.
Under water spray, five One Morse implants (One System Implant, Cannes France), 4.3 mm diameter and 12 mm length, were drilled a hole to the apical part of the screw cavity. A plastic tube was inserted in the hole. The implants were partially embedded in epoxy glue (Araldite 2012, Huntsman Polyurethanes, Everberg, Belgium) and sealed in a glass tube, allowing free spaces around the abutment connection and around the tube. Wax (Purple wax, GC Europe, Leuven, Belgium) was used to seal implant to glue, and to block the various gaps to be assessed.
Assay 1) abutment manual screwing (by one operator)
Assay 2) abutment manual screwing (by one operator), screw hole blocked with wax
Assay 3) abutment key screwing (35 Ncm), screw hole blocked with wax
Assay 4) abutment key screwing, screw hole and implant connection blocked with wax (as a negative test)
Assay number 3 was performed 10 times for one of the implants to assess the reproducibility of the method.
ANOVA of repeated measures was performed globally to test whether the measured slopes for the 4 assays were globally different. After a logarithmic transformation, data followed a normal repartition. Pairwise comparisons between each assay were tested taking into account Bonferroni correction at alpha = 0.05 (significance).
The slope determination reproducibility for 10 curves, given by the Kendall's coefficient (W) of concordance test, was 0.9 (Kendall test), p = 0.0004. Reproducibility of the method has been calculated by repeating 10 times the measure on one implant and the abutment screwed with a dynamometric key at a torque value of 35 N.cm, and screw hole blocked with wax; the coefficient of variation was 1.29%, which is very low and is usually associated with reproducible methods of measurement.
Dental implant-abutment assays
Pairwise comparisons between each assay
p = 0.0001*
p = 0.02*
p = 0.06
p = 0.0001*
p = 0.03*
p = 0.0004*
Clinically, existence of a gap does not mean necessarily bone loss. In particular, it was shown that micro-movement may have a higher influence than leakage . Furthermore, in this gas permeability model, the size of porosity cannot be assessed precisely. The smaller capillary measurable with gas permeability has an internal diameter of 10 μm (data not shown). This is much bigger than bacteria or even than endotoxin.
On the other hand, this assay only assesses the global tightness of the connection. The possible spaces around the implant collar which would not communicate with the inner part of the implant do not influence the result, though they can have a major clinical role. Despite this, leakage can be considered as a good marker of the machining quality.
Unlike in the color marker methods, this technique allows successive measures of the different interfaces (screw hole, collar) without re-opening the connection. This characteristic could be very useful for assessing different treatments of the implant connection such as screwing torque control, chemical stress or mechanical stress. Clinically, tightness could be improved by different ways apart from the quality of machining, for instance by the use of a sealent such as GapSeal® (Hager Werken), though the authors did not find any scientific publication about this product using PubMed.
Using gas leakage allows a precise physical and reproducible value independent of water-wetting properties of the materials tested. The global leak measure obtained can be compared, for simplification and calibration, to an "equivalent capillary" with a length-diameter couple.
This gas flow method appears to be a simple way to compare different implant systems. It seems to be better than the color marker techniques because it allows successive measures without disconnecting the abutment from the implant and is not subjected to entrapped air bubbles . This new method could be used for instance to assess the behavior of the connection before and after a mechanical stress mimicking the mastication strengths.
Gas permeability appears to be a new, simple, quantitative, reproducible and practical in vitro technique to assess dental implant-abutment leakage.
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