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ORIGINAL ARTICLE
Year : 2020  |  Volume : 10  |  Issue : 2  |  Page : 78-83

Retention force of all-zirconia, all-polyetheretherketone, and zirconia-polyetheretherketone telescopic attachments for implant-retained overdentures: In vitro comparative study


Department of Prosthodontics, Faculty of Dentistry, Mansoura University, Mansoura, Egypt

Date of Submission11-Apr-2020
Date of Acceptance12-Oct-2020
Date of Web Publication18-Dec-2020

Correspondence Address:
Prof. Radwa Mohsen Kamal Emera
Faculty of Dentistry, Mansoura University, El Gomhoria Street, Mansoura
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jdi.jdi_4_20

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   Abstract 

Aim: The aim was to evaluate and compare retention forces of all-zirconia (ZrO2), all-polyetheretherketone (PEEK), and ZrO2-PEEK Computer-Aided-Designing computer-Aided -Manufacturing (CAD-CAM) telescopic attachments for two implant-retained mandibular overdentures.
Materials and Methods: Fifteen identical acrylic resin models of the edentulous mandibular arch without alveolar undercuts were fabricated. Two implants were inserted in the canine region of each model. According to the material of telescopic attachment, models were divided into three groups: all ZrO2 (ZZ): primary and secondary crowns were made of ZrO2, all PEEK (PP): primary and secondary crowns were made of PEEK, and ZrO2-PEEK (ZP): primary crowns were made of ZrO2 and secondary ones were made of PEEK. Identical experimental overdentures were constructed for all models. For measuring the retention force, a universal testing machine was used to apply a pull-off test in presence of artificial saliva between the crowns. The achieved maximum values of retention force were recorded at the beginning of the study (initial retention) and after 540 cycles of insertion and removal simulating 6 months of the clinical service (final retention). Results were statistically analyzed, analysis of variance (P < 0.05).
Results: ZP group reported the highest initial force values for all groups. The final retention values of PP and ZP groups were significantly decreased than initial values, while the insignificant loss of retention was observed with ZZ group (P = 0.06). Independent samples test showed a significant difference in both initial and final retentions between each two groups.
Conclusion: Within the limitations of this study, ZrO2 is more preferred as a secondary telescopic crown against a primary ZrO2 one. Despite the loss of retention by time, PEEK can still be used as a secondary telescopic crown against ZrO2 or PEEK primary crowns regarding the acceptable initial and final retention values.

Keywords: Overdenture, polyetheretherketone, retention force, telescopic attachment, zirconia


How to cite this article:
Kamal Emera RM, Elgamal M, Altonbary G. Retention force of all-zirconia, all-polyetheretherketone, and zirconia-polyetheretherketone telescopic attachments for implant-retained overdentures: In vitro comparative study. J Dent Implant 2020;10:78-83

How to cite this URL:
Kamal Emera RM, Elgamal M, Altonbary G. Retention force of all-zirconia, all-polyetheretherketone, and zirconia-polyetheretherketone telescopic attachments for implant-retained overdentures: In vitro comparative study. J Dent Implant [serial online] 2020 [cited 2021 Jan 28];10:78-83. Available from: https://www.jdionline.org/text.asp?2020/10/2/78/303921


   Introduction Top


Several problems were reported by complete denture wearers such as lack of stability and retention of these dentures, decreased efficiency of chewing, and pain during mastication. These problems can be successfully overcomed using dental implants to fabricate the so-called implant-assisted overdentures.[1]

Since 1989, resilient telescopic attachments have been used to retain overdentures in the rehabilitation of the edentulous mandible. In case of severely atrophied edentulous mandible, two interforaminal implants with resilient telescopic attachments for overdenture retention appeared to be an effective and efficient treatment option with long-term success. Chiefly in geriatric patients, this concept may offer advantages concerning the handling, cleaning, and long-term satisfaction.[2]

Using ceramic materials for the fabrication of telescopic attachments was first described in 2000.[3] Zirconia (ZrO2) is a ceramic material that is widely used in recent years for medical devices, showing high biocompatibility, excellent mechanical strength, and wear resistance. The use of tooth-colored ceramic materials also is reported to have a positive psychological outcome on patients and promotes improvements in oral hygiene.[3]

