Wear investigation of implant-supported upper removable prothesis with electroplated gold or PEKK secondary crowns

Introduction

In the course of the aging population, the prevalence of periodontal disease and therefore tooth lost is continuously rising [1]. On the other hand, the wish to improve the masticatory function and aesthetic appearance is becoming more and more relevant in society. “Replacing the missing teeth using dental implants is a good treatment option with a high degree of success. As the dental implantology field develops and the number of implants placed worldwide increases” [2], there are various treatment options consisting of individual gaps up to the edentulous jaw. The “all-on-six” concept represents an alternative treatment option to the full denture for the upper jaw, whereby a removable, palate-free restoration is incorporated on six implants. This prothesis allows the patient a high level of stability during mastication, a good hold of the prosthesis and a natural feeling for the tongue due to the free palate area.

A telescopic prosthesis with a double crown system has proven to be a good treatment concept for elderly patients over decades [3], [4], [5], “because of their merits of incorporating proper distribution of force on the abutment, allowing for effective oral hygiene with the maintenance of periodontal health, achievement of good aesthetics and good survival rate” [6]. The prothesis covers partially and is supported by natural tooth and/or implants as anchoring elements. The primary crown is attached to the tooth or implant and the secondary crown to its prosthesis [7]. Traditionally high-gold alloys are used for primary and secondary crowns. Because of the steeply increased prices, non-precious alloys replaced the gold standard. This material requires precise manufacturing procedures to achieve an optimal fit between the contact surfaces of the primary and secondary crowns, which is responsible for a secure hold of the prothesis.

Due to the ever more accurate CAD/CAM technique, new materials gain relevance such as the methacrylate-free thermoplastic high-performance polymer (PEKK). The structure of PEKK has an aromatic ring with one ether and two ketone groups. It shows excellent biocompatibility, low bacterial adhesion, fracture resistance, shock-absorbing, low elastic modulus, good stress distribution, and wear resistance [8].

Wear is the progressive loss of material from the surface of a solid body, caused by mechanical causes of an opposing body. “Friction and wear occur due to relative motion between two contacting bodies. The main wear mechanisms are adhesion, abrasion, surface distress and tribochemical reaction” [9].

There are only few studies about PEKK as secondary crowns in a double crown system [10], 11]. There is one case report investigating more than one anchor [12]. This in vitro study investigates the wear of primary crowns of implant-supported prothesis using secondary crowns made from PEKK and gold.

This study aimed to clarify, whether PEKK secondary crowns in an implant-supported telescopic prosthesis are a good and economically priced alternative to the gold galvanic secondary crowns?

Materials and methods

A total of 20 implant-supported upper jaw protheses were constructed; 10 protheses with gold secondary crowns and 10 protheses with PEKK secondary crowns.

Regarding the “all-on-six” concept, each specimen was based on six implants (3.7 × 13 mm, TwinFit, Dentaurum Implants GmbH, Ispringen, Germany) with individual abutments. The framework and the abutments were made of a non-precious metal alloy (CoCr remanium star MD I/MD II, Dentaurum gmbH, Ispringen, Germany). All 120 abutments were finished and polished per hand.

Ten specimens contained 24 k gold secondary crowns manufactured by an electroplating process and the other 10 specimens contained PEKK (Pekkton^® ivory, Cendres+ Métaux SA, Biel/Bienne, Schweiz) secondary crowns produced by the CAD-CAM technology as a high-performance polymer (Figure 1).

Figure 1:

Experiment set-up. (a) Gold secondary crowns and (b) PEKK secondary crowns.

A universal base with six implants was used for all 20 samples: four implants had an angulation of an 20°-angle in oro-vestibular direction (position 13, 23, 25, 27); two implants were positioned in 90° to the base (position 17, 15) to approximate a clinical situation. The screwed abutments in 10 specimens were conical (with platform switching) and in the other 10 specimens, the abutments were plane (without platform switching, Table 1).

