Machinability of high-strength dental polymers and their performance as framework materials for all-on-four prostheses
Abdallah, Ali J.
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OBJECTIVES: To assess the viability of using high-strength polymers as framework materials for full arch implant-supported fixed prostheses, veneered with full-coverage restorations of different materials. The machinability, mechanical performance, and damping capacity of the polymer-based materials was of interest. METHODS: The two framework polymers – a polyetheretherketone (JUVORA™ Dental Disk, Juvora) (PEEK) and a fiber-reinforced composite (TRINIA™ CAD/CAM Disk, Trinia) (TR) – were characterized with Fourier-Transform Infrared (FTIR) Spectroscopy and energy-dispersive X-ray spectroscopy (EDS). Phase 1 consisted of a machinability study involving the merlon fracture test, which tested the milling success of PEEK and TR at 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, and 0.5 mm. 10 four-walled merlons of each thickness and material were milled out of CAD/CAM Disks (n = 100 merlons, n = 400 walls) using a 5-axis milling machine, inLab MC X5 (Dentsply Sirona, Germany). Milling success rate, fracture height, fracture length, fracture position, fracture direction, and chipping factor were assessed. In phase 2, 20 bars of dimensions 3.3 mm x 10 mm x 40 mm were milled from each of the two framework materials, PEEK and TR, and two veneer materials – a composite resin material (Shofu Disk HC, Shofu, Inc., Kyoto, Japan) (COM), and a high-translucency 3 mol% yttria-stabilized tetragonal zirconia polycrystal material (Cercon® ht, Dentsply Sirona, Bensheim, Germany) (ZR). Framework and veneer bars were bonded to each other in 4 framework/veneer combinations of 10 bilayers each: PEEK/COM (PCB), PEEK/ZR (PZB), TR/COM (TCB), and TR/ZR (TZB). Bilayer bars were loaded to failure in a 3-point bending test. Failure load, biaxial flexural strength, failure pattern and failure mode were documented. In Phase 3, 10 full arch fixed implant-supported frameworks were designed and fabricated in TR material over an epoxy resin model containing 4 implants in the second premolar and lateral incisor positions. 5 frameworks were veneered by COM in the canine to first molar region, while the other 5 were veneered by ZR. Four loading sites were designated per prosthesis in the occlusal surface of the first molars and the first premolars. Prostheses were loaded at the four occlusal sites in 5 cycles of loading and unloading. The damping capacity of the prostheses was calculated based on energy absorbed during loading and unloading. Displacement and permanent deformation values of the prosthesis structures were obtained from the load-displacement data. Prostheses were loaded to failure at the same sites, and failure load and failure mode were observed. RESULTS: The minimum machined thickness of PEEK and TR was 0.5 mm. There was no significant difference between milling success of PEEK and TR, but cumulative success rate was slightly superior in PEEK. PEEK exhibited a ductile response to machining damage, while TR showed a more brittle response. Chipping factor of PEEK was higher than TR eightfold, meaning TR showed an improved marginal integrity at 0.5 mm. Both materials showed concerning signs of machining damage with the milling parameters and tools used in this study. Bilayer bars with a TR framework withstood significantly higher loads at failure compared to bilayers with a PEEK framework. Bilayer bars with a ZR veneer withstood significantly higher loads at failure compared to bilayers with a COM veneer. The biaxial flexural strength of the four framework/veneer combinations could not be compared due to the occurrence of delamination in 3 of the 4 groups. The PZB group was the only group with fracture of both the veneer and framework without any delamination and exhibited a mean biaxial flexural strength of 46.15 ± 5.76 MPa. None of the bilayer bars with a TR framework exhibited framework fracture. In delaminated specimens, bilayer bars with a TR framework exhibited mixed adhesive-cohesive failure on both layers, while bilayer bars with a PEEK framework exhibited purely adhesive failure on the PEEK-cement interface. Full arch implant prostheses with a TR framework demonstrated elastic hysteresis in continuous cycles of cyclic loading, which is evidence of viscoelastic damping. Significantly higher energy absorption was observed in prostheses veneered with COM compared to ZR. Energy absorption decreased with increasing cycles of loading-unloading. Significantly higher maximum displacement was observed in prostheses veneered with COM compared to ZR, and in cantilever support compared to bounded support. Maximum displacement was inversely related to the thickness of the veneer and framework materials. Permanent deformation of the prosthesis was negligible after 10 cycles. The failure pattern of all prostheses presented as fracture in the veneer only and partial delamination of the veneer with mixed adhesive-cohesive failure mode. The mean failure load at ZR-veneered bounded sites was significantly higher than that of COM-veneered bounded sites. The mean failure load at bounded loading sites was significantly higher than that of cantilever loading sites. ZR-veneered prostheses demonstrated failure load values above 1000 N at all sites. CONCLUSION: The merlon fracture test is well-complemented by several quantitative and qualitative measures to assess the machinability of materials. Optimized tools and parameters for milling PEEK and TR should be investigated. Full arch implant prostheses with TR framework and ZR veneer are a viable option for fixed implant rehabilitation demonstrating damping capacity, adequate failure load values, and easy repairability.