Journal of Dental Implants

: 2019  |  Volume : 9  |  Issue : 1  |  Page : 24--29

Comparison between all zirconia, all PEEK, and zirconia-PEEK telescopic attachments for two implants retained mandibular complete overdentures: In vitro stress analysis study

Radwa M Emera1, Gilan Y Altonbary1, Samah A Elbashir2,  
1 Department of Removable Prosthodontics, Faculty of Dentistry, Mansoura University, Mansoura, Egypt
2 Department of Removable Prosthodontics, Faculty of Dentistry, Khartoum University, Khartoum, Sudan

Correspondence Address:
Radwa M Emera
Faculty of Dentistry, Mansoura University, El Gomhoria Street, Mansoura


Aim: This in vitro study was performed to evaluate and compare stresses applied to the implants retaining mandibular complete overdenture with telescopic attachments of different materials. Materials and Methods: Three identical clear acrylic resin models of completely edentulous mandibular arch were selected for this study. One implant was installed in each canine region of each model. The models were scanned to design telescopic attachment using computer-aided design/computer-aided manufacturing technology. According to material used for telescopic attachments fabrication, models were categorized as follows: All Zirconia (ZrO2) telescopic group (zz): Primary and secondary crowns were made of ZrO2, all polyetheretherketone (PEEK) telescopic group (pp): Primary and secondary crowns were made of PEEK, and Zirconia-PEEK telescopic group (zp): Primary crown was made of ZrO2, and secondary one was made of PEEK. Three identical mandibular complete overdentures were constructed. The secondary telescopic crowns were picked up within the intaglio surface of each overdenture. Four strain gauges were installed parallel to each implant. Bilateral and unilateral loads were applied, and strain values were recorded. Results: Comparison of recorded stresses values revealed a significant difference between groups where the highest stresses were recorded with all-PEEK group, followed by all-zircon group and the lowest stresses were noted with Zirconia-PEEK group for both bilateral and unilateral loading tests. Conclusion: Within the limitation of this in vitro study, it could be concluded that: telescopic attachments that fabricated from ZrO2 primary crown and PEEK secondary crown is a promising attachment regarding reductions of stresses transmitted to the implants retaining mandibular complete overdenture.

How to cite this article:
Emera RM, Altonbary GY, Elbashir SA. Comparison between all zirconia, all PEEK, and zirconia-PEEK telescopic attachments for two implants retained mandibular complete overdentures: In vitro stress analysis study.J Dent Implant 2019;9:24-29

How to cite this URL:
Emera RM, Altonbary GY, Elbashir SA. Comparison between all zirconia, all PEEK, and zirconia-PEEK telescopic attachments for two implants retained mandibular complete overdentures: In vitro stress analysis study. J Dent Implant [serial online] 2019 [cited 2019 Jul 18 ];9:24-29
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The use of implant supported overdenture has improved outcomes for edentulous patients compared with conventional denture. These include reduced residual ridge resorption, improved retention, and support of the prostheses resulting in a better quality of life, function, chewing nutritional status, and general health.[1] Telescopic crowns were initially introduced as retainers for dental prostheses. They are also known as a double crowns, crown and sleeve coping. These crowns consist of inner or primary telescopic coping, permanently cemented to an abutment and congruent detachable outer or secondary telescopic crown, rigidly connected to a detachable prosthesis. The secondary crown engages the primary coping to form a telescopic unit and serves as an anchor for the remainder of the dentition. The telescopic attachment provided support, retention, and prosthesis can be repaired without reconstruction of the entire superstructure, despite a localized failure.[2]

In prosthetic dentistry, metal alloys are most commonly approved materials due to their excellent physicomechanical properties, but some drawbacks like biocompatibility might be problematic, especially in combination with other metals in the oral cavity. The direct contact of different metals in the oral cavity, as well as metallic ions solved in saliva, may cause galvanic corrosion.[3] Using ceramic materials for the fabrication of double-crown attachments were first described in 2000.[4] Zirconia (ZrO2) is a ceramic material used for medical devices, displays good esthetics appearance, high mechanical strength, high biocompatibility, and resistance to wear.[5]

