The capability of hydroxylapatite plasma sprayed coatings to induce osteogenesis and then a faster primary stabilisation, is often compromised by the strength that those kind of coatings present.
The detachment from the implant surface may eventually compromise the implant stability and furthermore may lead to localised corrosion phenomena able to induce allergic reaction.
The evaluation of the fatigue life of such types of coatings has been always difficult because of the lack of systems able to reproduce at least partially, the biofunctional condition of employment, and to the lack of a reliable technique of evaluation.
An original test system has been realised in order to reproduce the mechanical stress induce by the bone tissue on the threads of the implant. The implant was mounted in a two-component acrylic resin having mechanical characteristics similar to those of the cortical bone.
AE technique was applied to continuously monitor the implant/coating/resin system during fatigue tests (smax = 300 N; smin = 0 N; = 1 hertz; 106 cycles).
Amplitude distribution analysis was used to determine the quality of the implant/coating interface.
None of the implants tested showed a fatigue limit beyond the 106 cycles.
The AE techniques allowed the monitoring of the coatings failure and confirmed the concerns about the real efficacy of this type of surface treatments.
Keywords - Acoustic Emission, dental implants, plasma spray coatings, hydroxylapatite, adhesion.
Endosseous dental implants are available with various surface characteristics ranging from relatively smooth machined surfaces to more roughened surfaces created by coatings, blasting by various substances, by acid treatments, or by combinations of the treatments. The main goal of such kind of processes is to realise a more efficient osteointegration between implant and bone tissue. Studies characterising these implants and surfaces include in vitro experimentation, animal studies, and human clinical trials. Both descriptive and functional testing of the bone-implant interface includes histomorphometrics and biomechanical testing such as torque removal values and push out/pull out strength.
There are various ceramic coatings available for dental implants. From a commercial standpoint, plasma-sprayed hydroxylapatite (HA) is the most popular. These coatings are typically partially amorphous after processing and contain crystalline phases other than HA. Plasma-sprayed HA and the other bioactive ceramic coating materials have been shown to enhance bone apposition as compared with uncoated metal implants. Some of the other available materials include the bioglasses, other calcium phosphates such as fluorapatite and tricalcium phosphate, and the inert ceramics such as alumina.
HA has a composition that is similar to the mineral content of the bone and is both biocompatible and osteo-conductive. In contact with the biological milieu, HA is resorbed and is substituted by the newly formed bone that anchors itself to the metallic substrate. This should guarantee secondary stability.
The velocity of reabsorption varies according to the grade of crystallinity and the chemical composition of the HA. However, the surface chemistry and topography of lower crystallinity might be favourable to cell attachment, but elevated medium pH might result in a cytotoxic effect that inhibits the proliferation of attached cells on coating surfaces.
Microbiologically, the HA-coated implant surface may be more susceptible to the formation of bacterial plaque. Additionally, critical variations exist between implant coatings that may affect long-term survival.
Implants with coatings of HA have been extensively studied by biologist with great emphasis being placed on the HA/bone interface, and there now exists much clinical experience. However, there is a dearth of information on the mechanical and structural aspects of the HA/metallic substrate bond.
Particularly critical seems to be the premature detachment of the coatings that may lead to an enhanced release of ions in the biological milieu and to long-term failure of the implant.
The evaluation of the fatigue life of HA coatings has been always difficult because of the lack of systems able to reproduce at least partially, the biofunctional condition of employment, and to the lack of a reliable technique of evaluation.
Implants are normally subjected to forces that are not axial. This may depend upon diverse factors like position of the implant in the jaw, biomechanics of oral function, parafunctional habits etc. In order to simulate as close as possible the in service condition and the effect of the surrounding bone tissue on the implants and thus on the coating, an original implant holder was realised.
The combination of Acoustic Emission (AE) during loading[3-7] and ad hoc mechanical testing makes possible to monitor cracks initiation and propagation at the metal/HA interface.
The main goal of the present work was to realise a reliable mechanical test able to reproduce the condition of stress to which coated implants are subjected and to collect information on the fatigue life of the coating itself.
Materials and Methods
Commercially available threaded titanium implants (fixture made of Ti grade 4, diameter 4.0 mm and 14 mm in length) were treated to obtain different surface finishes. The various treatments are summarised in Table 1.
Plasma spray technique, both in air (APS) and vacuum (VPS), was employed to obtain coatings with different characteristics. The types of deposits prepared are reported in Table 2.
The thickness of the deposits was generally within the interval 120-200 mm.
The crystallinity of the different deposits was evaluated through X-rays analysis, integrating in the interval 30° < 2 q < 35° the area of the peaks and then normalising such values with respect to the total area subtended to the spectrum[8,9].
