Breaking load: 4-unit bridges made of PEEK with one cantilever link

Investigation of the breaking load of differently veneered implant-supported 4-unit bridges made of PEEK with a free end pontic

Danka Micovic Soldierovic, Munich

To date, metal ceramics and zirconium oxide are the most frequently used materials for implant-supported fixed dentures. Due to the weak points of both materials (impaired aesthetics, allergies to metal or chipping and delamination of the veneering ceramic) and an increasing tendency to use metal-free restorations, thermoplastic materials (e.g. PEEK) are becoming an interesting option in implant-supported prosthetics.

A relatively new approach in this indication area is the use of the material class Polyaryl ether ketones (PAEK), which includes a variety of thermoplastics: polyether ether ketone (PEEK), polyether ketone ketone (PEKK) and aryl ketone polymer (AKP). PEEK is the most commonly used material in the PAEK family. It has excellent mechanical properties as well as high biocompatibility and chemical stability. Additionally, the elastic modulus of PEEK is similar to that of human bone (3-4 GPa), making PEEK restorations advantageous due to their cushioning and stress-reducing effects. Due to the above characteristics, PEEK is suitable for a wide range of indications, including frameworks for fixed and removable dentures, brackets for removable dentures, bite splints, temporary restorations, implant abutments, etc.

The various PEEK materials differ in their filler content, which is between 10-30% and influences the mechanical properties of the material. Since PEEK is an opaque (white to grayish or gingiva-colored) material that is not suitable for monolithic restorations, PEEK frameworks are usually veneered with composite materials using various techniques. The aim of this study was to determine the stability of differently veneered implant-supported PEEK bridges with a free end.

The 4-unit bridges with a cantilever link were made from two different framework materials (PEEK):

  • 1. milled PEEK frameworks (N=60), approx. 20% TiO2-Fillers and
  • 2. pressed PEEK frameworks (N=60), approx. 30% TiO2- Ink pen,

and three different veneering techniques:

  • a) conventional veneers (n=20/material),
  • b) digital veneers (n=20/ material) and
  • c) prefabricated veneers (n=20/material),

manufactured. The breaking load was examined before and after thermo-mechanical aging.

120 implant-supported 4-unit bridges were made from PEEK with a cantilever from the first premolar to the second molar. Half of the frameworks were milled from BioHPP blanks (bredent, Germany) using computer-aided design (CAD) and computer-aided manufacturing technology (CAM) (Ceramill Motion 2, Amman Girrbach, Austria) (Fig. 1).

The other half of the PEEK framework was pressed (for 2 press, bredent) (Fig. 2), divested and prepared for veneering.

Implants placed in the first premolar and first molar positions served as abutments, with the second premolar acting as a bridge pontic and a cantilever phalanx extending into the second molar area.

In order to achieve congruence in the shape of the different veneers, a master restoration was made with prefabricated veneers (Visio.lign, bredent). All other test specimens were veneered after this fully anatomical master restoration (Fig. 3).

To design the digital veneer, the master restoration and the framework were scanned (Ceramill Map 400, Amann Girrbach). After subtraction in the CAD software (Ceramill Mind, Amann Girrbach), the obtained stl. file was positioned in a bre.CAM.HIPC blank (bredent) and then milled.

The prefabricated veneers had to be individually adapted to the master restoration before bonding. For this purpose, a silicone key was made according to the master restoration and each veneer was ground manually using the silicone key as a guide. When bonding the pretreated veneers to the frameworks with a dual-curing luting composite (combo.lign, bredent), a transparent silicone mold was used to secure the position.

For conventional veneering, a transparent silicone mold was filled with veneering composite (crea.lign, bredent), pressed onto the framework (PEEK) and polymerized for 180s (bre.Lux PowerUnit 2, bredent). All bridges were then polished to a high shine and bonded to the titanium abutments following a specific bonding protocol.

Aging, breaking load measurements and fracture type analyses

Half of each subgroup (per framework material and veneering technique) was initially examined, while the other half underwent artificial aging in a chewing simulator (mechanical cycles: 1.200.000x, 50 N; thermal cycles: 6.000x, 5/55°C). Custom-made cobalt-chromium-molybdenum antagonists were used to apply force to each unit of the 4-unit bridges during mastication simulation and failure load measurements.

The breaking load measurements were carried out in a universal testing machine (Zwick 1445, Zwick/Roell, Ulm, Germany). The vertical force was applied at a crosshead speed of 1 mm/min. A drop of 10% below the maximum load was considered failure. The fracture patterns were then analyzed using a digital microscope (Keyence VHX-970F, Keyence, Osaka, Japan) (Fig. 4).

The veneering technology and the filler content of the PEEK material influenced the breaking load. Prefabricated veneers had higher breaking load values ​​compared to digital and conventional veneers, while digital and conventional veneers were in the same range of values. Regarding filler content, PEEK with 30% filler content had higher breaking load values ​​than PEEK with 20% filler content (Fig. 5). Thermomechanical aging had no influence on the breaking load.

The fracture types were divided into total (both framework and veneer material fractured), cohesive (fracture within the veneer material) and adhesive fractures (fracture between the framework and veneer) (Fig. 4). There were no significant differences between groups. No correlation could be found between the fracture types and the fracture load.

  • 1. All tested 4-unit PEEK bridges survived the chewing simulation and showed higher breaking load values ​​than the expected maximum chewing forces in the posterior region of up to 900 N.
  • 2. Artificial aging had no influence on the stability of the implant-supported 4-unit PEEK bridges.
  • 3. The use of the material PEEK with a higher proportion of TiO2-Fillers could improve the mechanical stability of the restoration.
  • 4. The veneering technique has a major influence on the long-term stability of implant-supported 4-unit bridges made of PEEK. The appropriate veneering method can improve the longevity of two-layer structures.

The results presented here are based on the following study:

Micovic Soldierovic D, Liebermann A, Huth K, Stawarczyk B. Fracture load of different veneered and implant-supported 4-unit PEEK fixed dental prostheses with a free-end unit. J Mech Behav Biomed Mater. 2022;129:105173. doi: 10.1016/j.jmbbm.2022.105173.

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