Polyetheretherketone (PEEK) represents a modification of the main thermoplastic high-performance polymer group polyether aryl ketone. It is a high-temperature thermoplastic polymer, consisting of an aromatic backbone molecular chain, interconnected by ketone and ether function group.[6] PEEK as well as ZrO2 represent both very biocompatible materials and are used for several applications, for example, for dental implants, provisional abutments, and fixed dental prostheses.[3]

Recent publication reported that PEEK is a suitable material for double crown system.[7] A new concept could be the combination of these two biocompatible materials, for example, ZrO2 and PEEK to produce metal-free dental prosthesis and telescopic crowns.[3] Consequently, the aim of this study was to evaluate stresses transmitted to the dental implant in case of using different tooth-colored materials for telescopic attachment fabrication.

 Materials and Methods

Fabrication of clear acrylic resin model

An impression was made for adult edentulous mandibular stone cast using polyvinyl siloxane (PVS) materia (speedex coltenE, Switzerland). Molten base plate wax was poured into the PVS impression; then, after complete hardening of the wax model, it was flasked, and three identical clear acrylic resin models were fabricated.

Simulation of oral mucosa

Residual ridge and retromolar pad area of clear acrylic resin model were covered by base plate wax with 2 mm thickness. A plaster index was created over the model and extended to buccal and lingual area of the model. After the plaster index was set, the wax was removed, and the plaster index was painted with separating medium. The internal surface of the index was filled with auto-polymerized silicone material, and then, the index was repositioned on the model with firm holding by rubber band till complete polymerization of silicone soft liner coating then the excess was removed with sharp scalpel.[8]

Fabrication of implant placement guide template

Implant placement guide template was fabricated from clear acrylic resin with two guide holes in the canine region according to the technique of Kuniyal and Vakil.[9]

Installation of implants in clear acrylic resin models

Dental milling machine (milling unit BF2, Bredent, GmbH, and Co Senden, Germany) was used to drill two vertical holes through the guide template in canines region bilaterally. Two dummy implants (Dentium Seoul, South Korea) with diameter 4.5 mm and length 10 mm were installed, and dual abutment (Dentium, Seoul, South Korea) with diameter 4.5 mm and gingival height 1.5 mm were screwed to the dummy implants.

Study group

According to the material of fabrication of the resilient telescopic attachment, the models were categorized as follows: All ZrO2 telescopic group (zz): The primary and secondary telescopic crown was made from ZrO2, all PEEK telescopic group (pp): The primary and secondary crowns were made from PEEK and Zirconia-PEEK telescopic group (zp): The primary crown was made from ZrO2, and secondary telescopic was made from PEEK.

The models were scanned to gain three-dimensional virtual image for designing resilient telescopic attachment using computer-aided design/computer-aided manufacturing (CAM) technology followed by the separate scan for each implant abutment (Shera eco_scan3, Germany) [Figure 1] and [Figure 2]. The primary crowns were designed with common path of insertion and 5 mm height (the gingival 3 mm was paralleled, and the occlusal 2 mm was tapered 4 degrees) [Figure 3]. The same parameters were maintained for all the groups. The computer numeric control data were sent to the CAM system, and the primary crowns were milled from semi-sintered ZrO2 blanks (Zenostar MO 2, Wieland dental, ivoclar vivadent) by the milling machine (Zenotec select hybrid) for ZZ and ZP groups and PEEK blocks (Bredent BioHPP Peek, Germany) for PP group. Any edges or sharp corners were rounded, and the primary crowns were polished and cemented to the abutments using zinc phosphate cement (Spofa Dental, A Kerr company, Czech Republic). For secondary telescopic crown construction, the model with the outer surface of primary crowns was scanned followed by separate scan for each primary crown to improve the quality of data. Parameters used for designing secondary crowns were; parallel walls with minimal thickness of 0.5 mm and an occlusal space 0.3 mm was built between primary and secondary crowns [Figure 4]. Mechanical projections were added to each secondary crown to help in mechanical interlocking to the denture base. Data were transferred to CAM system for milling the secondary crowns from semi-sintered ZrO2 blanks (Zenostar MO 2, Wieland dental, ivoclar vivodent) (Zenotic select hybrid) for ZZ group and PEEK blocks (Bredent BioHpp Peek, Germany) for PP and ZP groups [Figure 5].{Figure 1}{Figure 2}{Figure 3}{Figure 4}{Figure 5}

