Fracture load of differently veneered implant-supported 4-unit PEEK fixed dental prostheses with a free-end unit
Danka Micovic Soldatovic, München
Until today, the most commonly used materials for implant-supported fixed dental prostheses (FDPs) are metal-ceramic and zirconia. Due to shortcomings of both materials (compromised esthetics, allergies on metal or chipping and delamination of the veneering ceramic) and a growing tendency to use metal-free restorations, thermoplastic materials are becoming an interesting option in implant-supported prosthodontics.
A relatively new approach in this indication area is the use of the polyaryletherketone (PAEK) material class which includes a variety of thermoplastics: polyetheretherketone (PEEK), polyetherketoneketone (PEKK) and arylketonpolymer (AKP).
PEEK is the most commonly used material of the PAEK family. It has excellent mechanical properties and a high biocompatibility and chemical stability. Moreover, the modulus of elasticity of PEEK is similar to human bone (3-4 GPa) which makes PEEK restorations advantageous due to its damping and stress reducing effect.
The above mentioned characteristics make PEEK suitable for a wide range of indications, including frameworks for fixed and removable dental prostheses (RDPs), clasps for RDPs, occlusal splints, provisional restorations, implant abutments, etc.
The different PEEK materials
The different PEEK materials vary in terms of their filler content, which ranges between 10-30% and affect the mechanical characteristics of the material.
As PEEK is an opaque (white to greyish or gingiva colored) material that is therefore not indicated for monolithic restorations, PEEK frameworks are usually veneered using composite materials in different techniques.
The objective of this investigation was to determine the stability of differently veneered implant-supported cantilever PEEK FDPs.
Materials and methods
The fracture load of FDPs fabricated from two different framework materials:
milled PEEK frameworks (N=60), ca. 20% TiO2 fillers and
pressed PEEK frameworks (N=60), ca. 30% TiO2 fillers,
and three different veneering techniques:
- a) conventional veneerings (n=20/material),
- b) digital veneerings (n=20/material), and
- c) prefabricated veneers (n=20/material),
120 implant-supported 4-unit cantilever FDPs ranging from the first premolar to the second molar were manufactured. Half of the frameworks were milled from BioHPP blanks (bredent, Germany) employing computer-aided design (CAD) and computer-aided manufacturing (CAM) technology (Ceramill Motion 2, Amman Girrbach, Austria) (Figure 1).
The other half of frameworks was produced by pressing (for 2 press, bredent) (Figure 2), outbedded and prepared for veneering.
Implants placed in the position of the first premolar and first molar served as abutments, with the second premolar acting as a pontic and a free-end unit extending into the region of the second molar.
To achieve a congruency in the shape of the different veneerings, the master FDP was made using prefabricated veneers (Visio.lign, bredent). All other specimens were veneered according to this full-anatomy master FDP (Figure 3).
Figure 3: Veneering process (from the bottom: milled framework, applied visio.link layer, applied opaquer layer and fully veneered FDP).
In order to design the digital veneering, the master FDP and framework were scanned (Ceramill Map 400, Amann Girrbach). After subtraction in the CAD software (Ceramill Mind, Amann Girrbach), the obtained stl.file was nested in a bre.CAM.HIPC blank (bredent) and subsequently milled.
The prefabricated veneers had to be individualised to match the master FDP before bonding. To that purpose, a silicon key was made according to the master FDP and each veneer was manually ground using the silicon key as a guide. Transparent silicone mould was used as a position lock when bonding the pretreated veneers to the frameworks using a dual-curing luting resin composite (combo.lign, bredent).
For the conventional veneering, a transparent silicon mould was filled with veneering resin composite (crea.lign, bredent), pressed onto the framework and polymerized for 180s (bre.Lux PowerUnit 2, bredent).
Afterwards, all FDPs were high-gloss polished and adhesively bonded to the titanium abutments following a specific bonding protocol.
Aging, fracture load measurements and fracture type analyses
Half of each subgroup (per framework material and veneering technique) was examined initially and the other half underwent artificial aging in a chewing simulator (mechanical cycles:1,200,000x, 50 N; thermal cycles: 6,000x, 5/55°C). Individually made cobalt-chrome-molybdenum antagonists were used to apply force on each unit of the 4-unit FDPs during chewing simulation and fracture load measurements.
Fracture load measurements were performed in a universal testing machine (Zwick 1445, Zwick/Roell, Ulm, Germany). The vertical force was applied with a crosshead speed of 1 mm/min. A drop of 10% below the maximum load was considered as a failure. Thereafter, fracture patterns were analyzed employing digital microscopy (Keyence VHX-970F, Keyence, Osaka, Japan) (Figure 4).
The veneering technique and filler content of the PEEK material affected the fracture load. Prefabricated veneers showed higher fracture load values when compared with digital and conventional veneerings, whereas digital and conventional veneerings were in the same value range. Regarding the filler content, PEEK with 30% filler content presented higher fracture load values in comparison to PEEK with 20% filler content (Figure 5). Thermomechanical aging showed no effect on the fracture load.
Figure 5: Bar graph of fracture load [N] of different filled framework materials (20% and 30%) and veneering techniques.
Fracture types were classified as complete (both framework and veneering material fractured), cohesive (fracture within the veneering material), and adhesive fractures (fracture between the framework and veneering) (Figure 4). There were no significant differences between the groups. No correlation between fracture types and fracture load could be assessed.
- All tested FDPs survived the chewing simulation and showed higher fracture load values than the expected maximum bite forces in the posterior region of up to 900 N.
- Artificial aging showed no impact on the stability of implant-supported 4-unit PEEK FDPs.
- Using the PEEK material with a higher percentage of TiO2 fillers could improve the mechanical stability of the restoration.
- The veneering technique has a high impact on the long-term stability of implant-supported 4-unit PEEK FDPs. The appropriate veneering method can improve the longevity of bi-layered structures.