In the set-up, the six screwed abutments (with/without platform switching) equalise the deviation and line up to the pull-out direction of the wear simulator. The abutment height was 6 mm with 1° taper. The framework included an interconnector to attach the tertiary structure with the second part of the wear simulator rigidly. To generate a non-prestressed connection between the base and the framework, the secondary crowns (gold/PEKK) were lined (AGC^® Cem Automix System C. Hafner, Wimsheim, Germany) into the framework without tension and harden without movement.

In the first part of this in vitro study, each sample ran 10,000 cycles of insertion and separation in the wear simulator with the speed of V_max=2.27 mm/s for a 2–3 mm distance. The movement of the wear simulator was operated with a precision linear slide including a ball screw. It stopped at the predetermined maximum force (20 N). The separation process took place by reversing the direction of the rotation drive. An artificial saliva substitute (Glandosane^® neutral, Cell Pharm GmbH, Bad Vilbel, Germany) was used as a lubricant diluted with distilled water (2:1) during the whole process for a realistic influence to the wear in the oral cavity. Each abutment is moistened with a cannula (a total of six), which was affiliated to a pump system. Every 20 cycles fresh lubricant drips onto the abutment, rinses and moisturizes it with the saliva mixture. It should generate a ten-year use, if the patient removes the prothesis three times a day for cleaning [13] (Figure 2a). The progress of the wear process was recorded with the DASYLab12 software (National Instruments, Austin, Texas, USA) to calculate the needed forces to connect and disconnect the prosthesis from the primary crowns.

Figure 2:

Experiment set-up. (a) Wear simulator with full set up, (b) thermocycling, (c) wear simulator after thermocycling, and (d) scanning electron microscope.

The program controlled the direction of movement and the speed of the linear slide. The power and distance measurement were saved continuously with its cycle-number. Important was the highest value of the pull-off force.

To compare the impact of artificial aging of PEKK to gold, the study included thermocycling (Figure 2b). Each specimen with gold or PEKK secondary crowns ran through 10,000 cycles in 5 °C and 55 °C distilled water. The sample kept in a hot water bath for 60 s, before changing after 15 s in the air, to the next bath with cold water and vice versa. This supposed to simulate the effect of thermal stress to a material as described in several studies [14]. There is no documented standard temperature in literature, however a range of 0°–67° was recommended [15].

For the third part of the study, after thermocycling, the samples ran additional 10,000 cycles in the wear simulator as described above to investigate the influence on the artificial aging for both materials to the wear (Figure 2c).

Finally, the surface of the abutments (primary crowns) was investigated with the scanning electron microscope (Philips XL 30 ESEM, Philips, The Netherlands), to study the wear behaviour in relation to the material of the secondary crowns (Figure 2d). The samples were disinfected with alcohol and examined with the microscope to show optical signs of wear lines in quantity and quality with magnifications of 15×, 100×, 500×, 1,000× and 1,500× from both groups. An unused abutment was used as a reference.

Results

Over the whole study, the 20 specimens completed the double 10,000 cycles of insertion and separation and thermocycling with no screw loosening or bonding problems.

The mean pull-out force of PEKK initially increased from 14 to 22 N during the first 1,000 cycles and continued with stable values over the next 9,000 cycles (21 N).

The gold-specimens had no clear course and the values were varied between 15 and 19 N over the whole 10,000 cycles.

After thermocycling and reincorporation, a slight increase of pull-out force was observed for both materials in the first hundred cycles of 20–22 N (PEKK) and 18–20 N (gold). The force of PEKK-specimens fell back to their 21 N as shown before thermocycling. For the specimens with gold coping, the values increased between cycles 1,000 to 6,000 between 19 and 23 N. The mean values decreased for the last 4,000 cycles down to 15 N (Figure 3).

Figure 3:

Overview of the mean pull-out forces before (a) and after thermocycling (b).

All specimens with PEKK secondary crowns had a cleaner course than those with gold secondary crowns. Those had a lot value fluctuation in short periods of time.

The retention force changed with every reattachment of the base to the framework in the wear machine. There was no noticeable wear observed for PEKK considering the force. Only the gold secondary crowns showed a slight retention loss about 4 to 8 N during the last 4,000 of 20,000 cycles.