Fabrication of acrylic resin mandibular overdentures

Three identical mandibular complete overdentures were constructed. The secondary telescopic crowns were mounted over the primary crowns on the clear acrylic resin model, and the mechanical projections were covered with wax as a spacer. Each clear acrylic resin model was duplicated using PVS impression material. Then, the impression was poured with dental stone. Then wax rim was made, and the occlusal plane was adjusted to the level between the half of the retromolar pad area on the stone cast. Semi-anatomical artificial teeth were set, and the mandibular denture was waxed up, flasked, cured then finished and polished. Venting holes were performed in the fitting surface through the lingual flanges. Secondary crowns were fitted over the primary ones in the correct path of insertion and then picked up to the intaglio surface of the overdenture using autopolymerized acrylic resin.

Stresses evaluation

Installation of strain gauges: Four strain gauges were installed on the buccal, lingual, mesial, and distal sides parallel for each implant according to the following steps: Auto-polymerized silicon soft liner material was removed about 5 mm from the area around the implants using a sharp scalpel. The buccal and lingual side of alveolar ridge corresponding to each implant was prepared to be flat leaving only 2 mm thickness of acrylic resin. Two vertical grooves were made mesial and distal to each implant extended from the crest of the ridge to reach halfway of the model. Small holes were drilled in the acrylic resin base corresponding to each strain gauge to allow the wires pass through it. Four strain gauges (Kyowa electronic instrument co., Tokyo, Japan) type KFG-1-120-C1-11 L1M2R,1 mm length, with gauge resistance 119.6 ± 0.4, gauge factor 2.13% ±1.0%, adoptable thermal expansion 11.7 ppm/°C and temperature coefficient of gauge factor + 0.08%/°C were bounded by adhesive mesial, distal, buccal, and lingual to each implant. The lead wires were marked with a code to identify them during measurements.

Stress-strain measurement

Universal testing machine (Lloyd instruments) was run in compression mode at a crosshead speed 0.5 mm/s. A gradual unilateral/bilateral load of 70N was applied. Bilateral loading was performed using bar that was fixed on the central area between the two first molars on both sides and then loading was applied on the center of bar [Figure 6]. When unilateral loading was applied the point of load, the application was selected at the site of the central occlusal fossa of the first molar [Figure 7]. This was done for reproducibility and accommodation of the tip of the loading pin in the same location. The loading protocol was repeated five times for each loading application. The eight-channel strain-meter was used to collect data from the eight strain gauges attached to the model. The strain data were collected at rate of 2 Hz (2 reading/s) by the aid of KYOWA PCD software (PCD-30A software (Kyowa Electronic Instruments Co., Ltd., Tokyo, Japan)). All the measurement was repeated five times for each loading impact. The mean average was calculated. These procedures were done for all groups, and the stress was calculated from the following equation. The obtained data were tabulated and subjected to statistical analysis.{Figure 6}{Figure 7}

Stress = strain × modulus of elasticity.

The modulus of elasticity for the acrylic resin is 2.6 GPa.

Statistical analysis

Shapiro–Wilk test was used to identify the normal distribution of data. The data were nonparametric and violate the normal distribution. The peri-implant stress was presented by the median values. Wilcoxon signed ranks test was used to compare loading and unloading sides. Kruskal–Wallis test was used to compare recorded stress values between different groups, followed by Mann–Whitney test for multiple comparisons. P value is statistically significant if it was [10]

The current in vitro study was conducted to evaluate stresses transmitted to two implants retaining mandibular complete overdenture with all ZrO2, all peek, and Zirconia-PEEK telescopic attachments. The results revealed that under both bilateral and unilateral loading (both loading and unloading sides) the group of all PEEK (PP) recorded the highest stress values followed by all ZrO2 group (ZZ). While the group of Zirconia primary crowns and PEEK secondary crowns (ZP) recorded the least stress values.