- sand blasting
- chemical etching
|corundum 100 mm; P=4 bar HCl 15%, 3 weeks, 20°C Ti; thickness achieved 50-80 mm
|Table 1 : |
The coated screw of the implant was mounted in a two-component acrylic resin (Acryfix-Struers). The volume of the cylinder obtained was 10 mm of diameter and 30 mm in length (average dimension of the theoretical bone cylinder that surrounds an endosseous implant inserted in the jaw). The acrylic resin was chosen with mechanical characteristics similar to those of the cortical bone (E=14 GPa, n = 0.3).
powder; crystallinity 93%; grain size 60 and 30 mm
HA and pure Ti powder (grain size 35-40 mm) simultaneously
The embedded fixture was then placed in a cylindrical stainless steel holder, consisting of two separated parts (in order to better remove the tested implant) in which an inclined hole was realised. The implant was thus inclined respect to the vertical axis by 15°. The axial force was applied directly to the abutment that was fixed, through its internal screw, to the mounted part of the implant (Figure 1).
Fatigue tests were performed by applying tensile stress in the range of the masticatory loads (300 N). The number of cycles came out from the consideration that normally a person makes 3000 masticatory cycles per day. The fatigue test parameters are reported in Table 3.
The tests were monitored with Vallen AMSY-4 system. The system set-up is also summarised in Table 3. The threshold was maintained relatively low because of the attenuation of the signal between the coated implant and the transducers. The sensors were placed on the sample holder that worked as a waveguide. Expected cut on amplitudes was on the order of 15 dB.
|Fatigue test parameters
|150 kHz resonance frequency
|n = 1 hertz
|No. cycles = 106 cycles
|40 dB fixed gain
|Table 3: |
Results and discussion
The analysis of the AE data acquired during fatigue tests allowed some important consideration.
First of all, it resulted that none of the coatings last longer than 106 cycles. Table 4 resumes the results achieved by means of AE monitoring. The cycles to failure reported are the mean values obtained from the tests performed.
The sharp increase in the acoustic activity (Figure 2) indicates the rapid propagation of the crack at some point in the coatings or at the interface with the metal implant
Figure 2 represents the cumulative counts rate for a HA coating on HCl etched substrate with a crystallinity of 70% (VPS technique). The time t=0 corresponds at the cycle no. 887.648. It can be observed how the elastic energy release takes place in a very short interval of time.
||Cycles to failure
|Table 4 :|
Fig 2: Acoustic activity for VPS/HA/sb/70
The amplitude analysis of the signal involved in this phase shows values much higher than in any other period of the fatigue test (Figure 3) and with a peek greater than 65 dB. The corresponding number of hits is also significantly greater.
A 3D diagram, like the one reported in Figure 4, allow following the progress of the tests during the whole million cycles. In the case reported in the figure (VPS/HCl/HA) it can be seen how the most part of the AE activity is concentrated in the last section of the fatigue test and in the highest part of the loading cycle.
The behaviour of the most part of the coatings is very similar, even though some significant differences do exist in the fatigue life (see Table 4).
As a general rule, the coatings resist until a critical crack propagates in a catastrophic way. The detachment takes place at the coating/implant interface.
In the case of the HA coating deposited on a precoat of Ti the behaviour is completely different and can be attributed to a coalescence of microcracks that leads to a complete detachment. The distribution amplitudes involved at this stage is represented in Figure 5. Note how the peak is shifted versus low amplitudes (lower than 55 dB). Cumulative counts and energy distribution is also diverse and takes place less suddenly. It is reasonable hypothesise cracks formation and propagation at the titanium precoat/HA interface that precede the catastrophic failure of the HA coating.
From the analysis of the number of cycles to failure results that the coatings obtained through VPS by adding Ti powder to HA do not benefit of any improvement. VPS coatings on Ti precoat show a low number of cycles to failure (microcracks formation). The coatings obtained through APS do not show a good behaviour.
The etching of the surface with HCl seems to have some beneficial effect.
Anyway, none of the coatings resist above 106 cycles.
The AE analysis allows following the progression of the damage of the coating during the whole cycles of fatigue. It was possible also to establish the exact fatigue life of each coating with a good degree of approximation.
Material failure mechanisms can be related to AE events in term of their intensities and variations of intensities with fatigue cycle and stress level.
AE confirms to be a powerful technique able to give complementary information on the behaviour of materials.
Once again it results confirmed that the quality of the coatings is a critical factor that can affect the success of a dental implant and therefore need to be evaluated carefully.
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