The investigation of the surfaces of the primary crowns with a scanning electron microscope showed wear lines in pull-out direction on the vestibular surfaces of the abutments, no matter which materials were used. Each sample individually showed the greatest wear at different abutment positions, depending on it owns exact insertion direction. The wear was concentrated at the region of 17 or 27. Only the reference sample had no surface damages.

It can be distinguished between gold and PEKK by the quality and quantity of the wear lines on the surfaces. The gold-related abutments had more and deeper wearing lines on their surfaces, which can be seen on the overview picture with the 15 × magnification and the detailed picture with 500× (Figure 4).

Figure 4:

Wear behaviour of the primary crowns with a magnification of 15× (left) and of 500× (right): (a) Original abutment as reference, (b) primary crown with gold secondary crowns at the 27 region, (c) primary crown with gold secondary crowns at the 25 region, (d) primary crown with PEKK secondary crowns at the 27 region, and (e) primary crown with PEKK secondary crowns at the 13 region.

Discussion

The aim of the in vitro study was to investigate the different wear behaviour of PEKK secondary crowns in comparison to the gold standard.

For patient satisfaction, good prosthesis retention is the primary importance [16]. Over the time wear and loss of retention is a major problem due to tribological processes affecting the surface structure. Tribochemic reaction, abrasion, adhesion and disruption surface cause massive loss of force [17]. The major changes occurred during the first 2,000 cycles, which can be considered as the adjustment phase, which correspond to an approximate wearing time of 2 years [18].

Several studies investigated an acceptable retention force for patient comfort and no damage of the underlying structures such as teeth or implants. Kamel summarised that an “appropriate value is in the range of 2.5–10.0 N, but most studies have higher values” [6]. Pigozzo [19] observed retention forces with a mean values of 5–7 N, Stančić & Jelenković [20] 5–9 N, Weigl [21] 5.03 N, and Bayer [13] 5.5 N.

Stock [22] has reported retention forces between 9.6 and 38.2 N, which are above the recommended values. Most studies refer to experimentally simplified single stumps, which cannot interpret directly as a tendency concerning prothesis pull-out forces and therefore the clinical relevance [4]. In this study, six abutments for one framework were investigated with a mean pull-out force of 21 N (PEKK) and 19 N (gold), which is hardly find in the literature. Elkabbany examined the retention force of four implant associated PEEK- abutments and had values between 10.0 and 33.3 N [23].

Limitation of the study was, that only joining and separation force was measured, but not additional chewing loads. There is no Iso-normed set-up for wear processes, which makes it difficult to compare the studies [17]. Nonetheless many parameters affecting the retention force of double crowns are investigated in a lot of studies. In vitro parameters are abutment height, design, internal fit, manufacturing, inner- and outer crown material and test conditions. Some of the parameters have more important relevance than others. Clinical factors such as number, mobility, location and height are more important [16]. In this study design, six implants were evenly distributed in a parabola to simulate the tooth position of 17, 15, 13, 23, 25 and 27. That means that in the “all-on-six” concept a palate-free prosthesis can be used with homogenous force distribution without wobbling. However, it would also be conceivable that this would increase the retention force. Implants always have a rigid connection from the bone to the abutment. For this reason, a secondary crown is needed to achieve a passive fit and avoid mechanical stress on the implants. In this study a spacer as a secondary crown made of gold or PEKK is used to achieve the passive fit while cementation. The abutment height was 6 mm, which is in the range of most studies.

The retention increases with increasing the abutment height [24] and with decreasing the taper. In this study a 1° taper was used and is described necessary to obtain sufficient retention force [6]. The taper displayed higher influence on the retention force that the abutment height [25]. Wagner even says 1° taper had higher retention values than 0° [26]. This is another reason explaining the high retention values in this study considering six connecting abutments were used.