PEEK and ZrO2 belong to fundamentally different material chemistries. PEEK is a polymeric material, while ZrO2 is ceramic. Therefore, their performance will be even more dissimilar than would be suggested by the difference in properties. PEEK is a high-density thermoplastic polymer with a linear aromatic semi-crystalline structure that has exceptional physical and chemical properties as regards toughness, hardness, and elasticity. Furthermore, its low molecular weight, combined with the absence of metal, allows its use as an excellent biocompatible prosthetic denture material.[11] Zirconium oxide (ZrO2) that commonly known as ZrO2, was discovered by M. H. Klaproth (German chemist) in 1789. However, it was only introduced into the field of dentistry a few decades ago. It has since become a product of choice due to its superior properties. The phenomenon of transformation toughening of ZrO2 results in extraordinary bending and tensile strength, extremely high compact strength, in addition to its fracture and chemical resistance. For this reason, zirconium has been reported to have “self-repairing” properties thus preventing crack propagation. Moreover, it is a highly biocompatible material.[12],[13]

The higher stress values recorded by all PEEK group could be attributed to less flexure strength of PEEK (150–330 MPa) in comparison to ZrO2 (630–970 MPa) that may permit a limited movement within the attachment. This result is in accordance to a pervious study that was concerned with one treatment modality used for rehabilitation of mandibular Kennedy Class I cases, using two different materials, Co-Cr alloy and PEEK and their effect on the strain induced in the supporting structures for these telescopic-retained removable partial denture (RPD). Four primary copings made of metal and other four made of PEEK on abutment teeth (the first and second premolars bilaterally) where Co-Cr framework containing the secondary copings was constructed for metal primary copings and PEEK framework was constructed for PEEK ones.[14] The findings proved that PEEK telescopic retained RPD resulted in statistically significant higher strain in most of the channels when compared to Co-Cr alloy one. They explained this result by the much higher Young's modulus of Co-Cr alloy (220–230 GPa)[15] compared to that of the PEEK (3–4 GPa)[16] and concluded that Co-Cr alloy telescopic-retained RPD could still be considered a better choice for rehabilitation of the Kennedy Class I partial edentulous situations compared to PEEK one, where Co-Cr generated less stresses to the denture-supporting structures (the residual alveolar ridges and abutments).

The finding that the group of Zirconia primary crowns and PEEK secondary crowns recorded the least stress values can be explained by the cushioning effect offered by PEEK secondary coping in combination to the more harder ZrO2 primary one. A recent research evaluated the clinical and radiographic outcome of ZrO2 implants with PEEK restorations and concluded that PEEK restorations are a valid and alternative recommendation when using ZrO2 implants because of their cushioning effect and elastic modulus, which absorb occlusal forces and wear like a natural tooth, which could optimize and preserve osseointegration with time.[17] Moreover, stress transmitted to dental implants in this study regarding ZP group appeared to be comparable to that measured by Hegazy et al.,[18] who evaluated the stresses applied to implants through resilient telescopic attachment that was fabricated from metallic primary and secondary crowns, in contrast, PP and ZZ groups recorded more higher stress values. Regarding the higher stresses recorded with ZZ group, ZrO2 is three times harder (1200 HV), and its resistance to bending is 1400 Mpa.[19] Consequently, the combination of primary and secondary copings of ZrO2 resulted in direct transmission of chewing forces to the implants.


Within the limitation of this in vitro study it could be concluded that: Telescopic attachments that fabricated from ZrO2 primary crown and PEEK secondary crown is a promising attachment regarding reduction of stresses transmitted to the implants retaining mandibular complete overdenture.


According to results of this study, it is recommended to perform more laboratory studies to evaluate retention and wear behaviour of ZZ, ZP and PP telescopic attachments in parallel to in vivo studies to evaluate the clinical and radiographic outcome of these attachments.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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