04 May 2021

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Posterior Fixed Partial Dentures: how Polymeric Restorative Materials influence stress distribution

Giulia Palandrani

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Interim dental restorations are used during prosthetic rehabilitation in order to provide function, phonetics and aesthetics to the patient during a short period of the treatment while the final restoration is not finished. 

For many decades, the widely used material to manufacture the temporary restorations was acrylic resin ( AC) polymethyl methacrylate (PMMA), this material has some clinical disadvantages, such as low color stability and variable mechanical properties that depend on the handling conditions.

In the late-1990s, composite resins   (RC) were introduced in dentistry, and currently, they are widely used to manufacture temporary restorations and final restorations. composite resins contain fillers in their composition, presenting superior strength and color stability.

Another material that can be used to perform polymeric restorations is the polyetherether-ketone (PEEK), a semicrystalline linear aromatic polyacrylic polymer. In dentistry, this material is widely used as a temporary abutment for implants during prosthetic rehabilitation. However, it still presents some limitations regarding adhesion to the cement due to its low surface energy. In addition, this material can be used as an alternative for metal alloys  and PMMA for CAD/CAM restorations.

Materials and methods

A 3D model of partial right jaw from São Paulo State University database was selected in stereolithography file and exported to the CAD software. The abutment teeth (first molar and first premolar) were modeled using the BioCAD protocol containing 6 degrees of axial wall reduction with 1.5 mm of thickness, 1.5 mm of occlusal reduction and a shoulder finishing line. The connector presented a rounded shape, and the area was 4.2 mm2 for both abutment teeth. The cement layer was also modeled, with 100 μm thickness between the restoration’s intaglio surface and the teeth adhesive surface.

A static structural analysis was performed in the computer-aided engineering software, and the Maximum Principal Stress criterion was used to analyze the prosthesis and the cement layers of both abutment teeth. The materials were considered isotropic, linearly elastic, homogeneous and with bonded contacts. An axial load (600 N) was applied to the occlusal surface of the second premolar. 

Results and discussion

Regarding the FDP’s behavior, it was possible to observe that the region of prosthetic connectors concentrated higher stress magnitude regardless the restorative material. 

The molar’s connector showed a higher stress concentration than the premolar’s connector, regardless the restorative material. The highest stress peak was observed with the use of RC (129 MPa) compared to PEEK and AR.  The present study suggests that the acrylic resin should be used to reduce the stress magnitude in the connector region, thus preventing the possibility of a catastrophic failure. 

For the cement layer, the composite resins showed a promising behavior with the lowest stress concentration, followed by PEEK and then acrylic resin. In particular RC showed the lowest values in the occlusal region (7 MPa) and the highest values for the cervical margin (14 MPa) compared to PEEK (21 and 12 MPa) and AR (21 and 13 MPa). 

Conclusions

For CAD/CAM interim fixed partial dentures, the use of composite resins as a restorative material reduced the stress concentration in the cement layer, suggesting a beneficial response in the adhesive interface. The acrylic resin reduced the stress in the connector region, reducing the catastrophic failure possibility. PEEK presents an intermediate behavior between acrylic resin and composite resins mechanical response. 

Information: Influence of Polymeric Restorative Materials on the Stress Distribution in Posterior Fixed Partial Dentures: 3D Finite Element