Strength and flexibility of lithium disilicate bonded to polyetherketoneketone

Abstract

Statement of problem: Polyetherketoneketone (PEKK) is a high-performance polymer gaining popularity in dentistry for the fabrication of crowns, fixed partial dentures, removable partial denture frameworks, and frameworks for implant-supported fixed complete dentures. Despite a lack of performance data, lithium disilicate crowns have been bonded to retentive elements in PEKK frameworks.

Purpose: The purpose of this in vitro study was to compare the bond strengths and flexibility of lithium disilicate to PEKK or zirconia.

Material and methods: Forty-five PEKK, 15 zirconia, and 60 lithium disilicate beam-shaped specimens (12.5×2×2 mm) were fabricated. The ends of the PEKK beams were subjected to 3 different surface treatments before the application and light polymerization of a primer: 50-μm aluminum oxide airborne-particle abrasion, nonthermal air plasma, and argon-oxygen plasma. The zirconia specimen bonding surfaces were prepared with 50-μm aluminum oxide airborne-particle abrasion and the application of primer. Lithium disilicate specimens were etched with 4.5% hydrofluoric acid, and primer was applied. The lithium disilicate specimens were luted with an adhesive resin cement to the PEKK and zirconia specimens by using light-activated and chemically activated polymerization. Fifteen monolithic specimens of PEKK, lithium disilicate, and zirconia (25×2×2 mm) were also fabricated. All specimens were incubated overnight in 100% humidity before testing. Bonded and monolithic specimens were loaded in a universal testing machine, and 4-point bend tests were conducted until failure (n=15). The flexural modulus and strength were calculated and statistically analyzed with 1-way analysis of variance and Student-Newman-Keuls post hoc tests (α=.05).

Results: All bonded specimens failed at the adhesive interface. The zirconia-lithium disilicate bond strength was approximately twice that of the strongest group of PEKK (airborne-particle abrasion group) bonded to lithium disilicate (42 ±12 MPa and 24 ±13 MPa, respectively) and was approximately 9 times more rigid (71 ±19 GPa and 8 ±2 GPa, respectively). Monolithic PEKK fractured at 238 ±22 MPa, monolithic zirconia at 771 ±128 MPa, and monolithic lithium disilicate at 173 ±26 MPa. Monolithic PEKK was approximately 30 times more flexible than monolithic zirconia (6 ±1 GPa and 178 ±16 GPa, respectively). All values were statistically significantly different (P<.05), except for the bond strength between lithium disilicate and PEKK treated with airborne-particle abrasion and nonthermal air plasma and the flexural moduli of PEKK to lithium disilicate.

Conclusions: Bond strength between PEKK and lithium disilicate was significantly weaker than that between zirconia and lithium disilicate. Monolithic PEKK was significantly more flexible than monolithic zirconia.