Polyetheretherketone (PEEK) was developed by a modification of the main thermoplastic high-performance polymer group poly-ether-aryl-ketone. It is a thermoplastic polymer, composed of molecular chain from an aromatic backbone that is connected to each other by ketone and ether function group.[4] PEEK as well as ZrO2 exhibits high biocompatibility and can be applied for several dental uses, e.g., for provisional abutments, dental implants, and fixed bridges (FDP).[5] It was reported that PEEK is a suitable material for the double crown system.[6] The combination of these two biocompatible materials, PEEK and ZrO2, is a novel idea that goals to produce nonmetallic telescopic crowns.

This in vitro study aimed to investigate and compare the retention force of different CAD/CAM telescopic attachments (all ZrO2, all PEEK, and ZrO2-PEEK telescopic attachments) retaining mandibular implant overdenture.


   Materials and Methods Top


Fifteen acrylic resin models of edentulous mandible without alveolar undercuts were used for this study. Each model was covered by 2 mm layer of silicon soft liner material to simulate alveolar mucosa according to El-Charkawi et al.[7] Two implants with diameter 4.5 mm and length 10 mm (dentium implants) were installed in the canine region of each model using a guide template,[8] then two dual abutments with diameter 4.5, length 4.5, and gingival height 1.5 were screwed into the implants [Figure 1]. The resin models were duplicated for overdenture construction.
Figure 1: Two dual abutments screwed into the parallel implants installed in the acrylic resin model

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Models were divided into three groups; group ZZ (primary and secondary CAD/CAM crowns were of ZrO2), group PP (primary and secondary crowns were of PEEK), and group ZP (primary crowns were of ZrO2 and secondary crowns were of PEEK).

Designing and fabrication of telescopic attachments

Primary resilient telescopic crown construction

Each model was sprayed with a thin layer of scan spray (Shera Scan Spray) to avoid reflection then scanned (SHERA Eco-Scan 3) for designing a resilient telescopic attachment using the CAD/CAM technology. The following parameters were maintained for all groups; 5 mm height (2 mm gingival height was paralleled and the occlusal 3 mm was tapered 4°). The primary crowns were designed ensuring a common path of insertion [Figure 2]. The computer numeric control data were saved as STL files. The primary ZrO2 crowns were milled from semi-sintered ZrO2 blanks (ZrO2, Kataya) and the primary PEEK crowns were milled from BioHPP blanks (Bredent, UK). The primary crowns were cemented on an abutment using zinc phosphate cement.
Figure 2: The primary crowns were designed ensuring a common path of insertion

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Secondary resilient telescopic crown construction

Each model was scanned, followed by separate scans of each primary crown to improve the quality of data. The following parameters were applied for designing secondary crowns; parallel walls with a minimal wall thickness of 0.5 mm and occlusal space (0.3 mm) built in between the primary and secondary crowns [Figure 3].[2] Mechanical projections were added to the design of each secondary crown, to help in the mechanical interlocking of secondary copings to the denture base [Figure 4]. The designing data were saved and the secondary ZrO2 and PEEK crowns were milled.
Figure 3: Three-dimensional designing of secondary crowns with parallel walls and minimal wall thickness of 0.5 mm

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Figure 4: Mechanical projections were added to the design of each secondary crown, to help in the mechanical interlocking of secondary copings to the denture base

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Pick up of secondary crown procedures

The fitting surface was modified by making vent holes through the lingual flanges of each overdenture. Secondary crowns were fitted over the primary ones in the correct path of insertion and then picked up to the fitting surface of the overdenture using an autopolymerized acrylic resin. The excess resin material through the venting holes was removed using a diamond bur.