Both materials, gold and PEKK, have a precise manufacturing technique leading to a perfect fit and therefore a physical effect “creating a vacuum by separating flow resistance of the saliva” in capillary vessels [17]. That leads to a hydraulic impact, adhesion, and not to friction, when a lubricant is used [27]. In this case an artificial saliva with water was used over the entire process and every abutment was wetted individually every 20 cycles. This created the favourable tribological property, adhesion, because it increases retention with less wear than conus friction [16]. This internal fit is achieved by electroplating the gold as a second skin over the primary crown with silver lacquer spray paint as a chemical conductor. This only works for precious metals and the costs are correspondingly high.

An alternative is the high-performance polymer PEKK and the CAD/CAM technique. This technology allows “similar predictable results to be achieved as the gold standard and is suitable for clinical practice” [28]. Reciprocation is another major advantage to this technique, which allows an easy replacement of the secondary crown when it is damaged [29]. The file of the original design can be used again for new manufacturing. This is a relief for the patient, the dental laboratory and finally the repair costs can be reduced. More advantages of this technique are controlling the design features, reduce the work time, minimizing technical errors and sensitivity [6]. An optimal surface morphology can be achieved, which leads to a narrower joint gap and therefore a better fit with better adhesion and therefore a higher retention force [29]. A smooth surface with a minimal gap difference has a better adaption and stable retention values in the long term [30]. CAD/CAM technology has, in vertical marginal and the axial internal space, an accuracy of 20–80 µm, which allows the adequate fit and a marginal integrity of the secondary crown [31]. PEKK as a material is convincing with an accuracy of 51 µm [32].

Compared to the gold standard, PEKK has a lower specific gravity and functions as a shock absorber [33]. It also has a higher resistance to plastic deformation, which is visible in the curve course of the graphs of separation force values of PEKK. It has a homogenous trend and stable values around 21 N. Whereas the gold secondary crowns graphs show many deviations with unstable measurement values, because of tilting. There are also micro-pits seen in the SEM pictures, which can interlock micromechanically during the trail run and lead to unstable values. Due to its low hardness, gold can be easily be deformed. PEKK has a stable friction coefficient behaviour with “no sudden changes and has an impressive wear resistance” [34]. There is only a little wear found in the SEM pictures. A few wear marks as typical running-in line can be seen. Pedroso also made the discovery that the “friction increased steeply during the running-in phase” and is even higher compared to its related material from the PEAK-family, because of the higher filler gradient [34]. An increasing retention force is noticeable in the first 1,000 cycles of the PEKK specimens. However, it should be treated with caution as there are also inaccuracies between cementation and reinserting the framework back into the device, which can be seen after thermocycling. That phenomena does not happening in vivo, because the patient inserts the prothesis differently every time. But the experimental set-up is rigid and the movements are always repeated in the same way. An adaption of the interfaces until a critical limit of plastic deformation takes place and is also depending on the surface wear patterns [27].

The compressive strength of PEKK is similar to tooth dentine. It has lower costs, better stability and retention force than precious metals. The space between the primary and secondary crown can be influenced by the milling strategy [33]. A spacer can minimize the friction and thus reduce the possibility of transmitting extra force to the supporting structure or implant [6]. Maybe more spacer between the abutment and the secondary crown made from PEKK would be advisable for 6 anchoring elements to reduce the possible damage on the implant and have acceptable retention force values as recommended in the literature. Ultimately, it must also be made clear that 6 attachments have a higher retention force than a single stump. The force increases with each anchor. Another study, which is mentioned before, had retention forces between 10.0 and 33.3 N for 4 abutments and therefore my test results are exactly within this limitation [23].

Conclusions

PEKK secondary crowns have no disadvantage to gold in their retention force, gold secondary crowns even loose retention force over the last 4,000 cycles. Both materials show an increase of force after thermocycling, which is founded in the reincorporation of the samples into the wear machine. The PEEK samples have a smooth course run without deviations and show less optical traces of wear under the microscope on the related abutment surfaces.

The CAD/CAM technique is easy for manufacturing secondary crowns and has a good accuracy. PEKK is cheaper than gold and the secondary crowns can be replaced at any time, when damaged.

The retention force of gold and PEKK is higher than most studies with single stamps, but with 6 attachments the retention force should also increase.