Retention force measurements

A metallic cobalt chrome bar (2 mm, 15 mm, and 120 mm), with a grasping hook in the middle, was hovered by autopolymerized acrylic resin in the area of the second bicuspid and the first molar of each overdenture to facilitate the pull-off test[9] [Figure 5]. For measuring the retention force, models were placed and fixed in a universal testing machine (LLOYD instruments) [Figure 6] then a pull-off test was performed with a crosshead speed of 50 mm/min and a load cell of 3.5 KN. The achieved maximum values of retention force were recorded at the beginning of the study (initial retention). Overdenture with each combination of primary and secondary telescopic crowns was subjected to 540 cycles of insertion and removal representing 6 months of clinical service under the same conditions, i.e., moistening of the primary crown with artificial saliva (Glandosane, No. 9235461109, cell pharm, Bad Vilbel, Germany). Then, the pull-off test was performed again to evaluate the final retention.
Figure 5: A metallic cobalt chrome bar was hovered by autopolymerized acrylic resin in the area of the second bicuspid and the first molar of each overdenture to enhance the overdenture pull-off

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Figure 6: Model was placed and fixed in a universal testing machine and pull-off tests were performed with a crosshead speed 50 mm/min with a load cell of 3.5 KN

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Statistical analysis

Data were tabulated, coded, and then analyzed using the computer program Statistical Package for the Social Sciences version 23 (SPSS Inc., Chicago, Illinois, USA). Data were normally distributed as detected by the Shapiro–Wilk test. Descriptive statistics were represented in the form of mean ± standard deviation. Paired sample test was used to compare means of the initial and final retention values within each group. Independent samples test was used to compare means of retention values between each two groups. One-way analysis of variance was used to compare retention value means of three different groups, followed by the post hoc Tukey test. P < 0.05 was considered as statistically significant.


   Results Top


The mean values of telescopic attachment initial and final retention values for all groups are presented in [Table 1]; the highest mean initial retention value for all groups was observed in the ZP group where the mean initial retentive force was 21.3480 N and the final one was 16.1200 N. However, mean initial retentive forces of ZZ and PP groups were 14.6760 N and 15.3600 N respectively and their final retentive forces were 14.6360 N and 14.0800 N respectively.
Table 1: Comparison between initial and final retention values of telescopic attachment within each group I

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Comparison of mean values between initial and final retention values within each group using paired t-test is presented in [Table 1] showed that the final retention value for the PP and ZP was significantly decreased by time (P = 0.0001 and 0.0000 respectively), but no significant loss in retention force value was observed for the ZZ group where P = 0.06.

Independent samples test showed a significant difference in both initial and final retentions between each two groups as shown in [Table 2].
Table 2: Comparison of initial and final retention values between all groups

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   Discussion Top


One of the greatest challenges facing clinicians is to provide the prosthesis with sufficient retention. For maintaining patient satisfaction and clinical performance of the prosthesis, it is essential to maintain retention of the attachment system.[3] It was shown by previous studies that the double crown attachment retention force depends on precision fit, height of abutment, material of fabrication, and occlusal convergence.[10],[11],[12],[13] Therefore, the material and design will affect the selection of attachment type that guarantees a long term of function.[14],[15]

Different CAD/CAM materials may behave in a different manner when placed in function opposing one another.[16] The performance of double crown attachment made completely of ZrO2 or PEEK and that made of the combinations of these two materials is not clear. Consequently, this in vitro study was done to evaluate and compare these attachment retention values before and after simulation of clinical use. Evaluation of retention was done initially (initial retention) and after 540 cycles of insertion and removal representing 6 months of clinical use (final retention).

It was concluded that the accepted retention force of different attachment systems for implant-retained overdenture ranged between 5 N and 8 N throughout the long-term function.[17] Concerning the results of this study, it was found that the lowest measured initial retention force was 14.676 N for ZZ group. This value is higher than the lowest retention values to achieve sufficient patient satisfaction as reported in the literature for mandibular overdentures.[18],[19] The minimum value for retention force that was measured after simulation of 6 months of overdenture use was 14.08 N for PP group which is also considered a satisfactory retention value.

The highest mean initial retention values were observed in the ZP group where the mean initial retentive force was 21.3480 N. This result is in agreement with the in vitro study of Schubert et al.;[19] they found that CAD-CAM fabricated PEEK secondary crowns are able to provide stable and sufficient retention force values and could be considered appropriate and efficient to replace the electroformed secondary crowns.

This result was also documented by a recent clinical study of Emera et al.,[20] who evaluated the retention force of ZrO2-PEEK telescopic attachment for two implants retained complete mandibular overdenture. They concluded that in spite of the significant decrease of retention force after 1 year of overdenture use, PEEK may be considered an appropriate material for the fabrication of secondary telescopic crowns against ZrO2 primary ones concerning the acceptable initial and preserved retention force values.

Final retention values of PP and ZP groups were significantly decreased than initial values, while insignificant loss of retention was observed with ZZ group; this may be owing to wear and surface topography changes of the primary and secondary crowns opposing surfaces. It was reported that the frictional wear takes place throughout the function of telescopic attachment, represents common problem concerning retention.[21],[22]

This result is consistent with that of Emera et al.,[23] who evaluated the alterations in surface topography of all ZrO2, all PEEK, and ZrO2-PEEK telescopic attachments after simulated 6 months of using the overdenture. They concluded that the combination of PEEK and ZrO2 for the fabrication of telescopic attachment was accompanied with greater changes in surface topography (mostly in secondary crowns) compared to all ZrO2 and all PEEK telescopic attachments. This finding was explained by the different physical properties of the two materials where PEEK has a low modulus of elasticity (4 GPa) compared to ZrO2 (210 GPa), so PEEK restorations absorb occlusal loads and undergo wear. In contrast, the ZZ group showed the least wear values, and consequently its recorded initial retention values were maintained.[24]

A significant difference between the initial and final retention between all groups was shown due to significant wear occurred in all groups after simulating 6 months of overdenture use where simulation was done in the axial direction only that leaded to selective wear of certain to surfaces of attachments.[25] In contrast to intraoral conditions where lateral forces affecting the retainer are to be predictable throughout the chewing process were not simulated and neglected which is considered as one of the limitations of this study.[26]


   Conclusion Top


Within the limitations of this study, ZrO2 is more preferred as a secondary telescopic crown against a primary ZrO2 one. Despite the loss of retention by time, PEEK can still be used as a secondary telescopic crown against ZrO2 or PPEK primary crowns regarding the acceptable initial and final retention values.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
   References Top

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Merk S, Wagner C, Stock V, Eichberger M, Schmidlin PR, Roos M, et al. Suitability of secondary PEEK telescopic crowns on zirconia primary crowns: The influence of fabrication method and taper. Materials 2016;9:908-17.  Back to cited text no. 5
    
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Stock V, Wagner C, Merk S, Roos M, Schmidlin PR, Eichberger M, et al. Retention force of differently fabricated telescopic PEEK crowns with different tapers. Dent Mater J 2016;35:594-600.  Back to cited text no. 6
    
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El Mekawy N, Khalifa A, Abdualgabbar E. The influence of palatal coverage on the retention force and fatigue resistance of mini dental implant maxillary overdenture. J Oral Hyg Health 2016;4:200.  Back to cited text no. 9
    
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Engels J, Schubert O, Güth JF, Hoffmann M, Jauernig C, Erdelt K, et al. Wear behavior of different double-crown systems. Clin Oral Investig 2013;17:503-10.  Back to cited text no. 11
    
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Wagner C, Stock V, Merk S, Schmidlin PR, Roos M, Eichberger M, et al. Retention load of telescopic crowns with different taper angles between cobalt-chromium and polyetheretherketone made with three different manufacturing processes examined by pull-off test. J Prosthodont 2018;27:162-8.  Back to cited text no. 13
    
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Emera RM, Abdel-Khalek EA, Rashed M. Periodic retention evaluation of two implants retained complete mandibular overdenture with zirconia-PEEK telescopic attachments. IOSR J Dent Med Sci 2019;18:15-24.  Back to cited text no. 20
    
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Majcher A, Leśniewska-Kochanek A, Mierzwińska-Nastalska E. A method and a device for the evaluation of the retention of telescopic dental crowns. J Mech Behav Biomed Mater 2017;69:362-7.  Back to cited text no. 22
    
23.
Emera R, Elgamal M, Albadwei M. Surface wear of all zicronia, all PEEK and zirconia-peek telescopic attachments for two implants retained mandibular complete overdentures. In -vitro study using scanning electron microscope. IOSR J Dent Med Sci 2019;18:59-68.  Back to cited text no. 23
    
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25.
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26.
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2]



 

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